How does a subwoofer enclosure work?

[RENDOG]

So you take a 12" subwoofer and run an 80hz tone through it while it sits on your carpet and it doesn't make a loud sound. But then you make a squarish enclosure out of wood (sealed enclosure typically referred to as a direct radiator), cut a whole, drop the speaker in, and now you have a pretty loud sound. What exactly does the "box" do to the speaker that seems to create a louder sound vs with no box?

[Dave Non-Zero]

in that simplist of situations the box is there to stop the sound waves produced from the back of the speaker from cancelling out the sound waves produced by the front!
Others can tell you the more complicated version. its kinda beyond me.

[SeisTres]

It allows the "front" waves move the air and stops the "rear" waves from cancelling out the first wave. It also tunes the driver, so to speak, and controls the motion of the cone due to the dampening of the air being compressed inside the box or the way the ports allow to behave.

[J_Dunavin]

I look at it like an acoustic guitar. Without the body, it wouldn't sound like anything except some metal twangy strings.

[RENDOG]

I once heard that the wave hits the back of the box and then comes forward with greater "force". I would think that would change the amplitude(?) of the wave and that's why it sounds louder?

Then again, what really happens when you make a rectangular shaped box and the speaker is off to one side and it still sounds good? The internal air is the proper size for the speaker but the back of the box can be way closer to the speaker than with a deeper box.

I've also heard that you should not make a box a perfect square because you may have standing waves/resonant frequencies.

[Rune Bivrin]

It's really quite simple:

The driver cone vibrates back and forth in time with the sound. That moves the air, and that movement continues through the air and eventually arrives at your ears where it causes similar vibrations in your ear drums.

Without something to separate the front and back of the drivers, the air will just sneak around the edge of the driver and the net result is no sound.

However as sound - which is generally accepted as all vibrations from 20 to 20000 per second - moves at the speed of sound (really!), which is about 340m/s it will only perform this cancellation when the wavelength of the sound is larger than the distance from front to back of the driver.

To that end the driver is mounted on something called a baffle, which is a fancy word for a large board which makes the distance from front to back larger. This results in more bass. The design is known as an Open Baffle speaker.

For good low end you need a really large baffle, but someone quite ingenious realised that if you fold the baffle such that it forms an air tight box you get an effectively Infinite Baffle. The front and back waves never meet!

The downside to this is that the air inside that box will act as a spring on the driver membrane. This has to be taken into account when choosing driver, as the air spring will change the resonance of the system. It also wastes half of the power that goes into pounding the air inside the box.

Now, a lot of (if not most) boxes will have a small opening with a tube attached. The changes the behaviour at some frequenies and lets you use some of the previously wasted energy. This is called a ported box or a bass reflex box. That gives you better bass down to a particular frequency, below which it gets worse a lot quicker than the air tight system. That's where it "farts", and can easily get damaged by too much movement.

Oh, about standing waves and resonances:
Unless your box is very, very large that should be an non issue if you use something to line the interior with that absorb the sound waves. But making the box irregular will likely not make it worse, and certainly create a more interesting look...

[SeisTres]

While non parallel surfaces are recommended for speakers going into the upper ranges, eminence actually recommend a cube for their subs. Also, this is the reason why almost every commercial sub out there is either a cube or some kind of rectangular prism and this is also the reason why the tlah is a wedge shape and the dr's have angled top and bottom. Also, like it was said above, padding reduces the reflections inside the box.

[Monomer]

Best explanation I've read, aimed towards horn enclosures:

-The enclosure (horn) is built to better match the impedance from driver to air.


I may have botched it, it was in one of DSL's papers on (tapped) horns.




I hope (this) helps....

[SeisTres]

Best explanation I've read, aimed towards horn enclosures:

-The enclosure (horn) is built to better match the impedance from driver to air.


I may have botched it, it was in one of DSL's papers on (tapped) horns.




I hope (this) helps....

However, this really only applies to they way horns add sensitivity, which is indeed by matching the impedance of two different mediums in which there is energy transfer.

[Dave Non-Zero]

I thought the speaker enclosure was just a box for keeping the magic smoke in.

[LelandCrooks]

I'm with Dave.

[RENDOG]

It's really quite simple:

The driver cone vibrates back and forth in time with the sound. That moves the air, and that movement continues through the air and eventually arrives at your ears where it causes similar vibrations in your ear drums.

Without something to separate the front and back of the drivers, the air will just sneak around the edge of the driver and the net result is no sound.

However as sound - which is generally accepted as all vibrations from 20 to 20000 per second - moves at the speed of sound (really!), which is about 340m/s it will only perform this cancellation when the wavelength of the sound is larger than the distance from front to back of the driver.

To that end the driver is mounted on something called a baffle, which is a fancy word for a large board which makes the distance from front to back larger. This results in more bass. The design is known as an Open Baffle speaker.

For good low end you need a really large baffle, but someone quite ingenious realised that if you fold the baffle such that it forms an air tight box you get an effectively Infinite Baffle. The front and back waves never meet!

The downside to this is that the air inside that box will act as a spring on the driver membrane. This has to be taken into account when choosing driver, as the air spring will change the resonance of the system. It also wastes half of the power that goes into pounding the air inside the box.

Now, a lot of (if not most) boxes will have a small opening with a tube attached. The changes the behaviour at some frequenies and lets you use some of the previously wasted energy. This is called a ported box or a bass reflex box. That gives you better bass down to a particular frequency, below which it gets worse a lot quicker than the air tight system. That's where it "farts", and can easily get damaged by too much movement.

Oh, about standing waves and resonances:
Unless your box is very, very large that should be an non issue if you use something to line the interior with that absorb the sound waves. But making the box irregular will likely not make it worse, and certainly create a more interesting look...

Awesome, I understood 95% of this.

Ok, so when dealing with bass (20hz-100hz approx.), is the speaker making any sound on the actual surface or is it the air that it moves?

[SeisTres]

Sound (as humans perceive it) is the air moving. This is the way everything makes sounds. It moves in a harmonic motion and it agitates the particles (air) around it therefore transferring that motion from the object to our ear drums. Be it 20hz or 20khz, it is produced, transfered and received the same way.