Page 101 - Master Handbook of Acoustics

CHAPTER 5

Signals, Speech, Music, and Noise

S ignals such as speech, music, and noise are within the common experience of everyone. We

are profoundly familiar with the sound of speech; we hear speech virtually every day of our

lives. Speech is one of the keys to human communication, and poor speech intelligibility is extremely
frustrating. If we are lucky, we also hear music every day. Music can be one of the most pleasurable
and needful of human experiences. It would be hard to imagine a world without music. Noise is
usually considered as unwanted sound, often chaotic and disruptive to speech, music, or silence. The
close relationship of speech, music, and noise is explored in this chapter.

Sound Spectrograph

A consideration of speech sounds is necessary to understand how the sounds are produced. Speech is
highly variable and transient in nature, comprising energy moving up and down the three-dimensional
scales of frequency, sound level, and time. A sound spectrograph can show all three variables. Each

common noise has its spectrographic signature that reveals the energy that characterizes it. The
spectrographs of several commonly experienced sounds are shown in Fig. 5-1. In these
spectrographs, time progresses horizontally to the right, frequency increases from the origin upward,
and the sound level is indicated roughly by the density of the trace—the darker the trace, the more
intense the sound at that frequency and at that moment of time. Random noise on such a plot shows up
as gray, slightly mottled rectangles as all frequencies in the audible range and all intensities are
represented as time progresses. The snare drum approaches random noise at certain points, but it is

intermittent. The wolf whistle opens on a rising note followed by a gap, and then a similar rising note
that then falls in frequency. The police whistle is a tone, slightly frequency modulated.