Also found this interesting comment about 1/3 octave
amplification...Keely says the universe is built and runs from thirds.
The thread relates to Conch shells;
--------------
posted by Kari Pesonen;
I tested several different vessels (not seashells). I noticed max. ca.
15 dB amplification = increase in sound pressure level at certain
1/3-octaves.
Here you have the results shown in my jpeg-picture.
(select a fixed pitch font to align the columns well)
"Seashell noise tests" by Kari Pesonen (e-mail: kari.pesonen@hut.fi) (20
June 1999).
Test object: Glass jar, inner dia. ca. 80 mm,
length ca. 140 mm, mouth dia. ca. 65 mm,
mouth length ca. 10...15 mm.
A-weighted 1/3-octave spectra
(the analyzer, SVAN 912AE has digital A-weightning filter. Negative
levels at lower bands are valid).
Measured in a bigger office room. Main sound source: PC.
Point 1: At the mouth position without the jar.
Point 2: At the mouth of the jar.
Point 3: Inside (about at the center) of the jar.
Point 1 Point 2 Point 2
Band/Hz Level/dB Level/dB Level/dB
25 -12,2 -4,9 -11,0
31,5 -12,1 -6,0 -6,1
40 -5,2 -1,0 -1,1
50 4,4 4,6 4,3
63 -4,1 0,9 -0,1
80 1,9 0,1 -0,1
100 6,5 3,8 4,3
125 5,7 5,7 5,9
160 12,3 12,6 12,6
200 14,4 16,6 17,9
250 14,4 15,6 16,8
315 15,4 18,4 21,1
400 16,5 29,5 34,3
500 20,9 26,4 29,9
630 20,7 11,6 21,9
800 20,1 10,2 15,3
1000 16,6 14,4 14,2
1250 14,3 16,4 12,9
1600 15,3 23,8 17,3
2000 13,7 11,5 7,7
2500 14,2 13,2 8,5
3150 13,1 13,8 10,4
4000 12,8 13,7 12,2
5000 11,7 11,4 13,2
6300 9,1 8,6 10,2
8000 6,9 7,8 6,6
10000 6,0 5,7 6,1
12500 4,3 4,3 4,2
16000 2,7 1,7 2,2
20000 5,0 3,2 3,8
TOTAL 28,5 32,7 36,0 dB(A)
Max. pressure increase (amplification) at 400 Hz band. I measured
several vessels, but didn't save all results. This is a typical example
without a human head listening the jar. Other vessels gave both higher
and lower amplifications (at different bands).
--------------
Tom Breton wrote:
> Guys, I hate to tell you this, but there is no way that sea shell is
> amplifying anything (*).
>
> What it may be doing is preferentially blocking some frequencies,
> probably a bandpass kind of thing. And probably you accomodate the
> sound that's quieter on the whole, and perceive the passed frequencies
> as absolutely louder when they are just relatively louder.
>
> (*) For one thing, it's not even plugged in. }:)
If you are implying that one needs a source of external power for
pressure amplification, you are mistaken. The human ear canal behaves
like a transmission line that is terminated in an impedance which is
high relative to its characteristic impedance. Consequently, at the
freqeuncy of the line's quaarter wave resonance, the pressure at the
eardrum is considerably greater (approximately 10dB) than the pressure
at the ear canal entrance.
-------------
Indeed there is no need for electrical amplification. Not only in the
ear chanel, but also within the shell the sound pressure can increase
with respect to the sound pressure in the air around the head. Due to
resonances.
Note in addition that pressure is not the same as energy. Energy density
will however increase locally, not the total energy in the sound field.
Subjectively I can hear that sound pressure is amplified, certainly at
some frequencies, but measurements I did not take. We will need small
scale microphones etc. The theory will be the same as for Helmholz
resonators. See any textbook on acoustics.
------------
You all seem to be reading "amplification" as "pressure increase" or
"power concentration". I suppose that's not unreasonable, given that
we are talking about hearing. But I take amplification to mean
increasing the total power, and if seashells could do that they'd be
perpetual motion machines.
That resonance can't explain how a seashell works, otherwise the
perceived frequency would go down sharply as you moved it away from
your ear. In fact it rises, presumably because your ear no longer
partially blocks the seashell's opening. (Lowers the impedance of the
neck by increasing the area, if you must)
------------
You are misiing the point. I applied the "energy conservation"
principle. The energy in this case is the product of the acoustic
pressure and the acoustic velocity time the cosine of the phase angle
between them.
A standing wave can have a larger pressure amplitude at the surface of a
heavy material such as a shell or our flesh. The acoustic velocity goes
to a very small value and the phase can be changed. But our ears detect
sound pressure.
Hence the opportunity of "amplification".
-- Jerry Wayne Decker / jdecker@keelynet.com http://keelynet.com / "From an Art to a Science" Voice : (214) 324-8741 / FAX : (214) 324-3501 KeelyNet - PO BOX 870716 - Mesquite - Republic of Texas - 75187