Speaker cab 'wadding'

[DGBass]

I've decided to fully refurbish the 'porthole' bass cab I recently mentioned on another thread after finding out from its previous only owner it has had some interesting history. I'm happy about the porthole and port tubing aspect of what I hope to achieve however something thats always made me wonder is why some cab manufacturers use 'wadding' in their cab offerings and some don't. There doesn't seem to be a lot of consistency whether its in ported or sealed cab designs. Is there a specific scenario where 'wadding' is mandatory and is lack of 'wadding' in a cab just a cost cutting exercise? And does lack of wadding really make much difference, especially in ported cab designs? 

As examples, I once bought a Peavey MKIII bass combo which sounded very damped tone wise. It had a king size duvet shoved in the back and removing it totally changed the amp back to what I'd expect from a vintage Peavey combo. I also bought a TE 1110 ported combo which had about 20kg of four inch thick loft insulation in it. Again it was quite 'damped' sounding and removing the insulation changed the sound back to what I would expect a TE 4x10 to sound like. I've used Dacron  in some cab restorations in the past but never more than one inch thick wadding and I have noticed that it sometimes smooths out the bottom end quite nicely but it doesn't always have the same effect with different cab designs eg 2x10s or 2x12's. It seems to affect 1x15s most noticeably or that may just be my hearing. So, is it worth 'wadding' a 3 cu ft 'porthole' 1x15 cab with 1 inch thick wadding and what should that do to the tone if anything?     

Edited February 20, 2023 by DGBass
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[Bill Fitzmaurice]

Wadding, more properly referred to as damping, serves two purposes. One is to reduce internal reflections in the cab, which cause peaks and valleys in response. How much will give the best result varies from cab to cab, with between one and two inches usually being good.

 

The other purpose is to lessen midbass boom, which takes more material. In sealed cabs with high Q drivers, which have a tendency for boomy response, the cab might be fully filled. In extreme cases it may be not only filled but also compressed. Well designed ported cabs don't use high Q drivers, so they usually don't boom and don't require filling the cab to tame it. Of course not all ported cabs are well designed, so more than an inch or two of damping could be beneficial. A ported cab would never be stuffed and compressed, as that would mess with the cab tuning and could kill the low frequency output. 

 

How damping affects response and impedance can be software modeled by changing the Qa value. 100 is a bare cab, 50 is lined, 10 is filled, 5 is filled and compressed. It's an approximate result, as different materials have different indexes of absorption. It also is only accurate to roughly 2 to 3 octaves above the box frequency, as are all the results with speaker modeling software.

 

The only reason to not use any damping is cost. 🙄

[Phil Starr]

Whilst you have the cab apart have a look and see if there is any internal bracing of the panels. If there are large panels with no stiffening then even a thin strip of wood across opposite panels (side to side, back to front, top to bottom) will stiffen the box and reduce panel resonances. Something like a length of broom handle or a wooden batten fixed properly with screws/glue will do it. 

[DGBass]

2 hours ago, Phil Starr said:

see if there is any internal bracing of the panels

@Phil Starr It's an old school empty rectangular mdf box with no internal bracing and I suppose that was fairly common and typical of mass produced boxes in the early to mid eighties. I recall seeing battens fitted across the large rear panels of some Peavey cabs of this era and guess this might have been to reduce vibration from the biggest panel in a cab. It's not a very deep cab so there isn't huge scope(space) for fitting internal bracing once the large Fane driver is fitted. It's something to think about though and as its stripped down to bare wood at the moment it would be the ideal time to do that. I'll do some actual sound testing ( at rehearshal ) as it is and see how it sounds. If there are any serious cab vibrations that need attention I can consider trying battens 🙂    

[Phil Starr]

21 hours ago, DGBass said:

@Phil Starr It's an old school empty rectangular mdf box with no internal bracing and I suppose that was fairly common and typical of mass produced boxes in the early to mid eighties. I recall seeing battens fitted across the large rear panels of some Peavey cabs of this era and guess this might have been to reduce vibration from the biggest panel in a cab. It's not a very deep cab so there isn't huge scope(space) for fitting internal bracing once the large Fane driver is fitted. It's something to think about though and as its stripped down to bare wood at the moment it would be the ideal time to do that. I'll do some actual sound testing ( at rehearshal ) as it is and see how it sounds. If there are any serious cab vibrations that need attention I can consider trying battens 🙂    

Front to back is always tricky but side to side helps. The Peavey style vane braces on the rear panel help. It's good to try a test signal from an online signal generator through your bass amp and feel for vibrations in the panels with your finger tips. that locates the main resonances and tells you where to put the braces for best effect.

Edited February 22, 2023 by Phil Starr

[Bill Fitzmaurice]

A good front to back method is to have a brace or two attached to the baffle at driver attachment screw locations. That way the driver is more securely connected to the box as well.

[DGBass]

One thing I've considered is making too many modifications to what is a very old speaker in a box cab. Maybe part of the charm of an early eighties Trace Elliot mdf box is pronounced mid boom or very peaky responses to certain frequencies, if you like a 'pre-shaped' cabinet. I will definetely add some damping with Dacron to the interior panels. Bracing and vanes are a considerarion however its in the back of my mind that by bracing sidewalls and/or baffles and rear panels I may make the box much stiffer and less prone to certain inherent cabinet resonances that you get with big empty boxes. Could these mods change the cabs orignal sound by effectively cancelling out those resonances in the original big box construction? I have no illusions of it ever being a perfect flat response cab but how much of a change in sound could result by making the cab super stiff? Could it be a dramatic or more minimal change?   

