I asked this on an audio forum and got super technical responses... which I like, but the learning curve is killing me. Maybe you folks can dumb it down for me.
I have done a lot of sub enclosures in my life, but they have all been sealed with the exception of one ported box. My recipe is simple. The driver manufacturer says "0.9-1.25 cu ft sealed" so I build a 1.25 cu ft box and install a massive driver and give it 1.21 Jiggawats.
Now I want to do the opposite. I want to use a modest power level and an efficient smaller driver in an enclosure that makes the most of it. This is for my motorcycle. The fairing has two 6.5" Polk dB coax and they sound wonderful, but they really are lackluster below about 120hz. The hard bags on the flank seem like a great place to tuck a small-ish enclosure.
Head unit is designed as a receiver with a pretty stout amp. It's a Sony marine unit that puts out a (claimed) 40wrms x 4, and the performance I'm getting from the 6.5" suggest that is pretty accurate. The rear channel allows itself to be configured as a low-pass allowing you to hook directly to a sub. The owner's manual is not very helpful, but it appears that you can only hook up one sub to one of the rear speaker outputs, and it doesn't say if bridging is possible, nor does it say why it only says one sub. Like, there is no little sidebar that says "do not hook up two subs because explosion." For the purposes of this discussion, let's assume we are getting a total of 40wrms and I'm allowed to run one sub at 4ohms. I don't need to boom, I just want to fill in the 40-120hz range.
To me, this sounds like a job for a 4th order bandpass. So I downloaded a sub calculator and put in some T/S numbers for some 6.5" sub drivers and got really confusing results. One driver said I needed 0.5 cu ft in front and 1.2 in the back with a 60" long 4" diameter port. Then the next one said a 138" long by 6" diameter port... but only 0.09 cu ft in front and only 0.65 in back. I'm obviously not doing something right.
The first driver I'm looking at is a Dayton DCS165-4. The head unit is a Sony MEX-M100.
Can someone help me out with some suggestions? Maybe a book on the engineering of audio reproduction? Maybe a new brain so I can process all this?
I've only built bass guitar cabs but when I did I contacted the speaker maker directly and got some recommendations. Eminence is probably far better than Dayton though.
I will say though 87 db efficiency isn't exactly efficient, so it's gonna be a black hole of wattage. 40w may not be enough at all
First thoughts before I’ve had coffee and a chance to wake up:
you won’t get much because the sub isn’t playing into a room that it can (try to) pressurize, but into free air. But that being said, go with a dual voice-coil driver, them you can rum both rear channels into it. Yeah, you’ll need a cross-over and you’ll get stereo signals into it, but you’ll also get 80 watts.
Maybe you want a passive radiator design to get a little more out of it?
How much room do you have to play with? Have you thought about a tactile transducer???? AKA, butt shaker!
Subwoofer on a motorcycle? 
I guess if I was that concerned about high fidelity when riding I'd use earphones.
I have an Alpine 445 amp on my bike and I am having trouble finding speakers to handle it. I need little speakers for my fairing, 4". They can't handle the bass and will distort/shut down. I set the head unit DSP to "Vocal" and I can hear it OK. Anyway, speaker selection is probably more important, but if you have room for a sub, go for it and I'd like to see how it works out.
Slightly off topic, I bought a Klipsch sub with a blown amp for $5 at a thrift store. Put a 300W plate amp in it, and now I can shake the house.
stuart in mn said:
Subwoofer on a motorcycle? I guess if I was that concerned about high fidelity when riding I'd use earphones.
Not worried about hi-fi. I just want to fill in the lower frequencies that the 6.5s miss.
And headphones are illegal while driving. Strange that I'm allowed to wear a helmet with 1" of foam shoved in my ears, but earbuds? Not allowed.
Curtis said:
stuart in mn said:
Subwoofer on a motorcycle? I guess if I was that concerned about high fidelity when riding I'd use earphones.
Not worried about hi-fi. I just want to fill in the lower frequencies that the 6.5s miss.
And headphones are illegal while driving. Strange that I'm allowed to wear a helmet with 1" of foam shoved in my ears, but earbuds? Not allowed.
also strange that motorcyclists like to force their music loud as berkeley on everyone nearby
Antihero said:
I've only built bass guitar cabs but when I did I contacted the speaker maker directly and got some recommendations. Eminence is probably far better than Dayton though.
I will say though 87 db efficiency isn't exactly efficient, so it's gonna be a black hole of wattage. 40w may not be enough at all
You're right about the efficiency, but so far I haven't found many 6.5" offerings that don't suck. There are a few 6.5 subs around with 91+ dB efficiency, but the only one I found that fit the bill had an Xmax of 2mm and the T/S parameters suggested that it needed a huge enclosure.(relatively)
I'm guessing that the hard bags likely measure in the .75-.8 cu ft range but I'll have to take them off and fill them with water to accurately figure that out.
DrBoost said:
First thoughts before I’ve had coffee and a chance to wake up:
you won’t get much because the sub isn’t playing into a room that it can (try to) pressurize, but into free air. But that being said, go with a dual voice-coil driver, them you can rum both rear channels into it. Yeah, you’ll need a cross-over and you’ll get stereo signals into it, but you’ll also get 80 watts.
Maybe you want a passive radiator design to get a little more out of it?
How much room do you have to play with? Have you thought about a tactile transducer???? AKA, butt shaker!
I agree with everything here. My concern is with how the amp in the head unit is configured. The owner's manual shows a single driver on just one of the rear speaker outputs. Then there is a setting to send just LP to the rear. Since it is a head unit with 40wrms x 4, I wouldn't be surprised if you risk over-amping if you try to drive two channels on sub mode. And does the sub mode switch so there is only one that can be driven at a time? Will it send 40w to both? Will it bridge safely? Unfortunately, the owner's manual doesn't say. It just has a super short description that says you can hook a sub to one of the rear speaker outs. It doesn't show any other options, but also doesn't warn about getting creative.
