[califauna]

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: https://images-na.ssl-images-amazon.com/images/G/01/apparel/rcxgs/tile._CB483369110_.gif

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 separation of the woofers into separate compartments avoids 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).

Now, 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:

https://m.media-amazon.com/images/S/aplus-media/sc/9efaaa56-d306-42f7-9aff-c1720f18a0a9.__CR0,0,300,300_PT0_SX300_V1___.jpg
https://m.media-amazon.com/images/S/aplus-media-library-service-media/cc10c152-1ca8-4e27-9617-7f1fd119dfdd.__CR0,0,970,600_PT0_SX970_V1___.jpg
http://gadgetsin.com/uploads/2019/07/oontz_angle_3_raindance_waterproof_portable_bluetooth_speaker_2.jpg

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?
 

[richidoo]

Welcome to AudioCircle, Califauna!
I like your avatar pic 

The cabinet volume has a direct affect on the bass response in a sealed alignment. Decreasing the volume gives a big bass boost by raising the system Q. There is only one resonance at 58Hz but your are hearing the overtone at double that freq. Either something else is vibrating in sympathy, or room echo or whatever. If you tame the fundamental resonance then the 2nd harmonic will tame too. Maybe the divider is not very rigid and it flexed and caused the phantom harmonic peak.

My guess is that others like you disliked the boom at 58Hz so the designers removed the internal baffle to increase the volume to lower the Q and cure the peak in the redesigned D9 version.

You're asking all the right questions and you obviously have the mind for being a high end loudspeaker designer who (is supposed to) "do things right"    Being a boom box the priority is on making money more than sound quality, but they are to be lauded for trying to fix it. I can just imagine them telling the marketing department "We told you so!" 

The correct fix would be to maintain the separator baffle so that the two bass drivers have their own separate air springs, and increase the sealed volume to lower the system Q to reduce the bass peak and improve transient response. But that would require new plastic molds for the box.

Without the divider, and when it's playing the same center-panned bass on both drivers the peaks will be the same as now. But stereo bass signal the drivers will feel larger volume and that will reduce the bass peaking, so it probably sounds a little better on tracks with stereo bass.

You might be able to find replacement bass drivers with parameters that allow a lower Q with the same low volume with the divider in place. Playing around with a free loudspeaker box simulator like WinISD or others will tell you if that's actually possible, or if such a driver exists. You can look on Parts Express website, or Madisound for such drivers, they also publish the T/S parameters of the drivers which you can plug into the simulator to see a simulated FR of the new driver in the box.
https://audiojudgement.com/sealed-enclosure-closed-box/
Good luck!

[califauna]

Thanks your post got me reading up on some stuff.

Regarding Q, this was interesting, and a bit counterintuitive. I had figured that a larger enclosure would equate to lower resonant frequency given that there is less air-springiness from the larger volume, and therefore more bass at that F, but perhaps I am overestimating the contribution of the resonant frequency vs other factors, for example this 'Q' factor.

There is only one resonance at 58Hz but your are hearing the overtone at double that freq. Either something else is vibrating in sympathy, or room echo or whatever. If you tame the fundamental resonance then the 2nd harmonic will tame too. Maybe the divider is not very rigid and it flexed and caused the phantom harmonic peak.

I'm seeing no strong peak at 58 Hz interestingly.  I also figured that must be the fundamental though as it fits the fundamental of the harmonic series that the spikes seemed to be suggesting.

I have ruled out room acoustics. I quickly noticed the boominess when b-flat (and to a lesser degree A natural and B natural) are played on the different instruments. So I micd up two rooms and ran them through a spectrum analyser and confirmed the spikes at A natural/B-flat (I did a frequency sweep if I remember correctly). Also found the same result in open spaces.

Also I checked for reviews of the speakers online with frequency response analyses and found a couple. They both have spikes around 110Hz, 220Hz, corresponding to fundamental and parcials of A/B-flat. Eg:

https://youtu.be/Y__GH361JtQ?t=1032
https://youtu.be/4QSxBa7qO6Y?t=957

Without the divider, and when it's playing the same center-panned bass on both drivers the peaks will be the same as now.  But stereo bass signal the drivers will feel larger volume and that will reduce the bass peaking, so it probably sounds a little better on tracks with stereo bass.

