![]() This back pressure is transmitted by the membrane to the neck and outside. Because the nature of standing waves, a back pressure is created at the open chamber governed by the length of the tube, M. The acoustic tube of length M has its own resonance by forming multiple standing waves with frequency given by f = nc/2M for n = 1,2,3… By choosing appropriate value for M, needed low frequencies can be generated. When the device is 3D printed, you get a phone stand with tone control - which is otherwise almost impossible to do by electronic means. This membrane does the primary pressure wave filtering to remove the high frequency signals coming from the inlet. The membrane also attempts to smooth out high frequency pressure waves entering from the inlet through the neck. Because of the membrane, the volume in the open cavity (called as open chamber ) is varying depending on the pressure in the cavity. The biggest difference from the classical Hemholtz resonator is that the resonator is open at both ends and has a flexible vibrating membrane. The resonant frequency is governed by the quantities S, V, L and c, the speed of sound. Acoustics and audio play vital roles in our lives. This consists of a closed chamber (also called as cavity) with volume V, an opening with length L and cross section S. The idea is to start with a well-known acoustical device called Hemholtz resonator. stl file of the design for 3D printing with PLA filaments. If anyone is interested in printing this device, drop me a line. However the catch is that this method has to be done in the acoustic domain. The ear fills the missing details and perceives as bass band extension. This a trick to fool the ear into thinking that the sound heard actually contains full base band - though the signal reaching the ear just contains "fundamental-like" spectral peaks. The technique to extend and enhance the bass band signal is to employ the well known "missing fundamental" method. Any amount of software processing in the phone does not help. Improving the sound quality further to include bass extension is also very difficult, to say the least. This vented box design is so compact and elegant that adding a tone control is very difficult. There is a remarkable piece of engineering in the speaker module which converts the tinny sound from the piezo electric speaker to a more acceptable quality sound.
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