I made my own sub enclosure in the trunk for my convertible. It had sounded muddy for quite a while and I finally put my scope on it. I should have done it a year ago. The small amp was clipping quite badly. Mind you its quite small at 110 Wrms (@4Ohms) in bridged mode. So I tried my slightly larger amp at 150 Wrms (@4Ohms) in bridged mode and it did better but still clipped at the volumes I had been using. I was too dumb to realize I was killing this sub.

I bought a 500 Wrms (@4Ohms) amp and jury-rigged the wiring (too small a feed wire) just to try it. While it can blow away the other 2 without clipping, I thought it might be still louder than it was. This might be just the way it is, or an artifact of the reduced voltage available from the feed wire. At loud volumes the car buzzes. I can't win.

My real question is how much amp power should I really need for this. I want to hear my bass with the roof down while driving, I am not trying to impress anyone outside the car, its just for sound quality and clarity.

 

how much power a sub can take is DIRECTLY relative to the box.... you need to know the exact volume of the box.... for example my sub has 28.5mm of excursion one way... so I watch my sub while playing test tones and adjust my amp gain until I see it approaching xmax then I call it good...

unfortunately for me, my sub and amp are waaaaaay too much for my stock monsoon system. so after doing this I had to go back and turn my amp gain ALL the way down and then change my EQ on the monsoon deck where it goes in a diagonal line like this "/" with the bass all the way down and the mids and highs all the way up to even begin to sound balanced

 

What are you playing through the sub while watching the excursion. I found that a test tone has much more movement since all the energry is at one frequency, where music at the same volume and gain setting had much less movement.

 

The problem is that you are taking the battle ax approach in a convertible. Sealed box, shooting bass straight up right into the deck lid. Top up this probably sounds pretty good, because the car is essentially like a normal coupe.

Top down, the top pretty much blocks the trunk cavity off so you don't get much of anything. Especially now since you're not only firing the sub into the trunk lid but then expecting it to transmit through the top.

So as you've found out, the only solution is to pound the hell out of the sub, and when you do so now you're going to get the decklid / heat shields / stuff back there rattling all to hell.

There are a couple things I'd suggest to try:

1.) If your sub can hold it - keep the 500 watt amp, seems like you are going to need it given your wants.
2.) If your amp has it, try to set up a subsonic filter to limit the lower end of what the sub plays. If you have a way to control or limit the content below a certain frequency - say 30-40Hz, you might be able to keep the loose things from rattling.
3.) Alternatively, find out what is rattling and fix it. Dynamat, felt, whatever it takes.
4.) Take a different box approach. Shelf mounted wedge boxes that fire either the ports or drivers directly against the top when it's down, or boxes integrated in the cabin as a seat delete or some other approach.

Here's my setup. Wedge box, 4x8" subs, ports on the front low against the top when it is down. I only have 400 watts rms going to the four subs which is not enough. Just when it starts to sound good the amp is right at it's limit. But there is just enough bass the play loud and balanced with the top down. I can't boom but it is very well balanced. Top up sounds about like a dual 10" setup in a coupe.

 

You've pretty much nailed my problems.

I'll go ahead and install the amp.

It does have a subsonic filter at 15Hz I think.

I have put sound damper on everything in the trunk that is hidden, I really
don't want to touch the underside of the trunk lid. The rattling I have heard
is while I am in the garage so I canhear everything, I have to check it
outdoors.

Did you make your own box? I might make one, I would have to buy several
subs though. Do the 8" go low enough? How big are the ports?

 

On my car the first thing that rattled was the gas tank heat shield. After that maybe look at the license plate and then from there it's probably harder to deal with stuff like the top frame or trunk lid itself.

I did not build my box myself, it's a JBslim box with some modifications. I've worked on the tuning a little bit to get it down to around 80Hz. 80hz is not good, but it's the best I could do with the space and using 4 subs. The box is definitely missing the "feel" bass you get from a proper box in a coupe. Music still sounds fine but without the coupe reinforcement you lose the low low stuff.

Obviously you are good at box building, what you'd want to do is build a box up properly tuned to about 40-50hz or so, and put the ports as low to the ground as possible firing forward. I think it would be very easy using 2 subs, more of a challenge with 4, but it could be done. Then those subs would need every bit of those 500 watts you got.

But I am still not sure that such an approach would give you what you are looking for. A convertible with the top down is not going to sound like a coupe no matter what you do, and if you want it to be even close you're going to have to take a look at putting a box in place of one of the back seats.

