Blog 1: Fundamentals of Sound

In this lecture, we covered the basics of Sound, and what Sound is; therefore introducing us to the World of Digital Audio.

Frequency

Frequency is basically set by counting the number of cycles per second, and is measured in hertz (Hz).

One Cycle = 1Hz

The human ear, can detect sound from 20Hz to 20,000Hz (20kHz).

Animals have a much different hearing capability than humans do. The below chart was taken from Survival Life, which has an interesting article on how sound frequency causes pain. Hearing Capability

Wavelength

Where frequency measures the number of cycles per second, A Wavelength is the measurement of a complete cycle, therefore is inversely proportionate to frequency. The recognisable symbol for Wavelength is λ.

Below is a diagram from BBC Bitesize. The site is primarily for High School students, but nevertheless has fantastic information on wavelength that in turn is massively relevant for this module. A quote from their site follows which helped me understand Wavelength a little more:

The wavelength of a wave is the distance between a point on one wave and the same point on the next wave. It is often easiest to measure this from the crest of one wave to the crest of the next wave, but it doesn’t matter where as long as it is the same point in each wave. 

Wavelength

Amplitude

An Amplitude describes the amount of energy present in a Signal. As seen in the diagram under Wavelength, where the wavelength measures the length of the wave, the amplitude measures the height. The greater the amplitude, the louder the signal will be as it will put more pressure against the human ear. How loud a signal sounds, is measured in decibels (dB).

Amplitude is related to, but not the same as Volume.

Decibels (dB)

The decibel is based on the logarithm of the ratio between two numbers. It also describes how much larger or smaller one value is than the other. If the reference value is fixed then it can be used as an absolute unit.

dB = 10log10(P1/P2)

The decibel is strictly 10X the logarithm to the base ten of the ratio between the powers of two signals.

10log(2/1) = 3dB

The difference in dB between a signal with a power of 1 watt and one of 2 watts is.

If the dB is used to compare values other than signal powers, the relationship to signal power must be taken into account.

  • Voltage has a square relationship to power.
  • Ohms Law W=V2/R.
  • therefore to compare 2 voltages:
  1.    dB = 10log(V12/V22)
  2.    dB = 10log(V1/V2)2

The difference in dB between a signal with a voltage of 1 volt and one with 2 volts is.

20log(2/1) = 6dB

So a doubling in voltage gives rise to an increase of 6dB, and a doubling in power gives rise to an increase of 3dB.

A similar relationship exists to acoustical sound pressure and sound power.

Phase

complex wave = When multiple sound waves are combined they create a single wave.

destructive interference = Can be a result of two waves of equal amplitude and frequency but differing states of compression or rarefaction.

Phase becomes an issue when more than one channel is used to record a single source, such as stereo micing a guitar, multi-micing a drumset, or using a microphone/DI combo for bass. Recording “mono overdub style” avoids these issues, but doesn’t give you a dynamic stereo field. In fact, the problem most often manifests itself in when converting tracks from stereo to mono.

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