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Jerry Steiger Jerry Steiger is offline
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Default Simple Audio Test Chamber

It's been quite a while since I stopped following this group. I stopped
being the recorder for the local youth symphony and ran out of time to keep
up with audio matters, bu now I want to build a small audio test chamber for
work. I'm hoping the good folks here can help me a bit.

We make rugged hand-held computers for outdoor use. Our products are all
water proof. One of the challenges we always have is getting good speaker
output and microphone input. Good for the speaker means easily heard and
understood in somewhat noisy environments, meaning we need something like
80-90 dB SPL output. Good for the microphone means that we can record easily
intelligible voice input. Both are at a distance of a foot or two or three
with a frequency range of about 300 to 3400 Hz.

In the past we have tested speakers and microphones very simplistically,
using a sound level meter to monitor speaker output in a normal office
environment, but we would like to get a little more professional about it.
We had a summer student do some preliminary work, so we now have a reference
speaker (Event Tuned Reference 8) and microphone (Audio-Technica AT2020), an
Alesis RA 150 amplifier, an Edirol UA125 USB Audio Capture System, a small
box to link them all together, and some Matlab programs on the computer to
drive the system and plot the data.

I was thinking that it would be best to have a small enclosure in which I
could mount either a speaker (either the reference speaker or a test speaker
or one of our hand-held units acting as a speaker) and a microphone (either
the reference microphone or a test microphone or one of our hand-held units
acting as a microphone). I would line the inside of the enclosure with sound
absorbing panels so that we would be measuring mostly the direct response of
the speaker or the microphone, since that is what we would mostly be getting
in our outdoor environment. Am I on the right track? Is this something
reasonably easy for me to do?

How big should the chamber be? I was thinking of making the inside
dimensions 25"x59"x96". I chose those dimensions so I wouldn't have any
double or triple resonances but could use standard sizes of sound absorbing
panels. How much effort should I put into making the side dimensions to
spread the resonances? How close together can resonances be before they
become too problematic?

I would line the walls with 4" thick 24"x48" cotton panels filling most of
the sides and ends and 12"x12" foam corners on two of the long edges to
cover the gaps. I was thinking that I would put a hole in the long side
roughly 3 feet from one end that I could put removable plates in that hold
the reference speaker or the speaker under test. I would put another hole
and plate one meter from that hole on which I could mount my reference
microphone or the microphone under test. That would put my microphone
roughly 2 feet from the other end. Is that a reasonable way to position the
pieces? Should I fill the space behind the speaker and microphone with more
sound absorbing material? Would stacking sections of the pads work? Would
stacking the pads on the sides to leave just a narrow space a bit bigger
than our hand-held be worthwhile?

Would 1" MDF be stiff enough for the walls? Would I need to stiffen it with
some outside ribs? I can readily calculate the resonant modes of rectangular
plates. Do I need to worry much about the resonances? How many modes would I
need to look at?

Will the cotton panels (B.A.P. Bass Buster from www.acousticalsurfaces.com)
absorb enough sound to make this very roughly like an anechoic chamber at
the frequencies of interest? I don't need super precision, but I would like
to have measurements that correlate fairly well with what the user would
hear in actual use.



Jerry Steiger


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Chris Hornbeck Chris Hornbeck is offline
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Default Simple Audio Test Chamber

On Wed, 25 Jun 2008 21:15:10 -0700, "Jerry Steiger"
wrote:

It's been quite a while since I stopped following this group. I stopped
being the recorder for the local youth symphony and ran out of time to keep
up with audio matters, bu now I want to build a small audio test chamber for
work. I'm hoping the good folks here can help me a bit.

We make rugged hand-held computers for outdoor use. Our products are all
water proof. One of the challenges we always have is getting good speaker
output and microphone input. Good for the speaker means easily heard and
understood in somewhat noisy environments, meaning we need something like
80-90 dB SPL output. Good for the microphone means that we can record easily
intelligible voice input. Both are at a distance of a foot or two or three
with a frequency range of about 300 to 3400 Hz.

In the past we have tested speakers and microphones very simplistically,
using a sound level meter to monitor speaker output in a normal office
environment, but we would like to get a little more professional about it.
We had a summer student do some preliminary work, so we now have a reference
speaker (Event Tuned Reference 8) and microphone (Audio-Technica AT2020), an
Alesis RA 150 amplifier, an Edirol UA125 USB Audio Capture System, a small
box to link them all together, and some Matlab programs on the computer to
drive the system and plot the data.

I was thinking that it would be best to have a small enclosure in which I
could mount either a speaker (either the reference speaker or a test speaker
or one of our hand-held units acting as a speaker) and a microphone (either
the reference microphone or a test microphone or one of our hand-held units
acting as a microphone). I would line the inside of the enclosure with sound
absorbing panels so that we would be measuring mostly the direct response of
the speaker or the microphone, since that is what we would mostly be getting
in our outdoor environment. Am I on the right track? Is this something
reasonably easy for me to do?

How big should the chamber be? I was thinking of making the inside
dimensions 25"x59"x96". I chose those dimensions so I wouldn't have any
double or triple resonances but could use standard sizes of sound absorbing
panels. How much effort should I put into making the side dimensions to
spread the resonances? How close together can resonances be before they
become too problematic?

