Audio

Question 1

What are the terms used to describe the pushing together and pulling apart of molecules when transmitting sound?

Answer 1

Compression and rarefaction.

Question 2

What is an elastic medium?

Answer 2

A material where the molecules are free to move, such as in air or water, and therefore easily transmits sound.

Question 3

How would you describe one wavelength?

Answer 3

It is the distance between two points on the waveform exactly one complete cycle apart.
(There is another point on the waveform where the value is exactly the same, but the trend is going in a different direction, so this is not in the same part of the cycle. For example, a value just below the maximum will occur once just before the peak when the trend is upwards and once just after it when the trend is downwards.)
It is only a full wavelength if the value has gone through the complete cycle of positive and negative before returning to the same point, such as “zero” or the “reference line”.

Question 4

What is frequency?

Answer 4

Frequency is the number of times the complete cycle of a wavelength occurs in one second.

Question 5

How is frequency measured?

Answer 5

The frequency of a wavelength is described in “cycles per second (cps)” and the SI measurement for this is in “hertz”. A wave at 50cps is described as having a frequency of 50Hz.

Question 6

What is the frequency range for normal human hearing?

Answer 6

20Hz to 20,000Hz (20Hz to 20kHz)

Question 7

What is the wavelength range for normal human hearing?

Answer 7

From 56.5 feet (17.2m) for 20Hz down to less than 3/4 inch (17.2mm) for 20kHz.

Question 8

What is the relationship between wavelength and frequency?

Answer 8

For any waves travelling through a medium (e.g. air) the frequency and wavelength are inversely proportional, i.e. as one gets larger the other gets smaller.

Question 9

What is the word use to describe how audible sound is divided into groups of 8 notes?

Answer 9

An octave.

Question 10

What is the relationship between the frequency of the same note in different octaves?

Answer 10

The frequency increases logarithmically, as notes at 220Hz, 440Hz and 880Hz sound similar. Although the gaps between these notes increases exponentially, the octaves are still divided by 8 notes.

Question 11

What are harmonics?

Answer 11

Harmonics are whole number multiples of a fundamental frequency.
For example, for a fundamental frequency of 4,000Hz, 4,000Hz is the first harmonic, 8,000Hz is the second harmonic, 12,000Hz is the third, 16,000Hz the fourth and so on, and they are often referred to as odd-number or even number harmonics.

Question 12

If five instruments were playing the same note, how can a listener determine the type of instrument?

Answer 12

The sound produced by each instrument is made up of the same fundamental note (frequency) but different combinations of harmonics.

Question 13

What is the logarithm (log) of a number?

Answer 13

The log of a number denotes how many times the base number (e.g. 2 or 10) needs to be multiplied by itself in order to equal that number.
For example, using base 10, the log of 1,000 is 3 because 10 must be multiplied by itself 3 times to equal 1,000.
Base 2 is often used in computing (for binary calculations) while base 10 is frequently used in science and engineering.

Question 14

Why do we use logs (logarithms) in describing sound?

Answer 14

Logarithms are used in sound because values do not increase in a linear fashion. When values grow exponentially, such as frequencies, volume etc, the numbers can quickly become very large, that’s a lot of zeros. Logs enable us to concisely describe both small and very large numbers.

Question 15

What is a decibel?

Answer 15

A decibel (dB) is one tenth of a "bel".  The bel was named after Alexander Graham Bell, but was found to be too large to be used easily, so the decibel is used instead.
It measures the difference between two values of the same type, e.g. voltage or sound pressure level, compared to each other.  It is a logarithmic, not linear measurement.

Question 16

In decibels, what is the threshold of normal human hearing?

Answer 16

0 dB SPL

Question 17

How much does sound pressure reduce if you double the distance?

Answer 17

Sound pressure is reduced by 6dB every time the distance is doubled.

Question 18

What is the “inverse square law”?

Answer 18

When the distance from a sound or light source is doubled, the sound pressure or light intensity is reduced to a quarter. This is because the sound or light now has to cover 4 times the original area compared to before.

Question 19

What is the smallest perceptible change in volume?

Answer 19

A 1 dB change in volume (either up or down) is the smallest discernible difference, even though many would not notice it.

Question 20

What is the smallest change in volume likely to be noticed?

Answer 20

The smallest change in sound noticeable to most would be +/- 3dB, for example from 85 to 88 dB.

Question 21

What change in volume would be perceived as being twice as loud?

Answer 21

To appear to be twice as loud would need a change of +10 dB, for example from 85 to 95 dB.

Question 22

When do we use Decibel logs with base 10, and when logs with base 20?

Answer 22

Base 10 is used when comparing power quantity ratios, where a change in power to a factor of 10 is recorded as 10db. Base 20 is used for field quantity ratios, where the inverse square law applies when comparing sound pressure levels due to a change of distance or voltage.

Question 23

What is acoustics?

Answer 23

Acoustics is the study of sound waves and their qualities and characteristics.

Question 24

Name some topics covered in the study of acoustics.

Answer 24

How sound is generated and transferred, how it is affected by shapes, materials and surfaces, and how it is perceived by the brain.

