Page 22 - Master Handbook of Acoustics

CHAPTER 1

Fundamentals of Sound

S ound can be considered as wave motion in air or other elastic media. In this case, sound acts

as a stimulus. Sound can also be considered as an excitation of the hearing mechanism that

results in its perception. In this case, sound is a sensation. This duality of sound is familiar to those
interested in audio and music. The type of problem at hand dictates our approach. If we are interested
in the physical disturbance in the air in a room, it is a problem of physics. If we are interested in how
that disturbance is perceived by a person listening in the room, psychoacoustical methods must be
used. Because this book addresses acoustics in relation to people, both aspects of sound will be
considered.

Sound can be characterized by objective phenomena. For example, frequency is an objective
property of sound; it specifies the number of waveform repetitions per unit of time (usually 1 second).
Frequency can be readily measured on an oscilloscope or a frequency counter. On the other hand,
sound can be characterized subjectively. For example, pitch is a subjective property of sound.
Perceptually, the ear hears different pitches for soft and loud 100-Hz tones. As intensity increases, the

pitch of a low-frequency tone goes down, while the pitch of a high-frequency tone goes up. Harvey
Fletcher found that playing pure tones of 168 and 318 Hz at a modest level produces a very
discordant sound. At a high intensity, however, the ear hears the pure tones in the 150- to 300-Hz
octave relationship as a pleasant sound. We cannot equate frequency and pitch, but they are
analogous. Another duality exists between intensity and loudness. Similarly, the relationship between
waveform (or spectrum) and perceived quality (or timbre) is not linear. A complex waveform can be
described in terms of a fundamental and a series of harmonics of various amplitudes and phases. But
perception of timbre is complicated by the frequency-pitch interactions in the human hearing

mechanism as well as other factors.
The interaction between the physical properties of sound, and our perception of them, poses
delicate and complex issues. It is this complexity in audio and acoustics that creates such interesting
problems. On one hand, the design of a loudspeaker or a concert hall should be a straightforward and

objective engineering process. But in practice, that objective expertise must be carefully tempered
with purely subjective wisdom. As has often been pointed out, loudspeakers are not designed to play
sine waves into calibrated microphones placed in anechoic chambers. Instead, they are designed to
play music in our listening rooms. In other words, the study of audio and acoustics involves both art
and science. To learn the complexities of audio and acoustics, we begin with the science, keeping in
mind that our ears will ultimately determine the success or failure of our projects.

Simple Harmonic Motion and the Sine Wave

The weight (mass) on the spring shown in Fig. 1-1 comprises a vibrating system. Moreover, the