The phenomenon of diffraction is characteristic of absolutely any waves, for example, electromagnetic waves or waves on the surface of water. This article talks about the diffraction of sound. The features of this phenomenon are considered, examples of its manifestation in everyday life and human use are given.
Sound wave
Before considering the diffraction of sound, it is worth saying a few words about what a sound wave is. It is a physical process of transferring energy in any material medium without moving matter. A wave is a harmonic vibration of matter particles that propagate in a medium. For example, in air, these vibrations lead to the emergence of areas of high and low pressure, while in a solid body, these are already areas of compressive and tensile stress.
A sound wave propagates in a medium at a certain speed, which depends on the properties of the medium (temperature, density, and others). At 20 oC in air, sound travels at approximately 340 m/s. Considering that a person hears frequencies from 20 Hz to 20 kHz, it is possible to determinecorresponding limiting wavelengths. To do this, you can use the formula:
v=fλ.
Where f is the frequency of oscillations, λ is their wavelength, and v is the speed of movement. Substituting the above numbers, it turns out that a person hears waves with wavelengths from 1.7 centimeters to 17 meters.
The concept of wave diffraction
Sound diffraction is a phenomenon in which a wavefront bends when it encounters an opaque obstacle along its path.
A striking everyday example of diffraction is the following: two people are in different rooms of an apartment and do not see each other. When one of them shouts something to the other, the second hears a sound, as if its source is in the doorway connecting the rooms.
There are two types of sound diffraction:
- Bending around an obstacle whose dimensions are smaller than the wavelength. Since a person hears rather large wavelengths of sound waves (up to 17 meters), this type of diffraction is often found in everyday life.
- Change of the wave front as it passes through a narrow hole. Everyone knows: if you leave the door a little ajar, then any noise from the outside, penetrating the narrow gap of the slightly open door, fills the entire room.
The difference between the diffraction of light and that of sound
Since we are talking about the same phenomenon, which does not depend on the nature of the waves, the sound diffraction formulas are exactly the same as for light. For example, when passing through a slit in a door, one can write a condition for the minimum similar to that for diffractionFraunhofer on a narrow gap, that is:
sin(θ)=mλ/d, where m=±1, 2, 3, …
Here d is the width of the door gap. This formula determines the areas in the room where sound from the outside will not be heard.
The differences between sound and light diffraction are purely quantitative. The fact is that the wavelength of light is several hundred nanometers (400-700 nm), which is 100,000 times less than the length of the smallest sound waves. The phenomenon of diffraction is strongly manifested if the dimensions of the wave and the obstacles are close. For this reason, in the example described above, two people, being in different rooms, do not see each other, but hear.
Diffraction of short and long waves
In the previous paragraph, the formula for the diffraction of sound by a slit is given, provided that the wave front is flat. From the formula it can be seen that at a constant value of d, the angles θ will be the smaller, the shorter the waves λ will fall on the slot. In other words, short waves diffract worse than long ones. Here are some real-life examples to support this conclusion.
- When a person walks down a city street and comes to a place where musicians are playing, he hears low frequencies (bass) first. As he approaches the musicians, he begins to hear higher frequencies.
- The roll of thunder, which occurred not far from the observer, seems to him rather high (not to be confused with intensity) than the same roll a few tens of kilometers away.
The explanation for the effects noted in these examples is the greater ability of low frequencies of sound to diffract and their less ability to be absorbed compared to high frequencies.
Ultrasonic location
It is a method of analysis or orientation in the area. In both cases, the idea is to emit ultrasonic waves (λ<1, 7 cm) from the source, then reflect them from the object under study and analyze the reflected wave by the receiver. This method is used by man to analyze the defective structure of solid materials, to study the topography of the sea depths, and in some other areas. Using ultrasonic location, bats and dolphins navigate in space.
Sound diffraction and ultrasonic location are two related phenomena. The shorter the wavelength, the worse it diffracts. Moreover, the resolution of the received reflected signal depends directly on the wavelength. The phenomenon of diffraction does not allow one to distinguish between two objects, the distance between which is less than the length of the diffracted wave. For these reasons, it is ultrasonic rather than sonic or infrasonic location that is used.
