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Topic: Plagued by "inaudible sound" (Read 32757 times) previous topic - next topic
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Plagued by "inaudible sound"

I've tried searching for information on Google, but it's near impossible to find anything that doesn't say that sound above 20 khz is completely inaudible to the human ear. And even when I find a source that says that some people can hear above 22 khz or even higher, I can't find any information on what, exactly, "higher" is.

So, I'm not sure if this is the right place to be posting it, but as a last resort, I'm posting it here. I can hear sound above 20 khz. Is it possible to get equipment to measure the frequency of a sound source that isn't unreasonably expensive? And how high have humans been known to hear sound?


To start off, basically, this started when the fluorescent lighting in my Mathematics class was acting up. It was letting out a deafening high-pitched noise that didn't stop until the light bulbs in question were replaced. Every time I went to Math class, I'd always walk in expecting that high-pitched noise, and for a week or so before they fixed it, it was always there.

I've always had good hearing, but until the fluorescent lights started acting up, I didn't really think about it. Now I notice all of the "ultrasonic" sounds around me. For instance, those devices that you plug in that are supposed to emit a high-pitched noise that scares bugs away -- I can hear them. I can hear bats. I can even hear things that I never knew emitted noise like, for instance, certain LEDs.
I can't hear all LEDs, but some LEDs most definitely emit a noise. The first time I noticed was when I walked into the bathroom in the middle of the night and realized that I heard a high-pitch noise that alternated between off and on every second or so. I looked to find the source and noticed that my electric razor's charge light was blinking on and off at the same time that I could hear the sound go on and off. I also noticed that my DS Lite's charging LED emits a noise. In fact, the AC adapter that plugs into the wall, as well as many other AC adapters, emit a high-pitched noise when you plug them into the wall.
Another example is that this past weekend, I walked into my cousin's room and noticed a high-pitched noise. I thought that maybe it was his computer's fan, so I tried turning it off. The noise continued, until finally I found the culprit -- a surge protector. The surge protector had several LEDs showing the status and health of the device, and the noise stopped as soon as I unplugged it from the wall.

And the coup de grace, I can hear the new Compact-Fluorescent light bulb that I bought for my closet. And it's not a random, cheap, off-brand CF bulb, either; It's a GE CF bulb that complies with Energy Star's requirements. And, according to Energy Star's official website, the operating frequency of an Energy Star-compliant compact fluorescent light bulb has to be greater than or equal to 40 kHz.

At first I thought I was crazy, or perhaps had some weird form of Synesthesia. But it can't be Synesthesia because in all of the cases except for the compact fluorescent light bulb, I've heard the sound before I noticed the light. And in the case of my electric razor or my cousin's surge protector, I couldn't even see the tiny lights at the time of hearing the noise. But I've made sure to go in my closet, shut the door, and then turn on the light in the middle of the night when my hearing is at its maximum and I can definitely hear the CF bulb. It's not a tone of any kind -- it sounds more like white noise. However, it's so difficult to hear that I wouldn't have noticed it had I not known that they emit noise. When I turn on the light, it sort of just sounds like adding a very faint bit of white noise on top of the already white-noisiness of complete silence.


I didn't come here to complain about my annoyances, however. I came here because it's the only place I could find that might be able to answer my questions about ultrasonic sound. So these would be my questions:
  • Just how high have humans been recorded to hear?
  • Is the kind of equipment I'd need to test for noise this high-pitched prohibitively expensive?
  • What reasons would there be for an LED or an AC adapter to emit a high-pitched noise?
  • Are there any articles about noise above the 20 kHz range? (All I've found was one that talks about how the test subjects perceived a quality difference in a sample of music when >20kHz noise was played, even though they said that they couldn't hear the high-pitched sound that was being played.)
Thanks for your time,
Brian

Plagued by "inaudible sound"

Reply #1 –
The Nyquist Theorem says we can not hear anything above 20 kHz. I barely can hear 16kHz. Human adults above 30 years old have hearing loss they can't even reach that 16 kHz.

WHATEVER!!! But Nyquist was right in a certain practical way... There is a REASON why 22050 was USED. Now ask yourself why Nyquist used this samplerate and not a higher one?  If I can recollect it had to do about what we can hear and the enough safety margin to re-produce analogue sound without a brain to figure out it was digital. Something like that.

