Physics Multiple choice question?

Question

That was a multiple choice question from a past physics paper. The question intrigued me:

Sounds are made by vibrating objects. A certain object vibrates but person nearby cannot hear any sound. Which statrement is correct?
A) The amplitude of the sound waves is too large.
B) The frequency of the vibration is too high.
C) The sound waves are transverse.

Only one of the above answers is correct. Can you tell me the answer but I also want an explanation why. The explanation is very important since the first 2 options appear correct to me. Plz help!

Answer 1

A) isn't likely to be correct unless the person was deafened by the sound. For a person with undamaged hearing there is no "too large" an amplitude for hearing the sound. The sound could be too loud to make any sense of it, but that's another issue.
B) is plausible. Most humans hear up to about 20 kHz when young, and this cutoff frequency becomes lower with age and/or cochlear damage.
C) is the most interesting possibility. The sound would be inaudible if only transverse waves reached a sensing point of zero size, but such a point couldn't have a directional characteristic so it would also receive longitudinal (compression) waves. A finite-sized sensing area consisting of a flat flexible membrane with one-sided exposure to the air (like the eardrum) could be positioned parallel to the direction of propagation and compression. Then it would be swept by transverse waves, and pressure differences would travel across its outside surface and be transmitted into the inside space (middle ear) and heard. (Note this would not be true for a flat two-sided sensing area, which would have canceling pressure differences on the other side.) In addition, the external ear directs the sound toward the eardrum so longitudinal waves reach it. So C is ruled out.

Answer 2

A) Amplitude is a measure of how many molecules are affected by the vibration of a sound wave. The more molecules the larger the amplitude and the louder the sound. - So False, since sound increase with amplitude

C) Transverse waves are the displacement of the medium in a perpendicular to the direction of propagation of the wave. A ripple on a pond and a wave on a string are easily visualized transverse waves. - In longitudinal waves the displacement of the medium is parallel to the propagation of the wave. Sound waves in air travels via longitudinal waves like a slinky (area of high pressure and areas of low pressure). so false - since sound through air is longitudinal.

B)A vibration occurs over a single wavelength. Frequency is a measure of how many vibrations occur in one second. This is measured in Hertz (abbreviation Hz) and directly corresponds to the pitch of a sound. Optimally, people can hear from 20 Hz to 20,000 Hz (20 kHz), though with age the extremes of hearing are lost. - True - Since others are false this is true. But if object is vibrating at a extreme speed then it will be too high for the human ear to detect it (more that 20,000 cycles per sec), However this object is probably small since a vibration at this frequency will cause large object to shake apart. Most large objects that vibrate the vibration is too slow and the sound will be too low to hear it.

Answer 3

B) seems right to me, as the range of frequency that we can hear is limited, and shrinks as the person ages.

I can think of 2 other possibilities
1) the person is deaf
2) the vibrations are occuring within a vacuum