• TalkBass has been independent since 1998. Add your voice.
    Create a free account to reply to discussions, view embedded media, and browse with fewer display ads.
    Join free Log in
    Want zero display ads or expanded classifieds tools? Compare plans.

Double Bass Sealed cabinet theory question (Polytone content)

Chris Fitzgerald

Chris Fitzgerald

Student of Life
Staff member
Administrator
Gold Supporting Member
Oct 19, 2000
27,587
38,531
Louisville, KY
www.chrisfitzgeraldmusic.com
#1
(Apologies in advance for the novella)

I've had an old Polytone minibrute 12 amp - black velvet covering, three knobs, one switch - for forever and a day. It's been serving as my "keep at the office and let students borrow for school performances" amp for a while now, and it does fine for that. In recent years, the speaker has been making a sort of "chuffing/farting" sound at higher volumes, so I went ahead and ordered a replacement, an Eminence Beta 12A.

Upon trying to install it, I loosened the metal speaker grille (which looks like this one), only to discover that the four attaching bits that hold the speaker grille on were also the only thing holding the speaker itself in place (there were no screws attaching the speaker ring to the cabinet). The original speaker looked ancient but fine. I put the replacement in, put everything back the way it was, and turned it on and tried a DB through it.

At lower volumes, it sounded really pretty good, and better than the old speaker for sure. Once I turned it up, it started to get that classic Polytone boxy/nasal sound again, and the chuffing/farting sound returned - only this time, I was sure it wasn't the speaker. So the possibilities seem to be:

1) Because there are no screws, the speaker frame is rattling against the cabinet front; just find some screws and install them, and it'll be fine.

2) Because the cabinet is a sealed design, higher volume/pressure levels inside the cabinet are causing something to rattle. Find a way to port the extra air out, and this sound will stop.

3) Dude, it's a Polytone. Get over it.


So, for those in the know, what's the most likely answer? I plan to dig up some screws and figure out if that's the problem. But what if it isn't? Has anybody ever experienced trying to create an escape valve for the extra air that builds up inside a sealed cab? What would be the likely outcome of doing this? I've even thought of using some of the carboard spacers that came with the original speaker frame *under* the frame and then screwing it down, which would allow air to escape around the edge of the speaker. Is this likely to sound like poo?

In the end, of course, #3 always applies, but in the meantime why not have some fun?
 
#2
I think you either have to go all the way with a real port, or seal it tight. The problem is that there's actually a lot of pressure inside the speaker. If you let it out through a small hole, it will create turbulence which is what gives rise to the noises that you hear. To overcome that effect, the hole has to be big enough to reduce that speed down below roughly 1/10 the speed of sound, according to the rule of thumb from DIY speaker folklore. Then it becomes a regular port.

Root cause could be an air leak, or something vibrating in the innards of the amp. Chances are the problem is a leak between the speaker and the box, or around the mounting screw holes. You could revamp the box by gluing in some little blocks of plywood behind the baffle and then drilling proper mounting holes. You could beef up the gasket with some squishy weatherstripping.

For general vibrations, it sometimes helps to try different things -- laying the amp on its side, upside down, having somebody press on parts of it while you play. This can become an obsession, and consume a lot of your time.
 
#5
fdeck said:
Root cause could be an air leak, or something vibrating in the innards of the amp. Chances are the problem is a leak between the speaker and the box, or around the mounting screw holes. You could revamp the box by gluing in some little blocks of plywood behind the baffle and then drilling proper mounting holes.
Click to expand...

This sounds interesting but I'm not quite sure I follow. What is the baffle of which you speak? The little blocks of plywood idea sounds doable.
 
#6
I loved the sound of my old Polytone until it actually caught fire. Right there, just actually burst into flame. Couldn't fix the amp at that point then, to much burnt stuff, to expensive. That thing was always making racket with the bass when it got up to loud.

It was not really designed to play like we do now. The low end and volume expectations in 2010 are very different than in 1985. Eventually those things rattle themselves to death until they burn up.
 
#9
fdeck said:
More arcane terminology. The "baffle" is the front board of the speaker box that the driver mounts onto.
Click to expand...

That's what I thought, but wasn't sure. So you're suggesting putting small blocks around the circle in the baffle to raise the metal frame around the speaker cone, and letting the air escape around the rest of the circle? I was considering this, but wondered if there was a well known reason it wouldn't work.
 
#10
No, no. fdeck is suggesting that you put blocks on the inside surface of the baffle so that you can screw the driver down tight. You definitely do not want to let the air escape. That would ruin the "infinite baffle" design. The problem seems to be that you have air escaping. The idea is to stop it. I would take fingers' suggestion and put silicone sealer behind the driver where it meets the baffle or do as fdeck says and use weather stripping behind it. Using the silicone would make the driver more difficult to replace in the future, but it can be cut off relatively easily.
 
#11
Chris Fitzgerald said:
That's what I thought, but wasn't sure. So you're suggesting putting small blocks around the circle in the baffle to raise the metal frame around the speaker cone, and letting the air escape around the rest of the circle? I was considering this, but wondered if there was a well known reason it wouldn't work.
Click to expand...
I think he meant the opposite: that you glue plywood to the inside of the baffle at the locations where the speaker's mounting screws will penetrate the baffle. Presumably to give you more material for those mounting screws to bite into, thereby creating a better seal between the speaker and the baffle.
I would recommend using 10-32 T-nuts and machine screws, rather than wood screws. As well as split lock-washers under the screw heads and, if needed, flat washers under the lock-washers.
Watch that screwdriver. It's easy to slip and puncture the speaker's cone.
 
#12
Excellent idea for the T-nuts. If you're using them, you might not need the plywood blocks at all. It depends on the thickness of the baffle.
 
#13
Indeed, I meant blocks on the inside, to give the screws or tee-nuts something to grab onto, just in case the old plywood or particle board is a bit sketchy.

I had an old Polytone that a friend brought for me to fix. It was unrepairable. But he had used it in a touring band for years. The old foam grille was long gone, and he had replaced it with a cookie cooling rack that he got from St. Vinnies for 25 cents and spray painted black.
 

Latest posts

Top Bottom
Back