Foundations of Acoustics

Question 1

What is AIR PRESSURE?

Answer 1

The force that air exerts on the object it touches, normalised by its contact area

Question 2

What is the average air pressure?

Answer 2

101,000Nm^2 or 1.01x10^5Nm^2

Question 3

What type of wave is a soundwave?

Answer 3

Longitudinal wave

Question 4

What is the physical construction of a sound?

Answer 4

Vibrations caused by variations in air pressure over a short period of time

Question 5

Define a PURE TONE

Answer 5

The simplest form of sound, represented (temporally) by a sinusoidal wave

Question 6

What are the properties of a pure tone (in its temporal/sinusoidal representation)?

Answer 6

• Pressure as a function of time
• Periodic/regular
• 1 sine component

Question 7

What is the speed of sound?

Answer 7

343m/s

Question 8

Define COMPRESSION

Answer 8

Band of high pressure where there is a high density of air particles, corresponding to the sine peak

Question 9

Define RAREFRACTION

Answer 9

Band of low pressure where there is a low density of air particles, corresponding to the sine trough

Question 10

Define AMPLITUDE; how is it measured and does it effect sound?

Answer 10

• The degree of pressure fluctuation
• Measured from bassline to extrema
• Effects LOUDNESS - the higher the amplitude, the louder the sound

Question 11

Define FREQUENCY; how is it measured? What is its units? How does it effect sound?

Answer 11

• The amount of sinusoidal wave cycles per second
• Measured in Hz
• The physical correlate to pitch: x2 = ^8ve; (x2)(x1.5) = ^compound p5th

Question 12

Define PHASE: what range of values can it take?

Answer 12

The position along the wave cycle a sound starts, between 0 and 2π

Question 13

What is the formal definition (equation) for a SINE WAVE?

Answer 13

x(t) = A sin(2πft + ø)

Relative pressure = Amplitude x Sin(2π (frequency (Hz) x time (s)) + phase)

Question 14

Define WAVELENGTH

Answer 14

The distance between two peaks of a sinusoidal sound wave

the length of a full cycle

Question 15

What is the equation for WAVELENGTH?

Answer 15

λ = v/f

Wavelength (m) = Speed of the wave* (m/s) / Frequency (Hz)

• 343m/s in std atmospheric conditions)

Question 16

What is FOURIER THEOREM?

Answer 16

The theory that every periodic wave can be expressed by the addition of sine waves of differing amplitudes/frequencies/phases

Question 17

Define HARMONIC COMPLEX TONES

Answer 17

A wave built by adding multiple sine waves with increasing frequencies that are integer multiples of a common fundamental frequency

Question 18

Define a SAWTOOTH wave

Answer 18

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

Question 19

Define a SQUARE wave

Answer 19

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

Question 20

Define TEMPORAL REPRESENTATION

Answer 20

Visualising sound via fluctuations in pressure over time (vibrations)

Question 21

Define SPECTRAL REPRESENTATION

Answer 21

A visualisation of the sine components of a complex tone separated out into its constituent pure tones

Question 22

What is a Spectrogram?

Answer 22

A visual representation of a sound’s spectral properties over time as they change over time

Question 23

Define HARMONICS; what do they look like in spectral representation?

Answer 23

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

Question 24

Define PARTIALS; what do they look like in spectral representation?

Answer 24

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