JYA Tutorial Questions
Decibels
1) During a firework display, a rocket explodes high in the air. The sound spreads out uniformly in all directions [you can assume that there are no reflections from the ground]. When the sound reaches Mr Eno, who is 640m away the intensity I = 0.01 Wm -2 . Mr Fripp is 160m away, what is the intensity of the sound that he hears?
2) In a test to compare two audio systems the following readings were takes. System 1 produced an intensity I 1 = 90.0 dB and System 2 produced an intensity I 2 = 93.0 dB. What is the ratio I 1 / I 2 ?
3) The intensity of sound at a rock concert is 120dB and the intensity of the sound of an quiet flute is 67dB. How many times louder is the concert than the flute?
4) One of the key specifications of a cassette tape (remember them?) is what is called the signal-to-noise ratio. This compares the intensity of the signal to that of the background hiss (or ‘noise’). If the music have an intensity of 1.2 x 10-4 Wm -2 and the tape hiss has a value of 3.2 x 10-11 Wm -2 . What is the signal to noise ratio. Should you need it (do you?) the value of I 0 as the human threshold is = 1.0 x 10-12 Wm -2
5) The average sound intensity inside a busy restaurant is 3.2 x 10-5 Wm -2 . If the area of the average human ear is 2.1 x 10-3 m 2 how much energy goes into the waitresses head in one hour?
Doppler
6) At a football game, a stationary spectator is watching the half-time show. The show features a marching band who are moving at 0.83 ms -1 . If the speed of sound is 343 ms -1 and the trumpet emits a note of 784 Hz what is highest and lowest note heard by the spectator?
7) A high speed train is travelling at a speed of 44.7ms-1 (100mph) when the engineer sounds a horn. The horn has a frequency of 415Hz. If the speed of sound in air is 343 ms-1 what is the frequency of the note heard by someone when
a) The train is approaching
them?
b) The train is leaving them?
c) The train is right next to them?