In general, the frequency from 20 Hz to 20 KHz is a commonly employed frequency range utilized in an audio and video signal processing system that performs digital signal processing of acoustic or sound signals. A digital signal processing technique has a tendency to have wider dynamic range and characteristics, compared with an analog signal processing technique, therefore an original signal input can be more faithfully processed and amplified at a signal input section, a signal processing section and a power amplification section of the digital audio and video signal processing system. Unfortunately, however, sound reproduction of a speaker needs improvement.
Today's speaker system include a three-way type employing a tweeter for high-frequency sound reproduction, a squawker for medium-frequency sound reproduction and a woofer for low-frequency sound reproduction, and a two-way type employing the tweeter and the woofer. In order to improve low-frequency characteristics of the woofer, lowest resonance frequency should be set to the low frequency, and in such a case the diameter of a vibration plate must be large to improve the low-frequency characteristics. However, when the diameter of the vibration plate is large, volume of the speaker system becomes large as well, limiting installation environment. For this reason, small speaker systems will have a drawback that the sound signal of the low frequency cannot be faithfully reproduced due to the small volume of the speaker even in the case of receiving a high quality audio signal. In addition, the overall reproduction characteristics of the speaker cannot be improved even though the above method is employed to improve reproduction characteristics of the low frequency component of an audio signal by changing the diameter of the vibration plate of the speaker system.