Why don't you stuff guitar cabs?

[Mr. Foxen]

Stuffing sealed bass cabs is considered a total win for a bass cab's tone. Why not for a guitar cab? I'd figure the 'tone' is in the 'flaws', but I don't see why it should be any more so than for a sealed bass cab.

[Phil Starr]

Stuffing is quite a complex matter as there are at least three ways in which it affects the internal environment of the cab.

Heavy felts and rubbers mass load the panels and reduce resonances and the bits behind the cone reduce reflection of sound out through the thin paper of the cone itself especially if it is thick and its surface is uneven. (The felt that is, not the cone)

Fibrous stuffing in the air space behind the speaker helps to break up standing wave resonances especially if placed at the nodal points of such resonances.

Finally filling the air space slows the transmission of sound which increases the effective volume of the cab by about 10%.

If you get hold of the 'loudspeaker design cookbook' by Vance Dickason there is some extensive information on this.

Having said this opinion is really quite divided as to the advantages of 'stuffing' and what the best materials are and how much to use.

I guess the difference between bass and guitar usage is that bassist like their speakers to be more accurate than guitarists who depend upon reaonances to give a characteristic cab sound.

Edited June 13, 2010 by Phil Starr

[Mr. Foxen]

[quote name='Phil Starr' post='866108' date='Jun 13 2010, 04:18 PM']Finally filling the air space slows the transmission of sound which increases the effective volume of the cab by about 10%.[/quote]

I know this one has been disproven, BFM has pointed this out a few times, apparently it lowers the box Q but not the other benefits of increased size, although I still don't know what that means, aside from it cleans up midrange, rather than adding bottom like a larger box would.

[Phil Starr]

[quote name='Mr. Foxen' post='866195' date='Jun 13 2010, 05:35 PM']I know this one has been disproven, BFM has pointed this out a few times, apparently it lowers the box Q but not the other benefits of increased size, although I still don't know what that means, aside from it cleans up midrange, rather than adding bottom like a larger box would.[/quote]
If you get a look at the Loudspeaker Design Cookbook there are a collection of Graphs which represent data from experiments with different levels of stuffing and differing materials. They show both the lowering of Q directly from the impedance curves and the changes in frequency response created by stuffing. At low frequencies they tie in fairly with the idea of a 10%'ish increase in volume. I don't have the data in front of me but the concept of the transmission of sound being slower in dense stuffing comes from work on transmission line cabs and the phase inversions at the exit ports. Certainly the speed of sound is different in different materials and this is something you can look up in books of constants.

I'm not keen on stuffing high power cabs for an entirely different reason. All of the stuffings are also good heat insulators. Speakers only convert between 2 and 5% of the energy into sound the rest is dissipated as heat in the cab. A 500W speaker is radiating the same heat as 4or 5 large incandescent lights or a 500W electric bar fire. All this inside a tiny enclosed space. The back of a speaker magnet at full power will be hot enough to burn without any insulation. All this heat changes the resistance of the voice coil dramatically and the tolerances of every part of the system and the air inside the cab will have very different properties too. None of the thiele/Small figures will continue to hold true and the sound of the cab does demonstrably change if you push it hard over a long evening. Adding the weight of stuffing needed to change the characteristics of a cab will make these problems a lot worse and will also increase the chance of failure of the speaker quite dramatically. I'd need to be able to hear a great deal of sonic benefit to take this on.

I suppose I just wanted to alert you to the controversies about stuffing which theoreticians will pursue never mind the more practical people you will come across here. You won't get definitive answers to this one.

[Mr. Foxen]

[quote name='Phil Starr' post='867796' date='Jun 15 2010, 10:03 AM']A 500W speaker is radiating the same heat as 4or 5 large incandescent lights or a 500W electric bar fire. All this inside a tiny enclosed space.[/quote]

This would only be true is you were constantly feeding a speaker 500w, which with music you aren't.

[Phil Starr]

Of course that is true though if you are running high levels of distortion then you can get to within a significant %age of these figures.

Nevertheless heat remains one of the main reasons for failure of drive units and magnets do regularly reach temps of well over 100C in normal use. I know from the days when I manufactured speakers that their reliability and longevity was better without stuffing. Modern coils are much better than older ones at resisting thermal breakdown but why create a problem?

In addition some manufacturers do publish figures showing the loss of power due to rising temperatures when operated at continuous high levels.