I have a 4x8" setup with 400 watts and with the top down it sounds like a single 10" setup. So there is a huge disparity. The subs are still wanting a lot more power though. I'd like to hear it with a 1000 watt memphis, then it'd probably hit pretty hard. I didn't go that route because I have no desire to mess with the charging system or wiring.

I guess my point is that you're going to need to throw probably 4x as much power/driver at a vert than you would a coupe, and eventually you just run out of practicality.

You're probably best off with your setup and quieting down the rattles.

You did remove the top/trunk seperator, right?

 

I have probably overstated what I need with the roof down. I just want the bass to be there somewhat while driving. Yes, I did remove the separator piece. I will run the wiring for the bigger amp properly and try it for a while. I think I will make up a new box some time later.

What tuning were you actually doing? Changing the box itself?

 

JB's box has no ports. There are just slot openings in the front face of the box. This sounds really loud this way, because the tuning frequency is really high, upwards of 200Hz when 4 woofers are used. Used with a factory system it adds a nice punch and because it is so loud up high it sounds loud with the top down, especially for rock music where there isn't a lot of really low bass. But it was pretty obvious to me that the true bottom end was lacking off the bat.

I was dealing with aftermarket gear and don't want the subs pushing anything much over 120Hz, maybe 150Hz tops. I was more interested in filling out the real bottom end. So I added some plates that effectively halved the port cross section and created a ~5" long slotted port.

The problem with my setup is that there is not enough volume for each driver. Ideally, made a little bigger to allow for proper tuning - or just two really solid drivers and pound the hell out of them. The extra space would allow for proper port tuning. I really wonder if proper box optmization could make the same 400 watts I have to work with sound better with just two drivers, I'm just not willing to destroy the box to prove it out.

Whatever the case, in any form it is a major upgrade from stock, and the boxes are really well made, with no compromise in space. I'm just a bit of a tinkerer and wanted to do what I could to optimize it.

It's too bad you live in Ontario or you could take a listen.

Maybe I'll pick up a dB meter sometime to help publish some hard numbers.

I'd really like to see someone make some custom 8" driver fiberglass cavities fabbed into factory rear seat side panels. Something like that positioned down low in the cabin would probably be better than any compromise trying to work through the top. If I had any enclosure talent that is probably what I'd try.

The new mustangs have 8 inchers in the doors from the factory, and they sound pretty good. The bass is artificially boomy but it'd make 90% of people happy. It'd better because its quite an expensive option.

 

What actually holds that box in place? It looks like its just sitting there.

 

It wedges in place and is stable on it's own from falling, but it is necessary to fasten in down in one way or another. I ran a braided steel cable (you can see in pic) across and through a hook loop.

There are two bolt points on opposite sides used to hold some of the trim in place.

 

Test tones will do more excursion, I run an Adire Brahma and it will not distort untill after it blows. So I figure it's safer to tune my system using test tones. Even more so because I am running an Revolution 1500.1d to the Brahma 15" in about 3.5cubes ported in the low 30's... I can nail 147 steady with the gain all the way down on my amp and bass on the monsoon EQ all the way down as well, and that's playing lil jon not even burping... I have really been afraid to burp it because I don't think my interior can handle it....

 

See sig.......

While I offer two standard configurations (4-subs like Todd's or 2-subs w/ a place to mount the amp between the 2 outer subs and a cavity on the forward facing side...... where I have my external crossover mounted on my own box), I have built one for a customer w/ a custom configuration with 2 subs in the outer two sections and the ports (slotted openings) on the inner two sections, effectively doubling the air volume for each sub. This configuration still has area to mount an amp on the trunk-facing face between the subs, but the forward cavity is now used up as part of the "port" volume. I can also make the port slots smaller if you'd like. This should help the tuning situation as Todd has described.

In an effort to lower the tuning frequency of the box like Todd suggests, I've been "dreaming" of ways to use 1-1/2" PVC or something to create a long serpentine port inside the overall volume of the box, but haven't had time yet to lay it all out on paper. Also, if there was a way pipe this port around the top when it is down, there'd probably be alot less attenuation.

 

John, is there enough room to route side-firing ports out and around the top? Hrrm that could be pretty interesting.

 

The solution to fitting a longer port is simple, move both drivers to one side of the enclosure, this will allow you to use a fairly long streight length of PVC for your port, in your design you just have to use an elbow so it exits out the front of the box. While this won't work for a 4 driver configuration, it allows you to tune a 2 driver box fairly low (relatiely speaking). Just be sure you don't undersize the ports too much or you will have a lot of port noise.