Question
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Both the amplitude and the frequency of a sound wave decrease. What happens to the sound heard?
A) The sound is louder and has a higher pitch.
B) The sound is louder and has a lower pitch.
C) The sound is quieter and has a higher pitch.
D) The sound is quieter and has a lower pitch.

Answer
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Hint: Amplitude of a sound wave determines the loudness of that particular sound. Loudness of sound is a measure of the intensity of sound as perceived through human ears. Pitch of a sound wave gives an idea about the range and reach of sound. It is closely related with the frequencies of sound waves.

Complete step by step answer:
Sound waves are longitudinal waves with varying amplitudes and frequencies. Sounds get produced by vibration. Our phones in vibration mode are one of the best examples to understand the production of sound due to vibrations. When our phones vibrate, a sound is produced because of the to and fro motion of the phone touching the surface where the phone is placed. It is also interesting to hear different sounds when our phones are kept on different surfaces. Two important characteristics of sound waves are amplitude and frequency.
Amplitude of a sound wave determines the loudness of the sound. It is equal to the distance of maximum vertical displacement from the mean position as shown in the figure. It also refers to the compression and expansion of the medium through which the sound wave travels. Amplitude of sounds can be perceived as loudness by human ears. The higher the amplitude of a sound, the louder the sound is. The lower the amplitude of a sound, the quieter the sound is. Therefore, the loud sound produced by a siren has high amplitude whereas the feeble sound produced by a mosquito has low amplitude.

High amplitude (loud noise)
   

Low amplitude (quiet noise)


Frequency of a sound wave gives the rate of vibration of sound. It is equal to the number of vibrations in unit time. The value of frequency is measured in and is equal to, where is the time period of the sound wave. Time period of a sound wave is the total time taken by the wave to complete one cycle. Frequency of a sound is directly proportional to the pitch of the sound. Higher the frequency, higher the pitch. Lower the frequency, lower the pitch. It is interesting to notice that male voices have low pitch and female voices have high pitch. This refers to the fact that male voices have low frequency and female voices have high frequency. The following diagram explains the same.
High frequency (high pitch sound)

Low frequency (Low pitch sound)

Therefore, as amplitude and frequency of sound decrease, loudness and pitch of sound decrease.

So, the correct answer is “Option D”.

Note: Students can think about different sounds they come across in daily life. They can think about the sound of a stopping train, human voices, sound of an aeroplane, sound of birds, sound of ceiling fans, sound of electric vehicles, etc to ponder upon the loudness and pitch of different sounds. Students need to remember that both amplitude and frequency are directly proportional to loudness and pitch respectively. There need not be any confusion in selecting the correct answer from the options which look similar.