Shared space sealed enclosures

nigel1977 · Postby **nigel1977** » February 22nd, 2008, 3:19 pm

Serious question about sealed enclosures.

If I have a sub that needs .6cuft less its displacement of .2, in a sealed enclosure, would I need to build the enclosure 1.0 for 2 subs (0.6 + .2 + .2) or 1.6(.6 + .6 +.2 +.2), assuming the subs share the same space i.e. the enclosure is not partitioned.?

Would the subs each now see 0.6 or 1.2 if i choose the 1.6cuft option?

I have the common sense answer in my head, I just need to hear from the guys that build sealed enclosures.

Again, Im specifically asking the guys that build sealed enclosures.

jeff · Postby **jeff** » February 22nd, 2008, 4:17 pm

they would each see 0.6...
qtc and f3 would remain as it is for 1 sub were the box spec to be halved

nigel1977 · Postby **nigel1977** » February 22nd, 2008, 5:12 pm

How is the subwoofer able to see .6 in a 1.2 cavity?

Thats the bigger question. And thats the point of my dilema.

I beleived that the sub would see half of the volume, like you suggest, but the question to calm my fears is how?

From a logical perspective, Id say each of the subs see 1.2, simply because thats the air space behind them.

Or is my logic wrong?

hydroep · Postby **hydroep** » February 22nd, 2008, 5:31 pm

^^It depends on how they are powered and their phase. If each sub is wired for the same phase and the same power is being applied then acting together, they are exerting a force twice that needed for a .6 enclosure. So technically, each sub in this configuration sees only half of the true volume (1.2).

To understand this take this extreme example: if you power only one sub then that sub alone has to move the air in the box so it will see the 1.2 volume. Consequently its response curve will change. Applying more power to one sub also unbalances the equation and will cause one sub to see more volume than the other causing response curve changes as well.

These enclosures are susceptible to these type of phasing and power problems especially when the speakers start to age at different rates. The best thing to do IMO is to create two separate chambers in one enclosure. :cool:

southside connections · Postby **southside connections** » February 22nd, 2008, 5:40 pm

i think pressre vs volume will equate this out

jeff · Postby **jeff** » February 22nd, 2008, 6:47 pm

the proof lies in the equations of relations between the cms, vas and qts

Postby **- Rovin's car audio -** » February 22nd, 2008, 7:38 pm

last time i built a sealed box was about 2yrs ago when i had my type Rs so cant say much for using sealed boxes often since ppl dont request sealed them often

....but i would just use what cuft 1 single sub would & just double it if its 2 subs ......can just a separate board if u want ......

silent_riot · Postby **silent_riot** » February 22nd, 2008, 8:11 pm

i with hydroep 100% on this one.

nigel1977 · Postby **nigel1977** » February 22nd, 2008, 11:09 pm

To bring the example more solid.

2 identical subs, bought at the same time, broken in for the same period, fed by the same power from 2 identical amps bought at the same time blah blah blah.

How does the subwoofer differentiate that it gets .6 and the other gets .6?

In this particular case, its impossible to properly partition the enclosure into identical sized volume halves as the enclosure is an mdf/fiberglass combo.

I spoke to a subwoofer designer/manufacturer friend of mine awile ago, and he is also of the view that 2 subwoofers in a shared enclosure see the total air space each, and not half the airspace.

Im going to build the 1 cuft enclosure and try it for myself. after all, i have a SA-3056 sitting here looking at me.

will post some test results when i have them.

hydroep · Postby **hydroep** » February 23rd, 2008, 1:56 am

^^Hmmm. Using your example of equal power, aging etc. the two drivers are acting like one large driver with double the Vas because the total cone mass has doubled. All the other T/S parameters remain the same.

Hypothetically, if you were to mount this large driver in your box it would see the 1.2 cu. ft. of air volume. You would agree that each half of this large driver's cone (as with any single driver mounted in an enclosure) would be acting on 1/2 the air volume in the box no?

If you agree then it is fairly easy to understand that what you are simply doing is "splitting" the cone mass of the hypothetical large driver among two smaller drivers. (Remember the other T/S parameters remain the same). Together their combined cone mass "sees" 1.2 cu. ft. of air volume just like the single large driver but given that each one's cone mass is 1/2 that of the large driver's each one can only act on 1/2 of the total air mass which equates to .6 cu. ft. of the 1.2 cu. ft. enclosure.

Don't think it can be explained simpler than that really. Would be interested to see what your test results prove tho'... 8-)

silent_riot · Postby **silent_riot** » February 24th, 2008, 7:28 pm

Good explanation again, hydroep.

Nigel, just to use an analogy, imagine that you have an inflated balloon that represents the volume & pressure in the enclosure, and each of your hands represents each subwoofer.
If you press the ballon with one hand it is easier than than if you press it with your two hands at the same time, ent?
The conditions at 1.2 cu ft do not exist (for 1 sub) when you have something else (another sub) changing the pressure at the same time....and if both are performing the same action, then hydroep's explanation follows.

nigel1977 · Postby **nigel1977** » February 24th, 2008, 10:27 pm

Both your analogies are done assuming both subwoofers are in movement. wheat happens when the subs are are rest? Arent subs, especially thos that are crossed very low, usually at rest, until they are fed a frequency within their bandwidth?

Conventional theory suggests that the subs would see half the enclosure.
Again, I'll only be convinced when I have test measurements infront of me. Call me stubborn, but audiophile level car audio is anything but conventional. I should have a 1 cuft enclosure loaded with the subs by wed as per the manufacturer's specs. Yeah, the guy that built the speakers advised me to do a 1 cuft for both subs. Will post test results when I have them..

hydroep · Postby **hydroep** » February 24th, 2008, 10:45 pm

^^...

...

What does the subs being at rest have to do with anything? At rest they will not be producing any sound so that scenario is irrelevant.

You keep changing the conditions and to be honest I'm really not sure what you're asking anymore. Seems that testing is the only way you'll get answers to your question(s)...whatever they are. Good luck with that.

BTW kudos silent_riot. Your balloon analogy was actually quite simple and provides the layman with a good visual representation of what is going on in response to the second scenario... :cool:

silent_riot · Postby **silent_riot** » February 25th, 2008, 1:10 pm

Remember, theory and practise can be 2 entirely different things, eh.
I'm looking forward to the results...

hydroep · Postby **hydroep** » February 25th, 2008, 6:06 pm

^^Agreed 100%.

I hope then that he posts the results with respect to the question posed here (it seems important to him :wink:

)...:

nigel1977 wrote:Both your analogies are done assuming both subwoofers are in movement. wheat happens when the subs are are rest? Arent subs, especially thos that are crossed very low, usually at rest, until they are fed a frequency within their bandwidth?

If he wishes to make it absolutely clear, then of particular interest would be his answers to the following questions:

1. How much volume does each speaker sense while at rest?...

2. How does he measure how much volume each speaker senses while at rest (i.e. they are not being fed a signal)?

And...

3. Assuming that it were possible to answer 1 and 2, why does he want to know that anyway?

Just curious of course. ... 8-)