speaker recomandation

[catalin gramada]

Hi. There is a question bothered me for a while: Regards speakers vented from dust cap and also from the back .How can be recomanded a double vented speaker for a sealed cab ? what air spring load it is a box with some holes in it ?Never heard about covering the ventilation holes of a speaker to keep the cab sealed. Can this be done without damage?
I never buit a sealed box. I have two bass speakers from a big company. From some reasons I want to mount into a sealed to roll-off some fluby low end. The speakers are recomanded by factory to be used equaly in vented or sealed box at specifical volume. but leaks air from its vents holes and fabric dust caps so the box will not be sealed anymore. A closed cabinet maybe, sealed no way

[J M Fahey]

Donīt overthink it, that hole is microscopic and does not materially affect the "closedness" of the box, whether "infinite/sealed" or "tuned/bass reflex".

Even without that, you already have losses/leaks, no cabinet is absolutely tight; I mean if compared to, say, an air compressor tank.
You are not worried about keeping a "static" pressure inside (like in the compressor tank) but what happens under dynamic conditions (reproducing sound).

To be more precise: think the speaker moving mass (cone/coil/spider/dustcap) and springiness/elasticity: air compressed inside the cabinet + spider elasticity as a big tuned LC system , L being the moving mass and C the elasticity.

When you have the speaker in free air, moving mass (inductance) is a given value, elasticity (capacitance) is only suspension (+ edge) and speaker resonates at, say, 30 Hz.

Now you put it in a cabinet, mass is the same as before, elasticity is smaller because the "air spring" is stiffer, "capacitance" is smaller, resonant frequency rises.

It will not be a perfect lossless resonant system by any means, losses are important, so it wonīt have infinite Q but a certain "not-that-high" one, which could be modeled as a resistor in parallel with the resonant LC system.

Now you add a tiny hole in that "closed" cabinet.

It will be extremely lossy, because air through it will meet huge resistance, it will not behave as a little resonant circuit on its own but mainly as a resistor, lossy by definition.

How will it affect the closed speaker cabinet?

It will definitely not change main resonant frequency, just lower the system resonant Q, so speaker will not have, say, 32 ohms at resonant frequency but, say, 29 ohms, no big deal.

A Lab measurable but inaudible effect, and in any case, we add damping material *on purpose* inside cabinets to attenuate resonant peaks anyway, so .....

[Chuck H]

What he said^^^^^^^^^^^^^^

Implied above, but to explain it simply... The speakers movement is so fast that not enough air can escape small leaks during pressure and vacuum intervals (speaker cone moving back and forth) for it matter a whole lot. The speaker would have to be a lot slower for the pressures inside the cabinet to equalize. Slow enough that it wouldn't even be an audible frequency.

[bob p]

There are exceptions to the rules. I've got a Fender Bassman 10 that needs to read this thread, because he refuses to comply with the Thiele-Small rules.

That amp has 4x10 in a "vented" cabinet. It's actually a sealed cabinet with a few holes that are only 1" or so in diameter drilled into the baffle. Nothing noticeable happens when I play guitar, but when I play bass the cabinet will pressurize on low notes, and the cab will create a pumping sound as the cabinet depressurizes/equilibrates.

This is actually a well known problem with this amp. The cure is to either plug the holes and make the cabinet sealed, or to router-out the back panel to convert it to a traditional open backed Fender cab.

[bob p]

Leo's seen it too (post 43)

http://music-electronics-forum.com/t42422-2/#post428002

[Chuck H]

And that's a few pretty damn big holes compared to whatever leaks through a dust cap.

Maybe another solution could be to increase the diameter of the holes rather than cut into the back of the cab. Another half inch might be all that's needed. Hardly noticeable visually.

[bob p]

You're right. I was thinking about editing my post to say that there's a big difference between several 1" holes and "leaking" through a speaker. I don't doubt that increasing the diameter of the holes, or the number of the holes, might work. But there's not a lot of open real estate on the 4x10 baffle. I think that adding holes might weaken it structurally. I've thought about plugging them and going to a sealed 4x10 like a Marshall, or just opening up the back with a big square hole like a super reverb. Gimme a minute and I'll snap a pic...

[J M Fahey]

Yes, a well known effect of cabinet ports is "whistling", not an audiophool superstition but well studied even by Thiele-Small themselves.

Besides the main equations, thereīs some auxiliary ones to calculate "peak air speed through the vents" ; that and a certain critical combination of pipe length and diameter turns them into audible "tube organs" .

So if a large smooth wall tube "whistles", tiny rough surface ones must be much worse.

[bob p]

the holes are an odd size. 1.104" or so with a caliper.

I don't think Leo was reading up on Thiele-Small papers. My guess is that they just tried drilling some holes to see what happened with trial and error. It's not as if the CTS speakers have a Q that's in the range of good performance for a TS array.

[Chuck H]

If you decide to cap them, do it from the back so you can undo it without visual consequences if you don't like it. Replacing that baffle could be a PITA. It looks like one of those Fender cabs with the glued in MDF baffle. So, permanent-ish. Can't be removed and must be cut out and a new one retrofitted.

[Mick Bailey]

Run your cabinet/speaker figures through WinISD (no ports) then add a number of ports, each one corresponding to the annular gap around your voice coil(s), or whatever the effective hole size is through the speaker. See if this makes a difference to the response curve. I bet you can't see much, if any difference.

[bob p]

I don't do Windows.

[bob p]

If you decide to cap them, do it from the back so you can undo it without visual consequences if you don't like it. Replacing that baffle could be a PITA. It looks like one of those Fender cabs with the glued in MDF baffle. So, permanent-ish. Can't be removed and must be cut out and a new one retrofitted.

You're right about the baffle. PITA for sure.

[Justin Thomas]

I don't know where that came from and if it's a pop culture reference or just plain derision of Mr. Gates's products, but I personally know it from my favorite childhood cartoon, Voltron.

Justin

[Chuck H]

I don't know where that came from and if it's a pop culture reference or just plain derision of Mr. Gates's products, but I personally know it from my favorite childhood cartoon, Voltron.

Justin

I had to look it up. How was the operator supposed to look out?!?