Question 25

What three things happen to sound energy when it encounters an object or surface?

Answer 25

It is reflected, absorbed and/or transmitted to varying degrees, depending on the material, shape, angle etc.

Question 26

Name two types of reflections.

Answer 26

Direct or diffuse reflections.
Direct is also known as hard or specular reflection, and behaves like a mirror, reflecting most of the energy in a specific direction, according to the angle of approach.
A diffuse reflection is one where light is scattered back across a wide angle, such as from white painted plasterboard or a front projection screen.
Every material has a combination of direct and diffuse reflections, and the effect may also vary according to the wavelength.

Question 27

What is the difference between echo and reverberation?

Answer 27

With echo there are individual discrete copies of the original sound. With reverberation there are lots of reflections arriving at slightly different times, leading to an unclear overall sound.

Question 28

How can the absorption qualities of materials be used to improve sound quality in certain installations?

Answer 28

Using materials that absorb sound can cut down on unwanted reflections and improve sound quality. For this reason, spaces will often include carpet, acoustic tiles, curtains and upholstered surfaces.

Question 29

How would you define ambient noise?

Answer 29

Ambient noise can be described as anything other than the desired sound or signal. It can come from either within the room or external sources, and must be managed carefully in order to maintain the effectiveness of the sound system.

Question 30

What is the speed of sound in air?

Answer 30

The speed of sound in air is 344 metres per second (344m/s).
As a result of this, a note with a frequency of 10 cycles per second (10 Hz) would have a wavelength of 34.4 metres, 20 Hz would be 17.2m.

Question 31

What are the stages in the audio signal path, from origin to listener?

Answer 31

Sound travelling (normally through air) is received by a microphone as vibrations. 
The microphone transduces (converts) the vibrations into an electrical signal and sends it on, usually by wires, or perhaps via a radio link.  
The signal is processed (including amplification, perhaps echo cancellation etc).
The amplified signal ends up at an output device, some form of loudspeaker, where it is converted back into sound waves for the end user.

Question 32

What is transduction?

Answer 32

Transduction is the conversion of one form of energy to another, such as acoustic energy into electrical energy, or vice versa.

Question 33

Name some of the properties of a microphone that would need to be considered to ensure it was suitable for a specific application.

Answer 33

You would need to consider a number of factors, including the microphone type, size and mounting, directional pickup, sensitivity, frequency response and impedance.

Question 34

Name the two most common types of microphone technology and their main qualities.

Answer 34

Dynamic microphones are durable, can handle high sound pressure levels and do not require any power rover operate.
Condenser microphones are normally smaller and more sensitive to quieter sounds and higher frequencies. They do, however, need a source of power to operate.

Question 35

How does a dynamic microphone work?

Answer 35

In a dynamic microphone there is a coil of conductive wire attached to a diaphragm which is sensitive to changes in air pressure. The wire coil is housed inside a magnet, and as the diaphragm forces the coil to move, it induces a current. This very weak signal is called mic-level, and needs to be amplified before being sent to loudspeakers.

Question 36

How does a condenser microphone work?

Answer 36

A condenser microphone consists of two parallel plates separated by a very small distance. Power applied across the plates (known as “phantom power”) creates a charge between them, and the backwards and forwards motion of the front plate due to the sound waves received varies the electrical charge between the plates. Using the same phantom power, a small amplifier increases these minutes electrical waves up to a mic-level input.

Question 37

What is an electret condenser microphone?

Answer 37

An electret is a type of condenser microphone where one of the plates is already charged, removing the need for the higher voltage required by normal condenser microphones. As well as using phantom power, these mics can also run off tiny button batteries, and can therefore be much smaller than other types.

Question 38

What is phantom power?

Answer 38

Phantom power is the power supplied to electret and condenser microphones necessary for them to work, and to power the small built in amplifier needed to get their signals up to mic level. It is normally provided by the audio mixer, although separate power supplies can also be used, and can usually be switched on or off depending on the type of mic being connected. It’s usually in the range of 12-48 volts DC. Because the power is applied equally across both conductors, not only does it not damage dynamic mics, but there is also no need for any further cables.

Question 39

Name the five main designs of microphone according to placement.

Answer 39

Handheld mics are mainly for vocal use and are usually quite rugged in order to deal with their handling. Well suited to taking questions from the floor.
Gooseneck mics are mounted on desks, lecterns etc and can be angled toward the speaker.
Surface mount mics (boundary mics) are normally mounted on a conference table and take direct and reflected sound from the desk surface.
Lavalier mics (body worn) or headsets are frequently used by presenters and those moving around on stage, such as singers.
Shotgun or rifle mics are very focussed and are designed to pick up sound from a very narrow field, sometimes at a distance. They are often used on a boom for TV and film recording.

Question 40

Name five main polar patterns for microphones.

Answer 40

The polar pattern of a microphone describes its ability to pick up or reject sound from certain directions.
The five main patterns are:
Omnidirectional 
Cardioid
Hypercardioid 
Supercardioid 
Bidirectional.

