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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 1 response by gs5556

gs5556
1,218 posts
 
The microphone acts no different than your ear drum or speaker cone. How a single cone produces overtones is simple. Say you are listening to a 20 hz tone. The speaker cone moves back and forth 20 times a second. For a 100 hz tone it’s a hundred times a second.

What about the two tones played at the same time to produce a different sound? As the cone moves forward 20 times per second, it also moves back and forth 100 times per second. (Wave your hand back and forth with your arm still. Then move your arm while your are waving your hand. Then walk forward while waving your hand and moving your arm. The air displaced is a pressure representation of the combined motion).

The combination of pressure waves creates one wave at any point in time and over a certain time period it contains all the other waves (overtones) creating a particular sound. Add them all together as a function of time and there is your orchestra in your living room.
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