I would line the walls with 4" thick 24"x48" cotton panels filling most of
the sides and ends and 12"x12" foam corners on two of the long edges to
cover the gaps. I was thinking that I would put a hole in the long side
roughly 3 feet from one end that I could put removable plates in that hold
the reference speaker or the speaker under test. I would put another hole
and plate one meter from that hole on which I could mount my reference
microphone or the microphone under test. That would put my microphone
roughly 2 feet from the other end. Is that a reasonable way to position the
pieces? Should I fill the space behind the speaker and microphone with more
sound absorbing material? Would stacking sections of the pads work? Would
stacking the pads on the sides to leave just a narrow space a bit bigger
than our hand-held be worthwhile?

Would 1" MDF be stiff enough for the walls? Would I need to stiffen it with
some outside ribs? I can readily calculate the resonant modes of rectangular
plates. Do I need to worry much about the resonances? How many modes would I
need to look at?

Will the cotton panels (B.A.P. Bass Buster from www.acousticalsurfaces.com)
absorb enough sound to make this very roughly like an anechoic chamber at
the frequencies of interest? I don't need super precision, but I would like
to have measurements that correlate fairly well with what the user would
hear in actual use.


You're going to read much more useful and generally more positive
comments than my current one, so please forgive me if I jump into
the cold water with some of the "gotcha's":

First, just as a matter of good old Science, you'll need *something*
to act as a reference, if you really want to investigate.

Could be a reference speaker, or could be a reference mic, but neither
is cheap - and worse, neither is a "primary" reference. Without a
reference, everything you measure is only relative to ... something
else. Does that make sense?

Primary references tend to be impulse noise generators: pop a balloon
(not actually too wonderful), starter pistol (get your insurance in
order first), or Don Pierce's spark gap with camera flash discharge
(see the r.a.p thread - top recommendation, very elegant).


Second, if your box is dead enough, you don't care about dimensions.
If it isn't dead enough, you don't care about its results.

Third and lastest, above 300 Hz, measurements today are usually done
in a live room with a time window. Because impulses are challenging
dynamically - the ideal is an infint(insert some appropriate vowel)lly
short time of infinit(same damn vowel)lly large amplitude - judgement
calls must be made. Hence the strong recommendation to research
Don Pierce's recent posts, here on r.a.p IIRC.

Commercial implementations allow for a noise source with the
mathematical equivalent of an impulse, called an "MLS" (a
"maximum length sequence", not so helpfully named...) The
resulting hardware devices are generically called Melissa,
after my sister.

Fourth and post-mortem, it's voice range, so why do you care? Arf!

All the best fortune,
Chris Hornbeck
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Scott Dorsey Scott Dorsey is offline
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Jerry Steiger wrote:
I was thinking that it would be best to have a small enclosure in which I
could mount either a speaker (either the reference speaker or a test speaker
or one of our hand-held units acting as a speaker) and a microphone (either
the reference microphone or a test microphone or one of our hand-held units
acting as a microphone). I would line the inside of the enclosure with sound
absorbing panels so that we would be measuring mostly the direct response of
the speaker or the microphone, since that is what we would mostly be getting
in our outdoor environment. Am I on the right track? Is this something
reasonably easy for me to do?


It's easy to do if you only care about high frequencies. The hard part is
the door.

The lower the frequency of interest, the larger the chamber has to be.

Take a look at the IAC catalogue. They make portable chambers in all
varying sizes from very small to very large. You can try copying their
designs.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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hank alrich hank alrich is offline
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Default Simple Audio Test Chamber

Chris Hornbeck wrote:

Fourth and post-mortem, it's voice range, so why do you care? Arf!


There must be something critical about the communication link
represented by the laptop. If that's the case, then why not a headset
instead of the computer's mic and speaker?

--
ha
Iraq is Arabic for Vietnam
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Tony[_11_] Tony[_11_] is offline
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On Thu, 26 Jun 2008 05:15:27 GMT, Chris Hornbeck wrote:
snip
Primary references tend to be impulse noise generators: pop a balloon
(not actually too wonderful), starter pistol (get your insurance in
order first), or Don Pierce's spark gap with camera flash discharge
(see the r.a.p thread - top recommendation, very elegant).

snip

Chris - I've wanted to do this myself for quite some time. Following Don't lead would seem
to be the safest way, but I've "cleaned up" a lot and can't find that thread. Could you
suggest a URL, or some keywords, or a time frame? Thanks, Tony


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Chris Hornbeck Chris Hornbeck is offline
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On Fri, 27 Jun 2008 09:53:38 +1000, Tony wrote:

Chris - I've wanted to do this myself for quite some time. Following Don's lead would seem
to be the safest way, but I've "cleaned up" a lot and can't find that thread. Could you
suggest a URL, or some keywords, or a time frame? Thanks, Tony


OK, found it:

http://81.174.169.10/odds/impulse/

and the thread started 03/18/2008 in r.a.p

All the best fortune,
Chris Hornbeck
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On Fri, 27 Jun 2008 00:18:55 GMT, Chris Hornbeck
wrote:

and the thread started 03/18/2008 in r.a.p


Oops. Title was "Impulse testing a microphone - the result".
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On Fri, 27 Jun 2008 00:21:55 GMT, Chris Hornbeck wrote:

On Fri, 27 Jun 2008 00:18:55 GMT, Chris Hornbeck
wrote:

and the thread started 03/18/2008 in r.a.p


Oops. Title was "Impulse testing a microphone - the result".


Hey Chris - thanks - I got it. Cheers, Tony
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Jerry Steiger Jerry Steiger is offline
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"Chris Hornbeck" wrote in message
...

