Sound power and sound pressure level are two words in the audio industry, but we often confuse them, that is, where sound power should be used. As a result, we use sound pressure level to express them, which will easily lead to inaccurate data. In fact, these are two nouns with different meanings. Let's make a clear distinction.

What is the sound power?

Power is expressed by W in physical units. It usually refers to the amount of work done by an object in unit time (Joule J). Therefore, sound power is the amount of work done by a sound in unit time, which shows the sound producing ability of the sound source. Then its unit is very obvious, usually expressed in DBW. How do we get the sound power doubled and sound pressure level increased by 3dB?

In fact, there is a conversion formula: the increment of sound pressure level = 10logw / W0, where W0 is the reference value and W is the working power of the speaker at this time.

What is the sound pressure level?

The sound pressure level is usually expressed by dbspl, which is the atmospheric pressure perceived by the human ear diaphragm by the disturbance of sound to atmospheric molecules. The calculation method is a ratio of the sound pressure value of the sound and the reference sound pressure, and then multiplied by 20 times log, that is, SPL = 20logp / P0, P is the measured sound pressure value, P0 is the reference sound pressure value, so each time the sound pressure is doubled, the sound pressure level increases by (20log2) 6dB. According to this, we can calculate the acceptable loud pressure level of human ears.

Because the human ear can withstand a loud pressure of about 20Pa, and the reference sound pressure in the air is usually 2 × 10 ^ - 5pa, that is, the human ear can withstand a loud pressure level of about 120dB (20log20 / 2 × 10 ^ - 5), which is often referred to as the pain threshold, but this pain threshold is not applicable to all people, some people can reach 130 or even 140dB due to the working environment or natural ear structure and other reasons.

What is the relationship between sound pressure level and sound power?

There is a conversion formula between sound pressure level and sound power

LP=LW-10Log（4πr^2),

Where LW is a fixed value and R is the distance from the sound source to the test point.According to this formula, we can calculate the distance between the sound source and the test point, and the relationship between the sound pressure level and the sound power.

Inadvertently, we often fall into a misunderstanding, that is, when two or more speakers are superimposed, calculate their sound pressure level. Generally speaking, it is known that the sound pressure level of one speaker = 10logw + sensitivity. When two or more speakers are superimposed, some people directly use 10logw × n + sensitivity. (n represents the number of speakers, especially in calculating the linear array sound pressure level In fact, this calculation method is wrong. Because when two or more speakers are superposed, they are not simply superposed. Although they are superposed, the calculated result is only superposed sound power, not superposed sound pressure.

((a) represents the superposition of sound waves, (b) represents the cancellation of sound waves)

The sound waves will affect each other, and will be superimposed or cancelled. According to the principle of the figure above, we can conclude that when the sound with the same phase, the same sound source, the same amplitude and the same frequency is superimposed, the amplitude (energy doubled) is increased by 6dB. However, it is often strange that when two speakers of the same model are superimposed, they do not increase 6dB. That's because the distance difference is produced when the speaker is replayed and superimposed. These distance differences make some short wavelength frequencies interfere with each other, and the sound waves will be offset. The offset or incomplete offset of some waves reduces the energy of the waves, thus reducing the energy of the atmospheric pressure. Therefore, the sound pressure level between the two speakers is generally increased by about 4dB.