Plagued by "inaudible sound"

Reply #2 –
Quote from: Bourne on
The Nyquist Theorem says we can not hear anything above 20 kHz. I barely can hear 16kHz. Human adults above 30 years old have hearing loss they can't even reach that 16 kHz.


What exactly am I hearing, then? Perhaps I may be crazy with the compact-fluorescent, but I am 100% sure that I can hear ultra-sonic bug alarms, many LEDs, and noise from AC adapters.

Plagued by "inaudible sound"

Reply #3 –
The Nyquist theorem says absolutely nothing about what people can or cannot hear.

I can't answer many questions here. Physiologists and psychologists have made extensive investigations of human hearing. I'm sure the results are scattered through hundreds of published articles and textbooks. I'm reasonably sure some people's hearing goes above CD's cutoff (22050Hz) but probably not a great deal higher (24 or 26kHz?).

I read one Japanese study using recorded music of an instrument that produces frequencies of 30kHz and a little higher. The researchers made brainwave measurements that showed at least some people responding to these higher frequencies, although they had no conscious awareness of the fact. The high frequency drivers were mounted very near the subject's head.

Other studies have strongly suggested that starting somewhere not too far above 22kH, some people can sometimes "hear" higher frequencies, but not directly through their ears. The sound is conducted from outside through body tissues and along the bones.

Higher frequencies are absorbed by air to a much greater extent than lower frequencies. They tend not to travel very far, so detection must generally be near the source.

Something such as LED switching on and off probably emits a very broad range of frequencies, all at a very low level. The sound might be from some part of the power supply circuit that controls the blinking rather than from the LED itself. I suspect the mechanism is that something is made to vibrate due to being suddenly hit with a surge of power. Many of the sources you mention may well emit a wider range of frequencies than  your basic information on them suggests.

Plagued by "inaudible sound"

Reply #4 –
Quote from: AndyH-ha on
The sound might be from some part of the power supply circuit that controls the blinking rather than from the LED itself. I suspect the mechanism is that something is made to vibrate due to being suddenly hit with a surge of power.


That's very possible, as it tends to only be LEDs that are only on when charging, or in my razor's instance, the LED that lights up once it's done charging. Thanks for the info, too.

Plagued by "inaudible sound"

Reply #5 –
Quote from: MasaMuneCyrus on
So these would be my questions:
  • Just how high have humans been recorded to hear?
  • Is the kind of equipment I'd need to test for noise this high-pitched prohibitively expensive?
  • What reasons would there be for an LED or an AC adapter to emit a high-pitched noise?
  • Are there any articles about noise above the 20 kHz range? (All I've found was one that talks about how the test subjects perceived a quality difference in a sample of music when >20kHz noise was played, even though they said that they couldn't hear the high-pitched sound that was being played.)
Thanks for your time,
Brian


You've already had a wrong  reply - (Nyquist indeed!), and this one is going to be incomplete, but here are a few partial answers that may help you find your answer.

How high can humans hear? Don't know - but it is plausible that with a few billion humans there is a little tail to the distribution - I don't know how many genes determine the potential HF cutoff of a pair of ears though.

To measure the sound you would need a microphone extending above 20 kHz, and a means to record it with substantially more than 40 kHz sampling rate (that is what Nyquist does say).  There was a thread on bats some months ago that may prove interesting in this respect.  There is probably a way to do it for a few hundred $, but I don't have a certain recommendation. Make sure you can really hear test tones at 20 kHz before embarking on this though (see below).

I like the logic of your claim to hear the LED - but it is somewhat more likely to be another component in the charging circuit.  These chargers use some kind of DC-DC converter, usually with a little inductor which emits sound through magnetostriction.  They usually have variable frequencies in the range of a few kHz or up.  I can hear to about 17 kHz (or could last time I checked - but see below: I did some experiments while writing this) and can hear these easily - they are soft but not terrily high.  Often they are loudest (and perhaps also lowest) when trickle charging (I've not thought through why that is).

Your comment on synesthesia is interesting too, I had not known that it happens with sounds as such (a nice blue sound, or an orange flavoured sound - interesting thought; or what about the sound of the number 5 ...).

I become a bit sceptical when you mention 40 kHz.  Just because those lamps use a switching rate of 40 kHz does not mean they can only radiate at 40 kHz: there could be non-linearities that allow them to produce small amounts of ~20 kHz too (if something makes every second pulse a litte stronger for example).  While modern ballasts run in the 40kHz to 100 kHz range, older ones can be as low as 18 kHz. Mechanical compact fluorescents are easy to hear.

