Featured Can a 350 L box that hits 120 dB be lightweight?

[BrazilianBassBuster]

As shared in this thread, I plan on building a large BP6 box with more than 300 L. Since it is tuned to reproduce the 20-60 Hz range at 120 dB, it will need serious bracing. The He15 driver has an amazing peak-to-peak displacement of 5 liters, and it will be fully used.

So I'm afraid that the wood of the walls and bracing could add too much volume and make the box too heavy. So I thought about using a thinner wood and bracing it with many aluminum bars like this:

Below is an estimate of the box dimensions:

Doing a rough estimation of this as a rectangular parallelepiped measuring 50 x 70 x 100, I get a surface area of 3.1 m² and a volume of 350 m³.

Doing some calculations, I noticed the minimum wood thickness I can use is probably 15 mm. But one square meter of cheap 10 mm plywood already takes up 10 liters and 7 kg. With 15 mm and 3.1 m², the external wood occupies 46 Liters (10 L/m² x 1.5 x 3.1 m²) and adds 32.5 kg (7 kg/m² x 1.5 x 3.1 m²). That is too much.

So the idea is to use the aluminum bars every 20 cm, then place a second layer over it, every 30 cm, similarly to this image:

With the 20 cm spacing and 5 cm base, 25% of the internal surface would be covered with aluminum:

This technique is used to support solar panels. This is a multi-layer example:

Besides that, I believe I would also have to add some round pieces of wood screwing together the opposing walls:

Now I'd like some feedback, please.

Do those bracing ideas look effective?

With all of that bracing, should it be fine to go with 10 mm plywood instead of 15 mm, even at 120 dB sub-bass?

Has anyone ever had success with thin walls, such as 6 mm?

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[BrazilianBassBuster]

With some help from chat-GPT, I have developed a Python program that estimates the leakage caused by wall deflection. It uses a simplified model where the front and back chambers are cubes of different sizes. The program is attached as a text file and can be tested on onecompiler.com. Here is the output:

Cube of 37 cm with 1.5 cm plywood and 1 braces per wall.​

​

Cube volume: 50.65 liters​

Size of each unbraced squared piece of wall: 18.5 centimeters​

Deflection of a single flat wall: 0.000763 millimeters​

Total leakage due to deflection of the cube walls: 0.000665 liters​

​

Cube of 58 cm with 1.5 cm plywood and 3 braces per wall.​

​

Cube volume: 195.11 liters​

Size of each unbraced squared piece of wall: 14.5 centimeters​

Deflection of a single flat wall: 0.000288 millimeters​

Total leakage due to deflection of the cube walls: 0.000616 liters​

​

Percentage of subwoofer displacement leakage: 0.05%​

As the output shows, I had to use 15 mm plywood to keep the leakage below 0.1%.

 

[Jimi77 Premium Member]

That's a huge box. I'd go with baltic birch plywood to reduce weight.

 

[Old_Slapper]

You can build the box with 1/8" wood, then line it with 1/2" of fiberglass. You still use the aluminum supports, of you can use balsa wood that is wrapped with a couple layers of fiberglass. However, 1/2" thick fiberglass will not need much bracing.

 

[BrazilianBassBuster]

You can build the box with 1/8" wood, then line it with 1/2" of fiberglass.

Thanks, but have you ever seen someone doing this and achieving successful results?

I'm asking because I have already tried it and the results were the opposite of what I wanted. It became a super heavy box that I could hardly carry to the car by myself. Correct me if I'm wrong, but from my understanding, the 1/2" of fiberglass needs to be filled up with resin, doesn't it? The problem is that the resin is heavier than wood. The resin density ranges between 2x to 4x the density of plywood.

 

[Old_Slapper]

Thanks, but have you ever seen someone doing this and achieving successful results?

I'm asking because I have already tried it and the results were the opposite of what I wanted. It became a super heavy box that I could hardly carry to the car by myself. Correct me if I'm wrong, but from my understanding, the 1/2" of fiberglass needs to be filled up with resin, doesn't it? The problem is that the resin is heavier than wood. The resin density ranges between 2x to 4x the density of plywood.

Depends on the wood you use. I worked on a lot of wannabe racers who would strip their cars to drop weight, but then want a system. The Fiberglass was almost half the weight of the wood. When you're laying your fiberglass, don't use so much resin. You only need enough to dampen the fiberglass clothe. You don't need to drown the clothe in resin. Technically, you don't really need 1/2". About 4-5 layers will give you about 3/8". That, with some middle supports, would be a pretty solid box.

 

[Old_Slapper]

The best way to think about it is lifting a 12" fiberglass row boat. They weigh about 100 pounds and have a lot more fiberglass than what you would use for your box.