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Author Topic: Hetrodyning in the Orchestra  (Read 3925 times)
K0OD
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« on: December 30, 2012, 11:12:22 AM »

Trying to explain hetrodyning and other electronic principles to my son, a trumpet playing EE student.

One trumpet blows a 500 Hz note and a second one plays an 800 Hz note, you get a 1300 Hz note and a 300 Hz note, right?  (sum and difference)

One trumpet plays a note and second one simultaneously plays the same note, you get a 3 dB louder note, right?

I presume a real trumpet note isn't nearly clean enough to actually do that, or is it?
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AC5UP
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« Reply #1 on: December 30, 2012, 11:30:04 AM »

One trumpet plays a note and second one simultaneously plays the same note, you get a 3 dB louder note, right?

If the frequencies and phasing are perfect, maybe, but the chances of that are exceptionally slim. Remember too that musical instruments do not generate sine waves and the timbre of the horns will (probably) not be the same.

Now, if you want to make this interesting, put the two trumpeters on a moving train. One at the front, the other in the back, and as the train passes by at 62.37894432 miles per hour a dog sitting near the track hears.............
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K0OD
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« Reply #2 on: December 30, 2012, 11:58:40 AM »

Now both trumpets play a pure note but the second one starts 180 degrees later. Would you hear nothing?

What's wrong with that analogy?  Any musicians? 

How about an "Elmer" Bernstein?
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AA4PB
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« Reply #3 on: December 30, 2012, 12:10:35 PM »

Now both trumpets play a pure note but the second one starts 180 degrees later. Would you hear nothing?

What's wrong with that analogy?  Any musicians? 

How about an "Elmer" Bernstein?

The same reason's that AC5UP gave for in-phase not likely to be twice as loud.
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G3TXQ
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« Reply #4 on: December 30, 2012, 12:32:58 PM »

Now both trumpets play a pure note but the second one starts 180 degrees later. Would you hear nothing?

What's wrong with that analogy?  Any musicians? 

Either the two sources (the instruments) would have to be exactly co-located - impossible - or your ear would have to be at a point where the two waves arrived exactly anti-phase. Once you'd got your head in the correct position for the right ear, what about the left ear Smiley

Steve G3TXQ
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AC5UP
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« Reply #5 on: December 30, 2012, 02:37:46 PM »

...what about the left ear Smiley

And for those of the Hindu persuasion, the third eye.   Roll Eyes

BTW:  Long ago and far away when the world was in mono it wasn't unusual for an orchestra to be recorded using one mic strategically placed near the third row center of the audience. With quality equipment the results were better than AM radio or a 78 rpm disk could reproduce but still had a remarkably natural sound. Since that time we've progressed through Stereo to THX Surround to 7.1 Home Theater and all sorts of clever ideas that require dozens of microphones to all but remove the ambience of the hall. And will never be as perfectly phased, natural or balanced as one mic third row center. Which could explain why modern amplifiers have a button to add a digital delay that simulates what a concert is (allegedly) supposed to sound like.

Sometimes it boggles the mind.................  Or proves there really is such a thing as too much technology.   Grin
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WB6DGN
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« Reply #6 on: December 30, 2012, 03:12:54 PM »

Quote
And for those of the Hindu persuasion, the third eye.

Yes!  I've been pondering a similar question with the advent of multiple LED devices powered from a DC source.  How do you know that all those LED's outputs will be in phase so their outputs add?  Their output is, after all, electromagnetic energy.  Could they be 180 degrees out of phase so you saw no light?  Or sufficiently out of phase that the light output would actually decrease instead of being greater?  If not, why not?  All I've gotten from it so far is a headache.
Tom
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KE3WD
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« Reply #7 on: December 30, 2012, 03:14:08 PM »

Trying to explain hetrodyning and other electronic principles to my son, a trumpet playing EE student.

One trumpet blows a 500 Hz note and a second one plays an 800 Hz note, you get a 1300 Hz note and a 300 Hz note, right?  (sum and difference)

One trumpet plays a note and second one simultaneously plays the same note, you get a 3 dB louder note, right?

I presume a real trumpet note isn't nearly clean enough to actually do that, or is it?

Don't overlook the fact that the sum and difference tones will be at less amplitude.  

Musically speaking, there are also times when the difference note, much more so than the additive, which can be at such higher audio frequencies that we assume irrelevance, will be useful.  The Perfect 4th, Perfect 5th and Octave of the Tempered Scale actually reinforce the Root.  For example, the pipe organist often utilizes this by playing a P5 with one hand or the feet on the pedals, gaining an apparent root one octave lower than the root played, at half the amplitude.  The Classical Organists term this the, "Resultant".  Fast Forward to the Rock and Roll age and we find that Rock Guitarists often play what we now term as "5 chords" -- basically two note chords consisting of Root and Fifth, taking advantage of the same "resultant" situation and making for a chord with more apparent fullness or punch.  

In teaching music intonation, as in with the given example of the Trumpet (my first instrument after piano as a kid), I find it much easier to teach them to hear the Beats that result from heterodyning of two notes that are very close together in pitch rather than so far apart as your example.  

The tuning slide should be used to start with the slide further out than it should be to match the target tuning note, then, with the right hand, have the student slowly move that slide inwards, getting a little sharper incrementally, while listening for those beats.  

