She said size matters, but what about shape? - Polk Sub Enclosure

First - while I love my Supra board brethren, you guys have some real audio nuts and true experts over here.

I need to build a sub enclosure for a Polk MM1240 to be mounted in the typical driver foot well location of a Supra Saltare. It's gotta be a sealed box - I don't have room for a properly ported box down there, plus I prefer tight bass over loud. If it matters, I've got max rated wattage available.

Polk specs call for 0.88 cu. ft. (1520 cu. in.) and list overall external dimensions of 13.5"x18"x10" (using 3/4" material) - that won't fit. The best volume match I can come up with that fits is 14"x15"x10.5". Will this more cubic geometry have any negative effects on performance?

Also, do you guys generally put some type of spacers or feet on the bottom to allow things to dry out underneath?

Finally, a question on crossover points. The sub is rated at 22-200Hz, the mating cabins are 6.5" MM651's rated at 40-25k, and the towers are 6"x9" MM691's rated at 30-25k (and have a separate built in crossover). What would be a good starting point for crossover points for the sub, cabins, & towers?

Thanks, I'll hang up and listen...

I'm sure there will be several of the audio guru's that will answer your question, but my .02 fwiw would be to build it the size you have listed. it's a tad smaller, but i'm betting that I wouldn't notice any decrease in sound performance and if it's the room you have and the sub available, it kind of is what it is... stuff box with polyfil.
several on here have used king starboard to build boxes out of. I used mdf and coated the entire box in 3 or 4 coats of the oreilly's spray liner in a can stuff to help offer some waterproofing. others have had theirs coated in 'glass. it's whatever you are comfortable with. I spaced mine 2" off the floor with acrylic spacers. basically bought a small remnant of acrylic and cut it into spacers on either end/middle to get it off the floor. I then built a false front out of same acrylic and carpeted that all the way to the bottom. keeps box off the floor and ensures damp carpet is not in direct contact with the mdf box. if you plan on keeping the sub in the boat in that box for 10 years, might want to invest in the 'glass from a body shop. I'll probably change mine out in another year or so and would bet that this box will hold up every bit as good as a 'glassed box considering the time installed and cost 1/5 the price.

phil,dave or brian will have a much better take on crossover points and there are several threads and some videos that talk about setting them on the forums. good watching. if it were my boat, i'd probably start at 120 for the sub's high and 200 for all the other speakers low's and go from there with the tuning tips.

good luck and looking at your avatar, you did a real nice job on the resto.. looks like it's a real chick magnet :)

Syb - I'm no expert in Polk products but generally speaking as you reduce the size of a sealed enclosure you are creating more of an SPL alignment (handles more power-- and is more spiked). And if you increase the enclosure size, you typically see a woofer have more bass extension but yields less power handling. A couple of thoughts for you - .88 seems like a very small box to begin with for a 12" woofer. You sure its not in the 1.1 or 1.2cu range? And on the other hand, the small deviation shouldn't be that big of deal. I'd also suggest putting some poly fill into the enclosure as this will make the woofer feel like its in a slightly bigger enclosure.

re: speakers.

tower amp = HP mode
cabin amp = HP mode
sub amp = LP mode.

Sometimes guys switch to All Pass mode. This is the number one thing I check when tuning boats. If your cabin speakers for example see AP mode, the freq range is full bandwidth. This is hard for the speaker to reproduce. Swapping to HP knocks out the unwanted freq's and allows the speakers to be optimized in a bandwidth they are most efficient in. Same goes for Subs. make sure your amp is in LP mode.

starting points for you --- I stress starting points:

cabins 110Hz
Towers 90-100Hz
Sub 80Hz

We can probably really blast off to the moon with tech speak on all this but the right way to do this is look an RTA and see where the -3db down point is on each zone and make adjustments from there. using the targets I gave you are not going to help much because the knobs on any given amplifier are not clearly marked and not very linear in their adjustment. This is where the ears play a huge role.

