Hi.
I am wondering about the following.
If you build a subwoofer enclosure of normal size, ie 50x50x50cm.
Ie the average sized subwoofer. If this box is filtered with a fourth order filter at say 50Hz, will cabinet resonanses be a problem? I mean, will a MDF box resonate at the frequencies this sub can reproduce? I would guess that resonances would occur around the kilohertz area and up, not at 50Hz. At 100Hz the signal will be 24dB attenuated and at 200Hz 48dB and at 400Hz 72Db, ie gone.
What do you guys think and have anyone actually done any measurements on a woofer with regards to cabinet resonances?
What about putting absorbant materials in the woofer? All porous absorbers are only effective for medium to higher frequencies, so stuffing a subwoofer with anything does not really have any effect?
Last question. Standing waves. I have seen some people stating that standing waves occure between two parallell sides. I have always been taught that standing waves occurs between two points that are tangetial to each other. Creating angles only messes things up and makes the whole thing unpredictable to calculate. In any case, standing waves will occur when the distances between to points are 1/4 of a wavelength to the signal applied. And the only way to remove a standing wave is to absorb it. With all this in mind, all standing waves in a subwoofer will be far outside the frequency range of the speaker.
Many brainstormed ideas, I hope people will comment. I haven't studied this for 8 years now, so maybe my memory has faded a bit...
I am wondering about the following.
If you build a subwoofer enclosure of normal size, ie 50x50x50cm.
Ie the average sized subwoofer. If this box is filtered with a fourth order filter at say 50Hz, will cabinet resonanses be a problem? I mean, will a MDF box resonate at the frequencies this sub can reproduce? I would guess that resonances would occur around the kilohertz area and up, not at 50Hz. At 100Hz the signal will be 24dB attenuated and at 200Hz 48dB and at 400Hz 72Db, ie gone.
What do you guys think and have anyone actually done any measurements on a woofer with regards to cabinet resonances?
What about putting absorbant materials in the woofer? All porous absorbers are only effective for medium to higher frequencies, so stuffing a subwoofer with anything does not really have any effect?
Last question. Standing waves. I have seen some people stating that standing waves occure between two parallell sides. I have always been taught that standing waves occurs between two points that are tangetial to each other. Creating angles only messes things up and makes the whole thing unpredictable to calculate. In any case, standing waves will occur when the distances between to points are 1/4 of a wavelength to the signal applied. And the only way to remove a standing wave is to absorb it. With all this in mind, all standing waves in a subwoofer will be far outside the frequency range of the speaker.
Many brainstormed ideas, I hope people will comment. I haven't studied this for 8 years now, so maybe my memory has faded a bit...
Cabinet resonances would occur at maybe 500-1000 Hz, depending on its stiffness and mass. At least there will be none below 100 Hz unless you have really thin walls. The walls will move, but they will do so uniformly for all frequencies, and since the motion is spring-controlled (ie below resonance) it will be proportional to the pressure inside the box. Typically no problem.
Standing waves will occur at 345/(2*0.5)=345 Hz if the largest box dimension is 0.5 metres. This is 2.79 octaves above 50 Hz, and thus the signal will be attenuated by 67 dB at that frequency. No problem. If the walls are not parallel, the resonances will move. They will not disappear, though, they will just be harder to understand and calculate.
Finally, the function of suffing can be twofold. Firstly, it kills standing waves. You con't need to do that in this design. Secondly, it mankes the box appear larger, since the compression of air becomes isothermal, rather than adiabatic. This works well with closed boxes, but is bad for reflex boxes. Different materials, can have different effects with respect to standing wave attenuation and isothermalisation (is that a proper english word?), as I understand it.
HTH
Standing waves will occur at 345/(2*0.5)=345 Hz if the largest box dimension is 0.5 metres. This is 2.79 octaves above 50 Hz, and thus the signal will be attenuated by 67 dB at that frequency. No problem. If the walls are not parallel, the resonances will move. They will not disappear, though, they will just be harder to understand and calculate.
Finally, the function of suffing can be twofold. Firstly, it kills standing waves. You con't need to do that in this design. Secondly, it mankes the box appear larger, since the compression of air becomes isothermal, rather than adiabatic. This works well with closed boxes, but is bad for reflex boxes. Different materials, can have different effects with respect to standing wave attenuation and isothermalisation (is that a proper english word?), as I understand it.
HTH
I disagree there. Any energy that the driver expends moving cabinet walls back and forth is not being converted into sound and thus is wasted. Panel to panel bracing to elminate any free panel area wider than 8 inches maximum is required to conserve this energy and to keep any possible resonant frequencies well above the driver passband so that it will not excite them.
Agreed, standing waves are seldom a concern in modern cabinets that have internal dimensions generally smaller than 1/4 wavelength. Stuffing does not kill standing waves if they are occuring below 300 Hz or so, where stuffing is transparant to long wavelengths. Stuffing or lining is necessary above that, but not below. Using stuffing/lining with woofers was necessary when the average woofer crossed over at 800 Hz or higher, but not so with a sub that crosses over generally at 100 Hz or lower.
BillFitzmaurice said:
Expanding of the box (like blowing a ballon) does not dissipate energy (analogous to compressing the air inside the box doesn't). The energy is stored in the "springy" walls and is returned as the cone moves the other way so the energy is not lost. What is worse is that the walls will radiate sound in counter-phase to the sound radiated from the driver. This is waste of energy.
However, as long as the volume flow caused by the box is far less than that of the driver, it causes is little problem. This holds if the box oscillations are linear (which is reasonable) and so does not cause distortion and does not shake other things in the room (which is more more uncertain, or at beyond the control of the box designer).
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