It may have been the "increase in frequency" part that made the professor call it impossible. The heterodyne tone would
*decrease* in frequency the closer your whistle got to the center frequency.Ed
"Robert Baer" wrote in message news: snipped-for-privacy@corp.supernews.com... : Bob Cain wrote: : : > greysky wrote: : >
: >> One night I fell asleep while watching the late movie and when I woke : >> up, the channel had gone off the air, and there was the usual pattern : >> of circles on the tv, along with a rather annoying steady tone. : >> Curious to see how close I could whistle and perhaps match the tone : >> from the TV, I noticed a quite clear heterodyne tone produced in my : >> ears. The third tone would start out low (or was it high, I can't : >> quite remember because this was 25 years ago) and would increase in : >> frequency the closer I got to the center frequency. The next day I : >> asked my physics professor about the human ear's ability to produce a : >> heterodyne in this manner, and he stated flat out that it was : >> impossible. Now, I wasn't gonna argue with the guy because finals : >> were going to start and he held my grade in his hands and he was a : >> bit of a hothead.... but I couldn't figure out why he thought it wsa : >> impossible. Since then I've asked around a few more times, butthe : >> general idea ws that the human ear could not produce a heterodyne : >> signal - yet I quite clearly remember how easily my ears created one. : >> Now, 25+ years later, my ears are not as good as they one were - : >> tinnitus, some frequency selective deafness, and general ageism have : >> taken their toll and I can no longer produce the heterodyne signal. So : >> does anyone have any theories as to whether the human ear can produce : >> a heterodyne signal? If you are a young'un I innvite you to do this : >> experiment for yourself. All you need is a steady tone and the ability : >> to whistle. Use a tone generator set to about 1.5 Kilocycles coming : >> from a single speaker 3 - 5 meters away. Begin to whistle and slowly : >> try to match the tone generator and then go a bit higher and lower. : >> You should clearly be able to hear a third tone -the beat frequency- : >> varying as the inverse of your whistled tone. Give it a try and tell : >> us your results... : >
: >
: >
: >
: > Bob : Why is it that the "pictures" show as empty boxes, but when i : right-click to view image, *see* it and go back, the box is still empty?
Your computer is naff.
Are your ears a linear device? Can you tell the difference between a
0.2 pascal sound and a 0.4 pascal sound (i.e. 80 dB and 86 dB) with the same ease you can hear the difference between 0.22 Pa and 0.02 Pa (i.e. 80.8 dB and 60 dB), or 2 Pa and 2.2 Pa (i.e. 100 dB and 100.8 dB)?-- Army1987 (Replace "NOSPAM" with "email")
Are your ears a linear device? Can you tell the difference between a
0.2 pascal sound and a 0.4 pascal sound (i.e. 80 dB and 86 dB) with the same ease you can hear the difference between 0.22 Pa and 0.02 Pa (i.e. 80.8 dB and 60 dB), or 2 Pa and 2.2 Pa (i.e. 100 dB and 100.8 dB)?-- Army1987 (Replace "NOSPAM" with "email")
While neither the ears or any other sensor in existence are truly linear, the nonlinearity of human hearing has nothing to do with the beat frequency effect caused by the linear addition of two signals. You can see the effect clearly with a microphone and oscope while carefully keeping both in their essentially linear range. You do not even need to mix the two sounds in air, you could add the signal from two microphones in different anechoec chambers and see the effect, or simply add the signals from two signal generators, same beat frequency effect is clearly visible. But this has nothing to do with hetrodyne.
Serach for the thread "multiplying two signals" to find the last SED discussion of why addition is not the same as multiplication. Or better yet find a basic signal processing text and learn how nonlinear mixing (e.g. multiplication) produces sum and difference signals, and linear addition produces only a beat frequency.
It is possible to produce a true hetrodyn effect in air, commercial products are even available which use the effect to produce audible sound from the mixing of two ultrasonic sounds, but this requires that the two sounds be at frequencies and amplitudes where the propogation of sound in air is significantly nonlinear, well outside the range of human hearing, and is entirely different from the beat frequency effect which occurs in the linear hearing range.