You're going to read much more useful and generally more positive
comments than my current one, so please forgive me if I jump into
the cold water with some of the "gotcha's":


Your "gotcha's" are much appreciated! Sorry to be so slow to respond. Life
seems to be a little complicated right now.

First, just as a matter of good old Science, you'll need *something*
to act as a reference, if you really want to investigate.

Could be a reference speaker, or could be a reference mic, but neither
is cheap - and worse, neither is a "primary" reference. Without a
reference, everything you measure is only relative to ... something
else. Does that make sense?


Yes, but we don't need NIST traceable accuracy. I figure that a reasonably
good nearfield monitor and condensor mike like we've got are close enough.
We just want to know that one setup is really significantly better than
another.

Second, if your box is dead enough, you don't care about dimensions.
If it isn't dead enough, you don't care about its results.


I thought about that after I hit the send button. (Well, actually quite a
while after, while riding my bike to work the next morning.) I'm still
nervous about how much energy the sound absorbing pads are going to remove,
especially near the lower end of the sprectrum. The 4" Bass Buster pads
claim a Sound Transmission Loss using the ASTM E90 Test of .97 at 125Hz,
1.37 at 250Hz, 1.23 at 500Hz, 1.05 at 1KHz, 1.00 at 2KHz, 1.01 at 4KHz and
an NRC of 1.15, but I don't really know what that means for me.

Third and lastest, above 300 Hz, measurements today are usually done
in a live room with a time window.


We don't have a quiet room where I can get good data. I assume that the idea
would be to run multiple tests to reduce the S/N, but it seems like it would
be easier and quicker for me to build a relatively inexpensive chamber to
limit the effects of the outside environment.

Fourth and post-mortem, it's voice range, so why do you care? Arf!


Because voices are important! I don't get your point.

Thanks again,

Jerry Steiger


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Jerry Steiger Jerry Steiger is offline
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"hank alrich" wrote in message
...
Chris Hornbeck wrote:

Fourth and post-mortem, it's voice range, so why do you care? Arf!


There must be something critical about the communication link
represented by the laptop. If that's the case, then why not a headset
instead of the computer's mic and speaker?



It's not a laptop, it' more like a PDA or a smart phone on steroids. Just
fitting in a speaker is a problem, since we don't have near the volume and
area available that a laptop has. (We also don't have as much other stuff to
fit in either, but getting room for good audio performance is tough.)

Our customers can use a head set, but most of them don't want to. A lot of
them are surveyors. They don't like wired headsets because wires get snagged
by brush and they don't like wireless headsets because battery life is often
not good enough for a full days work. They work in the rain and either type
doesn't live very long in the wet. A good waterproof Bluetooth headset with
long battery life would be a nice accessory for us, but we don't have the
resources to design one ourselves and haven't found one from anyone else. If
you or anyone else has some leads I would love to hear about them.

Thanks!

Jerry Steiger




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Jerry Steiger Jerry Steiger is offline
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"Scott Dorsey" wrote in message
...
It's easy to do if you only care about high frequencies.


Since our little bitty speakers can't do much below 300Hz, I figured I was
getting into the range where it was doable.


The hard part is
the door.


I was planning to have small panels that I would mount with screws. That
will slow me down in testing, but this is something that we won't do all
that often or spend that much time on.

The lower the frequency of interest, the larger the chamber has to be.


Got any good rules of thumb or a likely source to find some? 300Hz means a
wavelength of 3.76 feet. Does the smallest dimension of my chamber need to
be greater than that or some multiple thereof?

Take a look at the IAC catalogue. They make portable chambers in all
varying sizes from very small to very large. You can try copying their
designs.


Thanks for the tip. I didn't find any that looked like a very good match for
what I am trying to do. They make a couple of small test chambers,the mac-1,
mac-2 and mac-3, but they only seem to be interested in isolating the device
under test from the outside, since they use sheet metal walls on the inside.

The mac-1 has roughly 2" insulation and they claim 23dB of reduction from
out to in at 250Hz, 29dB at 500Hz, and 36-38dB at 1KHz to 4KHz. The mac-2
has roughly 4" insulation and claims 40dB at 250, 45 at 500, and 52-57 at 1K
to 2K, so it does seem like my 4" cotton might be able to absorb quite a bit
of energy. More layers should be better, so maybe it isn't such a bad idea.
Or is it?

They don't seem to worry much about the "room modes". The mac-2 has inside
dimensions of 24x24x32 inches. Lots of reinforcing of nulls and peaks in
there!

I am thinking that Chris is correct, that if I get enough absorbtion at the
frequencies of interst, I don't need to worry much about the modes in the
chamber or the resonances in the walls. But can I get enough absorbtion?

Thanks!

Jerry Steiger



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Scott Dorsey Scott Dorsey is offline
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Jerry Steiger wrote:
"hank alrich" wrote in message
...
Chris Hornbeck wrote:

Fourth and post-mortem, it's voice range, so why do you care? Arf!


There must be something critical about the communication link
represented by the laptop. If that's the case, then why not a headset
instead of the computer's mic and speaker?



It's not a laptop, it' more like a PDA or a smart phone on steroids. Just
fitting in a speaker is a problem, since we don't have near the volume and
area available that a laptop has. (We also don't have as much other stuff to
fit in either, but getting room for good audio performance is tough.)


