Audio fundamentals and the time domain

Question 1

Describe a transverse wave.

Answer 1

Displacement of the medium is at right angles to the propagation of the wave
e.g. waves in water, vibration of a string

Question 2

Describe a longitudinal wave.

Answer 2

Displacement of the medium is parallel to the propagation of the wave (direction of travel)
e.g. a “slinky”, sound waves in air

Question 3

What is the speed of sound?

Answer 3

340 m/s

Question 4

What is sound faster through?

Answer 4

More dense media. e.g water, solid objects and varies with temperature.

Question 5

What is wavelength?

Answer 5

The length of one period (or cycle) of a waveform .

Question 6

What is frequency?

Answer 6

The rate at which the wavelength or period repeats.

Question 7

What does phase describe?

Answer 7

The position within a single cycle. Often described in terms of angles.
The phase relationship of 2 similar signals is also important when they are added together.

Question 8

Describe 2 waves ‘in’ phase.

Answer 8

They combine to double the amplitude (constructive interference).

Question 9

What happens if 2 waves are 180 degrees ‘out’ of phase?

Answer 9

Silence

Question 10

Describe standing waves.

Answer 10

In enclosed spaces (e.g. rooms, instruments), the reflection of sounds can combine to produce a phenomena known as standing waves.

Question 11

What is the unit for time in digital audio and DSP programming?

Answer 11

Samples - Divide seconds by samples-per-second
(e.g. 44100Hz sample-rate for CD quality audio)

Question 12

What is an important advantage of wire/tape based systems?

Answer 12

The ability to easily cut and rejoin recordings, e.g. sound editing.

Question 13

How were paper/plastic based tapes better than steel tape?

Answer 13

• Paper/plastic-based tapes had a clear advantage being more malleable and enabling cuts with a razor blade (or scissors) and rejoining with adhesive tape – known as the tape splice.
• The angle of the cut could be used to create a cross-fade.

Question 14

What type of wave could magnetic tape be compared to?

Answer 14

Longitudinal

Question 15

What is music concrete?

Answer 15

Found sound” as music - pioneered by Pierre Schaffer (late 40s/50s)

Question 16

Give 2 examples of uncompressed audio formats.

Answer 16

WAV (Microsoft Wave)
AIFF (Apple’s Audio Interchange File Format)
- Such formats have similar properties such as:
 sample rate, bit depth (resolution), channel count

Question 17

What should compressed formats never be used for?

Answer 17

Compressed formats, like MP3 and AAC should never be used for high-quality recording or editing, and only for final delivery (if this is appropriate) or mass online distribution.

Question 18

How can you use tape to create a cross fade?

Answer 18

Tape splicing at a 45°or 60°angle causes a natural (though short) cross-fade between parts.

Question 19

What is one way to avoid clicks?

Answer 19

One way to try and avoid clicks is to do edits on 
zero-crossings, i.e. only start / end a segment where the waveform crosses the +0dB line.

Question 20

Describe destructive editing.

Answer 20

Edits are applied to the original file.
Can be irreversible
Similar to tape editing (where the tape is actually cut).

Question 21

Describe non-destructive editing.

Answer 21

• Made possible by faster CPUs and realtime processing.
• No changes to the source files are needed.
• A “playlist” or edit decision list (EDL) is used to instruct which segments of which files should play, and what effects to apply on-the-fly (fades, EQ, reverb, etc.)