vern j. ostdiek donald j. bord chapter 6 waves and sound (section 6)

Vern J. OstdiekDonald J. Bord

Chapter 6Waves and Sound

(Section 6)

6.6 Perception of Sound

• In this section, we consider some aspects of sound perception—• how the physical properties of sound waves are

related to the mental impressions we have when we hear sound

• We will be comparing psychological sensations, which can be quite subjective, to measurable physical quantities. • A similar situation: “hot” and “cold” are subjective

perceptions that are related to temperature, which is a measurable physical quantity.


• To make things simple, we will limit ourselves to steady, continuous sounds.

• This frees us from having to include such effects as reverberation in a room—• That is, how the sound builds up, how it decays,

and so on

• The main categories that we use to describe sounds subjectively are pitch, loudness, and tone quality.


• The pitch of a sound is the perception of highness or lowness. • The sound of a soprano voice has a high pitch and

that of a bass voice has a low pitch. • The pitch of a sound depends primarily on the

frequency of the sound wave. • The loudness of a sound is self-descriptive.

• We can distinguish among very quiet sounds (difficult to hear), very loud sounds (painful to the ears), and sounds with loudness somewhere in between.

• The loudness of a sound depends primarily on the amplitude of the sound wave.


• The tone quality of a sound is used to distinguish two different sounds even though they have the same pitch and loudness. • A note played on a violin does not sound quite like

the same note played on a flute.

• The tone quality of a sound (also referred to as the timbre or tone color) depends primarily on the waveform of the sound wave.

6.6 Perception of SoundPitch

• Pitch is perhaps the most accurately discriminated of the three categories, particularly by trained musicians.

• It depends almost completely on the frequency of the sound wave: • The higher the frequency, the higher the pitch.

• Noise does not have a definite pitch, because it does not have a definite frequency.


• Pitch is essential to nearly all music. • There is a great deal of arithmetic in the musical

scale; • Each note has a particular numerical frequency.


• The figure shows the frequencies of the notes on the piano keyboard.


• There are seven octaves on the piano, each consisting of 12 different notes.• Within each octave, the notes are designated A

through G, plus five sharps and flats. • Each note in a given octave has exactly twice the

frequency of the corresponding note in the octave below.


• For example, the frequency of the lowest note on the piano, an A, is 27.5 hertz; • for the A in the next octave, it is 55 hertz; • it is 110 hertz for the third A, and so on

• The frequency of middle C is 261.6 hertz.


• Certain combinations of notes are pleasing to the ear, whereas others are not.

• Nearly 2,500 years ago, Pythagoras indirectly discovered that two different notes are in harmony when their frequencies have a simple whole-number ratio. • For example, a musical fifth is any pair of notes with

frequencies in the ratio 3 to 2.

• Any E and the first A below it have this ratio of frequencies, as do any G and the first C below it.


• The figure also shows the approximate ranges of singing voices and some instruments.


• For normal speech, the ranges are approximately 70 to 200 hertz for men and 140 to 400 hertz for women.• When whispering, you do not use your vocal cords,

and you produce much higher-frequency “hissing” sounds.

6.6 Perception of SoundLoudness

• The loudness of a sound is determined mainly by the amplitude of the sound wave. • The greater the amplitude of the sound wave that

reaches your eardrums, the greater the perceived loudness of the sound.

• The actual pressure amplitudes of normal sounds are extremely small, typically around one-millionth of 1 atmosphere.


• This causes the eardrum to vibrate through a distance of around 100 times the diameter of a single atom. • An extremely faint sound has an amplitude of less

than one-billionth of 1 atmosphere, and it makes the eardrum move less than the diameter of an atom.

• The ear is an amazingly sensitive device.


• There is a specially defined physical quantity that depends on the amplitude of sound but is more convenient for relating amplitude to perceived loudness.

• This is the sound pressure level or simply the sound level.


• The standard unit of sound level is the decibel (dB). • The range of sounds that we are normally exposed

to has sound levels from 0 decibels to about 120 decibels.


• Sound level does not take into account the irritation of the sound. • Your favorite music played at 100 decibels may not

sound as loud as an annoying screech of fingernails on a blackboard with a sound level of 80 decibels.


• The relationship between the amplitude of a steady sound and its sound level is based on factors of 10. • A sound with 10 times the amplitude of another

sound has a sound level that is 20 decibels higher. • A 90-decibel sound has 10 times the amplitude of a

70-decibel sound.


• The following five statements describe how the perceived loudness of a sound is related to the measured sound level. • These are general trends that have been identified

by researchers after testing large numbers of people.