B&K makes an artificial ear system and an artificial mouth that are
specifically designed for testing these things. They try very hard to
simulate off-axis responses of actual human body parts. They aren't
cheap, but they are surprisingly effective.

Also, Nokia has done a lot of research into human body simulation for
testing cellphones. A literature search might bring some of that up.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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Jerry Steiger wrote:
"Scott Dorsey" wrote in message

Got any good rules of thumb or a likely source to find some? 300Hz means a
wavelength of 3.76 feet. Does the smallest dimension of my chamber need to
be greater than that or some multiple thereof?


Look at some of the room mode calculators on the web. If you can make it
large enough that all of the major room modes fall below the lowest
frequency of interest, that's a start.

Take a look at the IAC catalogue. They make portable chambers in all
varying sizes from very small to very large. You can try copying their
designs.


Thanks for the tip. I didn't find any that looked like a very good match for
what I am trying to do. They make a couple of small test chambers,the mac-1,
mac-2 and mac-3, but they only seem to be interested in isolating the device
under test from the outside, since they use sheet metal walls on the inside.


Right, I think you're going to need something much larger than any of these.

They don't seem to worry much about the "room modes". The mac-2 has inside
dimensions of 24x24x32 inches. Lots of reinforcing of nulls and peaks in
there!


Right, that's not the right tool for the job... look for the larger chambers.
You can get away without a walk-in but you are going to need something larger
than a fridge if you care about 300 Hz.

I am thinking that Chris is correct, that if I get enough absorbtion at the
frequencies of interst, I don't need to worry much about the modes in the
chamber or the resonances in the walls. But can I get enough absorbtion?


If you want absorption at 300 Hz, you need several feet of fibreglass. And
yes, that is the solution.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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"Scott Dorsey" wrote in message
...
B&K makes an artificial ear system and an artificial mouth that are
specifically designed for testing these things. They try very hard to
simulate off-axis responses of actual human body parts. They aren't
cheap, but they are surprisingly effective.


Sorry, my answer only confused things even more! Our products aren't used
like normal voice cell phones. It would be like Maxwell Smart using his shoe
phone. The smallest are about one pound, the largest over two pounds. So I
guess the first would be like Maxwell using 99's shoe and the second would
be like him using the gentle giant's shoe. We actually planned on putting a
speaker in a smaller one so it could be used like a normal cell phone but
decided it didn't make sense for most of our customers. If we ever did that,
we might need to look into the artificial ear and mouth. Certainly we would
need to do considerably more testing to meet the certification standards of
the cell phone companies.

The microphone is used to record field notes or possibly for voice commands.
The speaker is used mainly for notifications and the normal computer beeps
and squeaks. I suppose people could play music over it to (thats how we used
to test them), but there are much smaller systems that would sound much
better and most everybody already has one, just like cell phones.

Sometimes the unit is mounted on a tripod with surveyor's instrument. In
that case it is roughly three feet off the ground and three feet away from
the surveyor's head.Sometimes it is mounted a pole, putting it a little
farther from the ground and closer to the surveryor, say 4 feet up and 2
feet away. Sometimes it is just held in a persons hand, so it could be as
close as a foot and a half or so. They are always used with the display and
keyboard pointing fairly close to straight at the user, so off axis response
beyond 30 degrees or so isn't very important. I figured I would just measure
the response straight on and not worry about off axis response, although I
could easily rotate the units under test if it became important.

Thanks again!

Jerry Steiger


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On Tue, 1 Jul 2008 21:19:52 -0700, "Jerry Steiger"
wrote:

The microphone is used to record field notes or possibly for voice commands.
The speaker is used mainly for notifications and the normal computer beeps
and squeaks. I suppose people could play music over it to (thats how we used
to test them), but there are much smaller systems that would sound much
better and most everybody already has one, just like cell phones.

Sometimes the unit is mounted on a tripod with surveyor's instrument. In
that case it is roughly three feet off the ground and three feet away from
the surveyor's head.Sometimes it is mounted a pole, putting it a little
farther from the ground and closer to the surveryor, say 4 feet up and 2
feet away. Sometimes it is just held in a persons hand, so it could be as
close as a foot and a half or so.


Sounds like a cool project. So you really need to measure
voice intelligibility, and likely with a pretty serious
and unpredictable noise background?

I say this because I see surveyors all the time working
right beside (or occasionally, in) the roadway. Traffic
goes by with noise levels of what? 75dB SPL (C)? 90db?
Could be even more with the dump truck or Harley option.

Another way of saying this is that I really wonder if the
model of measuring the hi-fi quiet-background aspects of
the box is going to matter to you at all. Maybe so, but
my guess is only very, very indirectly.

Voice intelligibility is often a surprisingly non-hi-fi
product, with severe bandwidth restrictions, dynamic
range compressions, sometimes even deliberately added
distortion (!) parts of the final mix.

Wiser folks have already suggested that this is a very
specific question for a very unusual use, so I'm only
riffing on their ideas when I propose that you consider
connection to a Bluetooth headset. Folks walk around
with these things all the time already, pretending that
they've been assimilated. Just a thought.


All the best fortune,
Chris Hornbeck


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Scott Dorsey Scott Dorsey is offline
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Jerry Steiger wrote:
"Scott Dorsey" wrote in message
...
B&K makes an artificial ear system and an artificial mouth that are
specifically designed for testing these things. They try very hard to
simulate off-axis responses of actual human body parts. They aren't
cheap, but they are surprisingly effective.


