acoustics

[RichD]

A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level. Their words came in, clear as a
bell, almost like being in the same room. I was surprised,
I thought the power would attenuate a lot faster.

Then I wondered if it also works the other way - could they
hear our conversations, just as crisp?

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

--
Rich

 

[Ethan Winer]

Rich,

'I see you, you see me' is pretty much a universal law.
Does that also hold for acoustics?

If someone is in front of a big "satellite dish" their speaking will be
focused toward you, and the dish will also collect sound from you and focus
it at them. The pattern will be highly directional, but it seems to me this
is indeed a reversible situation.

--Ethan

 

[Bob Masta]

On Tue, 25 Dec 2007 09:34:20 -0500, "Ethan Winer" <ethanw at
ethanwiner dot com> wrote:

Rich,

'I see you, you see me' is pretty much a universal law.
Does that also hold for acoustics?

If someone is in front of a big "satellite dish" their speaking will be
focused toward you, and the dish will also collect sound from you and focus
it at them. The pattern will be highly directional, but it seems to me this
is indeed a reversible situation.

This is the same example I was going to use, but with the opposite
result: The dish (parabolic reflector) focuses incoming parallel
waves to a point. If you put a point source at the focus, then the
output is a collimated beam. That's how headlight/flashlight
reflectors work. The output is not "focused" in the same sense
as when the dish is used as a collector, since there is no place
where the rays/waves focus to a point (assuming an ideal
parabolic reflector).

So in answer to the original question, the "I see you, you see me"
holds, in the sense that the process is not totally unidirectional,
but the efficiencies or sensitivities in each direction can be quite
different.

Best regards,


Bob Masta

DAQARTA v3.50
Data AcQuisition And Real-Time Analysis
www.daqarta.com
Scope, Spectrum, Spectrogram, FREE Signal Generator
Science with your sound card!

 

[Androcles]

"BobG" <bobgardner@aol.com> wrote in message
news:3f4f3de3-08f6-4b78-8f4f-1c786c15b14e@i29g2000prf.googlegroups.com...
: If they were having a conversation, they were conversing, not
: conversating.

On the other hand, there are five metacarpals.
The other side of the coin is the obverse, not the reverse or the converse.

 

[BobG]

If they were having a conversation, they were conversing, not
conversating.

 

[John Popelish]

RichD wrote:

A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level. Their words came in, clear as a
bell, almost like being in the same room. I was surprised,
I thought the power would attenuate a lot faster.

Then I wondered if it also works the other way - could they
hear our conversations, just as crisp?

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

I think it does, but only if the radiation pattern is also
reversed. For example, if someone stands at the focus of a
parabolic reflector, it very efficiently captures the radial
sound waves from their voice and produces a nearly plane
wave radiation pattern that travels long distances with
little dispersion. In effect, the parabola converts the
near center radial wave pattern to a far center radial
pattern (the waves act as if their center of radiation is a
lot further away than the speaker actually is) so the square
law attenuation rule still applies, but getting to twice the
distance to have wave strength fall to 1/4 means getting a
lot further away.

Now, think of this acting in reverse. Your voice radiates
in a spherical wave, so falls by square law, from your
mouth. Only a small fraction of that sphere is collected by
the parabola to reach its focus. The non-reversibility is
not the fault of the parabola, but your fault for not
radiating a reverse spherical wave pattern similar to what
you received. That kind of wave front would return to the
parabola and focus almost perfectly reversibly to the one
you received.

Now, if you add another parabola at your location, you will
send almost plane waves (spherical waves as if the center
were far from you), almost the reverse of what you received.

Replace both parabola with ellipsoids and the reversibility
is even better.
--
Regards,

John Popelish

 

[gearhead]

On Dec 25, 4:53 am, RichD <r_delaney2...@yahoo.com> wrote:

A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level.  Their words came in, clear as a
bell, almost like being in the same room.  I was surprised,
I thought the power would attenuate a lot faster.

Then I wondered if it also works the other way - could they
hear our conversations, just as crisp?

