Vern J. OstdiekDonald J. Bord

Chapter 6Waves and Sound

(Section 4)

6.4 Production of Sound

• In the remaining sections of this chapter, we will take a brief look at the three P’s of acoustics: • the production, propagation, and perception of

sound• The sounds that we hear range from simple pure

tones such as a steady whistle to complicated and random waveforms like those found on a noisy street corner. • Most of the sound we hear is a combination of

many sounds from different sources. • The loudness usually fluctuates, as do the

frequencies of the component sounds.

6.4 Production of Sound

• Sound is produced when vibration causes pressure variations in the air.• Any flat plate, bar, or membrane that vibrates

produces sound• A tuning fork is a nice example.

6.4 Production of Sound

• A dropped garbage-can lid, a vibrating speaker cone, and a struck drumhead produce sound the same way. • The tuning fork executes simple harmonic motion

and produces a pure tone. • The garbage-can lid and the drumhead have more

complicated motions and so produce complex tones or noise

6.4 Production of Sound

• The various musical instruments represent some of the basic types of sound producers.

• Drums, triangles, xylophones, and other percussion instruments produce sound by direct vibration. • Each is made to vibrate by a blow from a mallet or

drumstick.

6.4 Production of Sound

• Guitars, violins, and pianos use vibrating strings to produce sound.

6.4 Production of Sound

• By itself, a vibrating string produces only faint sound because it is too thin to compress and expand the air around it effectively. • These instruments employ “soundboards” to

increase the sound production.

• The electric versions of guitars and violins pick up and amplify the string vibrations electronically.• One end of the string is attached to a wooden

soundboard, which is made to vibrate by the string. • The vibrating soundboard, in turn, produces the

sound.

6.4 Production of Sound

• The figure shows a simplified diagram of the process used in pianos.

6.4 Production of Sound

• When a note is played, a hammer strikes the piano wire and produces a wave pulse. • The pulse travels back and forth on the wire, being

reflected each time at the ends.

• The soundboard receives a “kick” each time the pulse is reflected at that end. • This makes the soundboard vibrate at a frequency

equal to the frequency of the pulse’s back-and-forth motion.

• The sound dies out because the pulse loses energy to the soundboard during each cycle.

6.4 Production of Sound

• The strings on guitars are plucked instead of struck, giving the pulses a different shape. • This is partly why the sound of a guitar is different

from that of a piano.

• Violin strings are bowed, resulting in even more complicated wave pulses.

• In all three instruments, the frequency of the pulse’s motion depends on the speed of waves on the string and on the length of the string. • When a string is tuned by being tightened, the wave

speed is increased

6.4 Production of Sound

• The pulse moves faster on the string and makes more “round-trips” each second—the frequency of the sound is raised.

• Different notes are played on the same guitar or violin string by using a finger to hold down the string some distance from its fixed end. • The pulse travels a shorter distance between

reflections, makes more roundtrips per second, and produces a higher-frequency sound.

6.4 Production of Sound

• A similar process is used in flutes, trumpets, and other wind instruments.• Here it is a pressure pulse in the air inside a tube

that moves back and forth.

6.4 Production of Sound

• Initially, a sound pulse is produced at one end of the tube by the musician. • This pulse travels down the tube and is partially

reflected and partially transmitted at the other end. • The transmitted part spreads out into the air,

becoming the sound that we hear.

6.4 Production of Sound

• The reflected part returns to the mouthpiece end, where it is reflected again and reinforced by the musician. • The sound pulse is also inverted at each end: it

goes from a compression to an expansion, and vice versa.

• The musician must supply pressure pulses at the same frequency that the pulse oscillates back and forth in the tube. • This, of course, is the frequency of the sound that is

produced.

6.4 Production of Sound

• Different notes are played by changing the length of the tube—by opening side holes in woodwinds and by using valves or slides in brasses.

• The speed of the pulses is determined by the temperature of the air. • This is one reason why musicians “warm up” before

a performance.

• The air inside the instrument is warmed by the musicians’ breath and hands. • Hence the frequencies of the notes are higher than

when the air inside is cool.

6.4 Production of Sound

• The human voice uses several types of sound production and modification mechanisms.

• Some consonant sounds like “sss” and “fff” are technically noise: • They are hissing sounds produced by air rushing

over the teeth and lips.

• The randomly swirling air produces sounds with random, changing frequencies.

6.4 Production of Sound

• The vocal cords, located inside the Adam’s apple in the throat, are the primary sound producers for singing and for spoken vowel sounds.

6.4 Production of Sound

• When air is blown through the vocal cords, they vibrate and produce pressure pulses (sound) much like the reed of a saxophone. • This sound is modified by the shapes of the air

cavities in the throat, mouth, and nasal region.• Muscles in the throat are used to tighten and loosen

the vocal cords, thereby changing the pitch of the sound.

6.4 Production of Sound

• Moving the tongue or jaw changes the shape of the mouth’s air cavity and allows for different sounds to be produced. • A sinus cold can change the sound of one’s voice

because swelling changes the configuration of the nasal cavity

6.4 Production of Sound

• Perhaps you’ve heard someone speak who had inhaled helium.

• This is not a recommended exercise. • It is possible to suffocate because of lack of oxygen

in the lungs.

• The speed of sound in helium is nearly three times that in air. • This raises the frequencies of the sounds and gives

the speaker a falsetto voice.

6.4 Production of Sound

• Sound waves carry energy, as do all waves. • This means that the source of the sound must

supply energy. • Speaking loudly or playing an instrument for

extended periods of time can tire you out for this reason.

• For continuous sounds, it is more relevant to consider the power of the source, because the energy must be supplied continuously.

• Most instruments, including the human voice, are very inefficient; • Typically, only a small percentage of the energy

output of the performer is converted into sound energy.