Re: Speakers for High Frequency Sound

From: John Fields (jfields_at_austininstruments.com)
Date: 02/13/05 

Date: Sat, 12 Feb 2005 18:41:14 -0600

On 12 Feb 2005 12:18:13 -0800, pooua@aol.com wrote:

>>>>>One very important assumption you are making that is likely
>>>>>to be wrong is that no human can hear very much above
>>>>>20 kHz.
>[snip]
>>>>>there is no physical mechanism that would prevent a human
>>>>>from hearing higher frequencies.
>
>>>>---
>>>> Yes, there is. The mass of the tympanic membrane and the
>>>> sensitivity of the cochlear cilia.
>
>>>The tympanic membrane (the eardrum) can be bypassed;
>
>>---
>>Yes, but we're talking airborne sound here, aren't we?
>
>Must all perceived airborne sounds pass through the eardrum? I don't
>believe so. I know that nerve conduction is sometimes explained as
>physical contact through the skull, but I have always taken that as an
>example, not as a limitation. 

---
IOW, you don't want to be burdened with facts. 
If you were underwater and a transducer was pumping 15kHz energy into
the water, (a medium which much more closely approaches the density of
the human body) then you could claim bone conduction, but in air at
the intensities which would typically come from magnetostriction
causing dimensional changes in a horizontal output transformer, I
think not.
---
   
>I think we can hear airborne sounds (especially if they are intense)
>without the eardrum.
---
Certainly it's possible, but at the frequencies and intensities you're
experiencing, it's not likely.  I think what's more likely is that
you're experiencing an exaggerated example of the cocktail party
effect, especially if your  statement that you can pick out individual
flutes in the flute section of an orchestra is true. 
---
>>>But, what are the limits of the cochlear cilia? Certainly there
>>>are animals that can hear higher frequencies, and they use
>>>the same basic equipment as humans do.
>
>---
>>Yes, in the sense that Ferraris and Fords are both cars.  _But_ we're
>>not discussing non-human hearing, are we?
>
>At the moment, we are discusing physical limitations of humans hearing
>higher frequencies, as opposed to differences in biological species.
>The fact that there are animals that can hear higher frequencies than
>20 kHz strongly implies that there is not a physical limitation on
>humans hearing higher than 20 kHz. The limit is not physical, but
>specific. 
---
That's ridiculous.  We're humans and the limitations on our hearing
are physical and based on what our bodies have adapted to.  If they're
not physical, to what would you attribute the limitations? 
---
>It would then be possible for someone with an out-of-spec
>hearing apperatus to hear higher than 20 kHz, even if no one else
>could.
---
Yes, but then _their_ limitations would still be physical.  A thinner
tympanic membrane could, conceivably, vibrate faster than a thicker
one and transmit the vibrations, with less attenuation than normal, to
the auditory ossicles which could, conceivably, be less massive and
transmit, with less attenuation than normal, the more frequent
vibrations into the cochlea, which could, conceivably, enclose cilia
which could, conceivably, be thinner, shorter, or whatever would be
required to make them be able to respond to the faster vibrations and
send their signals into the eighth nerve into the brain for
processing.  Notice that everything leading up to the excitation of
the cochlear cilia is mechanical and it should be clear, even to you,
that the limitations imposed on our auditory system and, indeed, the
auditory systems of any species is physical. 
---
   