• For a particular person, the actual numerical values of the sound levels can vary somewhat from those listed. • Also some of the values given, particularly in

numbers 1 and 3 below, depend on the frequencies of the sounds.


1. The sound level of the quietest sound that can be heard under ideal conditions is 0 decibels. • This is called the threshold of hearing.

2. A sound level of 120 decibels is called the threshold of pain. • Sound levels this high cause pain in the ears and

can result in immediate damage to them.


3. The minimum increase in sound level that makes a sound noticeably louder is approximately 1 decibel. • For example, if a 67-decibel sound is heard and

after that a 68-decibel sound, we can just perceive that the second sound is louder.

• If the second sound had a sound level of 67.4 decibels, we could not notice a difference in loudness.


4. A sound is judged to be twice as loud as another if its sound level is about 10 decibels higher. • A 44-decibel sound is about twice as loud as a 34-

decibel sound. • A 110- decibel sound is about twice as loud as a

100-decibel sound. • This is a cumulative factor: a 110-decibel sound is

about four times as loud as a 90-decibel sound and so on.


5. If two sounds with equal sound levels are combined, the resulting sound level is about 3 decibels higher. • If one lawn mower causes an 80-decibel sound

level at a certain point nearby, starting up a second identical lawn mower next to the first will raise the sound level to about 83 decibels.


• It turns out that 10 similar sound sources are perceived to be twice as loud as a single source.


• The loudness of pure tones and, to a lesser degree, of complex tones, also depends on the frequency. • This is because the ear is inherently less sensitive

to low- and high-frequency sounds.

• The ear is most sensitive to sounds in the frequency range of 1,000 to 5,000 hertz. • For example, a 50-hertz pure tone at 78 decibels, a

1,000-hertz pure tone at 60 decibels, and a 10,000-hertz tone at 72 decibels all sound equally loud.


• At very high sound levels, 80 decibels and above, the ear’s sensitivity does not vary as much with frequency as it does at lower sound levels.

• One reason for this variation in sensitivity is that a considerable amount of low-frequency sound is produced inside our bodies by flowing blood and flexing muscles. • The ear is less sensitive to low-frequency sounds,

so these internal sounds do not “drown out” the external sounds that we need to hear.


• Sound levels are measured with sound-level meters.


• Most sound-level meters are equipped with a special weighting circuit (called the A scale) that allows them to respond to sound much as the ear does. • The response to low and high frequencies is

diminished. • The readings on the A scale are designated dBA.

• When operating in the normal mode (called the C scale) a sound-level meter measures the sound level in decibels, treating all frequencies equally.

• When in the A scale mode, a sound-level meter responds like the human ear and therefore indicates the relative loudness of the sound.


• Loud sounds can not only damage your hearing but also affect the physiological and psychological balance of your body.

• Since the beginning of humankind, the sense of hearing has been used as a warning device: • loud sounds often indicate the possibility of danger,

and the body automatically reacts by becoming tense and apprehensive.

• Constant exposure to loud or annoying sounds puts the body under stress for long periods of time and consequently jeopardizes the physical and mental well-being of the individual.


• The Occupational Safety and Health Administration (OSHA) has established standards designed to protect workers from excessive sound levels. Workers must be supplied with sound-protection devices such as earmuffs and earplugs if they are exposed to sound levels of 90 dBA or higher. • Some communities also have noise

ordinances designed to reduce the sound levels of traffic and other activities.

6.6 Perception of SoundTone Quality

• The tone quality of a sound is not as easily described as loudness or pitch. • Comparisons such as full versus empty, harsh

versus soft, or rich versus dry are sometimes used.

• The tone quality of a sound is very important to our ability to identify what produced the sound. • The sound of a flute is different from the sound of a

clarinet, and we notice this even if they produce the same note at the same sound level.

• The tone quality of a person’s voice helps us identify the speaker.


• The tone quality of a sound depends primarily on the waveform of the sound wave. • If two sounds have different waveforms, we usually

perceive different tone qualities.

• The simplest waveform is that of a pure tone: sinusoidal. • Pure tones have a soft, pleasant tone quality

(unless they are very loud or high pitched).


• Complex tones with waveforms that are nearly sinusoidal share the same characteristics. • Unlike frequency and sound level, a waveform

cannot be expressed as a single numerical factor.

• What determines the waveform of a sound wave?


• Any complex waveform is equivalent to a combination of two or more sinusoidal waveforms with definite amplitudes. • These component waveforms are called harmonics.

• The frequencies of harmonics are whole-number multiples of the frequency of the complex waveform.


• For our purposes, this means that any complex tone is equivalent to a combination of pure tones. • These pure tones, called harmonics, have

frequencies that are equal to 1, 2, 3, . . . times the frequency of the complex tone.


