In order to meet the requirements of high fidelity listening, the rated power should be determined according to the best listening sound pressure. We all have the feeling that the volume is low, the voice is weak, thin, dynamic, dull, low frequency is significantly lacking, and the fullness is poor, and the voice seems to shrink inside. When the volume is appropriate, the sound is natural, clear, mellow, soft, full, powerful and dynamic. But when the volume is too high, the voice is stiff, not soft, rough, and has the feeling of pricking ears. Therefore, the reproduction sound pressure level has a great relationship with the sound quality. The sound pressure level in the listening area should preferably be 80~85dB (A-weighted). We can calculate the rated power of the speaker and the rated power of the power amplifier from the distance from the listening area to the speaker and the characteristic sensitivity of the speaker.

Sound box: in order to make it able to withstand the impact of burst strong pulse in the program signal without damage or distortion. Here is an empirical value for reference: the nominal rated power of the selected speaker should be three times the power calculated theoretically.

Power amplifier: Compared with transistor power amplifier, electronic tube power amplifier requires different power reserves. This is because the overload curve of the electron tube power amplifier is relatively gentle. For the overloaded music signal peak, the tube power amplifier does not obviously produce clipping phenomenon, but makes the tip of the peak rounded. This is what we often call the flexible shear peak. However, after the transistor power amplifier is at the overload point, the nonlinear distortion increases rapidly, causing serious clipping to the signal. It does not round the peak but neatly cut it flat. Some people use the composite impedance composed of resistance, inductance and capacitance to simulate speakers, and test the actual output capacity of several high-quality transistor power amplifiers. The results show that when the load has phase shift, there is a 100W nominal power amplifier, and the actual output power is only 5W when the distortion is 1%! Therefore, the selection of the reserve of the transistor power amplifier:

High fidelity power amplifier: 10 times

Civil high-end power amplifier: 6-7 times

Civil mid-range power amplifier: 3~4 times

The tube power amplifier can be much smaller than the above ratio.

The margin between the average sound pressure level and the maximum sound pressure level of the system shall be determined according to the content of the broadcast program and the working environment. The minimum redundancy is 10dB. For modern pop music, disco and other music, 20~25dB redundancy is required, so that the sound system can work safely and stably.