>An analogous situation would be Vitamin C production. There is no
>physical limitation that would prevent humans from producing their own
>Vitamin C. Lots of animals produce their own Vitamin C, and they have
>the same basic machinery as we do for doing so. We can't, because of a
>mutation that shut down the genes necessary for Vitamin C production,
>but we still have the genes, and they could be re-activated. Besides,
>there are humans who are able to produce enough Vitamin C through
>spontaneous oxidation to keep them alive without an external supply.
---
*** vitamin C production, we're talking about acoustics.
---
>>>> I've done work trying to determine whether the nonlinearity of the
>>>> auditory system will allow beat notes which occur as a result of
>>>> exposure to the ear of ultrasonic signals which should result in
>>>> heterodynes being generated which can be heard, are heard
>>>>and, so far, the results have been negative.  That is, if the ear is
>
>>>>exposed to a pair of frequencies, both of which are frequencies
>>>>higher than can be heard, the beat note won't be heard either.
>
>>>That is interesting in its own way, but I don't believe that is
>>>directly applicable in this case. The sensitive person may not be
>>>hearing a beat note.
>
>---
>>Of course it's applicable.  The question you posed was whether there
>>was any physical limitation on human hearing and I gave the the
>>tympanic membrane and the cochlear cilia as examples.  I further cited
>>the heterodyne experiment as an example which indicated that if the
>>low-frequency beat note generated by two ultrasonic signals being
>>mixed in the ear wasn't being heard, then in all likelihood the
>>physical structure of the ear was preventing the high frequency
>>signals from propagating to the point where they could be mixed,
>>preventing the beat note from being heard.
>
>We already know that if a person cannot hear ultrasound, they could not
>hear the beat note from that pitch.
---
I arrived at that conclusion from the results of my own work, but if
you have evidence of prior discovery I'd like to hear about it. Can
you cite a reference to support your statement?
---
>The beat note is a perception
>generated in the brain from sounds it hears. There is no wave present
>at the beat frequency.
---
Au contraire!  Outside of the brain, take two sinusoidal signals, f1
and f2, multiply them, and the result will be that two new signals
will be generated: f1+f2 and f1-f2.  A filter will be needed to
isolate the desired signal from the melee in which it exists, and
that's the job of the brain, _but_ after the multiplication, that wave
will exist whether it can be detected or not.
---
 
>[snip]
>
>>>All the more reason to set up a test and measure it directly. I hate
>>>this guesswork.
>
>---
>>Then do it.  It should be easy enough, just an electret microphone and
>>an oscilloscope ought to do it.
>
>I am capable of hearing the sound I want to identify, and oscilloscopes
>are expensive. A microphone will pick up sounds, but how would I know
>if it is picking up the same sound that I hear? I'm sure that TVs
>produce sound at more than one frequency.
>
>What I need is something that will generate a calibrated tone that I
>can adjust until it matches the pitch I hear from a television. Then I
>will know that the two sounds are the same, and I will be able to
>determine the frequency. Additionally, it costs a lot less money to
>generate the audio signal.
---
You're assuming that the sound you hear is a simple sinusoidal signal.
It isn't, and depending on what you mix with it, what you interpret as
being the same frequency may well be something other than that.
---
>>> >I can also hear LCD screens, but that's at a lower pitch,
>>>>I think, and they are much quieter. I first noticed it when I
>>>>was in a nature park. It was very quiet outside, so as I
>>>>raised my digital camera up to take a picture, I could
>>>>distinctly hear the LCD screen.
>
>>---
>>>> you may have crosstalk between your vision and auditory systems.
>>---
>>>I suppose you would need to run a test to find out for certain?
>
>---
>>Yes, but _you_ would, not I...
>
>I already satisfied myself that I am hearing the sound. You are the one
>who is questioning whether I do or not.
---
Not at all.  I'm sure that you're hearing something, and all I'm
suggesting is that you get tested in order to determine whether there
might be a link between your visual and auditory systems which causes
you to hear what you see.
Just as an aside, you might want to think about how much 15.75kHz
your mom and dad were exposed to before you were born.
---
>>>>Now I am taking a college class in a room that has
>>>>3 television sets suspended from the ceiling. One
>>>>man saw me putting earplugs in my ears, and
>>>>asked if I could hear the televisions. It turned out
>>>>that he is able to hear some televisions (the one in
>>>>his college dorm), but he could not hear the
>>>>televisions in the classroom. As far as I can tell, I
>>>>am the only person in the room who hears those televisions.
>
>>---
>>>> It might be instructive to determine whether you can "hear" the
>>>> monitors with your eyes closed.
>
>>>I absolutely could hear the monitors with my eyes closed.
>
>[snip]
>
>---
>>Ok, but I don't understand the point of all of this.
>
>I am looking for a speaker that will produce sound waves from 12 kHz to
>50 kHz.
---
That's easy. I have, on hand, several transducers which can do that. 
Wanna buy one or two?
---
>I would like to find out what frequency of sound I am hearing from a
>television set.
---
That's a little more difficult, and from the attitude you've displayed
so far it seems there's little chance you'd defer to what you would
consider to be an opinion different from your own.
---
 
>The sound I hear is not imaginary or merely a psychological effect.
---
How do you know that's true?
---
> I actually hear a very loud sound that most people of any age report they
>have never heard. I am curious what this sound is, and why I am able to
>hear it so well when so few other people can.
---
Maybe you're just special?
-- 
John Fields
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