I'm assuming you have tried something like using Foobar2000 to play tones through headphones to get an impression of your hearing ability? Just add a "location" tone://Hz,seconds.  At 0dB on my laptop played through HD580s I just heard 19 kHz, but that is way down my response curve so I'm not doing that for long (far too high SPL). I could not hear a soft sound at 19 kHz. There was some noise along with the tone (you can still hear the difference between a pure tone and noise at the limit of hearing - that makes me puzzled about some of your comments: noise is hissy tones are not...).  That is cheap and easy if you have decent headphones (likely to work at 20 kHz). 
If your claim is right you should be able to hear a soft tone at 20 kHz.  If not you can end your experiments there.


If you measure, you'll have to make sure your measurement apparatus is linear enough so that if fed a mixture of ultrasonics it can faithfully record that without down-mixing into the audio.  I have no idea how hard that is to achieve at low cost.  A cheap starting point might be to try to record with a 48 kHz sound card (there might be one with a filter above 20 kHz, though below 24 kHz, of course) and a sensitive microphone that extends above 20 kHz (a dynamic mic that is only a few dB down at 20 kHz, should I would guess, still be OK at 22 kHz for example, in most cases). Record and use Audacity or some such to see what is there.
You'll need to check the details at each stage to make sure you are doing something sensible, and there are
lots of traps in the design of the experiment you want to do.  Unfortunately we can't do it for you, as it is you who claim to have the extraordinary hearing! 

In the examples you cite there could easily be normal sound, as well as the ultrasonics.  BTW I find TVs intolerable (UK, so PAL line frequency, yours may differ). 


Have fun,

Ken

Plagued by "inaudible sound"

Reply #6 –
I think you just hear high tones better/louder than the average person. And you're probably not hearing ultrasonic sounds.

E.g. I have an AC adapter and some other components that make a high pitch sound.

At a party once, there was a fridge emitting an annoying sound. The funny thing is that nobody noticed it at first. Initially only one person started complaining about that sound. And when other people started listening for it, they noticed it too. But once you hear it, you can't suppress hearing it.

Plagued by "inaudible sound"

Reply #7 –
Hearing bats is probably still somewhat inside the normal range of hearing. I do too, although only as single clicks, with no further details, if there are any. And once i've told a friend of mine what to pay attention to, and he heard them as well. He probably just didn't realize before, that bats were the source of that sound.

It depends on the bats though, some make clicks that are easier to hear than others.

Plagued by "inaudible sound"

Reply #8 –
Can I ask how old you are? High-frequency hearing changes rapidly with age, and there's a device called The Mosquito IIRC which is used in England to disperse groups of teens -- olds (like >19) can't hear it.

An audiometrist could, presumably, check your hearing, and if you're near a school of audiometry they might do it for free.

Really good hearing can be a mixed blessing.

Plagued by "inaudible sound"

Reply #9 –
Quote from: MichaelW on
Can I ask how old you are? High-frequency hearing changes rapidly with age, and there's a device called The Mosquito IIRC which is used in England to disperse groups of teens -- olds (like >19) can't hear it.

The samples available on the internet (it has been discussed here too) are quite audible to me. I'm >30.

Bourne, I just tested myself again and I could hear 17kHz just fine.

edit: I bet there are many hear in the same position. I'm by no means a rare exception and don't consider myself to have great hearing.
daefeatures.co.uk

Plagued by "inaudible sound"

Reply #10 –
You can always use software like Logic Pro to synthesize a "pure" sine wave at a known frequency and test your upper limit in real time.  You might find that those "ultrasonic" sounds you are hearing are actually well below 20kHz.

While it would be best to run at 96kHz or 192kHz sampling rates for maximum high frequency bandwidth, you can probably still learn a lot even if your system is limited to 48kHz or 44.1kHz.  No matter what, this won't be an ideal test because there are doubtless many filters between the samples and your ears which are reducing the amplitude of upper frequencies.  But at least you can get some idea of what you can hear.

Plagued by "inaudible sound"

Reply #11 –
Quote from: MasaMuneCyrus on
For instance, those devices that you plug in that are supposed to emit a high-pitched noise that scares bugs away -- I can hear them. I can hear bats. I can even hear things that I never knew emitted noise like, for instance, certain LEDs.

