Bob,

What kind of plug or sandwich would you recommend if one wanted to, as
closely as possible, terminate a tube with the characteristic impedence of
air?

Thanks to Arny for answering because I had no idea.

--Ethan

Ian,

I was thinking of the profiled stuff that diffuese too.

Sculpted foam doesn't diffuse either. Diffusion relies on having a
reflective surface, not an absorbent one.

--Ethan

On Tue, 14 Sep 2004 23:10:50 -0400, Porky wrote
(in article ):


"Laurence Payne" wrote in message
...
On Mon, 13 Sep 2004 10:07:23 -0400, Ty Ford
wrote:

I really have big problems with that statement. Dead space is seldom
linear.
Small dead space, when excited by a loud enough vocal (or any sound) can
still become quite resonant, thus imparting the telltale signs of a small
dead space. In fact, small spaces covered with acoustic foam actually
sound
sort of spongy when excited enough.

Are you just saying that dead space is unachievable?

Ever priced an anechoic chamber that was spec'ed for below 40 Hz? I would
say that space that is truly dead at all audio frequencies is not acheivable
in any practical home studio, unless the studioist is a multi-millionaire.
-) However, one can do pretty darn well for practical purposes with some
absorbant material and some creative thinking, especially if one is close
miking the vocalist. It isn't necessary to create a truly dead space, a
space that is fairly dead with no big peaks and dips in response at vocal
frequencies can make vocals sound very good.



All I'm saying (well maybe not all) is that slapping foam up everywhere is
NOT the answer. A balance of diffusion and absorption (OK trapping too, but,
I'm not a big fan) are all required to get a good space.

Then there's noise abatement. Adding more foam or diffusion won't keep your
neighbors happy and it won't keep the bus noises from getting into your
tracks.

The formulae for acoustics are pretty nailed down, but the application of
them is tricky. Acoustics don't scale (little room vs big room). Even two
seemingly identical rooms that have been designed and built by the same plans
may not sound the same.

It's back to the importance of one's ears at some point.

Regards,

Ty Ford



-- Ty Ford's equipment reviews, audio samples, rates and other audiocentric
stuff are at www.tyford.com

Art,

There's a lot of half truths buried in all the above information

I'll say the same. :-)

- There really isn't much difference from acoustic foam and many other
kinds.

There's a huge difference between the absorbing properties of open cell foam
and closed cell foam, as was explained earlier. The main difference is open
cell foam absorbs sound waves and closed cell foam doesn't.

acoustic foam is sculpted into wedges. That's what makes it work.

No, the wedges merely offer more absorbing surface, which increase
absorption at high frequencies at the expense of low frequencies. What
"makes it work" is the open cell structure, as was explained earlier.

When sound hits flat foam, it might absorb 40% of it (randomly chosen
number).

I'm glad you acknowledged that 40% is a random number, because it has no
basis in fact. In many parts of the world flat foam is far more common than
sculpted foam for acoustics purposes.

The great thing about pro-quality acoustic treatments is that the
frequency response affected is a known quantity. The thicker the foam, the
lower the note it will absorb, and flat frequency response is the goal.

Yes, though that's over-simplified. How materials are mounted and where
they're placed is a big factor too. And there are better materials than
foam. For example, the best bass traps I know of are based on rigid
fiberglass.

almost all small rooms have way too much bass reverberation. In music
studios, great care and expense is focused on diffusing sound at full
frequency

Diffusion aims to work at mid and high frequencies only. Absorption should
be broadband, but even the really good (and really expensive) RPG diffusors
don't do much below 500 Hz.

A wall of bookshelves filled with books makes an excellent diffusor.

A wall full of bookshelves is not a diffusor at all. Perhaps if you set the
shelf heights and orientation of the books "just so" you might get a little
low-quality diffusion. Other than that, a shelf full of books might absorb a
little at mid and high frequencies, but that's about it.

--Ethan

Ethan Winer wrote:
Ian,


I was thinking of the profiled stuff that diffuese too.


Sculpted foam doesn't diffuse either. Diffusion relies on having a
reflective surface, not an absorbent one.

And sculpted foam does diffuse because it is an imperfect absorber and
therefore reflects what it does not absorb.

Ian

Arny Krueger wrote:

"Bob Cain" wrote in message



Ethan, I think I've asked you this before but got diverted
in mid discussion so I hope you don't mind if I ask it again.

What kind of plug or sandwich would you recommend if one
wanted to, as closely as possible, terminate a tube with the
characteristic impedence of air?

Air.

Yep. An infinite amount of it in an infinitely long tube.
That presents practical difficulties.

Plugs don't work on the principle of matched impedances. The trick is to
implement a mismatched impedance in such a way that you don't create too
many standing waves.

Are you saying it is impossible to make a lumped acoustic
resistance with the same value as the characteristic
impedence of air (which is very nearly a pure resistance)?


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

"Bob Cain" wrote in message

Arny Krueger wrote:

"Bob Cain" wrote in message



Ethan, I think I've asked you this before but got diverted
in mid discussion so I hope you don't mind if I ask it again.

What kind of plug or sandwich would you recommend if one
wanted to, as closely as possible, terminate a tube with the
characteristic impedence of air?

Air.

Yep. An infinite amount of it in an infinitely long tube.
That presents practical difficulties.

Plugs don't work on the principle of matched impedances. The trick
is to implement a mismatched impedance in such a way that you don't
create too many standing waves.

Are you saying it is impossible to make a lumped acoustic
resistance with the same value as the characteristic
impedence of air (which is very nearly a pure resistance)?

