Blog Forums DIY Speakers and Subwoofers Boom box modification. Ramifications of removing internal divider.

last updated by Elliott 11 months ago
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    • #15131
      Hi, this is my first post here.

      I am in the middle of modifying a W-King D8 portable boom box (I’m adding some jack connectors to be able to connect external speakers). The speaker has a stereo pair of woofers and tweeters, divided in the middle of the speakers into separate so each stereo pair is in its own compartment. This is the D8:

      The speaker has a bit of a frequency response spike around 57.5Hz and 125Hz (B-flat 1 and B-flat 2 are noticeably boomy indoors, but not so much outdoors). When reassembling it, I thought about removing the division and another internal divider separating the electronics, to see if it might lower the resonant frequency of the system. However, based on the theory I have read so far, I also figured that the existing separation of the woofers into separate compartments should avoid the potential phase cancellation of any slightly out of phase left and right signal (bass, midrange, HF) which would occur if the woofers share the same enclosure space (stereo bass signals and drum signals are often slightly out of phase in much recorded music).

      However I’ve just seen a few speakers, including the newer version of this same speaker (W-King D9), where the stereo pairs are not separated in the speakers, and indeed share the same passive radiator:

      So, given the above, now I’m wondering how much of a difference the existing division in my D8 is likely to make regarding avoiding potential phase cancellation of low frequencies inside the box. Can anyone share some wisdom on this?

      Also wondering what the design rationale is with the speakers where the left and right woofers share the same single enclosure space, like the ones listed above. Is there an acceptance that some phase cancellation/destructive interference will occur inside the enclosure as a result of this? Is there some advantage conferred from only having one enclosure which compensates for this, perhaps lower resonant frequency of the drivers (due to larger enclosure volume), or something like this, perhaps giving a boost to bass signal at the expense of clarity?


    • #15146

      This is a weird one.  I can’t seem to tell is the W8 has a passive radiator on each side or a port on each side.  If it does, you do not want to remove the partition.  That can have negative effects of porting. And typically two separate cavities are better. Now it is unclear on what is causing the issue, but it might be internal resonance.  Half of 125 is 61.25, which isn’t that far from 57.5.  Typically resonance will occur at doubling the frequency.  What I would first try to do it line the internal cavities to see if it dampens it.  There’s only so much you can do about this, but you can try to lessen it’s impact. 

    • #15189

      Welcome to the forum!

      I agree with Nick, you don’t really want to be removing that divider, as you said there would be phase issues when the left and right drivers are playing different things (unless you wanted to make it mono).

      Either way there would be little to gain in removing the internal divide, removing it would give a little more internal volume, but not enough to make any noticeable difference, and while removing the divider may lower the frequency at which that resonance happens, it’ll likely also become a larger resonance since you are reducing rigidity. (Ported/passive radiator boxes generally end up with higher pressures than even sealed, unintuitive isn’t it?!), Meaning that rigidity really is key.

      I’m not sure damping would help too much at such a low frequency (for the 57.5hz peak), but if you were to try damping, try a relatively high density mineral wool as this typically works best for low frequency, but bare in mind, the more space taken up, the less efficient the bass becomes, too much space taken up and it might become peaky and one-note like.

      Another option that might be of more use and should take up less space than the thick wad of mineral wool is to add useful bracing, for instance it might be that the internal divider is flexing and could do with being replaced with a thicker divider, or gluing another sheet to it. However this may not be the cause of the resonance so other bracing might be useful, how rigid and thick are the main walls? Because that might actually be your main problem, how much does the box vibrate at these peaky frequencies? Try a tone generator and have a feel.

      Another but less likely option because of the two peaks almost being fundamentals, is that the peaks come from the bass driver’s response.

      Finally, it might still be worth lining the walls, since it may reduce the higher frequency peak at 125hz at least somewhat.

      Elliott Dyson – Elliott Designs (YouTube) – 3rd year MENG Student

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