The sound wave travels from water to air. sound propagation

Sound propagation requires an elastic medium. In a vacuum, sound waves cannot propagate, since there is nothing to vibrate there. This can be seen in a simple experiment. If we place an electric bell under a glass bell, as the air is pumped out from under the bell, we will find that the sound from the bell will become weaker and weaker until it stops altogether.

sound in gases. It is known that during a thunderstorm we first see a flash of lightning and only after a while hear thunder (Fig. 52). This delay occurs due to the fact that the speed of sound in air is much less than the speed of light coming from lightning.

The speed of sound in air was first measured in 1636 by the French scientist M. Mersenne. At a temperature of 20 ° C, it is equal to 343 m / s, i.e. 1235 km / h. Note that it is to this value that the speed of a bullet fired from a Kalashnikov machine gun (PK) decreases at a distance of 800 m. The muzzle velocity of the bullet is 825 m/s, which is much higher than the speed of sound in air. Therefore, a person who hears the sound of a shot or the whistle of a bullet need not worry: this bullet has already passed him. The bullet outruns the sound of the shot and reaches its victim before the sound arrives.

The speed of sound depends on the temperature of the medium: with an increase in air temperature, it increases, and with a decrease, it decreases. At 0 °C, the speed of sound in air is 331 m/s.
Sound travels at different speeds in different gases. The larger the mass of gas molecules, the lower the speed of sound in it. So, at a temperature of 0 ° C, the speed of sound in hydrogen is 1284 m/s, in helium - 965 m/s, and in oxygen - 316 m/s.

Sound in liquids. The speed of sound in liquids is generally greater than the speed of sound in gases. The speed of sound in water was first measured in 1826 by J. Colladon and J. Sturm. They carried out their experiments on Lake Geneva in Switzerland (Fig. 53). On one boat they set fire to gunpowder and at the same time struck a bell lowered into the water. The sound of this bell, with the help of a special horn, also lowered into the water, was caught on another boat, which was located at a distance of 14 km from the first. The speed of sound in water was determined from the time interval between the flash of light and the arrival of the sound signal. At a temperature of 8 °C, it turned out to be approximately 1440 m/s.

At the boundary between two different media, part of the sound wave is reflected, and part travels further. When sound passes from air to water, 99.9% of the sound energy is reflected back, but the pressure in the sound wave that has passed into the water is almost 2 times greater. The auditory apparatus of fish reacts precisely to this. Therefore, for example, screams and noises above the surface of the water are a sure way to scare away marine life. These screams will not deafen a person who is under water: when immersed in water, air “plugs” will remain in his ears, which will save him from sound overload.

When sound passes from water to air, 99.9% of the energy is reflected again. But if the sound pressure increased during the transition from air to water, now, on the contrary, it sharply decreases. It is for this reason, for example, that the sound that occurs under water when one stone strikes another does not reach a person in the air.

This behavior of sound on the border between water and air gave reason to our ancestors to consider the underwater world as a “world of silence”. Hence the expression: "It's dumb like a fish." However, even Leonardo da Vinci suggested listening to underwater sounds by putting your ear to an oar lowered into the water. Using this method, you can see that the fish are actually quite talkative.

Sound in solids. The speed of sound in solids is greater than in liquids and gases. If you put your ear to the rail, then after hitting the other end of the rail, you will hear two sounds. One of them will reach your ear along the rail, the other - through the air.

Earth has good sound conductivity. Therefore, in the old days, during a siege, "hearers" were placed in the fortress walls, who, by the sound transmitted by the earth, could determine whether the enemy was digging to the walls or not. Putting their ear to the ground, they also followed the approach of the enemy cavalry.

Solid bodies conduct sound well. Because of this, people who have lost their hearing are sometimes able to dance to music that reaches their auditory nerves not through the air and outer ear, but through the floor and bones.

1. Why, during a thunderstorm, do we first see lightning and only then hear thunder? 2. What determines the speed of sound in gases? 3. Why does a person standing on the bank of a river not hear the sounds that occur under water? 4. Why were the “hearers” who in ancient times followed the earthworks of the enemy often blind people?

Experimental task. Putting a watch on one end of the board (or a long wooden ruler), put your ear to its other end. What do you hear? Explain the phenomenon.

When completing task 22 with a detailed answer, write down the task number first, and then the answer to it. A complete answer should include not only the answer to the question, but also its detailed, logically connected justification.

