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Vern J. Ostdiek Donald J. Bord Chapter 6 Waves and Sound (Section 6)

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1 Vern J. Ostdiek Donald J. Bord Chapter 6 Waves and Sound (Section 6)

2 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.

3 6.6 Perception of Sound 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.

4 6.6 Perception of Sound 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.

5 6.6 Perception of Sound 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 6.6 Perception of Sound Pitch 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.

7 6.6 Perception of Sound Pitch 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.

8 6.6 Perception of Sound Pitch The figure shows the frequencies of the notes on the piano keyboard.

9 6.6 Perception of Sound Pitch 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.

10 6.6 Perception of Sound Pitch 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 hertz.

11 6.6 Perception of Sound Pitch 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.

12 6.6 Perception of Sound Pitch The figure also shows the approximate ranges of singing voices and some instruments.

13 6.6 Perception of Sound Pitch 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.

14 6.6 Perception of Sound Loudness 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.

15 6.6 Perception of Sound Loudness 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.

16 6.6 Perception of Sound Loudness 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.

17 6.6 Perception of Sound Loudness 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.

18 6.6 Perception of Sound Loudness 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.

19 6.6 Perception of Sound Loudness 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.

20 6.6 Perception of Sound Loudness 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.

21 6.6 Perception of Sound Loudness 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.

22 6.6 Perception of Sound Loudness 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.

23 6.6 Perception of Sound 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.

24 6.6 Perception of Sound Loudness 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.

25 6.6 Perception of Sound Loudness It turns out that 10 similar sound sources are perceived to be twice as loud as a single source.

26 6.6 Perception of Sound Loudness 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.

27 6.6 Perception of Sound Loudness 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.

28 6.6 Perception of Sound Loudness Sound levels are measured with sound-level meters.

29 6.6 Perception of Sound Loudness 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.

30 6.6 Perception of Sound Loudness 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.

31 6.6 Perception of Sound Loudness 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.

32 6.6 Perception of Sound Tone 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.

33 6.6 Perception of Sound Tone Quality 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).

34 6.6 Perception of Sound Tone Quality 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?

35 6.6 Perception of Sound Tone Quality 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.

36 6.6 Perception of Sound Tone Quality 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.

37 6.6 Perception of Sound Tone Quality 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.

38 6.6 Perception of Sound Tone Quality 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.

39 6.6 Perception of Sound Tone Quality 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.

40 6.6 Perception of Sound Tone Quality 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.

41 6.6 Perception of Sound Tone Quality 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

42 6.6 Perception of Sound Tone Quality 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.

43 6.6 Perception of Sound Tone Quality 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.

44 6.6 Perception of Sound Tone Quality 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.

45 Concept Map 6.2

46 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.

47 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.

48 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.

49 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.

50 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.

51 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.

52 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.

53 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.


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