## 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)