A glass of hot tea was left in a large cool room. Over time, the temperature of the tea caught up with the temperature of the surrounding air. How did the intensities of thermal radiation and thermal absorption of tea change in this case? Explain the answer.

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Possible Answer Sample

The intensity of thermal radiation decreased, the intensity of thermal absorption practically did not change.

Tea, on the one hand, radiates heat rays, on the other hand, absorbs heat radiation from the surrounding air. Initially, the radiation process predominates and the tea cools. As the temperature decreases, the intensity of heat radiation from tea decreases until it becomes equal to the intensity of absorption of heat radiation from the air in the room. Further, the temperature of the tea does not change.

When completing tasks 23–26, write down the task number first, and then the answer to it.

Assemble an experimental setup to study the dependence of the electric current in a resistor on the voltage at its ends. Use a 4.5 V current source, voltmeter, ammeter, key, rheostat, connecting wires, resistor marked R 1 .

On the answer sheet

1) draw the electrical circuit of the experiment;

2) setting with the help of a rheostat in turn the current strength c. circuits 0.4 A, 0.5 A and 0.6 A and having measured in each case the value of the electric voltage at the ends of the resistor, indicate the results of measuring the current and voltage for three cases in the form of a table (or graph);

3) formulate a conclusion about the dependence of the electric current in the resistor on the voltage at its ends.

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1) Scheme of the experimental setup

2)

3) Conclusion: with an increase in the current strength in the conductor, the voltage that occurs at the ends of the conductor also increases.

Task 24 is a question that requires a written answer. A complete answer should include not only the answer to the question, but also its detailed, logically connected justification.

The model of the boat floats in a jar of water. Will the immersion (draft) depth of the boat change (and if so, how) if it is moved from the Earth to the Moon? Explain the answer.

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Possible Answer Sample

Will not change.

The boat is submerged in water until the buoyant force acting on the boat from the side of the water balances the force of gravity. The depth of immersion (draught) of the boat is determined by the fulfillment of the condition: F tyazh = F vyt (1). The free fall acceleration on the Moon is less than on Earth. But since both forces are directly proportional to the free fall acceleration, then both forces F heavy and F vyt will decrease by the same number of times, and equality (1) will not be violated.

For tasks 25–26, it is necessary to write a complete solution, which includes writing a brief condition of the problem (Given), writing formulas, the use of which is necessary and sufficient to solve the problem, as well as mathematical transformations and calculations leading to a numerical answer.

We know that sound travels through the air. That is why we can hear. No sound can exist in a vacuum. But if sound is transmitted through the air, due to the interaction of its particles, will it not be transmitted by other substances? Will.

Propagation and speed of sound in different media

Sound is not only transmitted by air. Probably everyone knows that if you put your ear to the wall, you can hear conversations in the next room. In this case, the sound is transmitted by the wall. Sounds propagate in water and in other media. Moreover, the propagation of sound in different environments occurs in different ways. The speed of sound varies depending on the substance.

Curiously, the speed of sound propagation in water is almost four times higher than in air. That is, fish hear "faster" than we do. In metals and glass, sound travels even faster. This is because sound is a vibration of the medium, and sound waves travel faster in media with better conductivity.

The density and conductivity of water is greater than that of air, but less than that of metal. Accordingly, the sound is transmitted differently. When moving from one medium to another, the speed of sound changes.

The length of a sound wave also changes as it passes from one medium to another. Only its frequency remains the same. But that's why we can distinguish who specifically speaks even through the walls.

Since sound is vibrations, all the laws and formulas for vibrations and waves are well applicable to sound vibrations. When calculating the speed of sound in air, one should also take into account the fact that this speed depends on the air temperature. As the temperature increases, the speed of sound propagation increases. Under normal conditions, the speed of sound in air is 340,344 m/s.

sound waves

Sound waves, as is known from physics, propagate in elastic media. That is why sounds are well transmitted by the earth. Putting your ear to the ground, you can hear from afar the sound of footsteps, the clatter of hooves, and so on.

In childhood, everyone must have had fun by putting their ear to the rails. The sound of train wheels is transmitted along the rails for several kilometers. To create the reverse effect of sound absorption, soft and porous materials are used.

For example, in order to protect a room from extraneous sounds, or, conversely, in order to prevent sounds from escaping from the room to the outside, the room is treated and soundproofed. The walls, floor and ceiling are upholstered with special materials based on foamed polymers. In such an upholstery, all sounds subside very quickly.