Sorry, my answer only confused things even more! Our products aren't used
like normal voice cell phones. It would be like Maxwell Smart using his shoe
phone. The smallest are about one pound, the largest over two pounds. So I
guess the first would be like Maxwell using 99's shoe and the second would
be like him using the gentle giant's shoe. We actually planned on putting a
speaker in a smaller one so it could be used like a normal cell phone but
decided it didn't make sense for most of our customers. If we ever did that,
we might need to look into the artificial ear and mouth. Certainly we would
need to do considerably more testing to meet the certification standards of
the cell phone companies.


The artificial ear and mouth are just perfect for that sort of application;
they get used for walkie-talkies all the time, as well as headsets. But they
_are_ intended for close-talking applications.

The microphone is used to record field notes or possibly for voice commands.
The speaker is used mainly for notifications and the normal computer beeps
and squeaks. I suppose people could play music over it to (thats how we used
to test them), but there are much smaller systems that would sound much
better and most everybody already has one, just like cell phones.


The thing about the artificial mouth is that it will do a good job of
modelling someone talking into it, both in the correct manner and from
various different directions. If you need real intelligibility measurements
to show someone, this is critical. If you don't need objective measures
of intelligibility, and you can assume that folks will always be positioned
in the right place, you can just use a headphone driver for testing.

Sometimes the unit is mounted on a tripod with surveyor's instrument. In
that case it is roughly three feet off the ground and three feet away from
the surveyor's head.Sometimes it is mounted a pole, putting it a little
farther from the ground and closer to the surveryor, say 4 feet up and 2
feet away. Sometimes it is just held in a persons hand, so it could be as
close as a foot and a half or so. They are always used with the display and
keyboard pointing fairly close to straight at the user, so off axis response
beyond 30 degrees or so isn't very important. I figured I would just measure
the response straight on and not worry about off axis response, although I
could easily rotate the units under test if it became important.


If people are using the thing from three feet away, you need to be able to
measure response with a sound source three feet away. Because it may be
very different than the response up close, especially if you are using a
directional microphone.

My guess is that under typical surveying environment, at three feet away
from the mike the environmental noise problem is going to be severe. And
anything you do to make the mike more directional is going to make wind
noise problems worse.

Incidentally, let me commend the Gentex omni capsules to you. They are
very resistant to rain and mud. They also sell noise-cancelling mikes
for close-talking use, but that does not sound useful to you.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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"Jerry Steiger" wrote in message
. ..
It's been quite a while since I stopped following this group. I stopped
being the recorder for the local youth symphony and ran out of time to
keep up with audio matters, bu now I want to build a small audio test
chamber for work. I'm hoping the good folks here can help me a bit.

We make rugged hand-held computers for outdoor use. Our products are all
water proof. One of the challenges we always have is getting good speaker
output and microphone input. Good for the speaker means easily heard and
understood in somewhat noisy environments, meaning we need something like
80-90 dB SPL output. Good for the microphone means that we can record
easily intelligible voice input. Both are at a distance of a foot or two
or three with a frequency range of about 300 to 3400 Hz.

snip
I would like to have measurements that correlate fairly well with what
the user would hear in actual use.

Jerry Steiger


Following this thread I did my 'think my own thoughts' thing before
searching out state of the art info. I thought it would be a good thing to
have a standarised intelligibility teast. The audio equivalent of the
opticians sight chart. Unsurprisingly this has already been done with such
things as the STI test and there is equipment avaiable to take corresponding
measurements. eg

http://www.rksound.co.uk/goldline/go...ucts/dsp2b.htm

an interesting link that came up was a paper that also considered listener
comfort.

http://www.acoustics.org/press/141st/stefaniw.html

David F. Cox





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On Jul 2, 3:27*pm, "David F. Cox" wrote:
"Jerry Steiger" wrote in message

. ..





It's been quite a while since I stopped following this group. I stopped
being the recorder for the local youth symphony and ran out of time to
keep up with audio matters, bu now I want to build a small audio test
chamber for work. I'm hoping the good folks here can help me a bit.


We make rugged hand-held computers for outdoor use. Our products are all
water proof. *One of the challenges we always have is getting good speaker
output and microphone input. Good for the speaker means easily heard and
understood in somewhat noisy environments, meaning we need something like
80-90 dB SPL output. Good for the microphone means that we can record
easily intelligible voice input. Both are at a distance of a foot or two
or three with a frequency range of about 300 to 3400 Hz.


snip
*I would like to have measurements that correlate fairly well with what
the user would hear in actual use.


Jerry Steiger


Following this thread I did my 'think my own thoughts' thing before
searching out state of the art info. I thought it would be a good thing to
have a standarised intelligibility teast. The audio equivalent of the
opticians sight chart. Unsurprisingly this has already been done with such
things as the STI test and there is equipment avaiable to take corresponding
measurements. eg

*http://www.rksound.co.uk/goldline/go...ucts/dsp2b.htm

an interesting link that came up was a paper that also considered listener
comfort.

http://www.acoustics.org/press/141st/stefaniw.html

David F. Cox- Hide quoted text -

- Show quoted text -


While at Motorola a bought a smallish semi-custom box from these very
helpful folks.

http://www.gkacoustics.com/index.asp


I think it was ~ $1-2K. and ~2x 2x 3 ft . What ever it was fine for
300-4k (speech) cellphone & speakerphone/ accessories Quick pre
evaluation before a full B&K HATS artificial Mouth /ear in full
anachoic chamber. I used a "B word" ecm 8000 mic and M-Audio USB
interface.