I'm saying I don't know how, and all the people I know who do stuff like
this don't get ideal results. They end up with impedance mismatches that
they may try to minimize the effects of.

"Arny Krueger" wrote in message
...
"Bob Cain" wrote in message

Arny Krueger wrote:

"Bob Cain" wrote in message



Ethan, I think I've asked you this before but got diverted
in mid discussion so I hope you don't mind if I ask it again.

What kind of plug or sandwich would you recommend if one
wanted to, as closely as possible, terminate a tube with the
characteristic impedence of air?

Air.

Yep. An infinite amount of it in an infinitely long tube.
That presents practical difficulties.

Plugs don't work on the principle of matched impedances. The trick
is to implement a mismatched impedance in such a way that you don't
create too many standing waves.

Are you saying it is impossible to make a lumped acoustic
resistance with the same value as the characteristic
impedence of air (which is very nearly a pure resistance)?

I'm saying I don't know how, and all the people I know who do stuff like
this don't get ideal results. They end up with impedance mismatches that
they may try to minimize the effects of.

I would suggest that anything that would cause no acoustic reflection
back into the tube would imitate an infinite tube rather well. As I
understand it, the reason for postulating an infinitely long air filled tube
is simply to eliminate all effects that would be caused by reflection of the
wave. Whle it might be impossible to totally eliminate all reflection, I'm
sure one could get close enough for experimental purposes by experimenting
with combinations of highly absorbtive materials.

Ian,

And sculpted foam does diffuse because it is an imperfect absorber and
therefore reflects what it does not absorb.

I don't have the time or energy to define diffusion in depth for you, but
you can learn a lot about it at RPG's web site, as well as through Google.
Here's the basics, so you'll at least understand why foam does not diffuse:

Frequencies too low to be absorbed by foam are passed through, not
reflected. Frequencies too high may be reflected, but those frequencies
would be very high - above the range we think of when talking about
diffusion. Far more important, the simple act of reflecting is not
diffusion! If that were the case diffusors would be flat boards instead of
complex designs.

Read up some on this first, and then we can discuss it further if you'd
like.

--Ethan

Ethan Winer wrote:
Ian,


And sculpted foam does diffuse because it is an imperfect absorber and

therefore reflects what it does not absorb.

I don't have the time or energy to define diffusion in depth for you, but
you can learn a lot about it at RPG's web site, as well as through Google.
Here's the basics, so you'll at least understand why foam does not diffuse:


I am well aware of the basics.

Frequencies too low to be absorbed by foam are passed through, not
reflected. Frequencies too high may be reflected, but those frequencies
would be very high - above the range we think of when talking about
diffusion. Far more important, the simple act of reflecting is not
diffusion! If that were the case diffusors would be flat boards instead of
complex designs.

I never mentioned flat boards. Diffusion is simply the combination of
many randomly oriented reflections. The open cavities and non flat
surface of acoustic foam undoubtedly cause some diffusion. That was
my only point.

Ian

Porky wrote:


I would suggest that anything that would cause no acoustic reflection
back into the tube would imitate an infinite tube rather well.

Perzactly. There seems to be a practical materials problem
with regard to achieving that, however, from the responses
I've seen.

What I think could be made to work rather well, though is
some material with very low reflection coeficient over the
band of interest and thick enough that what LF remains to
pass through could be handled by an active absorber.

I'd wanted to make something like this for microphone
characterization but even if I could solve the termination
problem (and the full bandwidth planar source problem) I
couldn't see any way to get rid of reflections from
diffraction at the mic that would bounce back to it from the
tube wall.

But this won't help at all with a makeshift soundbooth. :-)


Bob
--

"Things should be described as simply as possible, but no
simpler."

A. Einstein

In article l7t1d.88372$yh.13377@fed1read05,
Jim Carr wrote:

If a picture falls off the wall in a house and nobody's there to hear it,
does it echo?

Is the picture a picture of a duck? If so, it definitely won't echo.
--
--------------------------------------+------------------------------------
Mike Brown: mjb[at]pootle.demon.co.uk | http://www.pootle.demon.co.uk/

"Mike" wrote in message
...
In article l7t1d.88372$yh.13377@fed1read05,
Jim Carr wrote:

If a picture falls off the wall in a house and nobody's there to hear it,
does it echo?

Is the picture a picture of a duck? If so, it definitely won't echo.

That was debunked on "Mythbusters", wasn't it?

Those guys are nuts, but for that kind of money, I
might be tempted to be nuts on TV myself.

Mike wrote:
In article l7t1d.88372$yh.13377@fed1read05,
Jim Carr wrote:


If a picture falls off the wall in a house and nobody's there to hear it,
does it echo?


Is the picture a picture of a duck? If so, it definitely won't echo.

If there's nobody there how do you know if/when it fell? Schrodingers
picture?

Ian

--
Ian Bell

Richard Crowley wrote:

"Mike" wrote in message
...
In article l7t1d.88372$yh.13377@fed1read05,
Jim Carr wrote:

If a picture falls off the wall in a house and nobody's there to hear it,
does it echo?

Is the picture a picture of a duck? If so, it definitely won't echo.

That was debunked on "Mythbusters", wasn't it?

Those guys are nuts, but for that kind of money, I
might be tempted to be nuts on TV myself.

I don't have cable, but that is one series that I've been told about
and sounds so interesting, that I want to by a collection when it
comes out on DVD.

--Dale

"Mike" wrote in message
...
In article l7t1d.88372$yh.13377@fed1read05,
Jim Carr wrote:

If a picture falls off the wall in a house and nobody's there to hear it,
does it echo?

Is the picture a picture of a duck? If so, it definitely won't echo.

Oh, a wise quacker, eh?