The "bug-scaring" devices emit a tone below 10khz I think.
I can hear bats too 
LED's can't emit noise.
Quote from: MasaMuneCyrus on
I can't hear all LEDs, but some LEDs most definitely emit a noise. The first time I noticed was when I walked into the bathroom in the middle of the night and realized that I heard a high-pitch noise that alternated between off and on every second or so. I looked to find the source and noticed that my electric razor's charge light was blinking on and off at the same time that I could hear the sound go on and off. I also noticed that my DS Lite's charging LED emits a noise. In fact, the AC adapter that plugs into the wall, as well as many other AC adapters, emit a high-pitched noise when you plug them into the wall.

Many AC adapters use switching power supply which is supposed to run above audible range, but as it ages, its frequency sometimes drifts in the audible range. Another possibility is that the adapter could emit sub-harmonic frequencies (which you can hear).
Quote from: MasaMuneCyrus on
And the coup de grace, I can hear the new Compact-Fluorescent light bulb that I bought for my closet. And it's not a random, cheap, off-brand CF bulb, either; It's a GE CF bulb that complies with Energy Star's requirements. And, according to Energy Star's official website, the operating frequency of an Energy Star-compliant compact fluorescent light bulb has to be greater than or equal to 40 kHz.
...When I turn on the light, it sort of just sounds like adding a very faint bit of white noise on top of the already white-noisiness of complete silence.

If you can hear just white noise then what you are hearing is for sure not the operating frequency of the bulb, but probably just random vibrating of some of its components (filament?)

If you want to check how high you can hear, and your sound-card is capable of 96khz or higher sampling rate (if you want to test above 22khz), then just generate some tones of different frequencies in audio software and listen to them.

Plagued by "inaudible sound"

Reply #12 –
Quote
What reasons would there be for an LED or an AC adapter to emit a high-pitched noise?


They're probably using a crappy switched mode power supply running at an audible frequency.

Plagued by "inaudible sound"

Reply #13 –
Thanks for the replies. I, too, am highly skeptical of the Compact-Fluorescent light, myself. Perhaps I could be hearing the compact-fluorescent bulb simply because it's heating up and making things glow, similarly to how sometimes you can "hear" really hot things (i.e., a candle, a piece of metal glowing orange) if you listen hard enough, though it's not really a tone, just a white noise of some sort.

In any case, I've listened to generated tones before and they got up to about 22 kHz before the speaker couldn't produce anything higher, and I could still hear it (albeit, just barely). I'd like to find some speakers that can play sound beyond what I can hear, then take it home and try it out at night in complete silence to definitively test my hearing. Unfortunately, I just tried the tone:// generated tones on Foobar on my PC here at home (both headphones and speakers) and it seems to have a cut-off point of about 22 kHz, as well (23 kHz is a lower pitch and I'm reasonably sure that 24 kHz doesn't play at all since 23 kHz is easier to hear than 22 kHz). Perhaps I could try my cousin's new computer with some good headphones this weekend.

Thanks for the replies, though. I'm still interested in why certain LEDs seem to give off noise (my cousin's surge protector is particularly loud, but in that case it could be the surge protector itself, and not the LEDs on it). Whatever the noise comes from, if an LED is incapable of producing it, it surely has to come from the mechanism that turns the LED on, because in the case of my DS Lite and Razor the noise only happens when the LED lights are on, and not when they're off.

Maybe I'll try looking around some more for what could be causing LEDs to do this. And I'll also have to find some speakers/headphones that are capable of producing sound at least up to ~22 kHz, to see if I can hear it.

Oh, and I'm 17, in response to the age question. So perhaps in a couple of years, I'll 'outgrow' the problem. 

Plagued by "inaudible sound"

Reply #14 –
Many headphones, even in-expensive ones, can deliver more high frequency to your ears than speakers. Using anything that has not been precision calibrated will not, can not, give you results that are anything more than an indication or impression, but perhaps that would be more satisfying than no information.

I could not work around CRTs without earplugs into my mid 30s. Television was often unpleasant, and if I did watch it was from as far away as the room would permit. Flourscent light fixtures were also often unpleasant. A few years of abusive living (audio wise) changed all that, but it was probably due for change due to age anyway. Hearing hf is a mixed blessing at best.

