Control the Frequency Response of a Loudspeaker by Changing Its Enclosure

Abstract

This article analyzes how to control frequency response of a loudspeaker by changing the volume of its closed box enclosure. The calculation is performed by the method of Thiele-Small on the basic of a pre-calculated loudspeaker, the parameters of which are given in third section. This became possible because of the simplification of the circuit on figure 1 to the form of circuit on figure 2. This allowed us to consider it as a second order filter (presence of two reactive elements). Obtained results are compared with corresponding characteristics of open-box enclosure of the same loud speaker, that was pre-calculated by the author too. Results are presented graphically in figure 3 and 4. As can be seen from them, the resonant frequency of the loudspeaker in the closed-box enclosure is higher than the resonant frequency of the loudspeaker in the open box. The result in the form of a ratio f_c/f_s is listed in table 2. Analyzing the obtained data, it can be noticed that with the change of the internal volume of the closed box (and hence its total quality factor), it is possible to affect both the resonance frequency and the peak amplitude values in these frequencies by changing the FR. The result shown in figure 3 and 4 is achieved by taking into account effect of radiation only on the one side of the driver (in the case of open-box enclosure). Closed box was calculating by taking into account both sides radiation of the driver. 

Shifting the resonance frequency of the system towards higher frequencies and increasing the sound pressure on the resonance generally worsens the FR of the loudspeaker (reduces the reproduction of low-frequency components of sound and increases the unevenness of the frequency). However, certain variants of this group of frequency characteristics may be useful depending on the reproducible frequency range and need of emphasize the low-frequency components (for example, in rock music). If you need a smoothed low-frequency sound, it is appropriate to use systems with low overall quality and increased internal volume or open-box enclosure. 

Therefore, the volume of the closed-box enclosure significantly affects the resonant frequency and the shape of the fre quency response of the loudspeaker. Reducing the volume of the enclosure of the loudspeaker leads to a decrease in its frequency range due to low frequencies and at the same time increase in the unevenness of the frequency response. The change in the resonant frequency of the system as the volume of the closed box enclosure decreases, the less the volume of the closed-box.