To matrix or to not matrix?

[Trist]

I'm building a new sub 15" to replace the one i just built.(because it was my first, and first are never that great)

Anyway, the last box was just single layer MDF 3/4", and dampend with pink fiberglass.

Anyway, here are my questions before i go ahead and build another one:

1. Does matrixing the box reduce sound quality?

2. Instead of matrixing the box, could i just double up the MDF, making it more rigid?

3. Would dampening the box with that pyramid like foam (kinda like they have in music studios) be better than pink fiber glass.

4. Are box demensions really that important? Does it have to be a big box with a certain length, width and height. Or can it just be a big box?

Well, thanks for you help.

 

[Hoffmeyer]

Hi Trist.
(in danish Your 'name' means sad ;-) )
1: A matrix does not reduce soundquality, it'll improve it, since it's reducing the distorsion comming from resonances within the enclosure, wich will make the enclosure vibrate. just make sure to take into account the volumn taken by the matrix-construction, and ad it to the Vb.

2: Doubling the MDF will make the enclosure more rigid, and therefore lower the amplitude of the distorsion/sound emmitted from the enclosure.

3: Different dampingmaterials has different accoustic qualities, so try all Your possibilities, and see wich You like the best.

4: Internal box-dimensions are important to the soundquality, especially in a subwoofer. There are different relations between the three meassurements, hight/width/depth, wich will reduce the constructive interference between resonances. My prefered formula is:
Width : 3.squareroot(Vb/1,9999)
Depth : 3.squareroot(2)*(3.squareroot(Vb/1,9999))
Height: 3.squareroot(4)*(3.squareroot(Vb/1,9999))

Happy building /;o)

 

[hifi]

in swedish...it means dull...

 

[Trist]

Thanks for the formulas, they should help alot.

Also, i've heard of people casing the box with sand. Does this help as well?

And also, for te box volume, is there a way to determin the box volume from the demensions of the speaker? Or do i just pick out a volume that i find adequate?

 

[Hoffmeyer]

Oh sand, You precious, so cheap, and yet so valuable.
Indeed it does help.
If You make a sandwich-baffle with MDF-sand-plywood, You'll have 3 different resonance-characteristics, where the sand, besides adding a lot of weight, converts motion/vibration into heat. Don't worry, the baffle won't get hot ;-).

Check out: http://www.linearteam.dk/
They have a little program, WinISD, that can help calculate the volunm of the speaker, just remember, when calculating the dimensions, to add the volumn for the woofer and the port.

 

[Peter]

No damping needed for subwoofers

As you are building a subwoofer which does not reproduce higher frequencies, you do not need damping material at all. All kinds of damping material have almost no damping effect at low frequencies. The only advantage of damping material in a subwoofer is that it increases the effective volume and therefore you can choose a smaller enclosure (minus 20-25 % volume, if tightly stuffed). A disadvantage is that the damping material introduces nonlinearities, which can do harm to bass precision. Be carefull with fiber glass. It can also do harm to your lungs.

Greetings from Berlin

Peter

 

[Trist]

becuase your all so helpful

OK, well, my new sub idea seems to be comming along quite well with all the helpful info. But i still got a couple more questions.

1. Is there any disavantages to having the sub down firing?
(i was told that sub frequencies were non-directional or something)

2. Is there any disavantages to having two, 3" or 2" ports, instead of a massive 4"?

Well, thanks again for the help

 

[Hoffmeyer]

Peter
I allmost agree with You.
Damping a subwoofer-enclosure will remove higher order harmonics, and will lower the amplitude of standing waves/frequencies.
As I wrote in a previous reply, different dampingmaterials has different characteristics. Some will have no effect in the lower regions, while some will.
I did some testing some years ago, with a transmission-line loudspeaker, in order so test the difference between sheep-wool and a synthetic dampingmaterial claimed to have the same accoustic qualities as sheep-wool. At the enclosure-resonance the synthetic dampingmaterial was damping the output 8dB more than sheep-wool. Off course i chose the latter.

Trist
If You're fireing the sub down, the cone will, due to its own weight, move off-center, and the suspension will be working more non-linear than intended.
It's correct that subfrequencies are non-directional, ore rather it's difficult to determin the direction of sustained subfrequencies.
What people tend to forget is that even subfrequencies has a start, i.e. a kick-drum starts with a kick/attact, mainly consisting of higher order frequencies, but the subwoofer is participating in this attack as well, and this attack is very directional.
Fireing the woofer down will only make the attack diffuse.

On Your second question I'll say no, as long as the air-velocety is sufficiently low.

 

[Peter]

standing waves in a subwoofer

Usually one does not have to consider standing waves in a subwoofer, because their frequencies are well above the crossover frequency. An example: If you have a volume of 0.5 m*0.5 m*0.5 m (=1.6 feet*1.6 feet*1.6 feet), resonances begin at about 340 Hz. For a 12 dB crossover at 100 Hz the fundamental standing wave is damped with -27 dB - absolutely no problem; damping material is not needed.
Of course one can shift the resonances towards lower frequencies by choosing larger dimensions. That is why I am not sure if one should really use different values for width, depth and hight of the enclosure instead of a design as compact as possible (a cubic one).

I do not think that higher harmonics of the frequencies reproduced by the driver can be surpressed by damping material, because the largest part of the distortions go directly from the woofer to your ears.

Greetings from Berlin

Peter