Calculating Best Air Volume for Speaker Enclosure

[wg1337]

Hi!
I have made a small enclosure for my portable audio system. At first it was for a different speaker, but now I simply replaced all electronics and it still sounds good. But I want better.
When I was making the box I didn't think about the air volume, just made a big box.
So what's inside?
- a 12V bike battery hooked to a 30W amplifier, that is hooked to 2x30W 16 ohm speakers.
Here are some pictures:
Speakers where originally in this enclosure
http://imageshack.us/photo/my-images/685/imag0043u.jpg/
http://imageshack.us/photo/my-images/683/imag0044ub.jpg/
And the box it self
http://imageshack.us/photo/my-images/714/imag0042ic.jpg/

So my question is how to calculate the best air volume for these speakers?
I checked some online calculators and they said I need 17l volume, maybe it is true, but maybe I made a mistake somewhere.
But if it is true then what would be the optimal volume?

 

[schip666!]

I haven't fiddled with speakers in a long time so this could be old news...

IIRC, enclosures serve two purposes. The main function is to prevent the back pressure from behind the speaker from canceling out the forward pressure from the front -- so just about any box or large panel will serve. The secondary function is to smooth the low frequency response of the system by 'balancing' the impedance around the speaker's resonance.

To accomplish the smoothing function one needs to know the resonance and compliance (the inverse of stiffness, kinda like resistance) of the speaker driver in question. Measuring resonance is fairly simple using a sine generator and a scope or voltmeter, but I never found an easy way to measure compliance -- so the exercise may become experimental anyway.

In any case this may be helpful:
http://books.google.com/books?id=1F...6AEwAA#v=onepage&q=speaker compliance&f=false

And, in return, where did you find "online calculators" and how did you use them?

ps...if the question really was just "What size is a 17 liter container?" ... Remember a liter is 1000 cubic-centimeters and the cube-root of 17000 is -- gimmeaminutehere -- 25.71 cm (about 10") per (in)side.

 

[wg1337]

I have read that vented box would give a 3dB gain, so probably will make this time a vented enclosure. It is really hard to find any information about these speakers, they want me to buy service manuals.
Btw, when I removed these speakers they had a capacitor on the + terminal. When I simply attached wire to capacitor (like it should be) then my cheap-end amplifier doesn't keep up and gives a bad sound, but when I remove the capacitor then it sounds a lot better. Speakers have 16 ohms, I think the capacitor changes the impedance.
Sadly I don't have a sine generator, only a voltmeter, those values (I think) are given by manufacturers, but everywhere it is said that all speakers need minimum dimensions.
Will read that book you gave, maybe will find something I should really know.
I tried some online calculators that google found, I used those Qs etc. from other speakers.

 

[schip666!]

Ah, vents...yes. Make your life even more complicated...

The various kinds of vents mostly serve to reduce the box size by tuning the system, c.f. Helmholz resonators. They also phase-shift some of the back-pressure so it adds to the forward wave, giving you a bit of gain in the low frequencies. 3db is nothing to sneeze at, but you need to know a lot about your whole system to design it correctly.

If the capacitor was in series with the speaker it was there to block DC from a single ended amplifier, and if by "bad sound" you mean no low end that would also be it's signature as it makes an LC high-pass filter. I doubt that a service manual would have the actual driver specs, for that you'd probably need to find the mfg of the driver and buy a few thousand units.

You could use a PC with Audacity or some other audio editing program as a signal generator. When I was fiddling with all this I had a sweep frequency generator synced to my scope so I could see a "real-time" response graph, but you can do it all by hand with a decent AC voltmeter. If you have the patience and a lot of scrap lumber you can just start making and testing boxes... Most likely your ears will be your best piece of test equipment.

 

[wg1337]

Here's a quick sketch on how I would like to make the now box:
http://imageshack.us/photo/my-images/3/schj.jpg/
Could you please tell me if this box could be a good one?
Speakers are 8cm, but actually are about 7cm, thinking of making vents about 3x7cm.
Maybe could suggest better dimensions?
Since I don't have a lot of wood then want to make the best box possible.
Btw, all dimensions are for the inner volume.
Maybe I really shouldn't make a vented box?
I have Logitech X-230, the woofer sounds really good (I think) and it seems it also is 30W, maybe should use it's dimensions?

 

[skeptic2]

The main function is to prevent the back pressure from behind the speaker from canceling out the forward pressure from the front

They also phase-shift some of the back-pressure so it adds to the forward wave, giving you a bit of gain in the low frequencies.

This is one thing I have never understood. It seems that if the front of the speaker is pushing the air, the back of the speaker would be pulling it and such a vent would cancel out the forward pressure. You do mention phase shift but unless the enclosure is very large it's hard to see where 180 deg of phase shift would come from.

 

[Studiot]

You really need to concentrate on the speaker type, before asking about the best enclosure.

I am assuming some sort of driven cone.

Speakers like this are of two types.

1)Those with an extremely flexible "roll surround" between the cone and the frame or chassis.

2)And those without this flexible mounting.
These are the cheaper, often paper sort of cones.

The point about this is that when the cone moves backwards it compresses the air inside the box.
Type (1) speakers rely on the compressibility of the air to act properly. They are best used in a sealed enclosure
Type (2) speakers use the flexibility of the cone+mounting to provide the restoring force and the compressibility of the air in the box is an embarrassment. These are best with a (tuned) vented enclosure.

go well

 

[AlephZero]

You do mention phase shift but unless the enclosure is very large it's hard to see where 180 deg of phase shift would come from.

Start by thinking about a speaker mounted at one end of a long pipe. Because the speaker cone is moving, this acts the same way as a pipe open at BOTH ends. (For a pipe closed at one end, the air at the closed end doesn't move!).

So, at the lowest resonant frequency of the pipe the air will be moving with a 180 degree phase difference between the two ends.

Bending the pipe into a U shape makes no difference to the vibration of the air inside, but now the displacement of the air at both ends is in the same phase, as viewed by somebody looking at the pipe from the outside.

You can think of the air in the pipe as acting the same way as a spring which is doing axial (not transverse) vibrations.

Now, imagine you partially block up the open end of the pipe. The result is that the air has to move faster through the smaller hole, so it has relatively more kinetic energy. That has the same effect on the adding some more mass to the end of the "spring", in other words it lowers the resonant frequency. You can keep the resonant frequency the same by making the pipe shorter.

You can select the length of the pipe and the size of the hole to make the end result look like a loudspeaker cabinet, not a U-shaped pipe.

In small loudspeaker cabinets there may be some plates (baffles) inside the box, to make the air flow along a longer path (similar to folding up the "pipe" several times).

 

[Studiot]

Maybe I really shouldn't make a vented box?

Since the Op is still online I should, perhaps, strengthen my comments.

You can actually damage a "roll surround" type speaker if you do not use a sealed enclosure.

 

[skeptic2]

Thank you very much AlephZero, now it makes sense.

 

[Chunkysalsa]

You can also stuff a box with stuff like polyfil to make the enclosure seem bigger. I know I did this for my subwoofer in my car to get rid of some low end boom.

Its been a while though so I really don't remember anything

 

[Averagesupernova]

Typically a ported enclosure will act as a low pass filter and a sealed enclosure acts as a high pass filter within the range of frequencies of interest. When someone describes the bass of a sound system as 'tight' that probably means the subwoofer enclosure is either sealed or a bandpass. A bandpass enclosure has one side of the speaker (correctly called driver I suppose) sealed and the other side ported. Interesting bass to be around and feel, but I can't say I like the sound.