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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 roberttcan

roberttcan
248 posts
 
A few things to consider here:

  1. You get placement information of instruments via two methods, volume and timing. If the right side is louder for a particular instrument, then that is where you perceive the instrument is coming from. That is the mechanism for placement of sounds that are continuous. The other mechanism is arrival time (between the left and right ear). That is used for transient sounds. A clap will arrive to one ear slightly before the other. Your auditory processing system is able to time that to pretty fine resolution and give you an idea of where it came from. Some posit that arrival time also plays a factor in placement of continuous sounds.
  2. The electromechanical system only stores and transmits sound waves (pressure variation). It does not transmit instruments, etc. It is your brain, knowing what a grouping of sounds mean, that is able to extract instruments and place them.
A single speaker will not convey on its own, any sense of space, but a room may create those cues (accurate or not), and your brain doesn't like an information vacuum so it will try to map what it hears onto what it knows.

I think you meant frequency and time domain. Amplitude is part of either frequency or time domain.
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