Sound Pressure Levels

[WashU77]

In a recent column: http://www.straightdope.com/columns/010406.html
Cecil mentions a sound level of 200 Decibels. Here's a pointless question. Why would this not me stated as 20 Bels? Does the 'Deci' part of the term not mean .1 of a Bel?

I'm guessing it's just common usage, like why are capcitors microfarad and picofarad, but never - to my observation - nanofarad...

Just pointlessly musing here.

-bs-

[akambe]

I recall an article in a Popular Mechanics a few years back that showed different technologies the military was considering for simulating nuclear blast damage to structures. One of the options was a HUGE array of HUGE speakers, bigger than anything in existence, that could send out pulses of pressure to blast houses & such. I figure, if it can blow apart a house, YES, it can blow YOU away.

[danvanf]

A decibel is an odd measurement, not like a foot or meter, it is a relationship between 2 values of power. In this instance 0db would be just below what a human could hear. So no the deci part isn't able to be replaced by pico, or kilo etc. I believe the formula for dB is dB=20Log(PowerLow/PowerHigh). If I remember correctly a 3dB gain requires twice the power.

Now to humbly be corrected by the masses. <Smile>

[Arnold Winkelried]

It would be technically correct to say 20 bels instead of 200 decibels, but the habit is to use decibels. In the same way, instead of saying 200 metres, one could say 20 dam (dekametres) or 2 hm (2 hectometres) but in common parlance dekametres and hectometres are almost never used.

Popular Mechanics is behind the times! Famous scientist Tryphon Tournesol made a sound-based weapon in the Tintin adventure "L'affaire Tournesol" in 1956. Fortunately for humanity, he realized that his work could be used for evil ends, and the microfilm containing the details of his work was destroyed.

[Arnold Winkelried]

danvanf, simulpost! After your comment I decided I should go back up my statement that 10 decibel = 1 bel.

Encyclopedia Britannica (http://www.britannica.com/eb/article?eu=30181&tocid=0) confirms it, and also says that a decibel is 10 times the logarithm of the power ration (not 20).

[danvanf]

I hate it when I am Soooo wrong. I type corrected... For pennance(sic) I give a link to some interesting dB drivil.
http://www.cc.columbia.edu/~fuat/cuarc/dB.html

[danvanf]

Thanks Arnold!
I hate it when I am Soooo wrong. I type corrected... For pennance(sic) I give a link to some other interesting dB drivil. http://www.cc.columbia.edu/~fuat/cuarc/dB.html

Yep it's 10, I got 20 from my audio days for Sound Pressure level. And further I'm no longer sure about 3dB being a doubleing of power, rather that it's the smallest decernable difference in sound.

Since I've been Soooo wrong I offer another dB link http://arts.ucsc.edu/EMS/Music/tech_background/TE-06/teces_06.html

[Camper]

I remember a comment in the Borland Turbo C++ that was in their sound() example. The comment was

True story: 7 Hz is the resonant
frequency of a chicken's skull cavity.
This was determined empirically in
Australia, where a new factory
generating 7-Hz tones was located too
close to a chicken ranch: When the
factory started up, all the chickens
died

I got to believe this is an urban legend, but the theory seems sound. Does the human skull cavity have a resonant frequency? If so what is it? I would not want to be the guinnea pig on this one.

[hazel-rah]

Originally posted by Arnold Winkelried
Popular Mechanics is behind the times! Famous scientist Tryphon Tournesol made a sound-based weapon in the Tintin adventure "L'affaire Tournesol" in 1956.

And in 1986 Kate Bush reported on the secret military efforts to create sonic weapons in her single "Experiment IV (http://www.heisjohn.com/kate/ex4l.htm)." Military sources denied this, claiming it was a gimmick to get longtime fans to purchase a greatest hits CD by putting one new track on it.

-fh

[ihrkelings]

On an old (obviously) "Outerlimits" the aliens kill the Earthings with a sound wave. I have perfected this technique and can now kill with a thought wave! But seriously, what about other uses of sound...I use the http://www.centerpointe.com holosync method for meditaton. It's technology allows me to achieve desired brain waves through sound. Also, subliminal sound waves have been used by our military (and others?) for crowd control etc. Anyone have further thoughts on this subliminal stuff?

