A sound wave is a mechanical longitudinal wave of a certain frequency. In the article we will understand what longitudinal and transverse waves are, why not every mechanical wave is sound. We find out the speed of the wave and at what frequencies the sound arises. Find out if the sound is the same in different environments and learn how to find its speed by the formula.
The appearance of the wave
Imagine a water surface, such as a pond in calm weather. If you throw a stone, then on the surface of the water we will see circles diverging from the center. And what will happen if we take not a stone, but a ball and we bring it into oscillatory motion? The circles will be constantly generated by the vibrations of the ball. We will see approximately the same as depicted in computer animation.
If at some distance from the ball we lower the float, then it will also oscillate. When oscillations diverge in space over time, this process is called a wave.
To study the properties of sound (wavelength, wave speed, etc.), the well-known toy "Rainbow", or Happy Rainbow, is suitable.
Let’s stretch the spring, let it calm down and shake it up and down sharply. We will see that there was a wave that ran along the spring, and then returned. This means that she was reflected from the obstacle. We watched the wave propagate through the spring over time. The particles of the spring moved up and down with respect to their equilibrium, and the wave ran left and right. Such a wave is called transverse. In it, the direction of its propagation is perpendicular to the direction of particle oscillation. In our case, the spring was the medium of wave propagation.
Now we’ll stretch the spring, let it calm down and pull back and forth. We will see that the coils of the spring are compressed along it. The wave runs in the same direction. In one place, the spring is more compressed, in another it is more extended. Such a wave is called longitudinal. The direction of oscillation of its particles coincides with the direction of propagation.
Imagine a dense medium, for example, a solid. If we deform it with a shift, a wave will arise. It will appear due to the elastic forces acting only in solids. These forces play the role of returning ones and generate an elastic wave.
Deforming with a shear fluid will not work. In gases and liquids, a transverse wave cannot propagate. Another thing is longitudinal: it spreads in all environments where elastic forces act. In a longitudinal wave, the particles come together, then move away, and the medium itself is compressed and rarefied.
Many people think that liquids are incompressible, but this is not so. If you press the piston of the syringe with water, it will squeeze a little. In gases, compression-tensile deformation is also possible. When you press the piston of an empty syringe, air is compressed.
Speed and wavelength
Let's return to the animation that we examined at the beginning of the article. We choose an arbitrary point on one of the circles diverging from the conditional ball and trace it. The point moves away from the center. The speed with which it moves is the speed of the wave crest. We can conclude: one of the characteristics of the wave is the speed of the wave.
The animation shows that the wave crests are located at the same distance. This is the wavelength - another characteristic of it. The more often the waves, the shorter their length.
Why not every mechanical wave is sound
Take the aluminum ruler.
It is resilient, therefore suitable for experience. Put the ruler on the edge of the table and press it with your hand so that it protrudes strongly. We press on its edge and sharply let go - the free part will begin to vibrate, but there will be no sound. If you push the ruler just a little bit, the vibrations of the short edge will create a sound.
What does this experience show? He demonstrates that sound arises only when the body moves fast enough, when the speed of the wave in the medium is high. We introduce another characteristic of the wave - the frequency. This value shows how many oscillations per second the body makes. When we create a wave in the air, the sound arises under certain conditions - at a sufficiently high frequency.
It is important to understand that sound is not a wave, although it is related to mechanical waves. Sound is a sensation that occurs when sound (acoustic) waves enter the ear.
Back to the lineup. When a large part is advanced, the ruler oscillates and does not make a sound. Does this create a wave? Of course, but this is a mechanical wave, not a sound one. Now you can define a sound wave. This is a mechanical longitudinal wave, the frequency of which is in the range from 20 Hz to 20 thousand Hz. If the frequency is less than 20 Hz or more than 20 KHz, then we will not hear it, although oscillations will occur.
Sound source
The source of acoustic waves can be any oscillating body, for this only an elastic medium, for example, air, is needed. Not only a solid body can vibrate, but also liquid and gas. Air as a mixture of several gases can be not only a medium of propagation - it is itself capable of generating an acoustic wave. It is his vibrations that underlie the sound of wind instruments. Flute or trumpet do not hesitate. This air is diluted and compressed, gives a certain speed to the wave, as a result of which we hear a sound.
Sound distribution in different environments
We found out that different substances sound: liquid, solid, gaseous. The same goes for the ability to conduct an acoustic wave. Sound propagates in any elastic medium (liquid, solid, gaseous), except for vacuum. In an airless space, let's say on the moon, we will not hear the sound of a vibrating body.
Most of the sounds perceived by a person spread in the air. Fish, jellyfish hear an acoustic wave diverging in water. If we dive under water, we will also hear the noise of a motor boat passing by. Moreover, the wavelength and wave velocity will be higher than in air. This means that the sound of the engine will be the first to be heard by a person diving underwater. A fisherman who sits in his boat at the same place will hear a noise later.
In solids, sound travels even better, and wave speed is higher. If you attach a solid object, especially metal, to your ear and knock on it, you will hear very well. Another example is your own voice. When we first hear our speech, previously recorded on a voice recorder or from a video, the voice seems a stranger. Why it happens? Because in life, we hear not so much sound vibrations from our mouth as the vibrations of waves passing through the bones of our skull. The sound reflected from these obstacles changes somewhat.
Sound speed
The speed of a sound wave, if we consider the same sound, will be different in different environments. The denser the medium, the faster the sound reaches our ear. The train can travel so far from us that the knock of wheels will not be heard. However, if you put your ear to the rails, we will clearly hear a rumble.
This suggests that in solids a sound wave runs faster than in air. The figure shows the speed of sound in different environments.
Wave equation
Speed, frequency and wavelength are interconnected. For bodies that vibrate at a high frequency, the wave is shorter. Low-frequency sounds are heard at a greater distance, because they have a longer wavelength. There are two wave equations. They illustrate the interdependence of wave characteristics from each other. Knowing any two quantities from the equations, we can calculate the third:
c = ν × λ,
where c is the velocity, ν is the frequency, λ is the wavelength.
The second equation of an acoustic wave:
c = λ / T,
where T is the period, i.e., the time during which the body performs one oscillation.