I will do that test when I put it back together.

So,what is likely to be the outcome if I remove the divider?  From what I have read so far, the result will be:

1.Some LOSS of clarity and volume due to some phase cancellation inside the enclosure from any out of phase stereo signal destructively interfering inside the enclosure (any correction or confirmation on this in particular is most appreciated).  This will depend on the degree to which signal arrives at the woofers out of phase with each other, and from what I understand, this does happen frequently with stereo recordings of bass and drums.
2. Some GAIN in clarity caused by lowering the Q of the system, meaning drivers oscillate for shorter after receiving signal.
3. Some LOSS of overall volume caused lowering of the Q of the speaker.

Anything else?

[dwmaggie]

Welcome to A/C.  I'm way up in Redding.  Have you considered stuffing the enclosures with acoustic fiberglass or specialized acoustic foam?  There are a couple of threads on here that make use of various materials and their purpose.  One thread is about rebuilding Bang and Olafson speakers utilizing foam for internal dampening.  I like your outside the box thinking.  j

[WGH]

I would keep it simple, stuff the cabinets with Kapok, it will remove the "boom" from the boombox. Kapok has a greater damping effect than fiberglass or other stuffings, a little goes a long way, experiment. I used it when I modded my Von Schweikert VR2's. Look for Kapok in fabric stores, it is an organic stuffing used in pillows and sofas.

More Kapok info:
https://www.audiocircle.com/index.php?topic=43753.0

[richidoo]

So,what is likely to be the outcome if I remove the divider?  From what I have read so far, the result will be:

1.Some LOSS of clarity and volume due to some phase cancellation inside the enclosure from any out of phase stereo signal destructively interfering inside the enclosure (any correction or confirmation on this in particular is most appreciated).  This will depend on the degree to which signal arrives at the woofers out of phase with each other, and from what I understand, this does happen frequently with stereo recordings of bass and drums.
2. Some GAIN in clarity caused by lowering the Q of the system, meaning drivers oscillate for shorter after receiving signal.
3. Some LOSS of overall volume caused lowering of the Q of the speaker.

Anything else?

IF there is FR peak caused by system Q way above .7 then removing the divider baffle would lower the Q and flatten the FR only when the bass signals L/R are not equal. When the bass signals L/R are the same, as in most popular music, the apparent volume to each driver will not change and the Q will remain high and no improvement in SQ.

OTOH, IF you are not measuring acoustic bass peak at resonance then too small box is not your problem and removing the divider may not offer any benefit. It could be that the divider part makes no difference in SQ or measured performance so the engineers eliminated it to reduce cost in next version.

If there’s no measurable peak in bass FR then could be they used a driver with high Q to boost bass due to weak motor and loose suspension in high Q drivers. This allows sloppy movement of the cone and overextension on bass transients, resulting in deeper louder bass at the price of greatly reduced transient reponse, aka loss of bass detail. Another advantage of driver with high Q is it’s cheaper because motor and suspension are weak.

Q is a concept that seems counterintuitive at first but with use it makes more sense and is useful in many disciplines of engineering, wherever there is resonance.

Stuffing will damp the motion of the cone where the suspension and amp cannot, but this works best in systems with good inherent mechanical and electrical damping. Adding acoustical damping to a system with weak mech and elec damping will likely just dull the transients further and reduce the bass extension, and if you add enough additonal damping will noticeably lower the SPL compared to the tweeter, if there is any. But it’s worth a try to add some and see what happens.

[califauna]

OK thanks for those posts. Did some reading and got my head around a few more things.