Plagued by "inaudible sound"

Reply #15 –
I hate that CRT noise, I can even tell if a tv is on in my house when I walk in the front door. The biggest noise-maker is an old ~27" magnavox, I can't stand watching anything quiet on it, just stuff that drowns out the noise.
we was young an' full of beans

Plagued by "inaudible sound"

Reply #16 –
I think this is akin to some people being able to hear differences between different speaker cables.   
It is known that humans can sense very low frequency sounds (even though we can't hear them), but high frequencies are a little different.

In any case, what you hear are most likely just an ordinary high frequency sounds produced by a switching PS or something similar that usually contain a coil, high frequency transformer, etc., as the sound emitter.    The human hearing at high frequencies is very directional and not very good at all, so usually you need to be in the right position to hear it, and turning your head just a few degrees in either direction could make the sound inaudible.

As far as I know hearing is usually determined by using simple tones, and what is normally accepted (20Hz - 20kHz with possible high frequency drop-off) is very accurate and there is no point debating it.

However, I have read about one experiment that was done with some people who had various degrees of hearing loss, such as they couldn't hear above 10, 12, and 15 kHz.  However, they could reliably differentiate between 2 pieces of music that were identical except that one had the high frequency components above the respective subject's hearing range filtered out.  This was in an old book (late '70's, I think) about acoustics and loudspeakers, so the experiment it mentioned must have been done quite a while ago. 

Now, anyone who knows a bit about filters should know that it's not possible to make a 'perfect filter' (forget about digital filters and signal processing -- that wasn't available back then) and that a low pass filter not only cuts off the high frequency components, but it will also have some impact on the frequency response below the cutoff frequency.  And on top of the frequency response, the filter will usually have a significant effect on the phase of the higher frequency components, and that may also be audible under certain circumstances.

So while I have no reason to doubt that the subjects in that test could distinguish between the unaltered and the low-pass filtered music, I have a feeling that it may have been due to something else then simply the presence or lack of the higher frequency sounds that were beyond the hearing range of the subjects.

We know a lot more about filters (LC) and digital filtering, so it would be interesting if a test like the  above could be repeated in a carefully controlled, double blind experiment.

Plagued by "inaudible sound"

Reply #17 –
Remember that the nonlinearity of every system can generate harmonics in the audible range.

If you have a bad audio card or bad headphone, you could hear some intermodulation distortions. (musicians know them as "tartini" sound)

maybe this is interesting to read:
Hypersonic sound

Plagued by "inaudible sound"

Reply #18 –
Quote from: Bourne on
The Nyquist Theorem says we can not hear anything above 20 kHz. I barely can hear 16kHz. Human adults above 30 years old have hearing loss they can't even reach that 16 kHz.

Why would you quote a theory you don't understand and have clearly never read?  The Nyquist–Shannon sampling theorem has absolutely nothing to say about anything that you assert it does.

To the OP... the trouble with your argument is asserting that you know the frequency of the sounds that you're hearing.  You don't.  They certainly aren't "ultrasonic" and you weren't born on the planet Krypton. 

If you care to really sample things, create a CD made of of sine-wave test tones (easily done in any audio editor).  Record tracks of 14kHz, 16kHz, 18kHz, 20kHz, and 22kHz.  The CD format can reproduce all of those without problem (and THAT is what Nyquist-Shannon posits).  Find a stereo system with decent components including speakers with a frequency response rating that goes up to 22kHz +/-3dB.  Look away from the system and have a friend pause and play each track with you holding up your hand every time you hear a tone.  Just how high you can hear should become pretty clear in no time.  Don't use headphones as most commercial headphones don't really reproduce sound all that well.

Plagued by "inaudible sound"

Reply #19 –
Quote from: MasaMuneCyrus on
Thanks for the replies, though. I'm still interested in why certain LEDs seem to give off noise (my cousin's surge protector is particularly loud, but in that case it could be the surge protector itself, and not the LEDs on it). Whatever the noise comes from, if an LED is incapable of producing it, it surely has to come from the mechanism that turns the LED on, because in the case of my DS Lite and Razor the noise only happens when the LED lights are on, and not when they're off.

Maybe I'll try looking around some more for what could be causing LEDs to do this. And I'll also have to find some speakers/headphones that are capable of producing sound at least up to ~22 kHz, to see if I can hear it.


Did you read my post?  Its not the LED making the sound, its the power supply connected to the LED.  You only hear the sound when the LED is on because the LED cannot be on without a power source.

Plagued by "inaudible sound"

Reply #20 –
Quote from: MasaMuneCyrus on
Thanks for the replies, though. I'm still interested in why certain LEDs seem to give off noise (my cousin's surge protector is particularly loud, but in that case it could be the surge protector itself, and not the LEDs on it).