As the beats get slower and slower, the note is getting closer to nailing the target pitch, until at some sweet point, the beats disappear altogether and the two notes reinforce, which means that the sound in the air will get a little bit louder (actually higher in amplitude as vs Loudness, which is really referring to an EQ situation and the Fletcher-Munson curve thing).  

It is also important to have the student keep moving the slide further inward than the sweet spot of being dead in tune with the reference pitch, just so they'll understand that the beats will begin to speed up again the further north of the reference they go.  

A very good thing to pick up and give the young student is one of those small quartz tuners.  For a Trumpet player, make sure that the Tuner is a Chromatic type and it is easier to use one that automatically changes pitches rather than one that is manually set to only one note at a time.  This kind of visual feedback, if used for ten minutes a day for awhile in their regular practice regimen can really speed up the process that a Trumpet player must develop, which is the ability to hear the next tempered note in a scale in their head *before* attempting to play said note.  The first drill here is to play the simple Bb Major scale (C scale on the Bb Trumpet) and endeavor to lock that tuning indficator dead center for each note played.  The Trumpet actually has a few intonation problems inherent in the instrument design that we must learn to overcome.  For example, the low D above Middle C is inherently sharp (fingering 1 and 3 valves) and that's why modern Trumpets all come with a Third Valve Slide on them, shoot that slide out to more easily play the low D in tune.  There is also a bit of "lipping" involved with nailing the Tempered Scale on the Trumpet.  Classical orchestral players also typically like to have a 1xt valve slide available, for the D above thirdspace C as well.  Then there are the uses of so-called "false fingerings" such as playing the top space E with 1 & 2 or just 4 by itself instead of Open.  

Get the lad a good electronic chromatic tuner.  The entry price is low indeed due to imports and availability.


73
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G3TXQ
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« Reply #8 on: December 30, 2012, 03:23:58 PM »

One trumpet blows a 500 Hz note and a second one plays an 800 Hz note, you get a 1300 Hz note and a 300 Hz note, right?  (sum and difference)
Not really! You would only produce separate 1300Hz and 300Hz tones if you had "multiplicative mixing" - that would require something in the mixing process to be non-linear.

What you would actually produce with additive mixing is a 650Hz sine wave ([f1+f2]/2) modulated with 150Hz sine wave ([f1-f2]/2).

When f1 and f2 are very close, the amplitude variation of the (f1+f2)/2 term is usually perceived as a beat at f1-f2.

Maths here:
http://en.wikipedia.org/wiki/Beat_%28acoustics%29

On-line demo here:
http://web.mit.edu/jorloff/www/beats/beats.html

73,
Steve G3TXQ
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WB6DGN
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« Reply #9 on: December 30, 2012, 03:51:45 PM »

Quote
Not really! You would only produce separate 1300Hz and 300Hz tones if you had "multiplicative mixing" - that would require something in the mixing process to be non-linear.

I thought the animal (human) ear, and, presumably the eye as well, were considered non-linear devices as the don't respond in linear fashion to all frequencies or levels within their range.  Am I mistaken?
Tom
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KE3WD
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« Reply #10 on: December 30, 2012, 04:33:51 PM »

You have to consider the medium as well. 

Sound traveling through air is subjected to the compliance of air. 

And air is a compressible fluid. 

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K0JEG
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« Reply #11 on: December 30, 2012, 05:41:48 PM »

Actually fairly easy to demonstrate with a piano, IIRC. Play 2 notes and watch the harmonically related strings vibrate in sympathy.
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K8AXW
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« Reply #12 on: December 30, 2012, 07:01:38 PM »

The last time I heard something like this was when a group of CB'rs was having a "fest" and they decided to all key their mobile rigs at the same time and see if some DX station could hear them with the combined 5,000 watts of RF.

5UP:  Hopefully the dog will hear someone yell, "Get the hell away from those tracks!"
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KA4POL
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« Reply #13 on: December 30, 2012, 10:06:03 PM »

You have to consider the medium as well. 
Absolutely right.

Quote
Sound traveling through air is subjected to the compliance of air. 
Right again.

Quote
And air is a compressible fluid. 

This is a contradictory statement. Air is a gas which is like all gases compressible. Fluids are not compressible. If they were you could not brake with your car brakes. The effect of gas in your brake fluid is: Bad or no brakes.
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KA4POL
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« Reply #14 on: December 30, 2012, 10:18:59 PM »

Trying to explain hetrodyning and other electronic principles to my son, a trumpet playing EE student.
Basically this is a very good idea. Practical examples are really making things understandable. But you are also running into problems. Sound is longitudinal waves whereas electromagnetic waves are transversal.
Quote
One trumpet blows a 500 Hz note and a second one plays an 800 Hz note, you get a 1300 Hz note and a 300 Hz note, right?  (sum and difference)
Again you will not arrive at the desired result because of reflections etc. In our real world it does unfortunately not work.
Quote
One trumpet plays a note and second one simultaneously plays the same note, you get a 3 dB louder note, right?

I presume a real trumpet note isn't nearly clean enough to actually do that, or is it?
There are harmonics to a note being played by an instrument. This is desirable for musical instruments. Pure tones seem to sound terrible. You could try this experiment using two electronic sound generators.
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