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"Chris Hornbeck" wrote in message
...
Sounds like a cool project. So you really need to measure
voice intelligibility, and likely with a pretty serious
and unpredictable noise background?


That pretty well nails it.

I say this because I see surveyors all the time working
right beside (or occasionally, in) the roadway. Traffic
goes by with noise levels of what? 75dB SPL (C)? 90db?
Could be even more with the dump truck or Harley option.


Your guess is as good as mine, probably better. I wonder if I could talk one
of the software guys into writing a sound logging program that we could
supply to chosen customers.

Another way of saying this is that I really wonder if the
model of measuring the hi-fi quiet-background aspects of
the box is going to matter to you at all. Maybe so, but
my guess is only very, very indirectly.


One reason to make the system work well in a quieter environment is that
customers usually decide whether to buy us or the competition based on how
the products work in the store. Our customers also work in much quieter
environments.

Voice intelligibility is often a surprisingly non-hi-fi
product, with severe bandwidth restrictions, dynamic
range compressions, sometimes even deliberately added
distortion (!) parts of the final mix.


These are subtle elements we may be able to get into after we have a better
working knowledge of the basics. Right now we have a long ways to go.

Wiser folks have already suggested that this is a very
specific question for a very unusual use, so I'm only
riffing on their ideas when I propose that you consider
connection to a Bluetooth headset. Folks walk around
with these things all the time already, pretending that
they've been assimilated. Just a thought.


Use with a Bluetooth headsets is in the product plan, but marketing still
wants a built-in microphone and speaker that works better than the ones on
our existing products.

Thanks again!

Jerry Steiger


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"Scott Dorsey" wrote in message
...
The thing about the artificial mouth is that it will do a good job of
modelling someone talking into it, both in the correct manner and from
various different directions. If you need real intelligibility
measurements
to show someone, this is critical. If you don't need objective measures
of intelligibility, and you can assume that folks will always be
positioned
in the right place, you can just use a headphone driver for testing.


It sounds like this would be a very good piece of test equipment for us to
have if we get really serious about this. I don't see it happening for some
time though, as I could imagine it sucking up large amounts of time.

If people are using the thing from three feet away, you need to be able to
measure response with a sound source three feet away. Because it may be
very different than the response up close, especially if you are using a
directional microphone.


Yes, I was thinking that I would just measure the microphone and speaker
response from three feet ( or a meter, since that is the "standard" distance
for measuring sensitivity), knowing that the response would only be better
at closer distances. Perhaps I should consider doing multiple tests to cover
more of the usage scenarios.

My guess is that under typical surveying environment, at three feet away
from the mike the environmental noise problem is going to be severe. And
anything you do to make the mike more directional is going to make wind
noise problems worse.


I hadn't thought about the interaction of wind noise and directionality in
the microphone. I was thinking that a directional microphone would be better
since the user is almost always in front of the unit and the noise is liable
to be coming from any direction. Just mounting the microphone in the unit
compromises its omnidirectionality by some amount. How much effect does
directionality have on wind noise pickup?

Incidentally, let me commend the Gentex omni capsules to you. They are
very resistant to rain and mud. They also sell noise-cancelling mikes
for close-talking use, but that does not sound useful to you.


Great! I hadn't run across them. An inherently waterproof microphone is a
distinct advantage. (I actually used Panasonic capsules that were exposed on
the front side in the first product we made, since they seemed to survive
our dunk testing without any problems, but we got nervous about their
long-term health and sealed them from water in later products.)

Thanks again!

Jerry Steiger




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"Scott Dorsey" wrote in message
...
Look at some of the room mode calculators on the web. If you can make it
large enough that all of the major room modes fall below the lowest
frequency of interest, that's a start.


When you say major, how high do I need to go? If it is just the first mode,
then about 2' will do at 300 Hz, but if I need to go the 9th mode, then we
are talking 16'.

Right, that's not the right tool for the job... look for the larger
chambers.
You can get away without a walk-in but you are going to need something
larger
than a fridge if you care about 300 Hz.


I can find space for something roughly refrigerator size .

I am thinking that Chris is correct, that if I get enough absorbtion at
the
frequencies of interst, I don't need to worry much about the modes in the
chamber or the resonances in the walls. But can I get enough absorbtion?


If you want absorption at 300 Hz, you need several feet of fibreglass.
And
yes, that is the solution.


OK, this sounds very promising. If I can get away with two feet of
fiberglass or cotton on all sides, then a 5'x5'x8' box would leave me with a
1'x1'x4' test space. Now all I need is a reasonably flexible way of mounting
and accessing the micrphones and speakers in the test area.

Thanks again!

Jerry Steiger


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Jerry Steiger wrote:
"Scott Dorsey" wrote in message

If people are using the thing from three feet away, you need to be able to
measure response with a sound source three feet away. Because it may be
very different than the response up close, especially if you are using a
directional microphone.


Yes, I was thinking that I would just measure the microphone and speaker
response from three feet ( or a meter, since that is the "standard" distance
for measuring sensitivity), knowing that the response would only be better
at closer distances. Perhaps I should consider doing multiple tests to cover
more of the usage scenarios.


It may not actually be better at closer distances.

