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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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Propeller Head Plaza Technical and scientific discussion of amps, cables and other topics. |
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In Reply to: It's old news (2005) - but I live in the woods... Tests confirm hearing above 22 kHz. posted by carcass93 on August 31, 2009 at 15:07:58:
It is true you can hear say 25KHz if it is loud enough, I used to be able to hear the 40KHz carrier that old burglar alarms used too. It was often 100 - 110dB SPL typically.
Loud enough is the key issue however.
I used to work with ultrasonic acoustic levitation systems for containerless manufacturing and got to play with / make sound sources that operated up to about 76KHz. One array of sources could produce 172dB SPL at 22KHz, enough intensity to levitate a sample of lead or with acoustic friction, light a cigarette.
Even though these were highly directional, hearing protectors were needed.
As the investigation here showed, at 88dB SPL, some of the people could detect the presence of a 24HKz tone.
It is also a far jump to interpret that as meaning this is important information to reproduce or that it would be audible in the presence of other signals.
Keep in mind that you can “detect†a tone in the 3-4KHz range 700,000,000,000 times easier than at 24KHz.
In your ears hear in frequency slices, called a “Barkâ€, about 1/6 octave wide in the mid band, two equal loudness tones within the same bark are heard as one tone. One tone is masked by the other or hidden from your ears detection like how masking systems which produce broadband noise, shortly becomes inaudible, but can still mask distant conversations in a office cube farm.
If you have a single tone one finds there is a ‘masking slope†on either side of it, in the mid band this slope is about –10dB per octave.
So, if one produced a tone at 2KHz and a second tone an octave higher, the higher tone has to be greater than –10dB relative to the lower, to be detected.
Your ears are not “flat†in response, they have a response that favors the 2-4KHz range with a roll off on either side. At the low end, your Eustachian tubes provide a “leak†to the back side of your eardrum, this prevents you from hearing excessive levels of very low frequencies so weather changes don’t deafen you. Up high your ears roll off also.
Refer to the equal loudness curves to see the response and change in response with level average ears exhibit.
http://hyperphysics.phy-astr.gsu.edu/Hbase/Sound/eqloud.html
This response affects masking thresholds tremendously.
For example at low frequencies, for example at 20Hz, your ears decreased sensitivity with lower frequencies, against a silent background, makes about 7% 3rd harmonic at 60Hz have the same apparent loudness as the fundamental at 20Hz. As a result most people have never heard distortion free bass.
Up high, one can see that the sensitivity to say 5KHz is about 20dB or 100 times greater than the sensitivity at 15KHz. This, combined with the masking curve means that against a silent background, the third harmonic of 5KHz at 15Khz, has to be about 7-8dB louder than the fundamental to be detectable against a silent background.
That is to say and as work by Earl Geddes and others suggest, that one is extremely tolerant of high frequency distortion because of your ears decreasing sensitivity the higher the components are.
In the case of 24KHz, lets pretend one has more than just 24KHz present like in the study.
Lets say you had music as the signal a broad band thing, lets say that you had energy around 8KHz. From the equal loudness curve one can see the threshold for detection is about +15dB SPL. We know from the test that at 24KHz, a few people were able to detect it at 88dB SPL so one can mentally extend the equal loudness curve too.
Using the masking slope and differing sensitivity, one finds that to be detectable, the 24K component has to be about 60dB ( a million times) louder to be detectable with an 8KHz signal.
The point I guess is that while one “can†hear above 20KHz, one would also find that even the sound of steam escaping has a spectrum which self masks higher frequency components to your ears above some frequency.
Any kind of musical signal or broad band sound made by a mechanical device will produce a spectrum which has the lower HF components having more energy than the highest HF components. This tilt, further increases the difficulty perceiving the very high components in the presence of lower frequency HF components
Now, a point to ponder. Work by Gary Kendall at Northwestern showed that in some cases a person could detect an interaural time difference as small as the equivalent of a 200KHz signal. This is not “frequency response†but the ability (with headphones) to detect a tiny time difference between ears, part of “stereo hearing in X, Y and Z†with only two points of reference..
Considering how poorly most loudspeakers preserve time, that has proven to be a good area to work in.
Best,
Tom Danley
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- Yes but........ - tomservo 09/3/0909:20:58 09/3/09 (0)
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