# Speed of Sound: Constant Irrespective of Source Speed

• PavanKumar
In summary, The speed of sound is constant and isotropic in a "nice" medium, but is not frame invariant. On the other hand, the speed of light is not only constant and isotropic in vacuum, but also frame invariant. This is what makes the speed of light more important. The simplest experiment that can prove this invariance is the Michelson-Morley experiment, which was designed to detect any differences in the speed of light due to different frames of reference. Other experiments have also shown the frame invariance of the speed of light to a high degree of accuracy. This is a key principle in Special Relativity and has been proven through various experiments.
PavanKumar
Hi,

As far as I know the speed of sound is also constant irrespective of the speed of the source. I guess, this is an obvious result of the property of waves. So, why is the constancy of speed of a light wave more important?

Please point out if there are any mistakes in my assumptions.

Hi PavanKumar, welcome to PF!

In a "nice" medium the speed of sound is isotropic, homogenous, and independent of the speed of the source. In vacuum the speed of light is also isotropic, homogenous, and independent of the speed of the source.

Where they differ is that the speed of light is frame invariant, whereas the speed of sound is not. It is the invariance, and not the constancy, that makes the speed of light so important.

Enclose
Hi DaleSpan,

However, I am still a little confused. What is the simplest experiment that can prove this invariance? I believe we can't use the Michelson-Morley experiment as one because all the measurement devices and the light source were in the same frame of reference.

PavanKumar said:
Hi DaleSpan,

However, I am still a little confused. What is the simplest experiment that can prove this invariance? I believe we can't use the Michelson-Morley experiment as one because all the measurement devices and the light source were in the same frame of reference.
The MMX is exactly what proves it and the fact that the device exists in one frame of reference is key. If light behaved like sound, the experiment would show that light's speed is different in different frames (not frame invariant) and identify the frame where it is c (the "stationary frame"). Similarly, you could use an MMX type setup for sound to measure wind speed/airspeed.

PavanKumar said:
What is the simplest experiment that can prove this invariance?
I would start here, with this excellent review article on experimental tests of SR:
http://www.edu-observatory.org/physics-faq/Relativity/SR/experiments.html#Test_Theories

"Robertson showed that one can unambiguously deduce the Lorentz transform of SR to an accuracy of ~0.1% from the following three experiments: Michelson and Morley, Kennedy and Thorndike, Ives and Stilwell. Zhang showed that modern experiments determine the Lorentz transforms to within a few parts per million."

PavanKumar said:
Hi,

As far as I know the speed of sound is also constant irrespective of the speed of the source. I guess, this is an obvious result of the property of waves. So, why is the constancy of speed of a light wave more important?

Please point out if there are any mistakes in my assumptions.

The speed of light is independent of the speed of the receiver as well as the speed of the source. This is different than the behavior of sound, the speed of sound is isotropic only in a special receiver frame, the frame in which the receiver is "at rest" with respect to the sound media.

The key difference for light is that there isn't any such "special" frame, the speed of light is always isotropic for any receiver / receiver frame.

## 1. What is the speed of sound?

The speed of sound refers to the rate at which sound waves travel through a medium, such as air or water. In dry air at room temperature, the speed of sound is approximately 343 meters per second, or 767 miles per hour.

## 2. Does the speed of sound change depending on the source's speed?

No, the speed of sound remains constant regardless of the source's speed. This is a fundamental property of sound waves and is known as the principle of Galilean invariance.

## 3. Why does the speed of sound remain constant?

The speed of sound is determined by the properties of the medium through which it travels, such as density and elasticity. These properties do not change based on the speed of the source.

## 4. Are there any factors that can affect the speed of sound?

Yes, the speed of sound can be affected by temperature, humidity, and altitude. In general, sound travels faster in warmer, less humid air and at higher altitudes.

## 5. How is the speed of sound measured?

The speed of sound can be measured using various techniques, such as timing the delay between a sound's production and its arrival at a distant location, or by using specialized equipment like a sound level meter or an oscilloscope.

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