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

--
Rich

Beware generalization.
One could easily come up with a scenario in which sound propagates
between two points with more attenuation in one direction.
Imagine a New York City deli owner closes up for the night. He pulls
down the sheet metal gate and retreats to the back of his store. A
friend comes up, puts his face six inches from the sheet metal barrier
and says something to the owner inside store, not yelling, just
raising his voice somewhat above conversational level. Having his
mouth right up against the gate causes the sheet metal to vibrate and
transmits the sounds inside the store. In the quiet of the store
owner can hear every syllable.
The store owner, standing in the cat food and paper goods aisle 16
feet from the sheet metal gate, responds. As humans are wont to do
when speaking, he uses the same tone of voice. Of course, by the time
the sound of his voice reaches the sheet metal surface sixteen feet
away it softens by a factor of a thousand compared to the power other
guy's voice hit the metal with. Not enough to overcome the damping of
the hinges between the segments of the sheetmetal gate. His friend
can hardly hear his voice, let alone make out the words.
Also, consider that the deli owner's friend is standing out in a
street surrounded by noises: traffic sounds, breezes blowing, people
talking. The whisper of a sound coming through the metal gate into
the noisy environment has such a low SNR that the friend standing
outside is lucky to hear anything at all.

It's still a crapshoot when highly trained architects and engineers
design a modern concert hall whether it will have any good acoustics
at all. Some of the best halls were designed generations ago using a
seat-of-the pants, intuitive approach. Acoustics are notoriously
unpredictable.

 

[Jamie]

BobG wrote:

If they were having a conversation, they were conversing, not
conversating.
Oh, that sounds so perverse!

--
"I'd rather have a bottle in front of me than a frontal lobotomy"

"Daily Thought:

SOME PEOPLE ARE LIKE SLINKIES. NOT REALLY GOOD FOR ANYTHING BUT
THEY BRING A SMILE TO YOUR FACE WHEN PUSHED DOWN THE STAIRS.
http://webpages.charter.net/jamie_5"

 

[Androcles]

"Jamie" <jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote in message
news:4Oacj.136$8I4.27@newsfe07.lga...
: BobG wrote:
:
: > If they were having a conversation, they were conversing, not
: > conversating.
: Oh, that sounds so perverse!

I'm not conversant with that term.
Please do not be so perversitative, conversate properly.

 

[Timo A. Nieminen]

On Tue, 25 Dec 2007, RichD wrote:

A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level. Their words came in, clear as a
bell, almost like being in the same room. I was surprised,
I thought the power would attenuate a lot faster.

Then I wondered if it also works the other way - could they
hear our conversations, just as crisp?

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

How well does "I see you, you see me" hold for a telescope? The optical
system is reversible, but that doesn't mean that both ends are equal.

In your example, it sounds like reflection from the next building and the
open windows. Your noise that reflects downwards from the open windows
will make it to street level, but since your room is likely to be quieter
than street level, you're probably talking more quietly than the people on
the street.

--
Timo Nieminen - Home page: http://www.physics.uq.edu.au/people/nieminen/
E-prints: http://eprint.uq.edu.au/view/person/Nieminen,_Timo_A..html
Shrine to Spirits: http://www.users.bigpond.com/timo_nieminen/spirits.html

 

[Uncle Al]

RichD wrote:

A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level. Their words came in, clear as a
bell, almost like being in the same room. I was surprised,
I thought the power would attenuate a lot faster.

Then I wondered if it also works the other way - could they
hear our conversations, just as crisp?

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

Whispering galleries and inside elliptic reflectors, yes. This is the
preferred way to build confessionals in church - at one focus - with
rentable scholarship rooms at the other focus.

Parabolic reflector (satellite dish), not so much. Incoming will be
focused but outgoing will be collimated.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
http://www.mazepath.com/uncleal/lajos.htm#a2

 

[Salmon Egg]

On 12/25/07 4:53 AM, in article
f269c480-fa62-4bcf-9c80-86016616752e@s19g2000prg.googlegroups.com, "RichD"
<r_delaney2001@yahoo.com> wrote:

A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level. Their words came in, clear as a
bell, almost like being in the same room. I was surprised,
I thought the power would attenuate a lot faster.

Then I wondered if it also works the other way - could they
hear our conversations, just as crisp?

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

--
Rich
It is not quite that simple. Most simple physical processes that are easily

understood is in the regime of linear physics. That includes "small signal"
acoustic and electromagnetic propagation..

In the EM case, it is possible to introduce nonlinearity in the form of a
Faraday cell that produces non-reciprocal propagation. Although probably
possible, my guess is that it is much more difficult to introduce that kind
of non-reciprocal propagation with longitudinal waves.

