Slot Loaded, Dual Opposed 8" Subwoofer (suitable for on wall/high shelf mounting)

[kiwifi]

I have started designing a subwoofer suitable for wall mounting. Like most, my room is not ideally shaped but I would like to locate a sub in each of the four quadrants of the room. This means locating at least one sub high up on a wall above a window.

To minimize cabinet vibrations the configuration is dual opposed. The slot directs the direct energy of the drivers into the room.




The cabinet has space for a plate amp to be fitted in the top, but I don't want to have to run power and signal up the wall, so I will use an amplifier closer to ground level. I will still need to fit a small access plate to the top to allow stuffing of the top section and a speaker connector. Construction is 12mm MDF, with two overlapping layers for the outside walls and the baffles.

The slot is just wide enough to install the drivers, which are Peerless 830586.


I am using these drivers because I happen to have four of them from another project. They are no longer available to buy since M&K went out of business, but the Peerless SLS 830667 would be a suitable alternative.

The overall dimensions for my design are very close my M&K MPS 2810 (MX-700) which uses the same Peerless 830586 (M&K 17040) drivers, so I expect the finished weight to be similar also.


[Discliamer: I am not an expert on slot loading!]
The frontal area of the slot determines the pressure loading of the drivers. In this case, the ratio of the total cone area to the slot opening is 1.65:1, so the pressure loading is relatively mild. I have seen ratios of between 2:1 and 3:1 recommended. Pressure loading the drivers increases the apparent mass of the cone which can lower the resonant frequency Fs. However, if the slot is to small then you start to hear the air as it moves in and out of the slot (similar to port noise), which is why the outer edges of the slot (and ports!) are rounded.
The depth of the slot must be kept to a minimum to prevent the cavity resonating at frequencies of interest. An eight inch deep slot is not going to be a concern at subwoofer frequencies.

You will notice that at the moment, the driver arrangement is Push-Push, both drivers face into the slot. In order make it Push-Pull, I needed access to the plenum (the back of the slot) otherwise the mounting screws for the inverted driver would be inaccessible. The other alternative was to mount the inverted driver from the inside of the baffle. I modeled this option, but determined that it compromised the structural integrity of the cabinet too much, so I am working on a removable plenum instead.

 

[Heinrich]

[Discliamer: I am not an expert on slot loading!]
The frontal area of the slot determines the pressure loading of the drivers. ... Pressure loading the drivers increases the apparent mass of the cone ...

The mass is true, but pressure not. Slot loading is a funny idea, but (in this context) useless. The proponents always forget about the radiation resistance, which leads deep into acoustical engineering. That's why.

So, what's left is mass increase. You could easily attach (reversibly) some extra lumped mass to the cone. Some plasticine, ... . Or even better, trust electronic equalization. As I said, "slot" isn't that elegant ...

 

[kiwifi]

The mass is true, but pressure not. Slot loading is a funny idea, but (in this context) useless. The proponents always forget about the radiation resistance, which leads deep into acoustical engineering. That's why.

Not so much interested in the mass loading aspect. Hopefully the slot will help direct sound into the room rather than towards the mounting wall.

I am still wading my way through this thread...

 

[digitalfrost]

If these are identical or close to Peerless SLS-8, I suggest you read my comments in this thread: https://www.audiosciencereview.com/...nd-or-measurements-peerless-sls-series.33333/

I found some TSP of 830586 here: https://www.diymobileaudio.com/threads/peerless-830586-initial-specs-wt3.78344/

And if the Qts of 0.7 is true, they are pretty useless chassis. I guess if the MPS 2810 was a real product you could try it, but my point is the base SLS-8 already has high Qts which makes it most unsuitable for anything but large closed boxes or a transmissionline, and these chassis seem even worse. Heck, you could use these for a dipole.

 

[Heinrich]

Not so much interested in the mass loading aspect. Hopefully the slot will help direct sound into the room rather than towards the mounting wall.

I am still wading my way through this thread...

Speaking of Hornresp, it may be interesting to simulate a slot, where it makes sense. Namely combined with increasing radiation impedance e/g in a room corner. Otherwise all the slotting is fully wasted. As I said, not all slotties are audio engineers. It's a fallacy. promoted by Nelson Pass (not an audio engineer, esp not w/ speakers), as I've learned recently. The report from NP is flawed and misleading as such. Finally it doesn't make any sense.

I was under the impression that you targeting bass, so 'directing sound' as you say is not really necessary, or possible.

Seems, again a quest ends up in unexpected results. That's science ;-)

 

[kiwifi]

I guess if the MPS 2810 was a real product...

The M&K MPS-2810 is definitely real! It was the professional version of the MX-700

 

[kiwifi]

I found a spec sheet for the MX-700 and the only difference between that and the "professional" MPS-2810 appears to be the addition of a balanced XLR input on the 2810.
Here is mine...

I may be trying to solve a problem that doesn't exist, but I felt that my 2810 would not do well firing into a shelf or a wall just above a window. That has given me the excuse to build a dual opposed version of the 2810. Whether it ends up being any better remains to be seen...

 

[kiwifi]

Here is the Push-Pull version of cabinet, with removable plenum...

Back


Front


With the left driver motor now mounted in the slot, I was able to reduce the width of the cabinet to almost exactly match that of the 2810.
External dimension for my design are now 368mm x 450mm x 268mm (MPS-2810 are 368mm x 445mm x 254mm)
A quick estimate of the internal volume is 21L for mine vs 15.8L for the 2810, that excludes the drivers and electronics.

The next step is to try and model effect of the slot in Hornresp

 

[Puddingbuks]

Why not copy this design?

Dynaudio 18s, dual opposed sub. Seems that this is way easier to make.

 

[kiwifi]

Why not copy this design?