Regarding ramifications of removing the divider in my D8 then, would the following revised summary be correct?:

1.Some loss of clarity and volume due to some phase cancellation inside the enclosure from any out of phase stereo signal destructively interfering (phase cancellation) inside the enclosure (any correction or further explanation of this in particular is most appreciated). This will depend on the degree to which signal arrives at the woofers out of phase with each other, and from what I understand, this does happen frequently with stereo recordings of bass and drums (any confirmation of whether all commercial music tends to have stereo bass signal summed to mono is appreciated).
2. Some gain in clarity caused by a lowering of the Q of the system, but only at instances where left and right signals differ (eg. out of phase, different volume, etc.)
3. Some loss of overall volume caused by a lowering of the Q of the speaker, but only at instances where left and right signals differ (eg. out of phase, different volume, etc.).

[califauna]

Can anyone chime in on those?

[WGH]

Is your W-King D8 portable boom box the same one that sells for $79 on Amazon?

https://www.amazon.com/dp/B07WNZS2FR?tag=techman0d-20&th=1&psc=1&keywords=wking%20d8&geniuslink=true

If it is I wouldn't mess with it. The D8 is designed to play real loud outdoors, not as a high fidelity indoor speaker. I don't think anything you do will change it's character. Have you opened it up yet?

[califauna]

If it is I wouldn't mess with it. The D8 is designed to play real loud outdoors, not as a high fidelity indoor speaker. I don't think anything you do will change it's character. Have you opened it up yet?

Not sure why you say this. From what little I know about speaker design and acoustics, I'm pretty sure doing stuff like removing dividers is going to change it's sound character in some way. In theory at least. Some of them have been suggested in this thread even. The question is, what way, right?

Also, I'm not trying to make it high fidelity (to any great degree), rather identify the cause of and if possible remove a frequency response spike around 115Hz, which makes B-flats played through the speaker on instruments sound boomy.  And learn something along the way.

Yes I opened it up a while ago.  Waiting on thoughts and opinions on likely ramifications of removing divider and suggestions on causes of 115Hz spike before putting it back together. Also waiting on two latching push-button switches to be delivered, which will be used to turn on and turn off the internal speakers when external speakers are in use, as per my idea.   
 

[charmerci]

When you turn up the volume, that whole cheap plastic box is an entire, vibrating box of resonating sounds.

[WGH]

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).

rather identify the cause of and if possible remove a frequency response spike around 115Hz

You are going to have to experiment, I looked online and there are plenty of positive youtube reviews but no modification review, yours will be the first.

All the positive reviews means the designer and manufacturer made a serious effort to make the best $79 boom box they could and probably prototyped many bad sounding boxes before settling on this design. The final result with the spike from 57.5Hz to 125Hz is a design feature and not a flaw.

Like charmerci says, the entire box is resonating at certain frequencies. The best solution is to line the inside of the bass section with No Rez. No Rez is a damping material developed specifically for loudspeaker applications for damping and eliminating enclosure resonances. Ask around here on AC, someone must have scraps they will send you. You don't need a full sheet at $54.95 unless you have other speakers you could use it in. Highly recommended!

[califauna]

Thanks for the above responses.

Yes it sounds pretty good generally and it's very loud for its size. The problem is when playing b-flat at the problem frequencies on instruments like guitar/piano. A bit boomy. I know the box wasn't designed specifically for live instruments, but that's what we use it for sometimes.

It's not a spike from 57.5Hz to 115Hz. It's a spike AT 57.5 AND 115Hz, corresponding to the first and second harmonics of a B-flat on 57.5Hz presumably.

Again, as it hasn't received any direct answers yet...regarding ramifications of removing the divider in my D8 then would the following revised summary be, in theory, correct?:

1.Some loss of clarity and volume due to some phase cancellation inside the enclosure from any out of phase stereo signal destructively interfering (phase cancellation) inside the enclosure (any correction or further explanation of this in particular is most appreciated). This will depend on the degree to which signal arrives at the woofers out of phase with each other, and from what I understand, this does happen frequently with stereo recordings of bass and drums (any confirmation of whether all commercial music tends to have stereo bass signal summed to mono is appreciated).
2. Some gain in clarity caused by a lowering of the Q of the system, but only at instances where left and right signals differ (eg. out of phase, different volume, etc.)
3. Some loss of overall volume caused by a lowering of the Q of the speaker, but only at instances where left and right signals differ (eg. out of phase, different volume, etc.).

