The pitch of the sound is the sound level of the previous master," I will teach you how to play sound" one book, chapter second, said P22:

" The level of the sound is the tone, is to describe the object vibration speed sign. The vibration of the object is the faster, the tone of voice is higher, we hear the voice of the tip; in turn, the vibration of the object more slowly, the pitch of the sound is lower, we hear the voice of the deep. How many times per second is called object vibration frequency, in Hertz ( Hz ). A second 50 object vibration, sound frequency is 50Hz; a second 1000 vibrations, sound frequency is 1000Hz."

As everyone knows the pitch of the sound is a high frequency is higher, the pitch of the sound more low frequency is low, but why? That's what as the basis? How to know the object 50 times a second vibration, sound frequency is 50Hz? What is it? After some research, this relationship is a physical formula can be found, it can be calculated. How to calculate?

Must first figure out what is the sound:

The sound is caused by air vibration object vibration generated by an up and down the pressure wave, the sound to the ear and hear the sound.

Figure 1, air pressure vibration forming up gas wave, transverse to the normal air pressure, normal pressure waves called sine wave, normal pressure waves below the called negative wave. The highest point is sine wave crest, negative wave nadir call trough.

Acoustic wave cycle: pressure from the starting point after the wave crests and troughs and then return to the starting point a week, the time required for sound waves cycle. A complete cycle is 360 degrees, the acoustic starting point is 0 degrees, to the crest of 90 degrees, the intermediate point is 180 degrees, the trough is 270 degrees, the end point of 360 degrees.

Acoustic amplitude ( Amplitude ): height is a sound wave amplitude, when the vibration amplitude is bigger, the pressure is bigger, the human ear feel pressure is greater, and hear the sound louder; instead, the amplitude is lower, the sound is more weak.

The acoustic velocity is 340 m / s.

Sonic does not have a fixed ＆#118alue, in dry air, sonic u = 331.3 + ( 0.606c ) m / S ( C = centigrade temperature )

Temperature 15 degrees when the sound velocity = 331.3+0.606x15 = 340.4m / S

The wavelength of the sound = speed / frequency

Wavelength ( Wavelength ): any two adjacent same phase distance. For example, the adjacent 0 degrees to 0 degrees, 90 degrees to 90 degrees, 180 degrees to 180 degrees, are equal to the wavelength.

Below using the above relations to calculate the frequency of the sound : look at the picture, figure 2 for the 1Hz sound wavelength, the wavelength of the sound of 3 10Hz.

( Fig 2 ) 1Hz sound wavelength of 340m / 1Hz, 1sec cycles 1 cycles

( Fig 3 ) 10Hz sound wavelength of 340m / 10Hz, 100ms cycles 1 cycles, 1sec cycles 10 cycles

And so on:

The voice of 1Hz wavelength of 340m / 1Hz = 340m, a cycle time is 1sec;

The voice of 10Hz wavelength of 340m / 10Hz = 34m, a cycle time is 100ms;

The voice of 100Hz wavelength of 340m / 100Hz = 3.4m, a cycle time is 10ms;

The voice of 1000Hz wavelength of 340m / 1000Hz = 0.34M, a cycle time is 1ms;

The voice of 10000Hz wavelength of 340m / 10000Hz = 0.034m, a cycle time is 0.1ms.

The same reason:

Frequency ( Frequency ), the number of cycles per second vibration of objects, called frequency. Hertz ( Hz ).

When a 1Hz voice, 1sec pressure and vibration 1 cycles;

When a 10Hz voice, 1sec pressure and vibration 10 cycles;

When a 100Hz voice, 1sec pressure and vibration 100 cycles;

When a 1000Hz voice, 1sec pressure and vibration 1000 cycles;

When a 10000Hz voice, 1sec pressure and vibration 10000 cycles.

From the above statistics to see:

When the sound of a second vibration more, it sends the higher the frequency; conversely, when the sound of a second to the number of vibrations are less, the emitted frequency is lower