What does unwanted tone mean?

In principle, there are two ways of canceling out sound. On the one hand, deliberately chosen soft or perforated surfaces can completely or partially swallow sound waves. On the other hand, it is possible to neutralize sound waves with counter-sound waves.

When sound is swallowed passively, its energy is converted into heat. One speaks of absorption. Substances with sound-absorbing properties are called ingestion substances. They are used to line recording studios, for example, in order to intercept all external noises. They also prevent the sound from being reflected on the wall. As a result, the room is completely without reverb and has no "own" sound. With technical means, reverb can be added to sound recordings afterwards in order to achieve the acoustic impression of any room.

Can you make sound disappear?

The other way to get rid of unwanted sound is a little more laborious. In theory it is very simple: A tone can be canceled by its counter tone. Sound consists of waves of air of different densities. So if one wave is combined with a second wave, which is their exact mirror image, they cancel each other out. To do this, however, they must have exactly the same pitch and volume. The principle applies to all waves, you can also observe it when there are waves in the water. Here, too, it is possible to extinguish one wave by another - the physicist speaks of interference.

Since unwanted noise almost always consists of a large number of overlapping tones of different height and volume, it is technically difficult to construct a device that precisely measures the noise and makes it disappear with an equally precise counter-noise. We will therefore have to wait even longer for the small box under the window that cancels out the street noise. The principle is successfully used to dampen machine noise from magnetic resonance tomographs. Magnetic resonance tomographs provide very precise images of the inside of the human body.



It doesn't work today, but in a few years we may be able to "switch off" street noise with counter-noise, at least what you hear from it in the tram or in the car. © dpa

To do this, the patient has to lie down in a tube that is part of the device. The noise here would be so great that many patients would panic. Thanks to the cancellation of the sound waves from the machines by counter-noise, only a faint humming can be heard.