Basic Acoustics

Question 1

What is Sound?

Answer 1

A longitudinal-wave disturbance in any compressible substance,
especially in air. Perceived as an object “heard” in the brain when these waves stimulate human ears.

Question 2

Sound propagation

Answer 2

how it travels from one place to another: waves

Question 3

Sound perception

Answer 3

how it affects the senses and emotions of a listener

Question 4

Longitudinal waves (compression waves)

Answer 4

wave motion and particle motion are parallel to each other (lengthwise waves on a slinky spring, sound waves).

Question 5

Transverse waves

Answer 5

wave motion and particle motion are perpendicular to each other (waves on a rope, electromagnetic waves, e.g. radio, light, x-rays).

Question 6

Wavelength λ (lambda)

Answer 6

The distance from one crest to the next (water waves) or from one compression to the next (sound waves)

Question 7

The speed of sound

Answer 7

344 meters per second: v = 344 m/s (dry air at room temperature (T = 20° C)

Question 8

Approximate speed of sound in other media

Answer 8

• carbon dioxide 270 m/s
• helium 1000 m/s
• water 1500 m/s
• steel 6000 m/s

Question 9

When the temperature is higher

Answer 9

the random molecular motions are faster, the molecules collide more often and pass the sound disturbance faster from one region to another.

Question 10

Speed of sound and air temperature

Answer 10

by every Celsius sound changes by 0.6m/s (21c = 344.6 m/s, 19c = 343.4m/s)

Question 11

Displacement amplitude

Answer 11

distance each bit of air moves to either side of its normal position during its vibration (usually on the order of millionths of a meter).

Question 12

Pressure amplitude

Answer 12

maximum increase of air pressure (above normal atmospheric
pressure) in a sound wave compression

Question 13

Musical time scales

Answer 13

long = entire song, section, or phrase
medium = individual notes or chords
short = individual sound vibrations (percussion)

Question 14

Frequency

Answer 14

measured in Hz, Frequency is the repetition rate of a vibration. In musical terms, one octave is a doubling of frequency.

Question 15

Frequency range

Answer 15

The range of audible frequencies covers about 10 octaves, and extends from approximately 20 Hz up to 20,000 Hz or 20 kHz (kilohertz).

Question 16

waveform

Answer 16

The waveform of a tone played on a musical instrument is usually obtained by converting the pressure fluctuations of the air into an equivalent electric voltage with a microphone. This voltage can be recorded or directly displayed with an oscilloscope.

Question 17

periodic sound

Answer 17

is perfectly steady and keeps repeating the same vibrational pattern indefinitely;

Question 18

transient sound

Answer 18

is not sustained, but quickly dies away (percussive sound).

Question 19

Phase

Answer 19

A complete cycle has 360 degrees of phase, Therefore the positive peak of the cycle is at 90 degrees, the zero point is 180 degrees, and the negative trough is at 270 degrees

Question 20

Wavelength = Speed divided by frequency. W = S/F

Answer 20

Assume a wave has a frequency (speed) of 100Hz.

Then W = 344/100
therefore: W= 3.44 metres

This equation can be used to measure any one of these three parameters, assuming that the other two are known.

Question 21

absorption

Answer 21

The drainage of sound energy into a surface

Question 22

reflection

Answer 22

Harder materials such as concrete will absorb very little energy because the densely packed particles in these rigid surfaces are not so easily jarred into motion and will instead bounce off

Question 23

normalised sine wave calculation

Answer 23

sin*degrees of phase/sin()
0 degrees = 0
90 degrees = +1
180 degrees = 0
270 degrees = -1

Question 24

diffusion

Answer 24

is when waves encounter a rough surface with bumps
about as large as the wavelength of the waves. The wave energy is scattered in many different directions by different parts of the surface. Irregularities that are ¼ of a wavelength, or greater, will start to cause diffusion.

Question 25

Multiple reflections

Answer 25

allow sound waves to travel from the source to a different room, for instance someone calling you from downstairs, however waves become weaker at each reflection, because part of their energy is lost to absorption.

Question 26

concave wall reflection

Answer 26

The performer must be a little closer than half the radius of the walls curvature so sound can spread nearly uniformly.

Question 27

Refraction

Answer 27

is a change of direction of wave travel due to difference in its speed from one place to another. (shouting against the wind will increase refraction)

Question 28

Diffraction

Answer 28

The bending of waves around small obstacles and the spreading out of waves beyond small openings. An ambulance siren can be heard around a corner.

Question 29

Diffraction through spaces

Answer 29

Waves coming through a narrow opening (compared to their wavelength) spread out almost equally in all directions. Waves coming through an opening that is large compared to their
wavelength most continue in a straight line

Question 30

The inverse-square law.

Answer 30

energy twice as far from the source is spread over four times
the area (squared), hence one-fourth the intensity. if we measure the intensity (i) of a sound at distance (d) from its source, at a distance of 2d, the intensity would be i/4. 4d therefore would equal i/16.

