1© Manhattan Press (H.K.) Ltd. Loudness, pitch and quality Intensity level and loudness Intensity level and loudness 11.2 Properties of sound.

© Manhattan Press (H.K.) Ltd. 1

• Loudness, pitch and qualityLoudness, pitch and quality

• • Intensity level and loudnessIntensity level and loudness

11.2 Properties of 11.2 Properties of soundsound


11.2 Properties of sound (SB p. 172)

Loudness, pitch and quality

sound human ear responded: 20 Hz – 20 000 Hzultrasonics: above this rangeinfrasonics: below this range (sources: earthquakes, thunder, volcanoes, heavy vibrating machinery)

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More to Know 3More to Know 3Go to

More to Know 4More to Know 4




(a) Loudness:- subjective sensation- determined by intensity of sound, sensitivity of ear

2

2Intensity

ra

a – amplitude of soundr – distance from source

(b) Pitch:- higher frequency, higher pitch




(c) Quality:- depends on waveform of sound

Audio-frequency generator

Piano Guitar



Intensity level and loudness

1. Intensity in decibels

dB 10

log 10

bel 10

loglevelIntensity

12

12

I

I

10-12 W m-2: threshold of

hearing for sound of 1 000 Hz

I: sound intensity

Unit: dB

Note: Intensity level increase of 3 dB corresponds to about doubling the sound intensity.

dB log 10 levelintensity Relative1

2

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More to Know 5More to Know 5





Intensity W m-2 dB

Threshold of pain 106 120

Thunder 105 110

Jet engine 104 100

Vacuum cleaner 1 60

Conversation 10-2 40

Rusting leaves 10-4 20

Threshold of hearing at 1 000 Hz

10-6 0





Threshold of hearing- minimum intensity of sound that normal human ear can hear

Threshold of pain- sound of intensity 1 W m-2 or higher which causes pain to our ears

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Common Error




2. Loudness of sound

- Human ear responds to sound over 120 dB- Ear is not equally sensitive to all frequencies

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Example 2Example 2

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Example 3Example 3


End


Ultrasonics in nature

Although ultrasonics is out of the audible range of human beings, it can be heard by some animals. For example, bats and dolphins use ultrasonics of frequencies 20 kHz and 150 kHz respectively for communicating.

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Infrasonics in nature

Some animals such as elephants, rhinos and whales can hear infrasonics. It is believed that this ability helps them to have an early signal about earthquake and other weather disturbances. Return to

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Audible intensity range

Different people have different ranges of audible frequencies. The range usually decreases with age.

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A sound of 0 dB does not mean that there is no sound anymore. It is the minimum sound that the human ears can detect and is equivalent to the threshold of hearing.

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Q: Q: A small source of sound emits energy uniformly in all directions. For a particular frequency, the intensity of sound at a distance of 1.0 m from the source is 1.0 × 10−5 W m−2, and corresponds to an amplitude of oscillation of the air molecules of 70 m. Assuming sound is propagated without any loss of energy, find(a) the intensity of sound,(b) the amplitude of oscillation of the air molecules at a distance of 5.0 m from the source. Solution



Solution:Solution:

m 14

7010011004

4 1004 3701001

)(Intensity (b)

m W 1004100105

01

05

01

101.0 :

21

2 5.0

1 m, 5.0 When

1011101.0 m, 1.0 When

1 )(Intensity (a)

25

72

27

25

2

2752

2

2

2

5

2

25

2

a..a

..................a.

...................aI

...

.I

.

.I

...............Ir

...............

rr

I

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Q: Q: A ship’s siren vibrates with displacement y where y = A sin200t(a) This sound causes the diaphragm of an eardrum of an observer 400 m away to vibrate. If the speed of sound is 340 m s−1, calculate

(i) the frequency of the sound,(ii) the number of wavelengths of this sound

between the siren and the eardrum,(iii) the phase difference between the vibrations

of the siren and the eardrum,(iv) the maximum energy of the eardrum if its

mass is 1.00 × 10−5 kg and the amplitude of its vibration is

1.00 × 10−8 m.(b) If the value of the speed and distance given were reliable only to 3 significant figures, comment on the confidence that could be placed in your answers to (a)(iii).

Solution



Solution:Solution:


(a) (i) Comparing the equation y = A sin200t with the wave equation y = A sint = A sin2ft

2ft = 200t Frequency (∴ f ) = 100 Hz

(b) If the value of the speed and distance were reliable to 3 significant figures, then the answer to (a)(ii) would be 118 and the phase difference in (a)(iii) would be zero.

J 100210001100210001212

21

21

eardrum ofenergy maximum efore ther motion, harmonic simplein vibratingis eardrum The (iv)

rad 1.220.6 eardrum theandsiren theof s vibrationebetween th difference Phase (iii)

61173.4400eardrum theandsiren ebetween th hs wavelengtofNumber

m 43100340h Wavelengt(ii)

168252222

...afmam

.

.fv

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1© Manhattan Press (H.K.) Ltd. Loudness, pitch and quality Intensity level and loudness Intensity level and loudness 11.2 Properties of sound.

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