Question 41

What is an omnidirectional polar pattern?

Answer 41

“Omni” means “all” and an omnidirectional mic picks up sound equally from all directions. Imagine the pickup area as the shape of a ball.

Question 42

What is a cardioid polar pattern?

Answer 42

“Cardioid” comes from the word for “heart” and a cardioid mic picks up sound from a heart shaped pattern, with the point extending forward, as most sound is collected from the front, virtually none from the rear.
A hypercardioid mic rejects even more from the side, but at the expense of some sound from the rear.
A supercardioid mic rejects even more from the side, making it very directional, but lets in even more sound from behind.

Question 43

What is a bidirectional mic?

Answer 43

Bidirectional mics (sometimes called a “figure of eight”) pick up sound equally from two opposite directions only.

Question 44

What is microphone sensitivity?

Answer 44

A more sensitive microphone produces a higher electrical output in the same sound environment then one that is less sensitive. Dynamic mics tend to be less sensitive than condenser microphones. Consider the sound pressure levels from your source, and distance from the mic to help identify the most appropriate solution.

Question 45

What is gain?

Answer 45

Gain is when a signal level is adjusted, usually upwards by amplification, perhaps to align with other input levels or to match an input requirement. A reduction in signal level is known as attenuation, while no change to the signal level is referred to as unity gain.

Question 46

What is negative gain adjustment known as?

Answer 46

Attenuation.

Question 47

How is gain described when the signal is passed through without being adjusted up or down?

Answer 47

Unity gain is when the signal is passed through without and signal level adjustment.

Question 48

What is a microphone’s output impedance and how should it compare to the audio device to which the mic is connected?

Answer 48

Impedance is the opposition to the flow of electrons in an AC circuit, such as a mic output. Professional mics have low impedance as this allows longer cable runs and makes them less affected by noise. Low impedance is usually 200 ohms or less, and the mic’s impedance should be a tenth or less than the impedance of the input it is connected to. Most professional inputs will be rated at least at 2,000 ohms.

Question 49

What are the two main technologies that can be used in order to use microphones wirelessly?

Answer 49

Wireless mics can use Infra Red (IR) but more commonly use Radio Frequencies (RF) and usually referred to as radio mics. The RF unit is often built into the casing of the mic, but can also be a separate device added onto a handheld, for example.

Question 50

What do we mean by line level, and what is the difference between the consumer and professional versions?

Answer 50

Line level is where most switching and audio processing is done before being sent to an amplifier ready for loudspeakers. Dynamic mics output at what is known as mic level, and need a preamplifier to get the signal up to line level (condenser mic outputs will have already been amplified once to get up to mic level). Professional line level is normal 1V, whereas consumer devices normally use 316mV, and can be identified by phono (RCA) plugs.

Question 51

What is the term used to describe the way sound moves through air?

Answer 51

Propagation.

Question 52

What is an audio mixer?

Answer 52

An audio mixer is a device for selecting, controlling and routing sound inputs (perhaps music or mics) to sound outputs (such as an amplifier and loudspeakers). Most audio mixers have more inputs than outputs, such as a 16x4 mixer or a large 64x16 unit.

Question 53

What is an automatic mixer?

Answer 53

An automatic mixer can be used to sense the presence of speech (perhaps in an audio conference) and automatically turn on (or fade up) microphone channels when speech is detected, and switch off (or fade down) when the channel is not in use, to prevent unnecessary noise or distortion. They are not suitable for use with music.

Question 54

What is the difference between a compressor and a limiter?

Answer 54

Compressors normally try to reduce the overall dynamic range of a signal in order to prevent distortion, and are designed to reduce signals that go above a pre-determined level.
Limiters are much more extreme versions of the same thing, designed primarily to work quickly to prevent equipment damage. For example, if a microphone was dropped, the sudden sound spike could blow an amplifier or speakers. The limiter cuts the sound before any damage is done.

Question 55

In audio processing, what is the difference between an expander and a gate?

Answer 55

An expander reduces the overall dynamic range by reducing sound levels below a certain threshold, and can therefore help prevent signal distortion.
A gate completely cuts out a channel if the sound level drops below a certain threshold, and can be used to turn off microphones when not being used, reducing unwanted noise.

Question 56

What are the three main types of filters used in audio processing?

Answer 56

A high-pass filter blocks lower frequencies, allowing only higher frequencies through.
A low pass filter block higher frequencies, and may be used perhaps to send only lower frequency sound to a sub-woofer loudspeaker unit.
A notch filter blocks a certain specific narrow frequency range, leaving all others to pass through.

Question 57

What is an equaliser?

Answer 57

An equaliser allows the individual gain control across a range of frequencies. While a simple bass and treble control is effectively a 2 channel equaliser, a graphic equaliser might have over 30 channels.

Question 58

Why might a delay unit be used?

Answer 58

An electronic delay might be used in a sound reinforcement system, to ensure that both direct and reinforced sound arrives at the listener at the same time, avoiding an echo. Delay would be increased the further the listener is from the original sound source, as otherwise the electrical sound signal would travel significantly faster than the original sound through air.