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CONCEPTS Applied and pure tones, but you can readily identify the source of the

very same musical note made by two different instruments. You
A Singing Glass recognize it as a musical note, not noise and not a pure tone. You
also recognize if the note was produced by a violin or a guitar.
Did you ever hear a glass “sing” when the rim was Th e difference is in the wave form of the sounds made by the two

rubbed? The trick to make the glass sing is to remove as

much oil from your finger as possible. Then you lightly instruments, and the difference is called the sound quality. How
rub your wet finger around and on the top of the glass does a musical instrument produce a sound of a characteristic

rim at the correct speed. Without oil, your wet finger will quality? The answer may be found by looking at the instruments
imperceptively catch on the glass as you rub the rim. that make use of vibrating strings.
With the appropriate pressure and speed, your catching
finger might match the natural frequency of the glass.
The resonant vibration will cause the glass to “sing” with VIBRATING STRINGS
a high-pitched note. A stringed musical instrument, such as a guitar, has strings that
are stretched between two fixed ends. When a string is plucked,

waves of many different frequencies travel back and forth on

the string, reflecting from the fixed ends. Many of these waves

quickly fade away, but certain frequencies resonate, setting up
5.6 SOURCES OF SOUNDS patterns of waves. Before we consider these resonant patterns
in detail, keep in mind that (1) two or more waves can be in the
All sounds have a vibrating object as their source. Th e vibra- same place at the same time, traveling through one another from
tions of the object send pulses or waves of condensations and opposite directions; (2) a confined wave will be reflected at a

rarefactions through the air. These sound waves have physical boundary, and the reflected wave will be inverted (a crest

properties that can be measured, such as frequency and inten- becomes a trough); and (3) reflected waves interfere with incom-

sity. Subjectively, your response to frequency is to identify a ing waves of the same frequency to produce standing waves.
certain pitch. A high-frequency sound is interpreted as a high- Figure 5.21 is a graphic “snapshot” of what happens when
pitched sound, and a low-frequency sound is interpreted as a reflected wave patterns meet incoming wave patterns. Th e

low-pitched sound. Likewise, a greater intensity is interpreted as incoming wave is shown as a solid line, and the refl ected wave
increased loudness, but there is not a direct relationship between is shown as a dotted line. The result is (1) places of destructive

intensity and loudness as there is between frequency and pitch. interference, called nodes, which show no disturbance and
There are other subjective interpretations about sounds.

Some sounds are bothersome and irritating to some people but (2) loops of constructive interference, called antinodes, which
take place where the crests and troughs of the two wave pat-
go unnoticed by others. In general, sounds made by brief, irregu- terns produce a disturbance that rapidly alternates upward and
lar vibrations such as those made by a slamming door, dropped downward. This pattern of alternating nodes and antinodes

book, or sliding chair are called noise. Noise is characterized does not move along the string and is thus called a standing
by sound waves with mixed frequencies and jumbled intensi- wave. Note that the standing wave for one wavelength will have
ties (Figure 5.20). On the other hand, there are sounds made by a node at both ends and in the center and also two antinodes.
very regular, repeating vibrations such as those made by a tuning
fork. A tuning fork produces a pure tone with a sinusoidal curved
pressure variation and regular frequency. Yet a tuning fork pro- Incoming Fixed end
duces a tone that most people interpret as bland. You would not wave
call a tuning fork sound a musical note! Musical sounds from
instruments have a certain frequency and loudness, as do noise A
Reflected
wave Fixed end

B
A
Node Node
Node Fixed end

B C

Antinodes

C FIGURE 5.21 An incoming wave on a cord with a fixed end
(A) meets a reflected wave (B) with the same amplitude and
FIGURE 5.20 Different sounds that you hear include (A) noise, frequency, producing a standing wave (C). Note that a standing
(B) pure tones, and (C) musical notes. wave of one wavelength has three nodes and two antinodes.

5-15 CHAPTER 5 Wave Motions and Sound 129