2 x woofers alignment questions

[sheep]

I've never tried to build a multispeaker sub before.

I want to use 2 8" woofers in a subwoofer box. That makes for a speedier output than 1 bigger, heavier cone driver.
If the box is closed, do I need to use double the volume as for a single woofer?
For a bass reflex alignment, do I need to change box or tuning if adding an additional woofer? If so, how would I do that?

[Boscoe]

Whether it be closed or vented double everything except port length.

[taloyd]

Originally Posted by sheep

I've never tried to build a multispeaker sub before.
I want to use 2 8" woofers in a subwoofer box. That makes for a speedier output than 1 bigger, heavier cone driver.

This is not true. The larger driver will likely have more swept area, and so will require less excursion for the same output, reducing distortion considerable.

However, if you are hung up on two drivers, use a configuration that maximizes that application, like a PPSL, or mounted on opposite sides of the enclosure, for force cancellation. If you DO put them on the same side, at least invert one for even-order distortion reduction.

In regards to your question - yes, two of the same driver will require double the box volume. Download Unibox and play with the tuning. Again, if you are hung up on two 8" woofers, look around carefully - most are woofers, not subwoofers. The Fs (natural resonance) of the driver will give it away - an Fs <40 Hz. is most likely a subwoofer.

The best advice? Follow an established plan design for a subwoofer. Then it'll definitely be successful!

cheers,
-Tal

[sheep]

For a bass reflex, do I also need to change the vent area?

What are the pros/cons of a single chamber vs separate chamber for each speaker?

[Boscoe]

Originally Posted by taloyd

This is not true. The larger driver will likely have more swept area, and so will require less excursion for the same output, reducing distortion considerable.

However, if you are hung up on two drivers, use a configuration that maximizes that application, like a PPSL, or mounted on opposite sides of the enclosure, for force cancellation. If you DO put them on the same side, at least invert one for even-order distortion reduction.

In regards to your question - yes, two of the same driver will require double the box volume. Download Unibox and play with the tuning. Again, if you are hung up on two 8" woofers, look around carefully - most are woofers, not subwoofers. The Fs (natural resonance) of the driver will give it away - an Fs <40 Hz. is most likely a subwoofer.

The best advice? Follow an established plan design for a subwoofer. Then it'll definitely be successful!

cheers,
-Tal

Two drivers = higher sensitivity? You double VAS so you don't really gain anything I guess.

Two drivers? Why not? 2 x 8" = 12" surface area wise so it really dosen't matter. Just keep an eye out for the parameters you won't get good results with a woofer like taloyd said.

Yes you also double to vent area but not the length. Imagine joining two boxes together.

Two separate enclosures? I guess the only positive is one slightly weaker driver won't get pushed by the other but this wouldn't be noticeable especially in a vented box, two different boxes is just more work and more materials.

[sheep]

I have downloaded Unibox 4.08. What does PPSL stand for?
The sub would cover up to say 140 Hz so maybe its really a 2 x woofer system for each side.
Anyways I have some 8" EPI woofers and I believe free air resonance is 20Hz.

[Boscoe]

Originally Posted by sheep

I have downloaded Unibox 4.08. What does PPSL stand for?
The sub would cover up to say 140 Hz so maybe its really a 2 x woofer system for each side.
Anyways I have some 8" EPI woofers and I believe free air resonance is 20Hz.

Yep that seems good and would be something I would go for, so you have 4 8" woofers in total?

PPSL is Push Pull something or other! Basically 2 drivers in a vented dox firing into a cavity which boosts performance.

There's a thread.

[chris661]

Originally Posted by sheep

That makes for a speedier output than 1 bigger, heavier cone driver.

No! Subwoofer drivers move incredibly slowly - at 50Hz, 10mm p/p travel gives an average speed for one stroke of 1m/s. This cone movement is slow. The speeds become much higher in the hundreds of Hz, where 10mm p/p at 500Hz needs 10m/s cone speed - a heavier cone will lag then overshoot etc.

I'd almost always take a larger driver than a bunch of smaller ones of equivalent cone area: a decent 10" subwoofer driver will move 1" without much problem. A 5" driver will have a lot of trouble matching this. 4 of them, while having the same cone area, will still be moving way too much for their size, and therefore go into distortion much earlier than the 10".

Chris

[sheep]

I have several EPI 8" woofers from speaker units.
I may try others with glass composite or even aluminum cones.
I believe 8" is more dynamic than to use a larger, heavier 12" woofer.
Even a 10" cone sounds slower.
Thanks for all the help.