I'd say, start here and come back and we'll walk you thru more.

Cheers

-Brian
Exile Audio

That is small for 12-inch woofers as a general rule but Polk actually recommends .88 cu.ft. internal displacement. If that matches the Thiele Small parameters and it produces a system Qtc of between .7 and .9 you should have a smooth, non-spiked SQ alignment.

David
Earmark Marine

Thanks guys.

Yeah, I was a little surprised to see the 0.88 cu.ft. in Polk's specs but figured the mfg would know best. I'll definitely add a little poly fill to the enclosure (as I already did for the towers). I knew to set the cabin amp to HP and sub to LP, but not sure about the towers since they have their own external crossover (xover point unknown?) for the 6x9 woofer and rated down to 30 Hz. You're right, the amp crossover knobs aren't clearly marked, but I'll give it a shot to start with your recommended settings. What's an RTA?

I'm pre-wiring and pre-building as much of the stereo as I can while the upholstery is being completed & just wanted to confirm the sub enclosure details. Once the vinyl's complete, I'll finish the sound install & see what I can do with adjusting for the best sound. Thanks again & in the words of Arnold - "I'll be back".

Mike,
RTA: Real Time Analyzer. A graphic display of the audio spectrum. Useful in setting up a complex equalizer or finding problems such as phasing issues. We have two versions, an Audio Control and a Neptune but it's not something that is necessary or even practical for setting crossover frequencies. The external crossover on the 6x9s is a passive crossover between the internal speakers and has no correlation to a typical active crossover in this application. You will still need to HP the amplifier on any tower speaker. Understand that the 30 Hz rating is unqualified so the speaker could be 20 dB down at 30 Hz so in other words that is a meaningless spec. A 6x9 is an infinite baffle speaker designed for use in an expansive trunk or door panel. When you place such as speaker in a comparably tiny pod the resonance shoots through the ceiling. It behaves much like a guitar string that is stretched as tight as it will go. The tone gets higher. So in this application the speaker will make little to no contribution below 100 Hz.

David
Earmark Marine

Quote:

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Originally Posted by sybrmike

First - while I love my Supra board brethren, you guys have some real audio nuts and true experts over here.

I need to build a sub enclosure for a Polk MM1240 to be mounted in the typical driver foot well location of a Supra Saltare. It's gotta be a sealed box - I don't have room for a properly ported box down there, plus I prefer tight bass over loud. If it matters, I've got max rated wattage available.

Polk specs call for 0.88 cu. ft. (1520 cu. in.) and list overall external dimensions of 13.5"x18"x10" (using 3/4" material) - that won't fit. The best volume match I can come up with that fits is 14"x15"x10.5". Will this more cubic geometry have any negative effects on performance?

Also, do you guys generally put some type of spacers or feet on the bottom to allow things to dry out underneath?

Finally, a question on crossover points. The sub is rated at 22-200Hz, the mating cabins are 6.5" MM651's rated at 40-25k, and the towers are 6"x9" MM691's rated at 30-25k (and have a separate built in crossover). What would be a good starting point for crossover points for the sub, cabins, & towers?

Thanks, I'll hang up and listen...

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I'm not a huge expert by no means, but have owned Polk subs and built boxes for them. Your best resource is a phone call to Polk. GREAT customer service!!! You can tell them how much room you have, your box size, etc., and they can give you the optimum operatiing frequency, etc. They gave me more info than I could digest, as i pretty much just wanted to know if it would work. Lol. Look them up on the web and call them...

Mike - an RTA will also show you visually the effects of an EQ. whats boosted etc. You can download one for an iPhone or droid for a few bucks. many have test tones (single freq) that will allow you to really take control of whats going on. It's going to depend how much you want to make yourself an expert on all of this. :>

You don't have to have one. It's just another tool in your tool belt so to speak.

-Brian
Exile Audio