---
Not to nit pick, but the correct term for a periodic variation of amplitude is 'tremolo'. 'Vibrato' denotes a periodic variation of pitch. If in doubt, see Google. Gotta watch them details, minutiae and particulars(DMP). :-) oc
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Not to nit pick, but the correct term for a periodic variation of amplitude is 'tremolo'. 'Vibrato' denotes a periodic variation of pitch. If in doubt, see Google. Gotta watch them details, minutiae and particulars(DMP). :-) oc
They'll be homodyning and heteroerging next thing you know.
dyne: the unit of force in the centimeter-gram-second system equal to the force that would give a free mass of one gram an acceleration of one centimeter per second per second.
erg: a centimeter-gram-second unit of work equal to the work done by a force of one dyne acting through a distance of one centimeter and equivalent to 10^7 joule
"Heterodyning needs a non linear device" I read it in wackypedia , so that must be right.
Nit pick all you want, oldcoot, this gaggle of neanderthals are not worth bothering with anyway, few of them can spell their own name. :-)
No.
John
| | No.
Fuckhead.
No, but I can hear echoes sometimes.
Actually I was writing about beating between two real sounds. Beating between tinnitus and real sound may or may not be similar.
Let me try an example to explain the difference between a beat frequency effect and hetrodyne mixing of two real sounds in air:
Consider two directional sound sources A and B directed so as to cross each other. Within the human hearing range where propogation is essentially linear, close to either source before the crossing zone you hear only the individual source A or B. In the crossing zone you hear both sources simultaneously, and a beat frequency caused by alternating constructive and destructive interference at the detector (ear or microphone) can be heard or measured. Neither sound beam is altered by this however, and beyond the crossing again only the individual sources can be heard, completely unmodified by the crossing. Turning one source on or off has no effect on the other. This is a beat frequency, not hetrodyne mixing.
Now crank both sources up into the nonlinear range, where sound pressure is not very small compared to atmospheric pressure (oversimplifying a bit). Since these levels will damage your hearing, we will listen with a microphone. Now, not just in the crossing zone but emmanating from the crossing zone are sum and difference frequencies. Now the sound beams are actually interacting, so that turning off either source causes the sound level of the other source beyond the crossing zone to increase, decreasing when both are on because some of the sound energy is actually being converted to the sum and difference frequencies. This is hetrodyne mixing.
Both effects are equally real, but they are not the same thing.
Ok, I misunderstood the meaning of hetrodyne, I thought it simply meant distorted beating sound, as if I played a G# and a A together on the electric guitar with distortion on.
-- Army1987 (Replace "NOSPAM" with "email")
But you're the one who's always wrong. By a factor of 1e14 this time.
John
...
I wonder if the lound, near ultrasonic, tone at 15.7 kHz coming from the TV set could have had anything to do with what you heard?
Mark Zenier snipped-for-privacy@eskimo.com Googleproofaddress(account:mzenier provider:eskimo domain:com)
-- But it isn\'t. All that\'s happening around zero-beat is linear constructive and destructive addition of the two signals due to the phase difference between them. If it were truly mixing, then f1 + f2 would be audible, which it isn\'t. Try it. Additionally, the ear isn\'t acting like an envelope detector, because no modulation is taking place.
-- Yup. Excellent catch!
-- Ah, but there\'s the rub... Tremolo generally refers to amplitude modulation, which is not what\'s happening when two signals are algebraically added instead of being multiplied, which is what we\'re discussing. Also, strangely, a \'tremolo bar\' on a guitar is used to change the pitch of the strings, sometimes periodically, not vary the amplitude periodically. But you\'re right, I should have been more explicit and expanded on what I meant instead of adding to the confusion. Thanks.
-- Well, genius, if you know a way to generate AM sidebands by using a linear device (or, more correctly, by using a totally linear system) I\'m sure we\'ll all be (me, anyway) on tenterhooks until you reveal your secret.
| >"John Larkin" wrote in message | >news: snipped-for-privacy@4ax.com... | >| On Fri, 04 Jan 2008 16:47:46 GMT, "Androcles" | >| wrote: | >| | >| | >| | >| >erg: | >| > a centimeter-gram-second unit of work equal to the work done by a force | >of | >| >one dyne acting through a distance of one centimeter and equivalent to | >10^7 | >| >joule | >| >
| >| | >| No. | >
| >Fuckhead. | >
| >
| | But you're the one who's always wrong. By a factor of 1e14 this time. | Even I can make the occasional typographic error converting superscript in this antiquated text-only drool bag known as Usenet.
Well, you elected to correct my typing, so it's only fair that I return the favor.
John
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