I am contemplating building a system for "light" PA. For smaller events, indoors, etc I observe that a well-designed vented system is not really being pushed anywhere near its limits.
But that system might also be working for 6 hours straight.
Assume a good-quality 12" PA woofer (with 2.5" VC) and 1" CD/horn, along with 18" subs (with 3" VC) in a 4 cu ft cabinet. Put them in sealed cabinets and play for hours on end.
Around what wattages will these motors start to heat up enough to be a problem?
Will an aluminum panel or fin radiator inset into the cabinet wall achieve any useful heat transfer?
But that system might also be working for 6 hours straight.
Assume a good-quality 12" PA woofer (with 2.5" VC) and 1" CD/horn, along with 18" subs (with 3" VC) in a 4 cu ft cabinet. Put them in sealed cabinets and play for hours on end.
Around what wattages will these motors start to heat up enough to be a problem?
Will an aluminum panel or fin radiator inset into the cabinet wall achieve any useful heat transfer?
Last edited:
PA loudspeakers are some of the most efficient available but even so they only convert about 2-3% of the input energy into sound, the rest is dissipated as heat. So at what power level does heat dissipation become a real threat to driver life in PA apps, it depends on the sustained energy level and duty cycle of the program material of course but my spit ball estimate would probably be at around 400 music watts into a 2" coil and 800w into a 4" coil.
I can see using a sealed cab for the mid-high box but it may be more trouble than it's worth for the sub, some of these modern drivers don't need very big ported enclosures anyway.
I can see using a sealed cab for the mid-high box but it may be more trouble than it's worth for the sub, some of these modern drivers don't need very big ported enclosures anyway.
EV published data on available long term sustained power for EVM12/15 in closed cabinets or open back, as in guitar combos, and the difference was a significant 20/25 %
I make Bass Guitar amplifiers, much prefer separate head and cabinet but when making 200-300W combos, I always make them vented.
Normal Bass "air movement" is enough to keep cabinets insides relatively close to room temperature.
Mind you, magnets get quite hot to the touch anyway.
I make Bass Guitar amplifiers, much prefer separate head and cabinet but when making 200-300W combos, I always make them vented.
Normal Bass "air movement" is enough to keep cabinets insides relatively close to room temperature.
Mind you, magnets get quite hot to the touch anyway.
Driver heat vent design makes a big difference, voice coil size is not a reliable indicator of thermal capacity.
If 3dB power compression is considered a problem (wasting an additional half your power on raising voice coil temperature and impedance), at about half their power rating (or less..) would be where it would start.
Yes, enough that Cerwin Vega now uses them on their small compression chamber 18" Folded Horn Earthquake bins.
Way back many years ago, I wrote an article about the two primary modes of loudspeaker failure: mechanical and thermal. Thermal failure is much less common than mechanical, simply because the driver is rarely working at its maximum output. However, this could be a problem if the amplifier is clipping - especially for mid and upper frequency drivers. We don't want to see flat tops on waveforms.
You should be much more concerned about over-excursion due to under-sized drivers or an inadequate number of drivers.
I've done sealed PA. Basicly subs don't work sealed, to start with they give you awsome output density (in large numbers and with huge amps - I was running 16*15" drivers for small PA). After 30mins or so you notice the bass getting 'flabby' and less loud so you turn it up... process continues until speakers get so hot they suffer adhesive failure. Also the air inside the boxes got so hot you could feel the heat through the metal handles. My mids (>500Hz) are sealed though and I have no issues there, much lower power levels though.
FWIW, I've gone back to sealed subwoofers, but with a modification: there's a pipe at the back of the cabinet which allows direct air exchange with the motor's pole vent.
I'm using a pair of Faital 18XL1800 and a bridged Powersoft T602. I'm certain I could get more SPL with larger cabinets, but these do the job for now. Plus, <30Hz extension in small venues can be a lot of fun!
Chris
I'm using a pair of Faital 18XL1800 and a bridged Powersoft T602. I'm certain I could get more SPL with larger cabinets, but these do the job for now. Plus, <30Hz extension in small venues can be a lot of fun!
Chris
Hi Chris do you have any details of your back of cab pipe/pics? how are you sealing against the pole vent?
Some sort of compressible foam between the 4” SCH80 (I’m guessing) and the backplate? Is there a noticeable whistle? I thought about this for the old lab horns, but haven’t tried it yet. The clearance is so small on those it would only need a wooden donut, not a pipe.
The cabs are fairly shallow, and the woofers are pretty deep.
I stacked 3x 18mm MDF donuts stacked and PVA'd together. Those were involved in the initial construction of the cabs*, so I knew they'd fit well.
* I'm not much of a woodworker, so I put the back panel down flat, placed the tunnel on top of that, and then the driver (mounted to the baffle) on top. Then I glued the sides on and threw a load of clamps at it.
There's a 60mm dia hole in the back of the cabinet (lined up with the woofer), and the tunnel has gasket strips at each end. Place the tunnel over the cabinet vent, put the driver on top, and tighten everything down. The driver's magnet compresses the gaskets and there's a good seal.
There is a noticeable whistle during large excursions with sine testing, but musical program material covers it well. NB - the whistle is at the back of the cabinets, so even less of a problem for the audience.
NB - I'd only recommend sealed subwoofers if the following is true:
- You have some very good drivers
- You have some very powerful amplifiers.
Sealed boxes are wasteful for PA use, but if you have the equipment to do it, they can be a lot of fun. Here's an example: https://www.diyaudio.com/community/threads/fun-in-a-theatre-3x-faital-18xl1800.386725/
Finally, it's worth noting that the Faital 18XL1800 lends itself well to this method of venting the rear of the driver, since there's only a pole piece vent on the back of the motor (there are other vents, and those exchange air internally for my build). Other drivers, like the B&C 18SW115, have vents on the backplate as well as the pole vent. Those would require further consideration.
Chris