If you compare Table 17.3 with Table 17.4, it may come as a shock. For example, standing in front of a speaker at a large-scale concert is equivalent to 110 dB. This means you have just less than 2 minutes before permanent damage may occur. If you work on a construction site, a pile driver may cause 130 dB, which allows you about one second!
The frequency of the sound also affects the way you will perceive sound. This frequency response is explained later in terms of resonance. It can be shown that a certain frequency
of sound is perceived to be more painful than an equal amplitude tone of a different frequency. Relative strength of sound
dB example of sound
Faint Soft
Moderate Loud 20 leaves rustling
30 whisper 40 quiet library
50 moderate rainfall 60 normal conversation dishwashers
70 trafic vacuum cleaners
80 alarm clocks
Over 85 dB for extended periods can cause permanent hearing loss Very loud (dangerous over 30 minutes)
90 lawnmowers power tools blenders hair dryers
100 snowmobiles mp3 players (at full volume)
110 concerts (in front of speaker) car horns sporting events
Uncomfortable (dangerous over 30 seconds) 120 jet planes (during take-off)
Painful and dangerous (use hearing protection or avoid)
130 jackhammers ambulances
140 ireworks gun shots customised car stereos (at full volume)
TABLE 17.4 Typical strengths of sounds from various sources H Sound intensity and the
threshold of hearing Sound is a wave that carries energy. We can calculate the amount of sound energy at a point by defining it in terms of intensity. The symbol for intensity is I; it is measured in watts per square metre (W m–2).
Sound intensity I = P
___ A
I = sound intensity (W m–2), P = power (W), A = area (m2)
The intensity of a sound is the rate of sound energy incident on 1 m2 at a right angle to the direction of motion of the sound.
