The loudness of sound depends upon both intensity and frequency. For a given frequency, an increase in intensity produces an increase in loudness, but the sensitivity of the ear is so different in the various frequency ranges that equal intensities produce far different sensations in the different regions. An audiogram for the normal human ear is given in figure.
The lower curve gives the faintest sounds that can be heard, and the upper curve gives the loudest sounds that can be heard without pain. It will be noted that the ear is most sensitive to frequencies between 2000 and 4000 Hz and that the sensitivity diminishes rapidly at higher and lower frequencies. In the above frequency range, it requires least energy to cause a sensation of sound. The intensity necessary for hearing in the regions near the high and low limits of audibility, is greater than what is necessary in the region of greatest sensitivity.
If the intensity becomes too great, the sensation becomes one of pain rather than of hearing. This is indicated by the upper curve which lies close to 106 m Wm-2 or 1 Wm-2 , putting 1 = 1 Wm-2 and I0 = 10-12
Wm-2 in equation (1-35), we get
= 10 log 1012
= 120 dB
A sound of intensity level greater than 120dB causes pain in the ear.
To understand how the intensity affects the frequency limits of audibility, consider the horizontal line at 20dB. At this intensity level, frequencies below about 200Hz and above 15,000Hz cannot be heard. At 40dB the lowest frequency to be heard is about 100Hz whereas the upper limit is about 20,000Hz. The area enclosed by the two curves representing the threshold of hearing and the threshold of feeling, is called the auditory sensation area.
The following table gives some common sound levels in decibels.
| Source |
Intensity level in dB |
| Threshold of hearing |
0 |
| Rustle of leaves |
10 |
| Whisper (at 1m) |
20 |
| Library |
30 |
| Quite home...... |
40 |
| Office, classroom...... |
50 |
| Normal conversation.... |
60 |
| Busy traffic..... |
70 |
| Ordinary factory....... |
80 |
| Niagara falls....... |
90 |
| Running train...... |
100 |
| Rock band with amplifiers |
110 |
| Threshold of pain, jet take off (at 60m) |
120 |
| Machine - gun |
130 |
| Rocket-launch |
180 |