You could read the answers people have already given in response to your questions, I think you'll find you cannot hear the LEDs in these applications.

Ken

Plagued by "inaudible sound"

Reply #21 –
Quote from: MasaMuneCyrus on
Whatever the noise comes from, if an LED is incapable of producing it, it surely has to come from the mechanism that turns the LED on, because in the case of my DS Lite and Razor the noise only happens when the LED lights are on, and not when they're off.


I did read the replies, and I mentioned that an LED "is incapable of producing [the noise]." To make it clear, I know that LEDs can't emit noise, but I'm interested in the fact that the only ones that 'appear' to make noise (i.e., the LED is not making noise, but something is making noise at the exact same time that the LED is turned on and it stops when the LED is turned off) are charging-related ones. And so I what it is that often makes noise when it powers an LED, but doesn't make noise if the LED is there. And to be clear, the said noise is coming from the product itself, not the AC adapter. I've mentioned before that sometimes the AC adapter itself makes noise, but in addition to that, certain products make the noise when they have to power a charging-indicator LED. And so again, while it's obviously not the LED that is emitting the noise, it's something inside of the product that ONLY makes noise when the product lights up the LED.

I like the idea of putting some sine waves on a CD and trying them out on a stereo; I hadn't thought of that. And yes, I'm not from Krypton; I didn't come here to gloat about some super-human ability or laugh at everyone, I came here to get some questions answered. I assumed that most of the replies would be littered with flames, but you guys have been nicer than expected and have definitely answered some questions. So thanks for that.

Plagued by "inaudible sound"

Reply #22 –
Transformers have physical wire wrapped around a magnet. When current flows through these wires, it can cause them to vibrate. When the current stops flowing, the vibration stops, too. Even devices with an external AC adaptor may have step-down transformers inside.

What you could use is a calibrated mic (expensive) or some regular mic where you can determine its actual frequency response.  Then you can record these various sounds that you're hearing and determine their frequency.

Alternatively, you could get a computer with more than 24kHz of bandwidth, and generate some tones above that.  You might still want a known microphone as confirmation, since your speakers, amplifier, and converters all have lowpass filters which might be dropping the amplitude of your test tones.

Plagued by "inaudible sound"

Reply #23 –
Quote from: MasaMuneCyrus on
Quote from: MasaMuneCyrus on
Whatever the noise comes from, if an LED is incapable of producing it, it surely has to come from the mechanism that turns the LED on, because in the case of my DS Lite and Razor the noise only happens when the LED lights are on, and not when they're off.


...but something is making noise at the exact same time that the LED is turned on and it stops when the LED is turned off) are charging-related ones. ...


I can confirm noticing this with my laptop, but only ever late at night, when it's switched off and charging. The noise can't be coming from the AC adaptor as it is under a pile of junk under my desk. In this case the noise is continuous, and its definately a high pitched tone. Despite my best efforts to protect it, my hearing is not at all good, i'm 18 but guitar playing, working with powertools in the holidays, concerts and a scarred ear drum (after it was perforated in an accident) have changed that, I hear a lot of whitish noise most of the time. I just did a test with foobar2000 and could hear 17800Hz clearly but could only just notice 18000, although this may be due to the crapness of my laptop. I did the test at about -30dB, but it still seemed quite loud so I was afraid to turn it up at 18000.

Plagued by "inaudible sound"

Reply #24 –
Quote from: aurgathor on
As far as I know hearing is usually determined by using simple tones, and what is normally accepted (20Hz - 20kHz with possible high frequency drop-off) is very accurate and there is no point debating it.
I wouldn't use the phrase "very accurate" when referring to these tests.  The tests report an average range, not the most extreme. Most human tests are more about overall statistics than establishing the extremes of individuals. MP3 testing shows that maybe 97% of people cannot hear problems with 128k, but that doesn't mean nobody can hear it.  If someone has a link to the details of the 20Hz - 20kHz tests, I would enjoy reading that.  I suppose it's too much to hope that the original research is online.

In my early 20's, I walked into a jewelry store and "heard" the ultrasonic alarm. It was a combination of a sensation of pressure and sound. I suppose it may have been set to a very high SPL. There may also have been subharmonics. My point is that most people cannot hear these things, or people wouldn't buy them to install in their store. But that doesn't mean nobody can hear them.