Foundations of Acoustics

Question 1

Define air pressure

Answer 1

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

Question 2

What is a pure tone?

Answer 2

The simplest form of sound corresponding to a sine wave

Question 3

What kind of motion does pressure have?

Answer 3

Regular, periodic motion

Question 4

What is the approximate speed of a sound wave?

Answer 4

343 m/s

Question 5

What is a compression?

Answer 5

A high pressure section of the wave corresponding to the peak of the sine wave - the low pressure regions are called rarefactions

Question 6

What is amplitude?

Answer 6

The amplitude corresponds to the degree of pressure fluctuation in the sine wave. It is measured as the distance between the baseline and the extrema of the wave.

Question 7

What is frequency?

Answer 7

The frequency parameter determines how many complete wave cycles fit within one second. Frequency has the units ‘Hz’

Question 8

What happens if you double the frequency?

Answer 8

Sounds an octave higher

Question 9

What happens if you multiply the frequency by 1.5

Answer 9

An additional perfect fifth higher

Question 10

What does phase determine?

Answer 10

The phase determines the starting point of the wave cycle. It takes values between zero and 2π

Question 11

What is the formal definition of a sine wave?

Answer 11

x(t)=Asin(2πft+ϕ)

x(t) is the relative pressure at time t;
A is the amplitude;
sin is a special mathematical function that you can find on a calculator, which takes a single number as an input and returns a number between -1 and 1;
π is a constant approximately equal to 3.14;
f is the frequency in Hz;
ϕ is the phase, taking values between 0 and 2π

Question 12

How do you calculate the wavelength?

Answer 12

λ=v/f

Question 13

What is a sawtooth wave?

Answer 13

A wave with very sharp corners

Question 14

What is a square wave?

Answer 14

It has rectangular corners

Question 15

What is Fourier theorem?

Answer 15

Fourier theorem tells us that every periodic wave can be expressed by adding together a collection of sine waves

Question 16

How can a sawtooth wave be created?

Answer 16

A sawtooth wave can be created by adding together infinitely many pure tones whose frequencies follow an ascending arithmetic progression

Question 17

How is a square wave produced?

Answer 17

Produced like a sawtooth wave, but with one simple change: all the even harmonics are removed.

Question 18

What is a harmonic complex tone and give two examples

Answer 18

A harmonic complex tone is defined as a wave built from combining many pure tones, each of which has a frequency that is a whole-number multiple of a so-called fundamental frequency. This sequence of whole-number multiples is called a harmonic series
Examples include - sawtooth and square wave

Question 19

What is a spectrum and how do you take a waveform to a spectrum?

Answer 19

Plot amplitude as a function of frequency, producing what we call the spectrum. The mathematical process of translating from the waveform to the spectrum is called Fourier analysis

Question 20

What are harmonics?

Answer 20

We call the upper frequencies harmonics

Question 21

What are partials?

Answer 21

If we had upper frequencies that didn’t correspond to integer multiples of the fundamental frequency, we’d call them partials.

Question 22

What is a spectrogram?

Answer 22

Here the horizontal axis corresponds to time, the vertical axis corresponds to frequency, and the colour corresponds to amplitude