Sound diffraction and examples of its manifestation in everyday life. Ultrasonic Location

The diffraction phenomenon is characteristic of absolutely any waves, for example, electromagnetic or waves on the surface of the water. This article talks about sound diffraction. 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 energy transfer in any material medium without moving matter. A wave is a harmonic oscillation of matter particles that propagate in a medium. For example, in air, these oscillations lead to the appearance of areas of increased and decreased pressure, while in a solid, these are already areas of compressive and tensile stresses.

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 ^o C in air, sound travels at a speed of approximately 340 m / s. Given that a person hears frequencies from 20 Hz to 20 kHz, it is possible to determine the corresponding ultimate wavelengths. To do this, you can use the formula:

v = f * λ.

Where f is the oscillation frequency, λ is their wavelength, and v is the speed of motion. Substituting the above numbers, it turns out that a person hears waves with lengths from 1.7 centimeters to 17 meters.

The concept of wave diffraction

Sound diffraction is a phenomenon that involves bending the wavefront when it encounters an opaque obstacle along its path.

A vivid everyday example of diffraction is the following: two people are in different rooms of the 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.

Sound diffraction can be of two types:

1. The envelope of an obstacle smaller than the wavelength. Since a person hears sufficiently large lengths of sound waves (up to 17 meters), this type of diffraction is often found in everyday life.
2. Changing the wave front as it passes through a narrow hole. Everyone knows: if you leave the door a little ajar, then any noise from outside, penetrating the narrow slit of the slightly open door, fills the entire room.

Difference of light diffraction from that for 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, you can record a condition for a minimum similar to that for Fraunhofer diffraction on a narrow slit, that is:

sin (θ) = m * λ / d, where m = ± 1, 2, 3, ...

Here d is the width of the door gap. This formula determines the areas in the room where no sound can be heard from the outside.

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 diffraction phenomenon is strongly manifested if the wave sizes and obstacles are close. For this reason, in the example described above, two people, being in different rooms, do not see each other, but they hear.

Short and long wave diffraction

In the previous paragraph, the formula for the diffraction of sound by a slit is provided, provided that the wave front is flat. It can be seen from the formula that for a constant value of d, the angles θ will be the smaller, the shorter the waves λ will fall on the gap. In other words, short waves diffract worse than long waves. The following are some life examples confirming this conclusion.

1. When a person walks along a city street and approaches a place where musicians play, he first hears low frequencies (bass). Approaching the musicians, he begins to hear higher frequencies.
2. The roll of thunder, which occurred near the observer, seems to him rather high (not to be confused with intensity) than the same roll several tens of kilometers from him.

The explanation of the effects noted in these examples lies in the greater ability of low frequencies of sound to diffract and in their lesser ability to absorb in comparison with high frequencies.

Ultrasonic Location

It is a method of analysis or orientation on the ground. In both cases, the idea is to emit ultrasonic waves (λ <1.7 cm) by the source, their subsequent reflection from the object under study and analysis by the receiver of the reflected wave. This method is used by man to analyze the defective structure of solid materials, to study the relief of the sea depths and in some other areas. With the help of ultrasonic location, bats and dolphins are oriented 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 diffraction phenomenon does not allow to distinguish between two objects, the distance between which is less than the length of the diffracted wave. For these reasons, it is ultrasound that is used, and not sound or infrasound locations.