Foundations of Acoustics
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What is AIR PRESSURE?
The force that air exerts on the object it touches, normalised by its contact area
What is the average air pressure?
101,000Nm^2 or 1.01x10^5Nm^2
What type of wave is a soundwave?
Longitudinal wave
What is the physical construction of a sound?
Vibrations caused by variations in air pressure over a short period of time
Define a PURE TONE
The simplest form of sound, represented (temporally) by a sinusoidal wave
What are the properties of a pure tone (in its temporal/sinusoidal representation)?
• Pressure as a function of time • Periodic/regular • 1 sine component
What is the speed of sound?
343m/s
Define COMPRESSION
Band of high pressure where there is a high density of air particles, corresponding to the sine peak
Define RAREFRACTION
Band of low pressure where there is a low density of air particles, corresponding to the sine trough
Define AMPLITUDE; how is it measured and does it effect sound?
• The degree of pressure fluctuation • Measured from bassline to extrema • Effects LOUDNESS - the higher the amplitude, the louder the sound
Define FREQUENCY; how is it measured? What is its units? How does it effect sound?
• The amount of sinusoidal wave cycles per second • Measured in Hz • The physical correlate to pitch: x2 = ^8ve; (x2)(x1.5) = ^compound p5th
Define PHASE: what range of values can it take?
The position along the wave cycle a sound starts, between 0 and 2π
What is the formal definition (equation) for a SINE WAVE?
x(t) = A sin(2πft + ø) Relative pressure = Amplitude x Sin(2π (frequency (Hz) x time (s)) + phase)
Define WAVELENGTH
The distance between two peaks of a sinusoidal sound wave the length of a full cycle
What is the equation for WAVELENGTH?
λ = v/f Wavelength (m) = Speed of the wave* (m/s) / Frequency (Hz) • 343m/s in std atmospheric conditions)
What is FOURIER THEOREM?
The theory that every periodic wave can be expressed by the addition of sine waves of differing amplitudes/frequencies/phases
Define HARMONIC COMPLEX TONES
A wave built by adding multiple sine waves with increasing frequencies that are integer multiples of a common fundamental frequency
Define a SAWTOOTH wave
adding infinite pure tones in an arithmetic sequence y = sin(x) + 1/2sin(2x) + 1/3sin(3x)…etc y = 1/n sin(nx) + 1/(n+1) sin((n+1)x)…etc
Define a SQUARE wave
adding infinite pure tones in an arithmetic sequence, omitting even harmonics y = sin(x) + 1/3sin(3x) + 1/5sin(5x)…etc y = 1/(2n+1) sin((2n+1)x) + 1/(2n+3) sin((2n+3)x)…etc
Define TEMPORAL REPRESENTATION
Visualising sound via fluctuations in pressure over time (vibrations)
Define SPECTRAL REPRESENTATION
A visualisation of the sine components of a complex tone separated out into its constituent pure tones
What is a Spectrogram?
A visual representation of a sound’s spectral properties over time as they change over time
Define HARMONICS; what do they look like in spectral representation?
When the upper tones of a complex wave are integer multiples of a common fundamental frequency, shown via equally spaced lines on the harmonic spectrum
Define PARTIALS; what do they look like in spectral representation?
When the upper tones of a complex wave are NOT integer multiples of a common fundamental frequency, shown via unequally spaced lines on the harmonic spectrum