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Can you HEAR an E-M wave ?

by smithpa9
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smithpa9
#1
Dec22-04, 05:30 PM
P: 33
In Ian Johnson's 1989 book, Measured Tones: the Interplay of Physics and Music, he says that in 1906 Reginald Aubrey Fessenden used an oscillating electric current to create a (first ever?) communication system for ships in the North Atlantic.

p. 283-4 says: "He chose a frequency which was too high to be heard--the actual value he settled on 200,000 Hz--and since then values between 1 and 10 times this number have become standard."

Am I reading that correctly??? Sounds like he was concerned that if he used a lower frequency, humans would be able to hear the transmission without use of the radio receiver. . . directly hearning the wave with their ears. How can you HEAR an electromagnetic wave, even if it was in a frequency range that our ears can hear?



Also, I posted a question in the Software group that didn't get an answer, and it might be more appropriate for a sound or acoustic expert that might have been attracted to this question. Here it is:

I'm studying a bit about acoustics and music by reading books on these topics. But without being able to hear the sounds, and see how they combine on an oscilloscope, it's hard to really learn the material.

I would like to find a computer program that would allow me to create musical notes of any frequency I want (not just those of a standard, equal temperment piano keyboard), combine them, and also see how the resultant sound would appear on a oscilloscope.

Anyone know of any program I could get that does those things?

thanks,
Paul
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T.Roc
#2
Dec22-04, 06:14 PM
P: n/a
paul,

have you searched under "fourier analysis"?

also, why are you interested in a tone generator that will produce off note frequencies if you are trying to learn music?

does not a speaker produce sound for our ears through the interaction of EM waves?

TRoc
smithpa9
#3
Dec22-04, 06:33 PM
P: 33
Thanks T.Roc. I'll try searching under fourier analysis.

The reason I'm interested in non-standard frequencies is that the "standard note" frequencies of the equal tempered scale in wide use today were not always what was used in the past, nor in other parts of the world. I want to be able to hear what other musical scales sound like, other than what's on a standard piano keyboard today. the "Just Scale" for example.

I also want to be able to hear for myself some of the phenomena that occur when listening to two notes simultaeously that are close together (beats) or far apart, or somewhere inbetween. In general, I want to be able to perform for myself some of the demonstrations that might be done in a class on acoustics.

Lastly, yes of course a speaker can produce sound using EM waves. The quote in Johnson's book makes it sound like he is concerned that if he used too low of a frequency in his crude radio transmitter, that people walking down the street would be bombarded with his radio transmission AND BE ABLE TO HEAR IT, without the use of a radio receiver, or speakers, or anything. Just using their ear, they could directly hear a radio transmission if the frequency was low enough.

Is there any truth to that? Or do you interpret that passage differently that I did?

Integral
#4
Dec22-04, 07:29 PM
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Can you HEAR an E-M wave ?

To hear something we must have mechanical oscillations with a frequency in the audio range. You cannot hear a EM wave because it is not a mechanical vibration. HOWEVER, EM waves can and do cause currents in conductors, these currents can induce mechanical vibrations which can be heard. The frequency of these vibrations will be either the fundamental of the the EM or a harmonic of the EM. Thus you can "hear" 60Hz hum, it is caused by mechanical vibrations caused by currents induced by 60Hz power. Any EM wave of sufficient strength in the audio frequency range can induce mechanical vibrations which can be heard.
smithpa9
#5
Dec22-04, 08:16 PM
P: 33
Wow. . . so Johnson's concern was a valid one. At least for anyone who was within a few meters of the transmitting equipment.

Thanks. :-)
brewnog
#6
Dec22-04, 08:47 PM
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Hearing 'beats' is easy, no need for any lab equipment at all.

Grab yourself a guitar, choose a string and keep plucking it. Then choose another string, and play around trying to get the pitch of that to *almost* match the pitch of the first. Pluck them at the same time, and keep playing around with the pitch of the second. You'll hear it pretty easily.
russ_watters
#7
Dec23-04, 01:03 PM
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My guess is he's talking about the carrier wave frequency coming through the speaker, but my EM theory is a little thin: if the transmission is sent at an EM frequency in the audible frequency range, will you be able to hear the carrier wave itself through the radio speakers?
pervect
#8
Dec23-04, 04:02 PM
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The early radio recievers no doubt used amplitude modulation and envelope detectors.

See for instance

this link

You would have components of the carrier in the demodulator with this sort of demodulator.

You'd have similar components with most demodulators that I can think of. Even an ideal synchronus demodulator will generate components of twice the carrier frequency, and most actual implementatiosn of the same will not be perfect and also generate some output at the carrier frequency).

(a brief description of a synchronus demodulator can be found here
an ideal mixer being fed frequencies of f1 and f2 is a multiplier which generates only f1+f2 and f1-f2 as output frequencies. Real mixers will have small components of f1 and f2 in the output of the mixer, hence my above remarks.
Integral
#9
Dec23-04, 05:08 PM
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Quote Quote by russ_watters
My guess is he's talking about the carrier wave frequency coming through the speaker, but my EM theory is a little thin: if the transmission is sent at an EM frequency in the audible frequency range, will you be able to hear the carrier wave itself through the radio speakers?
There are several reasons you do not want your carrier frequency near your data range. Amplifiers are indiscriminate in the band of frequencies they are designed for. So if you used a carrier of a similar frequency from the desired signal it would be impossible to separate them since you have to amplify the carrier along with the signal. Any information sent at the carrier frequency would be lost. So yes, if your carrier were in the audio range you would hear it in the speakers.

Also the carrier would limit the upper frequency of the signal, you cannot modulate a carrier with a signal of higher frequency then the carrier. For reasonable fidelity your carrier must be much higher frequency then the highest frequency in your signal.
T.Roc
#10
Dec23-04, 06:55 PM
P: n/a
paul,

i think you are confusing "scales" with "tuning". you can experiment with "eastern" scales (japanese, arabic, etc.) without changing your tuning. no matter where you go in the world, given an instrument of minimum complexity, you will find that they have tuned it to within a few degrees of A = 440hz.

all scales begin with the "tonic", and proceed harmonically from there. if you throw in a dissonant note for interest, such as in a blues scale, our rule says you must resolve this tension by bringing it back to consonance with the tonic. basically, eastern scales use what we term as "dissonant", and then harmonize it with other dissonant notes. music is mathematical, so just think of + & - #'s. 2 +'s = +, and 2 -'s = +.

try using steps 1, 4, 7, 9, 11 on the 13 step "system", starting with a sharp note.

TRoc


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