A question about the speed of EM and mechanical waves

[lxman]

I have known for many years that the speed of sound (usually quoted ≈340 m/s) and the speed of light (usually quoted ≈3*10^8 m/s) are vastly different. Doing some reading, I would seem to conclude that part of the reason for this is the fact that sound is a mechanical wave, propagated through whatever medium it exists in, whereas light is an electromagnetic wave. One key fact seems to be that EM waves can propagate in a vacuum, whereas mechanical waves cannot. To quote a rather corny phrase, "In space, no one can hear you scream."

What I am wondering, and I would appreciate someone pointing me in the right direction for information, is the following:

Waves, be they mechanical or EM, are measured by much the same characteristics, by what I see (wavelength, frequency, amplitude, etc.). That is not to say that they are the same thing, of course, but they would at least appear to be highly similar. I note that when I see a diagram of the electromagnetic spectrum, they usually deem to stop with low frequency/long wavelength radio waves. With mechanical waves, the typically quoted range of human hearing is 20 Hz to 20 KHz. Ultrasonic transducers typically operate in the range of 40-50 KHz (and at these frequencies, they still quote the speed of sound when calculating distance, which is what they are typically used for). This brings the two ranges within oh, say, 50-100 KHz of each other.

So what happens in this intermediate range? Do we hit a point where some energy state suddenly changes, and now we have left the speed of sound behind and now we are suddenly traveling at the speed of light? Is there some sort of gradual shift where I see the speeds shifting from one to the other?

Or is it a question of their propagation? Sound is usually created by, say, a vibrating membrane, whereas EM is typically produced via vibrating electrons? If that is the case, would it theoretically be possible to create mechanical waves in, say, the 500 KHz range? If so, would they be of any use, or would they not propagate well in air due to their high frequency?

 

[Pythagorean]

the type of wave has nothing to do with the frequency (you could have a mechanical wave an an electromagnetic wave of the same frequency). The type of wave depends on what's oscillating. If neutral matter is oscillating, you have just a mechanical wave. If a charged particle is oscillating, you get electromagnetic waves. Plasmas are an example that have both kinds of waves interfering with each other... since a particle can have both charge and mass, it can be susceptible to both kinds of waves.

But notice that a single electron oscillating back and forth will generate an EM wave, while a mechanical wave is energy propagating through an ensemble of particles and manifesting motion. They're quite different kinds of things.

 

[russ_watters]

lxman, your post reads as if light and sound are the same phenomena just with different frequencies. They aren't. They are completely different. So there is no transitional region where you go from one to the other. And the radio frequency range extends down to single digit Hz, so they do overlap: http://en.wikipedia.org/wiki/Extremely_low_frequency

Higher frequency mechanical vibration becomes difficult because unless the amplitude is very low, you risk ripping apart the medium you are trying to transmit through.

 

[nasu]

Ultrasonic transducers typically operate in the range of 40-50 KHz (and at these frequencies, they still quote the speed of sound when calculating distance, which is what they are typically used for). This brings the two ranges within oh, say, 50-100 KHz of each other.

The mechanical waves are not limited to the kHz range. Not even from a practical point of view.
Ultrasonic transducers for industrial and medical applications operate in MHz range. 20-50 MHz are quite usual.
Mechanical waves in solids may have, frequencies comparable with these of the visible light.

 

[PJC01]

I can understand your curiosity about the similarities and differences between electromagnetic and mechanical waves. You are correct in noting that these waves share many characteristics, such as wavelength, frequency, and amplitude. However, there are fundamental differences in their nature and how they propagate that contribute to their vastly different speeds.

First, let's address the fact that electromagnetic waves can propagate in a vacuum while mechanical waves cannot. This is because electromagnetic waves do not require a medium to travel through, as they are disturbances in the electromagnetic field. In contrast, mechanical waves are disturbances in a physical medium, such as air or water, and therefore cannot travel in a vacuum.

The speed of a wave is determined by the properties of the medium it travels through. In the case of sound, it is dependent on the density and elasticity of the medium. This is why the speed of sound varies in different materials. On the other hand, the speed of light is determined by the properties of the electromagnetic field, which is a fundamental force of nature. This is why the speed of light is constant and the same in all mediums.

As for the intermediate range between the speed of sound and light, there is no sudden shift or change in energy state. It is simply a gradual transition between the two types of waves. In this range, we can see waves that exhibit characteristics of both sound and light, such as infrasound and ultrasound. These waves can be used for various purposes, such as communication, imaging, and detection.

In theory, it is possible to create mechanical waves in the 500 KHz range. However, as you mentioned, they may not propagate well in air due to their high frequency. This is because high-frequency mechanical waves have shorter wavelengths and are easily absorbed or scattered by the air molecules. This is why ultrasound is typically used in medical imaging, as it can penetrate through the body without being significantly absorbed.

I hope this explanation helps to answer your questions about the speed of electromagnetic and mechanical waves. It is always fascinating to explore the similarities and differences between these two types of waves and how they play a crucial role in our understanding of the physical world.