Dynaudio 18s, dual opposed sub. Seems that this is way easier to make.

View attachment 455361

You are correct of course, but I like push-pull subs and the idea of protecting the drivers within a slot appeals to me.

 

[kiwifi]

Here are the HornResp simulated responses. This is my first time using this software, so hopefully my numbers are correct.

Firstly, a seal box with no slot, for reference...


The 114mm slot loaded version (as designed)...


If the slot was 50% wider (171mm)...


I can't make the slot smaller, but I can restrict the opening. In this case to a circle the diameter of the slot (114mm)...


The 50% wider slot offers little benefit over the current one.
Restricting the slot opening with a Port, lowers Fs but now the port resonance is so much more dominant than the drivers.

In all cases the effects of the slot occur at frequencies well above where the subwoofer will be used.

 

[Heinrich]

In all cases the effects of the slot occur at frequencies well above where the subwoofer will be used.

So, even a simulation shows, that the most prominent proponent of 'slot', Nelsomn Pass namely, and many other, are plain wrong in their claims. A myth, a fallacy, cargo cult?

When I suggested to use HornResp like many other before, I was thinking of taking advantage of the radiation impedance. Is it feasible to let the slot radiate closely, very close, slot-width close into a room corner? The room corner could possibly serve as a horn coupled to the slot, the only reasonable use for that slot.

 

[kiwifi]

So, even a simulation shows, that the most prominent proponent of 'slot', Nelsomn Pass namely, and many other, are plain wrong in their claims. A myth, a fallacy, cargo cult?

A lot of the slot loaded designs seem to be for much larger drivers or PA cabinets. In that case the slot dimensions are much larger, so the slot contribution will move into the subwoofer frequency band. Not necessarily a good thing for a HiFi sub though, I agree! In my case I was looking for a minimal contribution from the slot and a way to protect the dual opposed drivers.

By mounting the subwoofer on the wall I hope to be able to locate it at the 1/4 length & 1/4 height point. I am trying to even out my room response with the minimum of DSP. I have a couple of 12" subs for raw output.

 

[kiwifi]

It is easier to compare the responses by importing the SPL and phase data from HornResp into REW...


It should be relatively easy to flatten the response and extend the LF...

 

[Heinrich]

It should be relatively easy to flatten the response and extend the LF...

I don't expect this. The peak of ~10dB give or take will translate to increased distortion. Assume a 50Hz tone with some harmonic distortion, hence extra frequencies of 100Hz, 150Hz and so on. If the HD component hits the peak it is emphasized accordingly. 10dB makes a factor of 3. You cannot e/q that, as the e/q only acts on the input at 100Hz for instance, but not on the output (plus HD) at 50Hz. This is well proven to be true, sigh.

 

[A Surfer]

Well out of my depth here; however, I would like to say that I own two Sonos, dual opposed slot subs that seem to work well.

In truth they are more of low bass speakers as opposed to true subs. Smaller drivers from what I understand (and hear) are unable to reproduce actual sub bass with any meaningful amplitude.

I find the Sonos subs contribute a significant amount of good quality bass, but no, they don't compete with actual larger subs. Just before going Sonos, which I'm quite happy with, I had a nice speaker system comprised of a pair of Monitor Audio PL200s supported by a pair of SVS SB2000 Pro subs.

Having experienced very good subwoofer performance with the pair of well integrated SVS subs I have to acknowledge that the Sonos subs do not compete in true subwoofer territory, but they do absolutely contribute solid bass output.

My point in sharing this is to ask do people not believe that the engineers at Sonos know what they are doing and if so they elected to design dual opposed, slot based speakers. I believe they went this route as they are good audio quality, lifestyle speakers so aesthetics and room appeal was probably more important than producing true sub bass extension such as my SVS subs could achieve.

Again, the Sonos do contribute good quality bass and enough of it to be convincing down low. If I hadn't heard what a pair of well designed, 12" SVS subs could do I would not know what the Sonos actually miss reproducing. I expect the Sonos subs reproduce some sub bass extension, but not at any meaningful amplitude.

 

[Heinrich]

My point in sharing this is to ask do people not believe that the engineers at Sonos know what they are doing and if so they elected to design dual opposed, slot based speakers. I believe ...

This is an argument often seen. It isn't valid. Sonos produces what people (likely) want to buy. On which grounds people like a specific design must not be their concern. That for starters. Additionally to that, I raised several concerns, that are based on good plain engineering, and I've not seen any caveat yet. In terms off technology I mean.

It is only so, that there is some acknowledgement in certain circles, that hypes the style. The reasoning is weak to say the least, and it is easily disproven, as I did before.

You can do 'slot', but the merits are cosmetic only, low footprint and such, but the cost in sound quality is high, if you're after low distortion. I explicated on the conclusion above.

 

[A Surfer]

You may wish to do a little more reading here. Sonos has demonstrated that they have solid engineering chops. Nobody would ever deny that Sonos are lifestyle focused, including Sonos. That does not mean that they don't make great sounding speakers because they absolutely do.

My point wasn't meant to refute your extremely well articulated and researched argument against the use of a slot design. My point is correct thank you very much, because my point is that my two Sonos subs do produce meaningful and quality bass. That was my point.

The fact that there is a slot indicates that at least the Sonos engineers knew well enough how to use it to produce the sound profile they were designing for. Clearly you would never buy their products, good for you, you don't have to. Isn't that great?

 

[Heinrich]

You may wish to do a little more reading here.

No. The slot thing is cosmetic, Nelson Pass is wrong and misleading. Sorry.

 

[A Surfer]

No. The slot thing is cosmetic, Nelson Pass is wrong and misleading. Sorry.

Again, the slot thing really wasn't my point. So am I not hearing what I think I'm hearing from my subs?