TEACHER ENRICHMENT INITIATIVES

Activity 6B: Sound Waves

Sound is a sensation produced when vibrations initiated in the external environment strike the tympanic membrane. The waves travel through the air at a speed of 344m/sec (775mph) at 20° C at sea level. The speed at which sound waves travel becomes faster with increased temperature and altitude. Other media also conduct sound waves, such as bone, water and other fluids, but at different velocities. For example, sound waves travel at a speed of 1428 m/sec (3215 mph) in water.

The amplitude of a wave determines loudness, whereas pitch is correlated with the frequency or number of waves per unit of time. The greater the amplitude, the louder the sound; the greater the frequency, the higher the pitch.

The unit used for measuring the loudness of sound is known as a bel, a measure of air pressure changes. For convenience, 1/10 of a bel (or decibel) is normally used in describing noise levels associated with hearing. The threshold of hearing for humans is designated 0 decibels. Since the bel scale is logarithmic (and a given number of bels represents an exponent to the base 10), two bels (20 decibels) is 102(100 times) louder than threshold and six bels (60 decibels) is 106 (one million) times louder than threshold. Normal conversation measures around 60 decibels.

Sound frequencies audible to the human ear range from 20 to 20,000 cycles/second. The threshold of hearing varies with the pitch of the sound, greatest sensitivity occurring between 1000 and 4000 cycles/sec. Bigger vibrations create louder sounds.

According to the Environmental Protection Agency, a person exposed to 90 dB for a quarter of an hour or more in a working day, five days a week, will suffer some hearing loss over time. It is generally considered that a noise of 140 dB would be painful to humans.

Various activities ranked according to their decibel level

Jet takeoff at 60 meters

120 dB

Construction site

110 dB DEAFENING

Shout at 1.5 meters

100 dB

Heavy truck at 15 meters

90 dB VERY NOISY

Urban street

80 dB

Automobile interior

70 dB NOISY

Normal conversation at 1 meter

60 dB

Office, classroom

50 dB MODERATE

Living room

40 dB

Bedroom at night

30 dB QUIET

Broadcast studio

20 dB

Rustling leaves

10 dB

 

0 dB BARELY AUDIBLE

 

Keywords: 

Decibel, sound wave, frequency, tone, vibration, amplitude frequency

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Activity Code: 
6B
Unit Reference: 
Challenges and Changes: Sensitivity to Vision & Hearing Compromises
Lesson Reference: 
Lesson 6: How We Hear