The human ear and examples of natural heterodynes

A common way of tuning a musical instrument against a tuning fork or other reference is to get both of them sounding at once, listening to the beat frequency, and reduce it so that it first becomes a slow waah, waah, waah instead of a tone, and finally is no longer detectable at all. At that point the instrument matches the reference within some small fraction of 1 Hz. I suppose you could get a "tree falling in the forest with no one around" argument going about whether the human ear is "doing" the mixing, or merely "detecting" it.

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Bill Snyder   [This space unintentionally left blank.]

Bob

--

"Things should be described as simply as possible, but no simpler."

                                              A. Einstein

The tinnitus is a defect in the nervous system, the nerve ending is sending a false signal to the brain. Frequency selective deafness is a result of fine hairs in the cochlea breaking off. It is these hairs that vibrate in sympathy (resonate) with the sound and stimulate the nerve endings. They fracture with age and with use. Your physics teacher was right, ears do not whistle. Perhaps he misunderstood your question as I first did, I had it to read it through twice because you seem to imply that the ear causes sound to be emitted. There is no good reason why a young and healthy ear should not detect a heterodyne tone.

sending

As I recall, the fine hairs are regenerative recievers, so "a defect in the nervous system" could certainly include a miscalibrated amount of feedback.

It's known that tinnitus sufferers literally do have "ringing in the ears".

Let's see what Wikipedia says about it:

Not a hetrodyne, but a difference easily. The amplitude modulates with an apparent cusp (not sine wave) envelope. I would suppose the ears aren't as great at detecting the phase between cycles, making it appear to modulate.

Tim

-- Deep Fryer: A very philosophical monk. Website @

: Let's see what Wikipedia says about it: :

Trusting wackypedia, the crap anyone can write, is like trusting a politician.

: : > There is no good reason why a young and healthy ear should not : > detect a heterodyne tone. : : Not a hetrodyne, but a difference easily.

Let's see what wackypedia says about it.

"This article or section contains too much jargon and may need simplification or further explanation."

"heterodyning is the generation of new frequencies by mixing two or more signals "

: The amplitude modulates with an : apparent cusp (not sine wave) envelope. I would suppose

We'll ignore suppositions and "not sine waves" and go directly to simple sine wave addition.

Looks to me as if Greysky (whom you rudely snipped even it is his post I was answering) experienced heterodyning as he claimed. Yes a difference, and a heterodyne, easily.

Well actually they can and do!

For example:

Otherwise, just search for "otoacoustic emissions".

John

In the case of binaural beats, it'd be hard to argue that we are only detecting.

--
Army1987 (Replace "NOSPAM" with "email")

Yeah, and bullets can fly. Whistle: 1 a: a small wind instrument in which sound is produced by the forcible passage of breath through a slit in a short tube b: a device through which air or steam is forced into a cavity or against a thin edge to produce a loud sound 2 a: a shrill clear sound produced by forcing breath out or air in through the puckered lips b: the sound produced by a whistle c: a signal given by or as if by whistling3: a sound that resembles a whistle; especially : a shrill clear note of or as if of a bird

If I notice a puff of air coming from the side of your head then I'll probably conclude the cavity is filled with air and not vacuum :-)

I guess this is the definition you had in mind.

In particular, the ear can generate difference frequencies between two simultaneously presented acoustic sine waves which are detectable with a probe microphone placed in the ear canal.

It is also possible to detect an echo from a click stimulus using a probe microphone.

Both of these responses are diminished or absent when the cochlea is damaged by noise exposure or ototoxic drugs.

John

Definitions are indeed important, English is vague. However, the intent is generally understood from the context. : : In particular, the ear

: It is also possible : : Both of these

Which although interesting isn't relevant to Greysky's post. What is less interesting is that you didn't respond to Greysky's post, probably because like so many others you have a short attention span and prefer to snip, making it more difficult for others to know what Greysky's post was about and so it all crumbles into triviality while you spout what interests you and fail to answer the original post.

Speaking of heterodynes, there's a clever stunt anyone can do using radio frequencies instead of acoustics. It's a quick-and-dirty 'Theremin' using three broadcast-band radios:

Radio# 1 is set to any *blank spot* in the upper end of the band, anywhere around 1300-1600 khz. It is now prepared to receive on that 'clear channel'.