[Bill Fitzmaurice]

That's one way to look at it. The other is that they really didn't know what they were doing where cabinet design is concerned. Fender certainly didn't, not until the 1990s. Even today Fender isn't state of the art by any means. There are two reasons to brace. One is to silence panel vibrations that detract from the speaker sound. The other is to improve the efficiency of the speaker. Energy wasted causing panels to vibrate doesn't end up in the audience.

[Downunderwonder]

The other reason to brace is it lets you use a thinner panel for a significant overall saving in weight.

[DGBass]

16 hours ago, Bill Fitzmaurice said:

Energy wasted causing panels to vibrate doesn't end up in the audience.

I suppose thats also another way to look at it and I would agree there would be inefficiencies in a big box design. The question about whether the inherent sound of such a cab will change by extensive bracing and wadding is still open. Perhaps a little inefficiency is not a bad thing? Tube amps are a case in point. People still rave about their tube amp tone and buy tube amps despite them being highly inefficient. An 18mm MDF cab might not require as much bracing as a similar sized 12mm or 15mm cab but i'm sure it would make it more efficient. With this particular cab, anything added will also increase weight at this point as the basic cab structure is there already there. Testing with and without is likely the only way I'll find out and thats the plan at the moment🙂 

[Dan Dare]

On 21/02/2023 at 03:26, Bill Fitzmaurice said:

Wadding, more properly referred to as damping, serves two purposes. One is to reduce internal reflections in the cab, which cause peaks and valleys in response. How much will give the best result varies from cab to cab, with between one and two inches usually being good.

 

Bill, should you allow for the wadding when working out cab volume, or does it not affect it? Thanks.

[Bill Fitzmaurice]

1 hour ago, DGBass said:

The question about whether the inherent sound of such a cab will change by extensive bracing and wadding is still open.

That question was answered during the golden age of loudspeaker design, 1938-1973. The hi-fi guys knew it, the PA guys knew it. A few electric bass cab manufacturers knew it, those being manufacturers that also did hi-fi and PA, like JBL and EV. The mainstream musical instrument manufacturers didn't, operating in a vacuum as it were. JBL and EV never really cracked the musical instrument market, because they didn't make amps, and in those days amps and speakers were almost always sold as a set.

[DGBass]

4 minutes ago, Bill Fitzmaurice said:

That question was answered during the golden age of loudspeaker design, 1938-1973. The hi-fi guys knew it, the PA guys knew it. A few electric bass cab manufacturers knew it, those being manufacturers that also did hi-fi and PA, like JBL and EV. The mainstream musical instrument manufacturers didn't, operating in a vacuum as it were. JBL and EV never really cracked the musical instrument market, because they didn't make amps, and in those days amps and speakers were almost always sold as a set.

I just meant my particular MDF box cab🙂 And I guess you are saying the answer is Yes, it could make a difference to the sound although the yes word isn't specifically mentioned.

[Bill Fitzmaurice]

1 hour ago, Dan Dare said:

 

Bill, should you allow for the wadding when working out cab volume, or does it not affect it? Thanks.

it has no effect when used as lining, as it doesn't affect the low end where the cabinet defines response. When used as stuffing it does affect the low end, and should be taken into account during the design phase. If you want to make a sealed cab as small as possible stuffing is a must. However...

In 1995 a respected audio enthusiast named Ton Nousaine did some experiments with stuffing. Noting that it reduced a midbass response peak, similar in fashion that a larger box reduces a midbass response peak, he erroneously concluded that stuffing gave the same result as making the box larger. It does not. Stuffing and larger boxes both lower the speaker Qtc, which reduces a midbass peak. But a larger box also lowers the speaker frequency response and increases low frequency sensitivity. Stuffing does not. It actually reduces sensitivity. His error was due to not collecting adequate data to fully explain what he observed. Unfortunately he published his theory, which was widely accepted on the basis of his reputation, and the myth that stuffing a sealed cab gave the same result as making it larger was the result. That myth persists, to the extent that if you google 'stuffing loudspeaker' you'll find numerous references that continue to get it wrong, as they keep repeating the same flawed initial conclusion. The error would be obvious if they used loudspeaker modeling software to confirm what's going on, but they don't, because they don't know how.

Edited February 23, 2023 by Bill Fitzmaurice

[Bill Fitzmaurice]

29 minutes ago, DGBass said:

I just meant my particular MDF box cab🙂 And I guess you are saying the answer is Yes, it could make a difference to the sound although the yes word isn't specifically mentioned.

Yes, because what works in one speaker works in all speakers. The physics are the same, no matter what it's made of. MDF may not benefit from bracing as much as thinner, lighter materials, but it's not two inches of concrete either. OTOH bracing is what allows me to use cabs made of 3mm and 6mm plywood, with no ill effects either from a sonic standpoint or a physical standpoint, that being to my ancient arthritic spine. 😁