When sound encounters obstacles in the process of propagation, it will be reflected back and produce echo within a certain distance. The walls of the cinema are made uneven so that the sound cannot be transmitted to the human ear again after multiple reflections. Making the sound of the movie clearer is probably what it means!

The reason for the echo is that the sound emitted by the sound emitter is reflected back when encountering obstacles, but if the interval between the echo and the original sound is less than 0.1 seconds, then people will not be able to distinguish the original sound from the echo.

In other words, the distance between the sound emitter and the obstacle must be at least 17 meters before people can distinguish the original sound and the echo. In ordinary rooms, the distance is smaller than this distance, so the echo and the original sound are mixed together, and the human ear Can't tell, but it enhances the original sound.

So it sounds louder than outside. But when shouting at the mountain, the distance is more than 17 meters, so the echo can be heard. The walls of the cinema are made uneven, causing the echoes generated by each reflecting surface to cancel each other out, affecting the projection effect.

The condition for the human ear to be able to distinguish the echo is that the reflected sound has sufficient intensity and the time difference from the original sound must be greater than 0.1 seconds. The echo is heard most clearly when the size of the reflecting surface is much larger than the wavelength of the incident sound. That is when they are 17 meters apart.

20Hz~20000Hz, the range of human sensitivity to sound. In addition, there are ultrasonic and infrasound waves. 0.38～0.72μm, the range of human light perception. In addition, there are infrared rays and ultraviolet rays.

Note: When the time interval between the original sound and the echo is greater than 0.1 seconds, it can be distinguished by the human ear, which is called echo; when it is less than 0.1 seconds, it is reverberation (reverberation increases the volume, so the principle of the speaker It’s reverb).