Question 31

excitation

Answer 31

The action of setting an object into vibration

Question 32

Idiophones

Answer 32

sound is produced by the instrument ‘on its own’

Question 33

Membranophones

Answer 33

sound is produced by a vibrating membrane, stretched over an opening (percussion)

Question 34

Chordophones

Answer 34

string instrument

Question 35

Aerophones

Answer 35

sound is produced by a vibrating column of air

Question 36

Electrophones

Answer 36

(electronic instruments)

Question 37

Sympathetic vibrations

Answer 37

When a sound wave of one frequency strikes a surface (a violin, for example) it will vibrate naturally at the same frequency. For example, an A string at 440 Hz will cause an E string at 330 Hz to resonate, because they share an overtone of 1320 Hz (the third harmonic of A and fourth harmonic of E).

Question 38

Natural modes

Answer 38

a motion in which every part of the system oscillates sinusoidally at the same frequency.

Question 39

Resonance

Answer 39

means the comparatively large-amplitude vibration that results whenever the frequency of some driving force closely matches a
natural oscillation frequency of the system on which it acts.

Question 40

The Law of Superposition

Answer 40

As we add integer harmonics to a sine wave, the shape of the
wave changes.

Question 41

Vibraphone bars

Answer 41

help produce musically helpful frequency ratios. Thinning a bar at the center reduces the stiffness and lowers the frequency of the first mode to change the frequency ratio

Question 42

Acoustic reflection

Answer 42

The reason string instruments produce loud volume is because its sound reflects of the body of the instrument, causing the soundwave to invert and therefore doubling its amplitude.

Question 43

The Law of Superposition

Answer 43

if two waves in a positive phase occur at the same time and point, we can simply add their values to calculate their combined effect.
If two waves occur at opposite phases to each other, we subtract their values therefore resulting in nil.

Question 44

Open end tube

Answer 44

is when air (sound) can freely vibrate when traveling to the end of a tube. If both ends of the tube are open, the musical instrument is said to contain an open-end air column.

Question 45

Factors that affect the sound of a mic

Answer 45

polar response: how mics responds to sound from different directions
The frequency response: the accuracy with which the mic outputs frequencies across the audible range
The sensitivity: the measure of the strength of output derived from any given strength of input
The distortion and noise output figures

Question 46

Dynamic Microphone (velocity sensitive)

Answer 46

In a Dynamic Microphone, sound waves cause movement of a thin metallic diaphragm and an attached coil of wire. A magnet produces a magnetic field which surrounds the coil, and motion of the coil within this field causes current to flow, the amount of current is determined by the speed of that motion.

Question 47

Condenser (Capacitor) Microphone

Answer 47

In a condenser microphone, the diaphragm is mounted close to a rigid back-plate. A battery is connected to both pieces of metal, which produces an electrical charge. The amount of charge is determined by the distance between the two. This distance changes as the diaphragm moves in response to sound. When the distance changes, current flows in the wire as the battery maintains the correct charge. However the current is so small that it must be electrically amplified before it leaves the microphone.

Question 48

Electret Condenser Microphone

Answer 48

uses a material (Electret) with a permanently imprinted charge for the diaphragm. The major disadvantage of electrets is that they lose their charge after a few years and cease to work.

Question 49

Mic faults

Answer 49

Condenser require batteries or power from the mixing console to operate.
Dynamic mics require shielding from stray magnetic fields, which makes them a bit heavy,

Question 50

Overload characteristics

Answer 50

Any microphone will produce distortion when it is overdriven by loud sounds. With a dynamic, the coil may be pulled out of the magnetic field; in a condenser, the internal amplifier might clip.

Question 51

Noise prevention tricks

Answer 51

The basic technique, called shielding, or screening, is to surround the wires that carry the current to and from the mic with a flexible metallic shield, which deflects most radio energy.

Question 52

Balanced Lines

Answer 52

Use two cable cores to carry the signal, these are in inverse phase to each other. When one core signal is phase reversed, the signal now exists in phase with itself, and the noise is out of phase. Therefore the mic signal is preserved, but the noise is phase cancelled.

Question 53

Polar Patterns

Answer 53

The capability for mics to pick up sounds elsewhere from its front. There are 5 patterns: Omni, Bi-directional, Cardioid, Hyper-cardioid and shotgun.

Question 54

Decibel measurements

Answer 54

Are measured using a Logarithmic scale, whereby the scaling is in powers of ten.

Question 55

Intensity vs Pressure

Answer 55

Pressure is the measurable change in air pressure caused by a wave.
Intensity is the amount of energy directed at a given area per second.
intensity = pressure squared.

Question 56

Intensity and pressure equation

Answer 56

Intensity = log(?)x10
Pressure = log(?)x20
Therefore intensity is always half pressure and pressure is always double intensity

Question 57

Threshold of Hearing

Answer 57

intensity =1x10-12 watts/metre squared (known as 0dB SIL)
pressure =2x10-5 newton/metre squared (known as 0dB SPL)