Physics BR Sound Examples

[Natnow]

My version of BR does not have the answers to some examples so I was hoping you could help!

Ex 6.2b: A librarian asks you to be quiet. In going from a talk to a whisper, the intensity level of your voice decreases from 60dB to 20dB. By what factor does your voice correspondingly decrease in intensity?

A. Intensity decreases by a factor of 30
B. Intensity decreases by a factor of 40
C. Intensity decreases by a factor of 100
D. Intensity decreases by a factor of 10,000

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[pepocho]

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[t5Nitro]

It's D. For every 10 decibels change, there is a factor of 10 intensity change. So a change of 40 decibels (10, 10, 10, and 10) is 4 factors of 10 (10^4).

Edit: pepocho beat me to it.

 

[Natnow]

Thank you so much!

I have another question:
Ex 6.3b A bat emits an ultrasonic squeak in the direction of an insect and notices that the reflected sound is of lower frequency than the emitted pulse. What could BEST explain this frequency shift?
A. Bat is moving towards stationary insect
B. Insect is moving away from a stationary bat
C. Either the bat or insect is moving toward the other
D. Either the bat or insect is moving away from other

I am between B and D, but more inclined to pick D based on Doppler effect of the TWO objects moving away from each other for the overall frequency of the emitted sound to decrease. I feel like you can't really determine that either the bat is moving away or the insect is moving away, thus it could be either based on the decrease in frequency. If you could please explain that will be very helpful!

Thanks for the help!

 

[orangetea]

Thank you so much!

I have another question:
Ex 6.3b A bat emits an ultrasonic squeak in the direction of an insect and notices that the reflected sound is of lower frequency than the emitted pulse. What could BEST explain this frequency shift?
A. Bat is moving towards stationary insect
B. Insect is moving away from a stationary bat
C. Either the bat or insect is moving toward the other
D. Either the bat or insect is moving away from other

I am between B and D, but more inclined to pick D based on Doppler effect of the TWO objects moving away from each other for the overall frequency of the emitted sound to decrease. I feel like you can't really determine that either the bat is moving away or the insect is moving away, thus it could be either based on the decrease in frequency. If you could please explain that will be very helpful!

Thanks for the help!

Your inclination between B and D is correct but if you think about it D actually encompasses answer B as well. Just intuitively thinking about it why is the frequency lower? Well the object is probably farther OR from a different perspective I could be walking away from the object (or flying in the bat's example) which makes the observed frequency to be lower. I would go with D because although B is correct D is the BEST answer.

 

[Natnow]

This was a BR question I came across in a Biology passage regarding the ear and sound.
I am having trouble understanding the answer explanation, I thought I could use the Physics dB and intensity conversion but I'm having no luck. If someone could please explain how they would arrive at the correct answer D, it would be greatly appreciated.

72. The human ear normally responds to a range of sound covering about 120dB. The loudest sound pressure level that can heard over this range is:
A. Ten thousand times less than the reference
B. One million time less than the reference pressure.
C. Ten thousand time greater than the reference pressure.
D. One million times greater than the reference pressure

The passage states that the reference pressure is 20 micro Newtons/m^2. And the sound pressure level = 20log base 10 (test pressure/reference pressure)