Featured Building an openable subwoofer box!?

[BrazilianBassBuster]

Why do I want an openable subwoofer box?

The lack of space is the physical constraint that makes reproducing the lowest frequencies in a car so difficult. The 20-40 Hz waves are too long, so to tune the loudspeaker at this frequency we need a lot of air in the enclosure to act as a long mechanical spring.

Of course, loudspeakers with stiffer suspensions will require smaller enclosures, but then any decent car subwoofer already has a very stiff suspension. Well, we can double the stiffness of any loudspeaker if we use two units in an isobaric compound configuration. That will divide in half the enclosure space needed, but this won't help much in the 20-40 Hz for two reasons: (1) the port dimensions will remain the same (in this frequency range, port size may take even more space than the actual air spring space), and (2) an isobaric arrangement lowers the system's efficiency in 6 dB, so it requires quadrupled power to achieve the same result (4x), which you may not be able to provide, or your loudspeakers may not support.

Conversely, with doubled space for the sub-enclosure, we will gain +6 dB in efficiency. Usually, this is done by using two drivers side-by-side, which will also double your system's power handling and your air displacement potential, allowing an extra +3 dB if you double the power. This is why enclosure space is so important.

OK, but the car has lots of space, so what's the problem with using it? The problem is that the used space becomes unavailable for its original purpose. If the box uses the space from the back seats, then we can't have people sitting there. If it uses the trunk, then we can't carry objects there anymore. But what if we built a special box that could be opened to put stuff inside it? OK, I won't put people inside a subwoofer box, but maybe it is fine to put the spare tire inside it? Perhaps the 16-inch Dahon foldable bicycles and a foldable electric scooter?

How can the box be opened?

I have thought about 3 options:

Option A) A simple "trunk bed" system:

Option B) Use a "guillotine door" system to close the box:

Option C) A door that slides over the box, possibly made of glass:

Option D) A rounded small class lid that opens up:

(in reality, the curved glass in the above image is not openable, but you can imagine it opening up like in the previous image)


Initial thoughts:

Option A seems to be the easiest one to build. Because I can easily buy a ready kit with that metallic joint and the piston used to lift the lid:

I actually sleep in a bed that is just like the one in that picture.

However, all of the 3 ideas have the same challenge, which is to avoid vibration (resonance) and air leakage. In that sense, option D seems to be the most promising one. Because the curved glass is much better in holding resonance than the flat wood. Also, it allows me to create a box that goes up to the car's roof. And if another piece of glass is added on the back of the box, then I might still be able to use the car's rearview mirror. Just need to keep those glasses clean. And the curved glass is kind of easy to buy as well:

So, what are your thoughts about all of those ideas? Has anyone ever tried something similar?

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[Jimi77 Premium Member]

Bad ideas IMHO. The "door" needs to be airtight and maintain airtightness while the sub is banging away. Additionally, you run a risk of damaging the sub if you put stuff in the enclosure. You also alter the airspace in the enclosure when you put stuff in the enclosure. Perhaps you should build an enclosure that would be easy to remove. The other option that comes to mind is to go with an infinite baffle set up. Won't perform like a 6th order BP, but is the most space friendly.

 

[Popwarhomie 5,000+ posts]

Foam weatherstrip tape. I have used it for years for 4th, 6th orders, flat wall baffles, and ports to be able to remove without damage. Seals just fine.


Removed my 2 18 baffle for a 4 12 baffle while keeping the shell intact.

 

[BrazilianBassBuster]

Hi @Jimi77 , thanks for your reply. I'll answer your points from the simplest one to the hardest.

Perhaps you should build an enclosure that would be easy to remove.

The whole point is to have a huge enclosure with at least 350 liters, and maybe as much as 1000 liters. That's not a removable size. A huge enclosure can make 140 dB at 20 Hz inside the car and 120 dB outside the car, but a small one will always be way behind. Usually, they don't even hit 100 dB at 20 Hz. And yet the smaller enclosure tends to be heavy enough that you won't want to remove it frequently. Especially if you live in an apartment (like I do).

Additionally, you run a risk of damaging the sub if you put stuff in the enclosure.

Isn't that the same risk that is faced in any sealed or ported enclosure placed in the car trunk?
Isn't that problem resolved by using a good subwoofer grill like the one below?

You also alter the airspace in the enclosure when you put stuff in the enclosure.

Yes, but when you are using a huge volume, the object will have little impact. For instance, the design I want is BP6 with 50 L + 200 L and the door would be on the 200 L side. If I add two bicycles and a foldable scooter, the total amount of space occupied will be no more than 10%. We can simulate that volume reduction using WinISD and the difference is so small. I think we would hardly notice any difference.

And maybe with this approach, I could double it, since I have two He15 drivers. That means it would have 400 L of free space when opening the door. The bigger the enclosure, the less it cares about the objects you put inside it.

The "door" needs to be airtight and maintain airtightness while the sub is banging away.

Yes, but what if the door is stiff enough and gets applied with enough pressure all around its contact area after being closed?

