A source produces a sound of 30 mW. What is the sound intensity at a distance of 2 m from the loudspeaker?
SAMPLE ANSWER 11E The sound energy is travelling through an area of a sphere of radius 2 m.
A 4pr 2 4p(2)2 50.24
I P A 30 103
6 104 W m2 50.24 5.97 104
The sound intensity at a distance of 2 m from the loudspeaker is .
Fig 11.22 2 m
THRESHOLD OF HEARING
The threshold of hearing (I0) is the smallest sound intensity detectable by the average human ear at a frequency of 1 kHz.
Its value is 1 1012 W m2 . Frequency response of the ear and frequency limits of audibility
The human ear responds differently to sounds of varying frequency. It is most sensitive to waves of frequency between 2 kHz and 4 kHz.The frequency limits of audibility are the highest and lowest frequencies that can be heard by a normal human ear. The range is 20 Hz to 20 kHz.
SCIENCE,TECHNOLOGY & SOCIETY
Frequency limits of audibility The upper limit of audibility decreases as we age. A product that appeals to many young people is the high-pitched ringtone available for lots of mobile phones only audible to young people.They may be inaudible to your teacher depending on his or her age! Another use of the same phenomenon is where some shopping centres have speakers that emit high-pitched notes that are designed to be inaudible to adults but act as a deterrent to youths loitering outside the shop entrances. The question as to whether this infringes the human rights of young people is one that is currently being debated.
Speakers outside a shopping centre emit high-pitched notes to discourage young people from loitering
188 INVESTIGATING PHYSICS
Different animals have different ranges of frequencies of audibility. For example dogs can hear notes of higher than 20 kHz; a dog whistle produces such a note, which is why it is inaudible to humans.