If you're going to do it from three feet away, buy one of the Tannoy dual
concentric loudspeakers and try it. You can buy one of the nicer Series 600
monitors for good money, or buy a used i6 for cheap, and they will probably
do what you want. The nice thing about these speakers is that they are as
close to constant directivity as you can get, and the pattern is the same
in all directions; it has radial symmetry.

Do the measurement outside, in an open field, high off the ground on
a ladder if possible. You will be able to do good measurements at lower
frequencies more effectivity in an open field than with a small chamber.
Even voice frequencies.

My guess is that under typical surveying environment, at three feet away
from the mike the environmental noise problem is going to be severe. And
anything you do to make the mike more directional is going to make wind
noise problems worse.


I hadn't thought about the interaction of wind noise and directionality in
the microphone. I was thinking that a directional microphone would be better
since the user is almost always in front of the unit and the noise is liable
to be coming from any direction. Just mounting the microphone in the unit
compromises its omnidirectionality by some amount. How much effect does
directionality have on wind noise pickup?


It is a substantial difference. A pressure microphone will be much less
sensitive to wind noise than a cardioid. And yes, mounting the pressure
mike in a housing means the pattern will be altered and you'll have issues
with wind noise on the housing too.

Incidentally, let me commend the Gentex omni capsules to you. They are
very resistant to rain and mud. They also sell noise-cancelling mikes
for close-talking use, but that does not sound useful to you.


Great! I hadn't run across them. An inherently waterproof microphone is a
distinct advantage. (I actually used Panasonic capsules that were exposed on
the front side in the first product we made, since they seemed to survive
our dunk testing without any problems, but we got nervous about their
long-term health and sealed them from water in later products.)


The Panasonics are interesting because they have a Helmholtz resonator in
front of the diaphragm, with a very small entrance hole. Because the hole
is so small, it's hard for water to get into it. Soapy water, though, will
penetrate it very easily.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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"David F. Cox" wrote in message
m...
Following this thread I did my 'think my own thoughts' thing before
searching out state of the art info. I thought it would be a good thing to
have a standarised intelligibility teast. The audio equivalent of the
opticians sight chart. Unsurprisingly this has already been done with such
things as the STI test and there is equipment avaiable to take
corresponding measurements. eg

http://www.rksound.co.uk/goldline/go...ucts/dsp2b.htm


Thanks! I will look into this product.

Jerry Steiger


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"Kevin T" wrote in message
...
While at Motorola a bought a smallish semi-custom box from these very
helpful folks.

http://www.gkacoustics.com/index.asp


I think it was ~ $1-2K. and ~2x 2x 3 ft . What ever it was fine for
300-4k (speech) cellphone & speakerphone/ accessories Quick pre
evaluation before a full B&K HATS artificial Mouth /ear in full
anachoic chamber. I used a "B word" ecm 8000 mic and M-Audio USB
interface.


I assume that you bought the "Stone Booth in a Bag". Very interesting! I'm
thinking I will go with something more elaborate and semi-anechoic, but this
certainly would be a lot simpler.

Thanks!

Jerry Steiger



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Jerry Steiger wrote:
"Scott Dorsey" wrote in message
...
Look at some of the room mode calculators on the web. If you can make it
large enough that all of the major room modes fall below the lowest
frequency of interest, that's a start.


When you say major, how high do I need to go? If it is just the first mode,
then about 2' will do at 300 Hz, but if I need to go the 9th mode, then we
are talking 16'.


16 feet is a little large... eight feet is probably enough. You should be
able to do the math to see the relative importance of the various modes.
How many modes you care about depends on how flat a response you really need;
by the ninth mode the peaks aren't very big at all but they are still there.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."


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"Scott Dorsey" wrote in message
...
If you're going to do it from three feet away, buy one of the Tannoy dual
concentric loudspeakers and try it. You can buy one of the nicer Series
600
monitors for good money, or buy a used i6 for cheap, and they will
probably
do what you want. The nice thing about these speakers is that they are as
close to constant directivity as you can get, and the pattern is the same
in all directions; it has radial symmetry.


The 600 sounds like a great choice. It looks like they are discontinued. If
I can find a single, I should be able to talk my boss into springing for
one. He may not want to spring for a pair, which is the way they mostly seem
to be available. I'll check into the i6 as well.

Do the measurement outside, in an open field, high off the ground on
a ladder if possible. You will be able to do good measurements at lower
frequencies more effectivity in an open field than with a small chamber.
Even voice frequencies.


That will work for the next couple of months, but then the rains come! I
should do some outdoor testing now to use as a baseline for chamber testing.

Thanks again!

Jerry Steiger


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Jerry Steiger wrote:
"Scott Dorsey" wrote in message
If you're going to do it from three feet away, buy one of the Tannoy dual
concentric loudspeakers and try it. You can buy one of the nicer Series
600
monitors for good money, or buy a used i6 for cheap, and they will
probably
do what you want. The nice thing about these speakers is that they are as
close to constant directivity as you can get, and the pattern is the same
in all directions; it has radial symmetry.


The 600 sounds like a great choice. It looks like they are discontinued. If
I can find a single, I should be able to talk my boss into springing for
one. He may not want to spring for a pair, which is the way they mostly seem
to be available. I'll check into the i6 as well.


There is a guy selling i6T models on Ebay right now for very cheap. You
will have to take the 70V transformer out, but that's not a big deal.

I think the 600 has been replaced with the Precision 6, which has a goofy
supertweeter on the top that won't matter at all in your application.