The real problem arises when you consider noise and clutter. That would
apply to both acoustic and EM propagation. Suppose you are looking at source
of light against a noisy background. Look for an LED generating a signal in
front of the sun. That is a tough job for a naked eye. But if you add
spatial and spectral filtering, your signal to noise ratio can be greatly
increased.

On the other hand, if you are near the LED looking away from the sun toward
a dark background, it would be much easier to see a signal from a similar
LED transmitter without adding technology.

Bill

 

[Ian Iveson]

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

Wind direction may have an effect on sound but not on light.

Not true for light anyway. What about one-way mirrors?

Ian

 

[Don Klipstein]

In <3mecj.41296$S37.9053@fe3.news.blueyonder.co.uk>, Ian Iveson wrote:

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

Wind direction may have an effect on sound but not on light.

Not true for light anyway. What about one-way mirrors?

One-way mirrors are actually not directional. They are mostly
reflective and slightly transmissive, and installed between rooms with
greatly different illumination levels. If you flip the mirror around,
the effect of that "partially silvered" surface will not change.

- Don Klipstein (don@misty.com)

 

[Phil Allison]

"RichD"

A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level. Their words came in, clear as a
bell, almost like being in the same room. I was surprised,
I thought the power would attenuate a lot faster.

Then I wondered if it also works the other way - could they
hear our conversations, just as crisp?

** Almost certainly not.

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

** It ain't true of optics or acoustics.

Egs. A peep hole device allows you to see who is outside your door while
being unobserved, similarly a periscope or telescope allows an observer to
be unseen.

A small hole in a wall of a room will allow you to hear what is happening
outside if you press your ear against it - however, very little of your
voice energy escapes the room through that hole.





....... Phil

 

[Chris Hornbeck]

On Tue, 25 Dec 2007 11:08:38 -0500, John Popelish <jpopelish@rica.net>
wrote:

Now, if you add another parabola at your location, you will
send almost plane waves (spherical waves as if the center
were far from you), almost the reverse of what you received.

The Sante Fe, NM Childrens Museum has a wonderful outdoor
example of this. Two parabolic dishes, doubtlessly C-band
castoffs, face each other across an open space of maybe
fifty feet, centers at a young person's head height.

In front of each dish is a post-mounted yoke, into which you
place your chin. Back-to-back conversation, outdoors, at
about fifty feet and normal conversational volume - it's
big fun.

Much thanks,

Chris Hornbeck

 

[Ethan Winer]

Bob,

The output is not "focused" in the same sense as when the dish is used as
a collector

Yes, but the person away from the collector isn't a "large" source either.
So it still seems it would be 100 percent reciprocal, no?

This is like half of a pair of reflectors like you see in parks, where two
people can be hundreds of yards away and talk as if they're next to each
other.

--Ethan

 

[John Larkin]

On Tue, 25 Dec 2007 09:34:20 -0500, "Ethan Winer" <ethanw at
ethanwiner dot com> wrote:

Rich,

'I see you, you see me' is pretty much a universal law.

Not if I'm in the dark, and you're in the sun. Or if I'm hiding in the
bushes and you're not. Or if I have a telescope but you don't.

John

 

[Arny Krueger]

"RichD" <r_delaney2001@yahoo.com> wrote in message
news:f269c480-fa62-4bcf-9c80-86016616752e@s19g2000prg.googlegroups.com

A few days ago, I was in a room on the third floor, the
window was open, some people were conversating
down at street level. Their words came in, clear as a
bell, almost like being in the same room. I was
surprised, I thought the power would attenuate a lot
faster.

Then I wondered if it also works the other way - could
they hear our conversations, just as crisp?

I recall a discussion of optics, and someone remarked:
'I see you, you see me' is pretty much a universal law.

Does that also hold for acoustics?

Yes, acoustical reciprocity exists.

 

[Rich Grise]

On Tue, 25 Dec 2007 15:30:57 +0000, Androcles wrote:

"BobG" <bobgardner@aol.com> wrote in message

: If they were having a conversation, they were conversing, not :
conversating.

On the other hand, there are five metacarpals. The other side of the
coin is the obverse, not the reverse or the converse.

No, actually, the obverse is the "front" of the coin, usually considered
to be "heads", and the "tails" side is the reverse.

Hope This Helps!
Rich