[Jerry Steiger]

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

[Chris Hornbeck]

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

[hank alrich]

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

[Jerry Steiger]

"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

[Scott Dorsey]

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."

[Jerry Steiger]

"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

[Tony[_11_]]

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

[Chris Hornbeck]

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

[Chris Hornbeck]

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".

[Tony[_11_]]

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

[Jerry Steiger]

"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

[Scott Dorsey]

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."

[Jerry Steiger]

"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

[Scott Dorsey]

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."

[Jerry Steiger]

"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

[Scott Dorsey]

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."

[David F. Cox]

"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

[Kevin T]

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.

[Jerry Steiger]

"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

[Jerry Steiger]

"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