Introduction to Speech Acoustics 1

1. The nature of sound

Sound is variations in air pressure detectable by the human ear. Pressure varies through time at a particular point, and over space at a particular time, as molecules of air collide (condensation) or move apart (rarefaction).

Fig. 1: Diagrammatic representation of fluctuations in air pressure such as those caused by a vibrating tuning fork.

2. Waves and energy

Movement, or variation at a particular point, can be plotted as a waveform on a graph, as in Fig. 1.

N.B. (i) The relation of waveform to molecule movement.

The variation in pressure at a given point gives a sine wave for pure sounds involving simple harmonic motion (SHM).

The amplitude (the amount of maximum displacement from zero) of the wave reflects the highest pressure involved, and therefore the acoustic energy. N.B. Amplitude is not connected to frequency.

Relations between frequency (F), period (P), wavelength (w) and speed of sound (c):

3. Types of sound sources

a. Tuning fork - periodic

b. Vowel sounds - quasi-periodic. Fig. 3:schwa waveform

c. Flow of water - continuous random noise

d. Fricative - random within certain constraints. Fig. 4:

e. Hammer hitting table - transient

f. Stop consonant - transient + noise. Fig. 5:/t/ waveform