1, the appearance of sound

Everything that makes a sound vibrates. When the vibration stops, the sound will also stop.

Sound is produced by the vibration of objects, but not all the sounds produced by vibration can be heard by human ears.

2. Transmission between voices

The transmission of sound needs medium, and vacuum cannot transmit sound.

(1) All gases, liquids and solids can transmit sound. These substances as media are called media. Even if astronauts on the moon talk face to face, they still need to rely on radio. That's because there is no air on the moon, and vacuum can't spread sound.

(2) Sound travels at different speeds in different media. Generally speaking, solid >: liquid > air.

The speed of sound in the air is about 340 meters per second.

Step 3 echo

In the process of sound propagation, the sound that people hear again when they encounter obstacles is called echo.

The distinguishing condition between echo and original sound: the echo reaches people's ears more than 0. 1 second later than the original sound. Therefore, the sound must be reflected by obstacles with a distance exceeding 17m, so that people can hear the echo.

Below 0. 1 sec, the reflected sound can only strengthen the original sound.

Use echo to measure the depth of the sea or to detect how far an object is from an obstacle.

4. Music

The sound made by an object when it vibrates regularly is called music.

Three elements of musical sound: tone, loudness and timbre.

The sound level is called tone, which is determined by the vibration frequency of the speaker. The higher the frequency, the higher the pitch. (Related concepts: Hertz Hz)

The size of sound is called loudness, which is related to the amplitude of speaker vibration and the distance from the sound source to the human ear.

The quality of sound produced by different vocal bodies is called timbre. Used to distinguish different sounds.

5. Noise and its sources

From the physical point of view, noise refers to the sound made by the vocal body when it vibrates irregularly. From the perspective of environmental protection, all sounds that hinder people's normal rest, study and work, as well as those that interfere with people's desire to hear, belong to noise.

6, the division of sound level

People use decibels to divide sound levels. 30 dB -40 dB is an ideal quiet environment. If it exceeds 50dB, it will affect sleep; If it exceeds 70dB, it will interfere with the call and affect the work efficiency. If you live in a noise environment above 90dB for a long time, it will affect your hearing.

7. Ways to reduce noise

It can be weakened at the sound source (noise elimination), during propagation (sound absorption) and at the human ear (sound insulation).