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. Find out the speed of the wave and the frequencies at which sound occurs. Let's find out if the sound is the same in different environments and learn how to find its speed using the formula.
Wave appears
Let's imagine a water surface, for example 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 bring it into oscillatory motion? The circles will be constantly generated by the vibrations of the ball. We will see approximately the same as shown in the computer animation.
If we lower the float at some distance from the ball, it will also oscillate. When fluctuations diverge in space over time, this process is called a wave.
To study the properties of sound (wavelength, wave speed, etc.), the famous Rainbow toy, 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 a wave appeared, which ran along the spring, and then returned back. This means that it is reflected from the obstacle. We observed how the wave propagated along the spring over time. The particles of the spring moved up and down in relation to their equilibrium, and the wave ran left and right. Such a wave is called a transverse wave. In it, the direction of its propagation is perpendicular to the direction of oscillation of the particles. In our case, the wave propagation medium was a spring.
Now let's 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 stretched. Such a wave is called longitudinal. The direction of oscillation of its particles coincides with the direction of propagation.
Let's imagine a dense medium, for example, a rigid body. If we deform it by shearing, a wave will arise. It will appear due to the elastic forces acting only in solids. These forces play the role of restoring and generate an elastic wave.
You can't deform a liquid by shear. A transverse wave cannot propagate in gases and liquids. Another thing is longitudinal: it spreads in all environments where elastic forces act. In a longitudinal wave, the particles approach each other, then move away, and the medium itself is compressed and rarefied.
Many people think that liquidsincompressible, but this is not the case. If you press on the plunger of the syringe with water, it will shrink a little. In gases, compressive-tensile deformation is also possible. Pressing the plunger of an empty syringe compresses the air.
Speed and wavelength
Let's return to the animation that we considered at the beginning of the article. We choose an arbitrary point on one of the circles diverging from the conditional ball and follow it. The point moves away from the center. The speed at 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 crests of the wave are located at the same distance. This is the wavelength - another of its characteristics. The more frequent the waves, the shorter their length.
Why not every mechanical wave is sound
Take an aluminum ruler.
It's bouncy, so it's good for the experience. We put the ruler on the edge of the table and press it with our hand so that it protrudes strongly. We press on its edge and release it sharply - the free part will begin to vibrate, but there will be no sound. If you extend the ruler just a little bit, the vibration of the short edge will create a sound.
What does this experience show? It demonstrates that sound only occurs when a body is moving fast enough when the wave speed in the medium is high. Let us introduce one more characteristic of the wave - the frequency. This value shows how many vibrations per second the body makes. When we create a wave in the air, sound occurs under certain conditions - when enoughhigh frequency.
It is important to understand that sound is not a wave, although it is related to mechanical waves. Sound is the sensation that occurs when sound (acoustic) waves enter the ear.
Let's get back to the ruler. When the larger part is extended, the ruler oscillates and makes no sound. Does this create a wave? Sure, but it's a mechanical wave, not a sound wave. Now we 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 vibrations will occur.
Sound source
Any oscillating body can be a source of acoustic waves, it only needs an elastic medium, for example, air. Not only a solid body can vibrate, but also a liquid and a gas. Air as a mixture of several gases can be not only a propagation medium - it itself is capable of generating an acoustic wave. It is his vibrations that underlie the sound of wind instruments. The flute or trumpet does not vibrate. It is the air that is rarefied and compressed, gives a certain speed to the wave, as a result of which we hear the sound.
Spreading sound 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 empty space, say on the moon, we will not hear the sound of a vibrating body.
Most of the sounds perceived by humans are airborne. Fish, jellyfish hear an acoustic wave diverging through the water. We, if we dive under the water, will also hear the noise of a motor boat passing by. Moreover, the wavelength and wave speed will be higher than in air. This means that the sound of the motor will be the first to be heard by a person diving underwater. The fisherman, who is sitting in his boat in the same place, will hear the noise later.
In solids, sound travels even better, and the wave speed is higher. If you put a hard object, especially metal, to your ear and tap 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 alien. Why is this happening? Because in life we hear not so much sound vibrations from our mouths as 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 go so far from us that the sound of the wheels will not be heard yet. However, if you put your ear to the rails, we can clearly hear the rumble.
This suggests that sound waves travel faster in solids 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 can be 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, you can calculate the third:
с=ν × λ, where c is the speed, ν is the frequency, λ is the wavelength.
Second acoustic wave equation:
s=λ / T, where T is the period, i.e. the time for which the body makes one oscillation.
