Dec 1990 P2 Q10
(Long question)
(a) A loudspeaker and a microphone are set up facing each other several metres apart. Explain how the vibrations of the cone of the loudspeaker produce sound waves in the air and how these waves are transmitted through the air to the microphone.
(b) The separation of the loudspeaker and the microphone in (a) is 6.8 m. When the cone of the loudspeaker vibrates at a frequency of 200 Hz, there are exactly 4 complete waves in the air between the loudspeaker and microphone.
(i) Calculate the speed of sound as it travels in the air between the loudspeaker and microphone.
(ii) Estimate the number of waves between the loudspeaker and microphone when the frequency is 2.00 kHz.
(iii) State the effect on the sound heard by a normal healthy ear of changing the frequency from 200 Hz to 2.00 kHz.
(c) Imagine that you have been asked to carry out an experiment to determine the speed of sound in air.
(i) Make a list of the apparatus you would use.
(ii) With the aid of a diagram, give a brief description of the experiment you would carry out.
(iii) State one precaution you would take to ensure that your value for the velocity of sound is as accurate as possible.
Answer:
(a) When the cone of the loudspeaker vibrates, air molecules near it also vibrates at the same frequency. This vibration by the cone (forwards and backwards) causes the air particles to come nearer and then further away alternately producing compressions and rarefactions which move forward, resulting in sound waves. These alternating compressions and rarefactions reach the microphone, causing the cone of the microphone to vibrate as well.
(b)
(i) wavelength of the sound wave = distance / num of waves
λ = 6.8 /4
λ = 1.7 m
Using v = fλ,
speed = 200 Hz * 1.7 m/s
speed = 340 m/s
(ii) As the frequency of sound waves increses, the wavelength decreases speed remains the same, i.e. frequency * wavelength remains constant.
Since frequency increases by 10 times, the wavelength decreases by 10 times and hence the number of waves in the same distance increases by 10 times.
Thus, the number of waves = 4 * 10 = 40.
(iii) At 200 Hz, a humming low pitch sound will be heard. As the frequency increases, only the pitch increases. The loudness of the sound remains the same.
(c)
(i) Apparatus required: A gun, a stop watch, measuring device, and the help of an observer.
(ii)
In an open space, my friend an I will stand at a measured distance, for example, 300m, apart from each other. I will fire the pistol. On seeing the flash/smoke from the pistol, my friend will start the stopwatch, and on hearing the bang, stop the stopwatch. The speed of sound will be the distance divided by the time taken.
(iii) Switch directions and redo the experiment. This way, the effects due to wind can be nullified.
