# Speed of Sound Experiment: Hear the Results

• Varsha Verma
In summary, the question posed is whether a listener will be able to hear the sound from a car with a speaker attached to the back, blasting high volume music towards the reverse direction, as the car accelerates and reaches speeds faster than the speed of sound. The answer is that a listener would not be able to hear the sound, as the sound waves would be stretched to inaudible levels due to the Doppler effect. Furthermore, the experiment would require special considerations regarding the frequency and decibel levels in order to accurately determine if the sound can be heard.f

#### Varsha Verma

Imagine a car which has a speaker attached to the back. Speaker is blasting very high volume music towards the reverse direction of the car. Now the car starts accelerating and in a few seconds starts moving faster than the speed of sound. Will a listener be able to hear the sound now. Will the sound waves start traveling towards the direction the car is traveling when the sound barrier is crossed.

https://s33.postimg.cc/41gpb6mbj/car_sound_experiment.jpg

A Physics Enthusiast
Imagine a car which has a speaker attached to the back. Speaker is blasting very high volume music towards the reverse direction of the car.

so what do you think will happen ?
What research have you done to try and find some answers ?

Imagine a car which has a speaker attached to the back. Speaker is blasting very high volume music towards the reverse direction of the car. Now the car starts accelerating and in a few seconds starts moving faster than the speed of sound. Will a listener be able to hear the sound now. Will the sound waves start traveling towards the direction the car is traveling when the sound barrier is crossed.

https://s33.postimg.cc/41gpb6mbj/car_sound_experiment.jpg

I didn't understand the second part of the question clearly. But I think the overall question is clear to me.

I think you know how to use Doppler effect equation, otherwise you won't post the question ^^'. But it seems that you don't know were to start.

Before reading the Spoiler, I encourage you to think hard on analysing the problem! I know it is sometimes hard. Start with asking what do you want? Then what do you know? and then how can you solve this problem? -- Don't be afraid of searching on the internet. If you failed after a long time, look at the spoiler.

First, you need to know how much can a human hear! The human hearing ranges in frequency between 20Hz and 20KHz (Above or lower than this is impossible to hear).

Second, you need to make the speaker emit one type of frequency (Otherwise the experiment is flawed, since each frequency can be shifted out of the hearing range with a different "difference")

Third, now just use Doppler equation for sound ->> Here, If you do not remember the meaning of the equation. Read the General section from Wiki.

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I believe I understand the question and I believe you need some sort of special device to pick up the completely distorted "music".

As you move away, the sound, which is simply vibration still exists, but the wavelength will be stretched to inaudible level.

Varsha Verma
I believe I understand the question and I believe you need some sort of special device to pick up the completely distorted "music".

You don't need a device that record sound beyond 20Hz-20KHz. You just need a device that record 20Hz-20KHz. If the recorder didn't pick up the sound at a certain speed then the sound is out of the range, this is basically because the experiment does not require from us to quantitatively say how much the frequency has shifted, but merely to state whether a human can hear it or not.

Since we know the speaker is emitting the sound at the time of observation, we just need to state whether the recorder can pick or cannot pick the sound. (We can graph the change of frequency with time to make it more convincing, since the car is accelerating).

There is something I didn't consider before:

(dB)

Human ear can hear sounds at the specific range 20-20K, but the minimum dB changes with frequency! This makes the experiment more complicated and the experimentalist must take into account these possibilities while preparing the experiment. I am not sure how complicated is the experiment! Is it a thought experiment or a real experiment. Anyway, if it is a real experiment there are a lot to consider. But the main issue I think is with the frequency.

Varsha Verma