Radio# 2 is *manually tuned* (not digital) and is the "transmitter". It is placed in close proximity to Radio# 1 for close coupling. Back- to-back usually gives the strongest coupling. Tune Radio# 2 slowly through the lower portion of the band until its local oscillator* (LO) is heard as a strong 'rushing' sound on Radio#1. Leave it set on that spot. This unmodulated carrier is now the victim of our attention. :-)

Radio# 3 is a Walkman type, also *manually tuned* (not digital). It is brought into close proximity to Radio# 2 for close coupling. Tune it slowly thru the lower portion of the band until its LO is heard as a whistle 'beating' (heterodyning) against Radio# 2's LO. Tune for zero beat. You'll hear a a "Weeeoooop..." as the tone approaches zero beat. Now you can play the 'Theremin'.

Squeeze and massage Radio# 3 (the Walkman), which causes its LO frequency to vary enough to produce a large, warbling shift of the beat note, which is heard on Radio# 1. 'Tremolo' (amplitude shift) is done by rapidly moving the Walkman away from / toward the other pair. With minimal practice you can play 'spook music' rivaling anything heard in sci fi genres or by the Beach Boys.

*In American practice at least, the local oscillator (LO) frequency is pretty much standardized as 455 khz, which means it tracks 455 khz above the received signal. F'rinstance, if you're listening to a station at 900 on the dial, the set's LO is running at 1355khz.

google "SOAE" and you'll fine stuff like...

John

Yes, the LO tracks to produce a constant frequency for the IF amplification stage prior to detection of the audio signal. No matter what station the radio is tuned to, the IF strip only sees one frequency (which is why heterodyning is done, of course). Interesting that moving the Walkman rapidly is producing Doppler shift, so it is motion dependent and not position dependent.

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com... : On Thu, 03 Jan 2008 08:41:20 GMT, "Androcles" : wrote: : : >

: >"greysky" wrote in message : >news:Kn%ej.2661$ snipped-for-privacy@nlpi070.nbdc.sbc.com... : >: 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... : >: : >: Greysky : >

: >The tinnitus is a defect in the nervous system, the nerve ending is sending : >a false signal to the brain. Frequency selective deafness is a result of : >fine hairs in the cochlea breaking off. It is these hairs that vibrate in : >sympathy (resonate) with the sound and stimulate the nerve endings. : >They fracture with age and with use. : >Your physics teacher was right, ears do not whistle. Perhaps : >he misunderstood your question as I first did, I had it to read : >it through twice because you seem to imply that the ear causes : >sound to be emitted. : : google "SOAE" and you'll fine stuff like... : :

: : John How much should I fine it? Does 50 bucks sound ok?

Sorry, simply summing two signals to obtain an amplitude beat frequency is not hetrodyne.

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heterodyne: 1. To generate new frequencies by mixing two or more signals in a nonlinear device such as a vacuum tube, transistor, or diode mixer. (188) Note: A superheterodyne receiver converts any selected incoming frequency by heterodyne action to a common intermediate frequency where amplification and selectivity (filtering) are provided. 2. A frequency produced by mixing two or more signals in a nonlinear device. (188)

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In order to get sum and difference sine waves via the hetrodyne effect, nonlinear mixing is required. This is explained in pretty much any primer on the subject, and has been hashed to death here on SED in the past few years. With sound levels and frequencies within the range of human hearing the propogation of sound in air is linear, and therefore hetrodyning cannot occur. Greysky's physics professor was right.

Nobody will ever convince you of the existance of things that you are determined to not believe. And nothing will likely ever improve my typing. As disabilities go, I prefer mine.

John

Get two sinewave generators and two loudspeakers and try it. You certainly *will* hear the near-zero-beat difference frequency. The ear is acting as an envelope detector.

Try it.

John

No, the effect is not Doppler at this small scale. The 'tremolo' is just amplitude shift caused by varying distance. The frequecy shift of the Walkman's LO is due to minute component movements caused by squeezing/'massaging' the Walkman.

What is needed to mix two frequencies and generate a sum or difference? A non-linear device. I'm pretty certain that the ear becomes nonlinear, particularly at moderate to high sound levels. Look at the mechanics in the middle ear.

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Paul Hovnanian	paul@hovnanian.com
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Procrastinators: The leaders for tomorrow.