[charmerci]

The designers put in the dividers there to eliminate resonances, by removing them you make it more flimsy. But if you no-rez the interior, you might or might not have a better  box than without them. However, no one knows until you do it with and without the dividers. You're on new ground.

[califauna]

There is some new ground here as you have pointed out, as there aren't any online accounts of modifications of this speaker, but in theory there are predictable differences that arise from removing a divider between two speaker chambers, doubling the enclosure size but also bringing all the speakers into one chamber, so we're not in terra incognita here in all regards. Some of these theoretical differences have been pointed out by others here. I've tried to summarize these above.

The more I look at frequency response curves for Q values, resonant frequency curves and the like, the more I'm thinking these spikes at B-flat/A are the resonant frequency of the system as a whole and are so pronounced because of a low damping/high Q value, so adding some padding/stuffing seems to make sense. 

Again, any confirmation of the predicted outcomes, in theory, is appreciated, as they haven't received a direct answer yet.

Regarding padding, my understanding is that adding padding/lining/stuffing:

1. Absorbs some of the back wave, thereby reducing enclosure panel vibrations and additionally reducing the propagation of the backwave outside the box, thereby reducing (for low frequency signal) both destructive interference (phase cancellation) and constructive interference outside the box, and inside the box to some extent.
2. Lower the Q of the system, thereby reducing the frequency response spike at the resonant frequency of the driver + system, reducing efficiency, improving transients, reducing vibrations, and lowering overall volume.

[WGH]

...but in theory there are predictable differences that arise from removing a divider between two speaker chambers, doubling the enclosure size but also bringing all the speakers into one chamber, so we're not in terra incognita here in all regards.

You are so focused on removing the center partition that you completely disregard the the side passive radiators.

"In the same way as a ported loudspeaker, a passive radiator system uses the sound pressure otherwise trapped in the enclosure to excite a resonance that makes it easier for the speaker system to create the deepest pitches (e.g., basslines). The passive radiator resonates at a frequency determined by its mass and the springiness (compliance) of the air in the enclosure. It is tuned to the specific enclosure by varying its mass (e.g., by adding weight to the cone). Internal air pressure produced by movements of the active driver cone moves the passive radiator cone. This resonance simultaneously reduces the amount that the woofer has to move."¹

The passive radiators are tuned to the driver and the air behind it. Removing the divider completely changes the math.

If the speaker really has a narrow spike at 57.5 and 115Hz does it come from the active drivers, the passive radiator or the cabinet? Hard to tell without measurements. Room EQ Wizard (REW) is a free measurement software program that will help define where the speakers problems are located. Without measuring you are just shooting in the dark hoping to hit the target. A steep learning curve but worth it.
https://www.roomeqwizard.com/
There are plenty or REW forums and tutorials to get you started.

Or you can watch the Alan Ross W King D8 Review to see a frequency response graph
https://www.youtube.com/watch?v=tiYJ1riJa_s




Now compare that graph to an in-room measurements of GR Research speakers using REW

https://www.audiocircle.com/index.php?topic=178270.0


¹ https://en.wikipedia.org/wiki/Passive_radiator_(speaker)

[califauna]

You are so focused on removing the center partition that you completely disregard the the side passive radiators.

The ramifications of removing the centre partition is the reason is the main reason I posted the thread!

I was told in a reply above in this thread that if I removed the divider the frequency response of this speaker would only change for asymmetrical signal between left and right channels.  I took that to apply to the action of the passive radiators as well, but I think you're right that I have neglected to consider the passive radiators enough in general. It just occurred to me to read up on the resonant frequency of passive radiators in sealed boxes as a possible cause of the peaks. That would make sense as they would be in the right place to add bass around probably the low end of the speakers frequency response.

Thanks for that Alan Ross link. The peaks (115Hz and 230Hz) show up in another Alan Ross review of the same speaker (https://youtu.be/Y__GH361JtQ?t=1030). I also already carried out frequency response testing of my own and confirmed response peaks there, which suggested to me a FR peak with a fundamental harmonic at 57.5Hz. Boominess at 115Hz and 230Hz is additionally very noticeable by ear, especially for notes played on instruments.