Do the measurement outside, in an open field, high off the ground on
a ladder if possible. You will be able to do good measurements at lower
frequencies more effectivity in an open field than with a small chamber.
Even voice frequencies.


That will work for the next couple of months, but then the rains come! I
should do some outdoor testing now to use as a baseline for chamber testing.


I regularly do microphone and speaker testing outside in a meadow, since
I don't have a large enough chamber to do anything useful at lower frequencies
or in the far field. It works surprisingly well, and I have done comparisons
between my backyard and the big chamber at Wyle Laboratories in DC, and
they are pretty close down to around 75 Hz. And my backyard is a lot cheaper
than what Wyle charges.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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"Scott Dorsey" wrote in message
...
16 feet is a little large... eight feet is probably enough. You should be
able to do the math to see the relative importance of the various modes.


I should be able to do the math, but I'm afraid I don't know what math it is
I need to do. I don't know what the underlying physics is for the importance
of the modes. Is there a text I can read to set me straight?

Thanks!

Jerry Steiger



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Jerry Steiger wrote:
"Scott Dorsey" wrote in message
...
16 feet is a little large... eight feet is probably enough. You should be
able to do the math to see the relative importance of the various modes.


I should be able to do the math, but I'm afraid I don't know what math it is
I need to do. I don't know what the underlying physics is for the importance
of the modes. Is there a text I can read to set me straight?


The F. Alton Everest book on small studio acoustics is good, but the
math is also in the big Master Handbook of Acoustics.

There are also a bunch of room mode calculators out on the web. Here is one:
http://www.marktaw.com/recording/Aco...WaveCalcu.html
I haven't tried this one, but it's not all that hard to do with a desk
calculator so a java applet should be easy to do...
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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"Scott Dorsey" wrote in message
...
Jerry Steiger wrote:
"Scott Dorsey" wrote in message
...
16 feet is a little large... eight feet is probably enough. You should
be
able to do the math to see the relative importance of the various modes.


I should be able to do the math, but I'm afraid I don't know what math it
is
I need to do. I don't know what the underlying physics is for the
importance
of the modes. Is there a text I can read to set me straight?


The F. Alton Everest book on small studio acoustics is good, but the
math is also in the big Master Handbook of Acoustics.


I got Everest's "Acoustic Techniques for Home and Studio" at the OSU
library. The Master Handbook of Acoustics apparently disappeared many years
ago, although they thought they had it till I tried to check it out. The
basic room resonance calculations are in the small book, but nothing on the
relative levels of the modes. He makes the point that for very high
frequencies, the walls are no longer flat and parallel, but that is about as
far as it goes. For the 300 to 3000 Hz waves I'm interested in this
obviously isn't a problem.

I checked out several other books and found some interesting and helpful
material, but nothing that explained the relative level of different room
modes.

I wonder if the expectation that higher modes are lower level is just due to
the typically higher absorbtion at higher frequencies. I haven't been able
to come up with any other explanation.

Thanks again. Sorry to take so long to reply. Other work and family matters
have kept me out of contact.

Jerry Steiger




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"Scott Dorsey" wrote in message
...
There is a guy selling i6T models on Ebay right now for very cheap. You
will have to take the 70V transformer out, but that's not a big deal.


I will look into this, although I suppose those particular speakers are long
gone.

I think the 600 has been replaced with the Precision 6, which has a goofy
supertweeter on the top that won't matter at all in your application.


Aah, I thought the supertweeter was the tweeter and ignored them This gives
me another possibility.

Thanks again!

Jerry Steiger


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I suspect that the "relative levels" issue isn't typically addressed because it's very
much a function of the 3D location of both source and listener, and to the relative
positions of any damping material in the room - way too complicated. I'm sure your
suggestion wrt frequency dependent damping is also relevant. But you can get some kind of
an idea from the relative areas of the parallel plates that cause each mode, and the
proportion of that area that is damped effectively for its mode frequencies.
Floor-to-ceiling modes typically have the biggest are, but the ceiling can often be
treated well, and sound sources and mics positioned to avoid the worst modes.
Cheers, Tony

On Mon, 28 Jul 2008 23:02:49 -0700, "Jerry Steiger" wrote:
"Scott Dorsey" wrote in message
...
Jerry Steiger wrote:
"Scott Dorsey" wrote in message
...
16 feet is a little large... eight feet is probably enough. You should
be
able to do the math to see the relative importance of the various modes.

I should be able to do the math, but I'm afraid I don't know what math it
is
I need to do. I don't know what the underlying physics is for the
importance
of the modes. Is there a text I can read to set me straight?


The F. Alton Everest book on small studio acoustics is good, but the
math is also in the big Master Handbook of Acoustics.


I got Everest's "Acoustic Techniques for Home and Studio" at the OSU
library. The Master Handbook of Acoustics apparently disappeared many years
ago, although they thought they had it till I tried to check it out. The
basic room resonance calculations are in the small book, but nothing on the
relative levels of the modes. He makes the point that for very high
frequencies, the walls are no longer flat and parallel, but that is about as
far as it goes. For the 300 to 3000 Hz waves I'm interested in this
obviously isn't a problem.

I checked out several other books and found some interesting and helpful
material, but nothing that explained the relative level of different room
modes.

I wonder if the expectation that higher modes are lower level is just due to
the typically higher absorbtion at higher frequencies. I haven't been able
to come up with any other explanation.

Thanks again. Sorry to take so long to reply. Other work and family matters
have kept me out of contact.

Jerry Steiger

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