Problem 1: Sound intensity level One member of the choir sings with the sound in
ID: 1523309 • Letter: P
Question
Problem 1: Sound intensity level
One member of the choir sings with the sound intensity level of 5 dB measured by the listener in the
audience. The choir has ten members. What is the sound intensity level measured by the same listener
if all members of the choir sing at once, their voices are in the phase and have the same amplitude?
(i) 10 dB.
(ii) 15 dB.
(iii) 20 dB.
(iv) 25 dB.
(v) 50 dB.
Problem 2: Speakers
The two speakers are placed in the opposite corners of the room and emit the sound with the same
amplitude, the same phase and the same wavelength of 80 cm. Alice is standing in the middle of the
room and listens to the sound emitted by speakers. Alice now moves towards one of the speakers by 20
cm and then again by another 20 cm. What does she hear?
(i) The sound gets quiet at the first stop, and then even more quiet at the second step..
(ii) The sound gets quiet at the first stop, and then completely silent at the second step.
(iii) The sound gets silent at the first stop, and then again loud at the second step.
(iv) The sound gets silent at both stops.
(v) The sound stays loud at both stops.
Explanation / Answer
Problem 1 : Sound intensity level
We know that, SIL (in dB) = 10 log10 (I / I0)
(5 dB) = 10 log10 (I / I0)
The choir has ten members. Then, we have
10 (5 dB) = 10 log10 (I / I0)
The sound intensity level measured by a same listener which will be given as :
using a formula, we have
(SIL)new = 10 log10 (I / I0)
(SIL)new = 10 (5 dB)
(SIL)new = 50 dB
Problem 2 : Speakers
Alice is standing in the middle of room & listens to the sound emitted by speakers. Alice now moves towards one of the speakers by 20 cm and then again by another 20 cm. What does she hear?
(v) The sound stays loud at both stops.