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Electrical/mechanical representation of instruments and space

Help, I'm stuck at the juncture of physics, mechanics, electricity, psycho-acoustics, and the magic of music.

I understand that the distinctive sound of a note played by an instrument consists of a fundamental frequency plus a particular combination of overtones in varying amplitudes and the combination can be graphed as a particular, nuanced  two-dimensional waveform shape.  Then you add a second instrument playing, say, a third above the note of the other instrument, and it's unique waveform shape represents that instrument's sound.  When I'm in the room with both instruments, I hear two instruments because my ear (rather two ears, separated by the width of my head) can discern that there are two sound sources.  But let's think about recording those sounds with a single microphone.  The microphone's diaphragm moves and converts changes in air pressure to an electrical signal.  The microphone is hearing a single set of air pressure changes, consisting of a single, combined wave from both instruments.  And the air pressure changes occur in two domains, frequency and amplitude (sure, it's a very complicated interaction, but still capable of being graphed in two dimensions). Now we record the sound, converting it to electrical energy, stored in some analog or digital format.  Next, we play it back, converting the stored information to electrical and then mechanical energy, manipulating the air pressure in my listening room (let's play it in mono from a single full-range speaker for simplicity).  How can a single waveform, emanating from a single point source, convey the sound of two instruments, maybe even in a convincing 3D space?  The speaker conveys amplitude and frequency only, right?  So, what is it about amplitude or frequency that carries spatial information for two instruments/sound sources?  And of course, that is the simplest example I can design.  How does a single mechanical system, transmitting only variations in amplitude and frequency, convey an entire orchestra and choir as separate sound sources, each with it's unique tonal character?  And then add to that the waveforms of reflected sounds that create a sense of space and position for each of the many sound sources?

77jovian

Showing 2 responses by 77jovian

77jovian OP
386 posts
 
Still thinking about this.  Let me give another example. 

Like a synthesizer, you could combine a series of pure, sinusoidal, tones in a particular combination of fundamental and overtones with varying amplitudes and fine-tune it until it sounds like a flute.  Why do we hear the sound of a flute instead a bunch of sine waves at varying frequencies and amplitudes?  Why do we hear the whole instead of the components?

To reverse the example, is there a single whole sound that is the sound of the components of an orchestra, if you get my meaning?  Why do we hear the single sound as the sound of many instruments?
77jovian OP
386 posts
 
Thank you for the very helpful comments. 

Here's what I distill from the comments.  The graph of a waveform with axes of frequency and amplitude can describe a sound, but it's the changes in the waveform over time that provide meaning to the sound.  So the envelope of a note over time that includes, perhaps, a percussive attack and it's changing frequencies and their amplitudes as the note decays that provide clues to what instrument is making the sound.  And the listener's brain recognizes the "flute-iness" of the sound over time, or whatever instrument is playing.
OK, I get that.  But I'm not sure that adequately explains how we discern multiple instruments from a single changing waveform or the position of the instruments in 3D space.  I'll have to think more about that.
I acknowledge those of you who have pointed out the role of two ears spaced apart from each other.  But I think there must be more to it.  After all, there are guys like David Pack, a wonderful musician and record producer who has been totally deaf in one ear for the last 40 years.  His work is not devoid of spatial information.  Nor is a single, full-range speaker incapable of all spatial presentation.

Apologies to those of you who think this is a simple-minded discussion.  I find it deeply profound and an astonishingly complex interaction of physical and cognitive phenomena, worthy of reflection.  Well, there are those who stand at the rim of the Grand Canyon and think, "hey, it's a water-carved canyon, so what?"
This discussion reminds me how awesome is technology that is capable of reproducing this extraordinarily intricate physical process to a degree that sublime enjoyment is possible. 

Conversely, it's equally awesome to realize that the brain is capable of perceiving and appreciating this magic from the cheapest,]flimsiest nickel-sized speaker in a cheap cell phone.  Maybe some people who are satisfied with the sound from their cell phone just have vivid pattern-recognition skills.

Finally, as to the ad hominem posts, you should be ashamed.


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