Sound diffraction
Oto-kun
Dad, on the way back from Disneyland yesterday, we drove on the highway, right? There was a part where there were walls on both sides, but those are there to reduce car noise, right?
father
That's right. Yesterday, I drove on the metropolitan expressway, and there are places where sound barriers have been installed at the edges of the road to reduce the impact of automobile noise on the surrounding buildings and residences.
Oto-kun
How effective are soundproof walls?
father
This is a difficult question. Let me explain it to you little by little. Even if you put up a sound barrier, the noise doesn't completely go away. The cars that are visible without a barrier become invisible when a wall is erected, but the sound diminishes yet still reaches you. This phenomenon, where sound bends around obstacles, is called 'sound diffraction.' Light, on the other hand, does not bend around obstacles and cannot reach the other side, but sound can.
Oto-kun
"It's the same as when you block the light from the sun or fluorescent lights, and a shadow becomes clear. But why is it that sound can bend around obstacles while light cannot?"
father
Actually, that's the point. You know that both light and sound are waves, right? The wavelength of light is about one millionth of that of sound. Since the wavelength of light is shorter than one-thousandth of a millimeter, it is much shorter compared to the dimensions of objects around us. Therefore, when light hits an object, the area behind it becomes a shadow. However, a single strand of hair might not create a noticeable shadow because light can bend around it.
Oto-kun
So, if the sound is 1/1000 of something multiplied by 1,000,000, then 1000 mm means the wavelength is 1 meter?
father
The frequency of sound with a wavelength of 1 meter is about 300 Hz, which corresponds to the average frequency of a female voice. The wavelengths of sound and light that humans can hear or see have a range. I mentioned that light is shorter than 1 mm by a factor of 1000, but to be precise, visible light ranges from 0.4 to 0.8 μm, which includes the colors of the rainbow from red to violet. As I think I mentioned before, the frequency of audible sound ranges from 20 Hz to 20 kHz, which corresponds to wavelengths of 17 mm to 17 m. You know that sound travels at about 340 m per second, right? Since 1 Hz is one vibration per second, the wavelength of a 1 Hz sound is 340 m, for 10 Hz it is 34 m, and for 10 kHz it is 34 mm.
Oto-kun
I see, the wavelength of sound is long, so it can go around obstacles.
father
That's right, although sound has a longer wavelength compared to light, high-frequency wavelengths, which are shorter, are at most a few centimeters to tens of centimeters. Therefore, with soundproof walls that are several meters in size, the area behind the wall is in shadow, and sound does not reach there, just like light.
Oto-kun
I see, I understand. Conversely, low-frequency sounds can travel several meters, so they can become larger than the dimensions of the wall, allowing them to overcome the wall and wrap around to the other side.
father
That's right. Therefore, even if sound barriers are installed, the low rumbling sound is not significantly reduced, and they are less effective against low-frequency sounds. However, the noise generated by vehicles typically has a frequency characteristic that peaks around 2kHz, where human hearing is most sensitive. Sound barriers are effective at these frequencies, which is why they are widely used in areas with high noise levels, such as near highways surrounded by buildings and residences. When designing sound barriers, it is important to understand the frequency characteristics of the noise in that location and to ensure that the barriers are effective at the receiving points, such as buildings and residences. The noise level at these receiving points is regulated by national noise regulations, which set standards for acceptable noise levels, so it is necessary to design sound barriers to ensure that these levels are not exceeded.