Do you think a few latches similar to the one below can accomplish that?

Do you think it is an impossible task or just a difficult technical challenge?

The other option that comes to mind is to go with an infinite baffle set up.

Yes, that's another thing I have always thought about. But like you said it's limited because there are no ports, so it isn't efficient in the 20 Hz range.

 

[Jimi77 Premium Member]

With weather stripping and clamps it can work.

 

[BrazilianBassBuster]

Foam weatherstrip tape. I have used it for years for 4th, 6th orders, flat wall baffles, and ports to be able to remove without damage. Seals just fine.

Yes. Thanks for the image. I guess the difference is that you are using screws, which I would have to replace. I might buy some latches to do an experiment.

 

[BrazilianBassBuster]

I have found those 90-degree latches that look good for a flat lid:

However, I think the ones below, are more appropriate for a lid with curved glass:

Because those are stronger, easily adjustable, and work on a flat surface, which would be the case:

And by the way, I just found an infinite baffle in this short video (see the full video too):

Unlike what I said before, with 3000w, this 24-inch sub can hit about the same SPL output as the He15 in BP6. This is because it has a super low Fs of 16 Hz, so it can hit 20Hz at -4.5 dB in an infinite baffle setup, with the advantage of having a minimal group delay (10 ms against 70 ms of the He15 in BP6). The specs say 92 dB/w while WinISD calculates 88.7 dB/w, which is enough.

The advantage of having no box seems great, at first. However, I'm not sure about the disturbance that it would cause to the people around. So maybe having it inside a 500 L enclosure would be better? The sealed box would give the same output as the IB setup with the advantage of cone excursion control below 20 Hz.

Then, again, the idea of having an openable box still seems advantageous.

 

[Jimi77 Premium Member]

Most infinite baffles vent into the trunk. I wouldn't feel comfortable with the motor exposed like that.

 

[spokey9 5,000+ posts]

True Ib is the way if you want 20hz and lower output

 

[BrazilianBassBuster]

Most infinite baffles vent into the trunk. I wouldn't feel comfortable with the motor exposed like that.

Yes. Unfortunately, people care too much about the looks. I guess that's why they installed it that way. They said it will get a grill down there but I doubt it will hold the impact of a speed bump.

True Ib is the way if you want 20hz and lower output

Correct me if I'm wrong, but we can't hear below 20 Hz, right? I know we can feel it, but hey we do already feel 20 Hz pretty hard, don't we? Does 10 Hz improve the experience in any way?

Or did you mean that IB systems can hit 20 Hz more easily? Well, for me the chart below proves otherwise.

• Green line: HS-24 at 3000w in a 500 L enclosure with BP6 with 620 L (taking the ports into account)
• Orange line: HS-24 at 3000w in IB setup (simulated with a 50k liters box)
• Yellow line: the He15 driver I have at 2000w and using a total of 330 L (taking the ports into account)

All three designs required a high-pass at 15 Hz to avoid damage from excessive cone excursion; using 2nd order for IB and 4th order for BP6. In WinISD, placing the HS-24 in a 500-liter box produces the same result as 5k L (IB simulation). Actually, the 500 L box is slightly better, and without the need for a high-pass filter.

For me, the conclusion remains the same: BP6 always takes much more bass from whatever subwoofer. It works best for drivers with low Q, but the high Q drivers also speak louder in BP6.

Anyway, I have three IB15 drivers from Acoustic Elegance that would fit nicely in the spare tire well of my old Corsa Wagon. That would be a good way to test the theoretical superiority of sound quality. Those are ideal for IB since they have high Q and low Fs of 20 Hz. That's one option in case I get too lazy about box construction. At 20 hz, they would play 6 db lower than the single He15 in BP6, but the three IB15's would require a total of just 300 w to reach full excursion (100 w each), versus 2000 w from the He15. I'm not sure about the neighbors, but I'm ok with doing it just for experimental purposes. Because it's a very old car that should have already been trashed.

 

[spokey9 5,000+ posts]

Correct me if I'm wrong, but we can't hear below 20 Hz, right? I know we can feel it, but hey we do already feel 20 Hz pretty hard, don't we? Does 10 Hz improve the experience in any way?

It really depends on the music you listen to...but it's a different thing sitting in a vehicle that can hit 14-18hz with authority

 

[Jimi77 Premium Member]

Our (my) hearing drops off pretty quickly at ~30hz. It's more a sensation than hearing when you get that low.

 

[spokey9 5,000+ posts]

Our (my) hearing drops off pretty quickly at ~30hz. It's more a sensation than hearing when you get that low.

Everything in the 20's is more feel than actual hearing

 

[Jimi77 Premium Member]

Everything in the 20's is more feel than actual hearing

I find it painful to be honest, like somebody is stabbing a pencil into my ear.

 

[spokey9 5,000+ posts]

I find it painful to be honest, like somebody is stabbing a pencil into my ear.

I like it I think the magic happens 20 and lower