• For example, the figure shows the waveform of a complex tone with a frequency of, let’s say, 100 hertz. • It is equivalent to a combination of the three pure

tones shown of frequencies 100, 200, and 300 hertz.


• This means that we could artificially produce this complex tone by carefully playing the individual pure tones simultaneously. • This is one method that electronic music

synthesizers can use to create sounds.


• The tone quality of a complex tone depends on the number of harmonics that are present and on their relative amplitudes. • These two factors give us a quantitative way of

comparing waveforms.

• A spectrum analyzer is a sophisticated electronic instrument that indicates which harmonics are present in a complex tone and what their amplitudes are.


• In general, complex tones with a large number of harmonics have a rich tone quality. • Notes played on a recorder or a flute contain only a

couple of harmonics, so they sound similar to pure tones.

• Violins and clarinets, on the other hand, have more than a dozen harmonics in their notes and consequently have richer tone qualities.


• The waveform of noise is a random “scribble” that doesn’t repeat. • This is because noises are composed of large

numbers of frequencies that are not related to each other: they are not harmonics.

• “White noise” contains equal amounts of all frequencies of sound. • It is so named because one way to produce “white

light” is to combine equal amounts of all frequencies of light.

• The sound of rushing air approximates white noise


• The existence of higher-frequency harmonics in musical notes explains why high fidelity sound-reproduction equipment must respond to frequencies up to about 20,000 hertz. • Even though the frequencies of musical notes are

generally less than 4,000 hertz, the frequencies of the harmonics do go up to 20,000 hertz and higher.

• Since our ears can’t hear any harmonic with a frequency above 20,000 hertz, it isn’t necessary to reproduce them.• To accurately reproduce a complex tone, each

higher-frequency harmonic must be reproduced.


• The study of sound perception brings together the fields of physics, biology, and psychology. • The mechanical properties of sound waves interact

with the physiological mechanisms in the ear to produce a psychological perception.

• One of the challenges is to relate subjective descriptions of different sound perceptions to measurable aspects of the sound waves.


• The hearing apparatus itself is one of the most remarkable in all of Nature. • It responds to an amazing range of frequencies and

amplitudes and still captures the beauty and subtlety of music.

• At the same time, it is very fragile.

• We need to learn more about how our sense of hearing works and how it can be protected.

Concept Map 6.2

Summary

• Waves are everywhere. They can be classified as transverse or longitudinal according to the orientation of the wave oscillations.

• The speed of a wave depends on the properties of the medium through which it travels. • For continuous periodic waves, the product of the

frequency and the wavelength equals the wave speed.

Summary

• Once waves are produced, they are often modified as they propagate. • Reflection and diffraction cause waves to change

their direction of motion when they encounter boundaries.

• Interference of two waves produces alternating regions of larger amplitude and zero-amplitude waves.

• The Doppler effect and the formation of shock waves are phenomena associated with moving wave sources. • The former also occurs when the receiver of the

waves is moving.

Summary

• Although sound can refer to a broad range of mechanical waves in all types of matter, we often restrict the term to the longitudinal waves in air that we can hear. • Sound waves in air are generally represented by

the air-pressure fluctuations associated with the compressions and expansions in the sound wave.

Summary

• Sound with frequency too high to be heard by humans, above about 20,000 hertz, is called ultrasound. • Ultrasound is used for echolocation by bats and for

a variety of procedures in medicine.

• The sounds that we can hear can be divided into pure tones, complex tones, and noise.

Summary

• Sound propagation inside rooms and other enclosures is dominated by repeated reflection, called reverberation. • This causes individual sounds to linger after they

are produced. • Moderate reverberation causes a positive blending

of successive sounds, such as musical notes, but can adversely affect the clarity of speech.

Summary

• Sound is produced in many different ways, all resulting in rapid pressure fluctuations that travel as a wave. • Different musical instruments employ diverse and

sometimes multiple sound-production mechanisms, including vibrating plates or membranes, vibrating strings, or vibrating columns of air in tubes.

Summary

• We use three main characteristics to classify a steady sound we perceive: pitch, loudness, and tone quality. • The pitch of a sound depends mainly on the

frequency of the sound wave.

• The loudness of a sound depends mainly on the amplitude (or sound level) of the sound, but it is also affected by the frequency.

Summary

• The tone quality of a sound depends on the waveform of the sound wave.

• The waveform of a complex tone depends on the number and amplitudes of the separate pure tones, called harmonics, that comprise it.

vern j. ostdiek donald j. bord chapter 6 waves and sound (section 6)

Documents

sound section

aspects of sound perception

definite pitch

high pitch

low pitch

perception of highness

definite frequency

different sounds