[Arjuna34]

Originally posted by Camper
I remember a comment in the Borland Turbo C++ that was in their sound() example. The comment was

True story: 7 Hz is the resonant
frequency of a chicken's skull cavity.
This was determined empirically in
Australia, where a new factory
generating 7-Hz tones was located too
close to a chicken ranch: When the
factory started up, all the chickens
died

I got to believe this is an urban legend, but the theory seems sound. Does the human skull cavity have a resonant frequency? If so what is it? I would not want to be the guinnea pig on this one.

A chicken's skull is far too small to resonate at 7 Hz. For example, if you've ever blown across a jug to produce a tone, you were vibrating the jug at it's resonant frequency. A jug the size of a chicken's head would be in the hundreds of Hertz.

Arjuna34

[GalaciDaliciducleic Acid]

It would be interesting to hear more about the effects of certain specific sounds and frequencies on the human body. I'm thinking in particular of Survival Research Laboratories, a group of artists/anarchists/brats based in San Francisco (natch) who stage "performances" in which various skillfully-designed robots and assorted machines attack; drill; crush; shoot fire, sparks, and metal balls; and generally whale on each other (and audience members who've foolishly opted for the front row).

SRL got into trouble with the local officials once (not the only time, the others involving fake bombs, etc.) for dicking around with a modified jet engine that spewed out apocalyptic amounts of acoustic power in the extremely-low-frequency (into the subsonic) range. Not only did this scare the living crap out of people for miles around (EARTHQUAKE!!), but evidently (surprise!) seriously fucked with the inner ears of those in the vicinity. Mark Pauline (head brat of SRL, in a very entertaining documentary called The Pleasures of Uninhibited Excess) claimed that he lost almost all equilibrium and basically couldn't stand up for several hours afterward. Naturally, this didn't stop them from using the damn thing in their upcoming performance... seasoned SRL fans generally wear earplugs (not to mention protective gear) anyway.

So (apart from the fact that everybody reading this should rush right out and try to find an SRL video - it's no fun 'till something burns to the ground!), any further fascinating insights?

[Duck Duck Goose]

Originally posted by Camper
I remember a comment in the Borland Turbo C++ that was in their sound() example. The comment was

True story: 7 Hz is the resonant
frequency of a chicken's skull cavity.
This was determined empirically in
Australia, where a new factory
generating 7-Hz tones was located too
close to a chicken ranch: When the
factory started up, all the chickens
died

I got to believe this is an urban legend, but the theory seems sound. Does the human skull cavity have a resonant frequency? If so what is it? I would not want to be the guinnea pig on this one.
This factoid is widely posted on the Web, including (partially) on the Urban Legends website, under "Astounding Avian Anomalies".

http://www.urbanlegends.com/afu.faq/listing.cgi?selection=avian
The resonance frequency of a chicken's skull cavity is ~7 Hz.

And OTOH there's this.

http://www.interpac.net/~plntpuna/siriusa/VOD/vod-vol-2No-1.htm

Voice of the Dolphin

Vol. 2 No. 1 March 17, 1998

Low Frequency Active Sonar, the Whales, and the Navy

It is reported that the sound source for LFAS has a range of 100-1000 Hz. What are the power levels at lower and higher frequencies than this? For example, since it is known that 7 Hz sounds will homogenize chicken brains, 4-5 Hz will rip mesenteries, 11 Hz will cause feelings of dread, and 14 Hz cause vomiting in humans, then there may well be other effects in the 0-100 Hz side bands of this huge sound projector that the Navy has yet to report on.


I don't know enough about this to know whether this is true. Will 4-5 Hz "rip mesenteries"?

http://www.m-w.com/cgi-bin/dictionary
Main Entry: mes·en·tery
Pronunciation: 'me-z&n-"ter-E, -s&n-
Function: noun
Inflected Form(s): plural -ter·ies
Etymology: Middle English mesenterie, from Middle French & Medieval Latin; Middle French mesentere, from Medieval Latin mesenterion, from Greek, from mes- + enteron intestine -- more at INTER-
Date: 15th century
1 a : one or more vertebrate membranes that consist of a double fold of the peritoneum and invest the intestines and their appendages and connect them with the dorsal wall of the abdominal cavity b : a fold of membrane comparable to a mesentery and supporting a viscus (as the heart) that is not a part of the digestive tract
2 : a support or partition in an invertebrate like the vertebrate mesentery
- mes·en·ter·ic /"me-z&n-'ter-ik, -s&n-/ adjective

(Interesting to note that a Google search for "Australian chicken factory" turned up, among other things, "The History of Female Prostitution in Australia". :D

I love the Internet.