Owner's manual
If you scroll to p34, here is exactly what it says about the subwoofer thing: "You can use a subwoofer without a power amplifier when it is connected to a rear speaker cord. Note: use a subwoofer with an impedance of 4 to 8 ohms."
That's all you get. It shows a diagram with two speakers, one sub, and the fourth speaker wire doesn't go anywhere.
AngryCorvair said:also strange that motorcyclists like to force their music loud as berkeley on everyone nearby
Fortunately, my pipes are so loud that no one can hear my music, so it all works out.
Dr. Hess said:
AngryCorvair said:also strange that motorcyclists like to force their music loud as berkeley on everyone nearby
Fortunately, my pipes are so loud that no one can hear my music, so it all works out.
And I'm usually singing at the top of my lungs to whatever music is playing.
Kinda interested in this myself as I may want to put a sub in my daily driver miata at some point.
In reply to Dr. Hess :
So which saves more lives: the loud pipes or the loud music? 
In reply to thatsnowinnebago :
Loud pipes, definitely.
In reply to Dr. Hess :
Haha, I was hoping for a "belt and suspenders" type situation.
Yeah, that owner’s manual is near useless. Maybe call crutchfield. They are supposed to hve great support.
I would try connecting the fronts as shown, but the two rear channels to the sub, one to each voice coil. The radio will be putting out full range, and a cross over at the speaker will allow low pass
this is a tough situation.
How about pulling a trailer behind the bike with a couple folded 18s and a kilowatt?
DrBoost said:
Yeah, that owner’s manual is near useless. Maybe call crutchfield. They are supposed to hve great support.
I would try connecting the fronts as shown, but the two rear channels to the sub, one to each voice coil. The radio will be putting out full range, and a cross over at the speaker will allow low pass
this is a tough situation.
I think what I might do is take some 6x9s I have and hook them both up to the two rears, then put it in "sub" mode and see if they both still work. Then I'll push them hard for a while and see if it trips a thermal protection or something. Wise to do that while the warranty is fresh :)
The head unit itself has a low pass for the rears in "sub" mode, so (while not the cleanest way) at least it just sends 40w of low pass instead of 40w of full range and attenuates the highs at the speaker.
I just know that bass requires a lot more oomph to control a larger driver with more excursion, so that may be why the don't show the possibility for two subs.
DrBoost said:
How about pulling a trailer behind the bike with a couple folded 18s and a kilowatt?
Fantastic.... but I think the 23-amp stator might be a bit blindsided by that.
Curtis said:
I'm guessing that the hard bags likely measure in the .75-.8 cu ft range but I'll have to take them off and fill them with water to accurately figure that out.
Hey Curtis,
If you give me a napkin sketch of the bag dimensions and a rough depth, I can give you a volume pretty quick.
If you can give me a sketch like this: 
I can turn out this for ya, which will be accurate enough for your sub:
So my 30 second doodle is 574/1728, or 0.33 cu ft.
I will do that, but it's a pretty complex/curved thing. Its swoopy in all three dimensions, plus the door is swoopy all by itself.
The Parts Express website gives the following information;
Optimum Cabinet Size (determined using BassBox 6 Pro High Fidelity suggestion)
- Vented Volume = 0.5 ft.³
- Vented F3 = 42 Hz
That doesn't give you full information; I would want to know recommended port length and port diameter. Customer support at Parts Express might actually be worth talking to as we as hitting up the forum there and asking. Not sure how helpful they will be for someone brand new. If you knew someone with speaker box modeling software, this would get a lot easier. There are websites that can give approximations like what PE shared with a bit more information on a recommended port although those sites are probably best taken with a grain of salt.
Are you okay with cutting up one of those bags? They look like they were not inexpensive and they definately go with the bike (vs, say, hacking up a knock-off Pelican case to be a mountable sub).
They are the factory bags, so not cheap to replace... especially if it turns out to be a bust. I don't mind cutting a hole in the backside for a port since I could seal it up easily if I take it out.
Sounds like you guys are talking about making the bag itself into an enclosure... which I don't mind, I just think it would be easier to make it a standalone box, especially because the seal is dubious. It has a rubber gasket which keeps it from rattling, but it's not sealed. It prevents water intrusion by the design of the lip, not necessarily the rubber gasket. Long story short, I don't think the box itself will seal well against sound waves unless I epoxy it shut.
My original idea was to cut some framing from 1x lumber to fit inside the bag, skin the frame with luan, then glass the inside and out with a few layers. Basically make a custom fiberglass laminate enclosure that fits in the bag.
I think you would need the sub driver itself firing out of the bag as well as the port. That's two holes and one is rather large. I would want some kind of "marine rated" sub driver for this which I'm sure exists and then cover it with a metal mesh grill for protection. Driver inside the bag with just a port exiting would really damp the acoustic output, I think.
What about my knock-off Pelican case replacing the top box on your bike and build a sub out of that? It'll be ugly but it'll probably work and not cost you a nice quality / expensive bike case.
pres589 (djronnebaum) said:
I think you would need the sub driver itself firing out of the bag as well as the port. That's two holes and one is rather large.
What about my knock-off Pelican case replacing the top box on your bike and build a sub out of that? It'll be ugly but it'll probably work and not cost you a nice quality / expensive bike case.
If it were a bandpass box, just the one chamber is ported. The driver could live inside with the port on the outside.
I just paid good money to put a brand new tour trunk on the back so I had room to pack for two people... knowing that I was going to give up a half a cube or so for my sub idea.
