porting a compression driver enclosure

[Damo s]

I'm building a new pair of tops very soon. The compression driver will be contained in its own sealed enclosure. I'm wondering if its worth adding a port at the back to aid cooling especially since they are likely to be used for long periods.

I was thinking of perhaps a single 30mm port so it can breathe a bit.

good idea or waste of time?

 

[turk 182]

are you porting the driver or the box that it's contained in?

 

[Damo s]

Porting the box its contained in

 

[turk 182]

is the back of the box compartment needed for structural integrity? other than that the back would only be an esthetic piece on a horn assembly. if cooling is a concern you could use an expanded metal mesh or perforated plate.


back in me "touring days" horns where mounted is what was called a sled or sleeve with removable covers, but your speaking of a horn in an enclosure containing other drivers correct?

 

[Damo s]

The horn enclosure is separated off from the mid bass driver but all part of the same speaker as such. I could just cut out a decent size slot or 2 and put mesh over them but it needs some amount of panel there I would say for integrity of the speaker as hole. Ideally it needs somewhere for the connector plate to be mounted in that compartment since I'll be bi-amping these speakers.

I had to separate the enclosures to keep the size of the mid bass part to spec.

 

[turk 182]

only potential side effect could be cavity resonance.


for long term thermal stability wouldn't heat sinking the driver be more advantageous?

 

[Damo s]

I don't know since I'm new to this stuff.

 

[conanski]

A compression driver and horn don't need an enclosure at all so you can do whatever you like with the back of it, an '80's PA would often have the hi freq horns in an open back box just to make them easier to stack on top of everything else since a naked horn+driver is an awkward shape. Technically the open back would help cool the driver but these things rarely see more than 10-20w continuous so there isn't a lot of heat to dissipate.

 

[Damo s]

I would have thought it might need more than 10 or 20 watts in order to keep up with 4k of sub.

 

[turk 182]

well if this is for PA use...scrap the heatsink comment i made, the additional weight would add to transport load.
it is a good idea to ensure adequate cooling so yeah venting the compartment is likely to be advantageous but as for a cavity resonance developing i would think that an A/B comparison of closed versus open(or vented) would reveal any audible adverse effects.that would be relatively easy to try with two like cabinets,no?

 

[turk 182]

sidebar comment with respect to open backed horn sleeves...it allows for easy driver or diaphragm replacement in an on site scenario.

 

[Damo s]

well if this is for PA use...scrap the heatsink comment i made, the additional weight would add to transport load.
it is a good idea to ensure adequate cooling so yeah venting the compartment is likely to be advantageous but as for a cavity resonance developing i would think that an A/B comparison of closed versus open(or vented) would reveal any audible adverse effects.that would be relatively easy to try with two like cabinets,no?

Good plan.

The speaker is a deep trapezoid shape so I'm hoping there wont be very much resonance anyway. Reflections from any facing front to rear panel area (372mm apart) the resonant frequency is somewhere around 930hz so well below the xover point (about 2k), hopefully. I guess I'll find out once built and measured

 

[Damo s]

I'm building these:

Speakerplans.com

But with the height adjusted for one driver so its enclosure is 25 litre tuned to 60hz, and a rear firing port instead of the front firing pair. A panel between the bass and CD driver to separate the enclosures. The low extension doenst really matter since the signal to them will be >110hz. The port is more for some ventilation for cooling purposes. They may be OK as a full range but not really required or designed for that use. I found the port air velocity starts going out of control below 100hz with the rated 600rms signal, but again not really an issue as the subs will be doing the work there.

 

[turk 182]

it wouldn't be a resonance supplied by the horn driver that would be the source, it would be from the fact that low frequencies (that tend to be omnidirectional) would serve to excite a cavity resonance but for the size/volume of your horn compartment i'd hazard it would likely be be higher in frequency and at"war" volume not likely audible.

 

[Damo s]

OK thanks

 

[ndcraig]

it wouldn't be a resonance supplied by the horn driver that would be the source, it would be from the fact that low frequencies (that tend to be omnidirectional) would serve to excite a cavity resonance but for the size/volume of your horn compartment i'd hazard it would likely be be higher in frequency and at"war" volume not likely audible.

Years ago I put an omni measurement mic in that cavity to see what was actually happening in there as we were using dozens of open-backed HF cabinets and the on-stage crew were concerned (for feedback reasons).


There was no prominent peak in the LF band and the several that were there were excited more by the backline instruments cabs on stage instead of the PA itself.

The cavity is occupied by a horn and large driver essentially breaking it up into 4 wedge-shaped regions that were too small to "grab" any of the LF from the PA. The mids were highly directional and you couldn't even tell they were on from behind them when outdoors so none of them were getting in there.
It was only when drums, keys and guitar fired up that the mic picked up anything significant that would peak -and they were far drowned out by the backline itself and the resonances from their open-backed cabinets bouncing around backstage.

We stapled a "placebo" pad of carpet underlay to the inside top of the cavity just to show the stage crew that we took care of the "problem"

 

[conanski]

I would have thought it might need more than 10 or 20 watts in order to keep up with 4k of sub.

The CD will only get 1/10th of the power the mid driver sees or 60w in this case and that is a peak number, the continuous(rms) level would be a lot lower again. But that is enough power to produce 125-130dB of output which would balance nicely with 140dB of sub.

A panel between the bass and CD driver to separate the enclosures

That is unnecessary in this type of cab.

 

[chris661]

Completely necessary, if the midbass driver wants a particular cabinet volume and opening up the HF section would result in too much.


Chris

 

[Damo s]

Yeah that's the problem I had. The relatively small volume size (25 litres) along with needing somewhere to have the required size port to keep air velocity under control, where the end of the port was a diameter distance away from any internal wall / surface. It took me quite a while on win ISD to come up with the simulated plan and that was one of things i had to do.

Making the box bigger just dropped the output around the xover region. 25 litres ended up being a good compromise between response, air velocity, phase, suitable port size / location etc.

 

[conanski]

So why not just make it a sealed enclosure?