[Duck Duck Goose]

I'm looking for the "14 Hz causes vomiting in humans" factoid. Can't find it anywhere. I'm having trouble with the difference between decibels and frequencies and resonance and ultrasound and infrasound and hertz. (Hertzes?) Google "effects sound frequencies humans vomiting" brings up this--

http://www.acoustics.org/137th/altmann.html
In spite of the fuss made about infrasound in the military press, at 1-20 Hertz there is really no profound effect on humans. I have found no hard evidence for vomiting or uncontrolled defecation, even at levels of 170 dB or more.

<snip>

With its low audio frequency of 460 Hertz (close to the concert pitch of 440 Hz of the well-known tuning fork), the sound level was 137 decibels, about the ear-pain threshold, at 30 m distance.
[using three terms in the same sentence...] :rolleyes:

"Hz" means "pitch", like on a piano? So it's conceivable that if you played a really, really low note for chickens, they would die? Or would they just not be able to hear it in the first place?

http://www.ornithology.com/lectures/Senses.html
The frequency range of sound waves a single bird species can receive is narrower than that of mammals. Birds are less sensitive to the high and low ends of their range than mammals, but in the middle frequencies it is similar. However, birds are about 10x as sensitive to rapid fluctuations in pitch and intensity than humans. Birds can distinguish between frequencies that differ by 1% or less and they can distinguish between sounds separated in time by only .6 to 2.5 milliseconds.

Google. "acoustic weapon vomiting". All these websites agree that certain frequencies (or is it decibels?) can make you vomit, or kill you, but so far I don't see any actual numbers. Is this Classified Information?

http://www.bbc.co.uk/science/playground/theremin6.shtml
Sound as a weapon?
In the same way that the theremin uses two oscillating frequencies to produce an audible tone, there has long been suggestion that this same theory might be applied to non-lethal weapons. Scientists in the USA are known to be experimenting with acoustic heterodyning to produce a non-lethal weapon of tremendous precision. The theory is that if two sound waves are aimed to cincide within an individuals body, the resulting vibrations would cause extreme discomfort and even vomiting, yet leave the target with no permanent injuries.
http://www.hrw.org/about/projects/arms/memo-acoustic.htm
Depending on the frequencies, intensities (decibel level), and modulations employed, acoustic weapons could cause permanent or temporary physical damage, including damage to internal organs, interference with the workings of the central nervous system, and thermal injuries (burns). Other effects noted by the U.S. military, acoustic contractors, and experts include tissue destruction, hemorrhaging, spasms, acoustic fever, vomiting, choking respiration, "intolerable sensations mainly in the chest," "significant decrement in visual acuity," incontinence, postexposure fatigue, and diffuse psychological effects.

I sense that I am only making things worse. There's never an acoustical engineer around when you need one...

[Chronos]

Frequency is measured in hertz (a hertz is just one over a second), and is what is commonly called pitch. Wavelength is related to frequency, in that the frequency times the wavelength is the speed of sound in that medium. Decibles measure the intensity of a sound, or the loudness, in other words. Resonant frequencies are specific, so if something resonates at 7 Hz, it probably won't resonate at 6 Hz or 8 Hz. Even then, if a sound is too quiet, it won't do anything, so there's a minimum intensity for a hypothetical sonic weapon to be of any use. Here, the exact value doesn't matter, as long as it's above some certain threashold, so if a 150 decibel sound is effective at something-or-another, then a 160 decible sound will also be effective.

[Duck Duck Goose]

Thank you. :) I think I've got it now--if I want to make a chicken's head explode, not only do I have to find a way to generate a sound that resonates at the frequency of 7 Hz, I also have to find a way to make it a certain loudness, too.

My next question is, if I invented a Chicken Exploder Acoustic Weapon, would there be a market for it? Besides the U.S. Armed Forces, I mean? :D

[Arjuna34]

Duck Duck Goose, amid all that internet searching, you missed the relevant link right here at home:

Does subsonic sound cause acute anxiety? (http://www.straightdope.com/classics/a1_280b.html)

Arjuna34

[Duck Duck Goose]

Thanks for the link, Arjuna, but I don't want to make the chickens anxious--I want their heads to explode.

Although, admittedly, having their heads explode would probably make them anxious...

:D