Analog Sound

Electrical Representation

  • Represent sound pressure as a voltage on a wire

  • The classic: telephone

  • Allows for transmission, processing

Microphone

  • Turn sound into voltage: usually with microphone

  • Microphone varies resistance, capacitance or voltage (reversed speaker) depending on air pressure differential between front and back

  • End result is a voltage representing instantaneous sound pressure

  • Microphones are bad: noisy, nonlinear devices; often limiting factor in sound chain

Speakers

  • Turn voltage into air pressure change

  • Wire solenoid attached to paper cone like this (demo at :30 in linked video) or this

  • Typically in a resonant cavity (speaker cabinet)

  • Speaker solenoid roughly tracks change in current through the wire, which makes things complicated (impedance matters)

  • Need wavelength to be long for low frequency to move enough air: big speaker "woofer"

  • Need response time to be fast for high frequency: tiny speaker, maybe piezoelectric — "tweeter"

Attenuation / Amplification

  • Simplest transformation

  • Attenuation: Sound out linearly less than sound in

  • Easy to attenuate in all the obvious ways

  • Amplification: Sound out linearly greater than sound in

  • Amplification usually requires electronics

Signal Path

  • We now know how to build something like a telephone or record player or stomp box:

    • Use a microphone to convert air pressure to voltage

    • Maybe process the voltage somehow: store it somewhere or modify it with circuitry

    • Use a speaker to convert voltage back to sound

Distortion

  • Ideally, electric signal exactly represents sound pressure

  • In practice, the signal path may introduce distortion

    • Nonlinearity: the signal doesn't accurately track the sound pressure

    • History: the past signal influences the current signal

  • We will talk about "harmonic distortion" (THD) at some point

  • Some "distortions" are deliberate, because we are used to hearing distorted sounds and so they "sound good"

Feedback

  • "Feedback" is a classic oscillation effect:

    • Sound coming out the speaker and back into the microphone interacts with speaker + microphone + air as a resonance

    • The resonant frequency depends on the distance between microphone and speaker (air delay), and on the frequency response of the loop

    • If the loop has net positive gain at some frequencies (amplification)…

Limitations

  • Representation of analog sound as an electrical signal is potentially awesome: high accuracy in time, can represent very high and low frequencies well

  • In practice, there are problems:

    • Any "noise" (unwanted signal) is also very accurately represented. It is easy to accidentally generate voltage noise in an analog system, which will be accurately represented / amplified as well

    • Analog signal storage devices are clunky, and don't work well: records, tapes, etc

    • Manipulating electrical signals requires complex, expensive and special-purpose electronics

    • "Audiophiles" love this stuff, so you have to deal with them (could be worst problem)

Last modified: Tuesday, 31 March 2020, 7:57 PM