"Stewart Pinkerton" wrote in message


On Wed, 24 Dec 2003 17:28:39 -0600, "Rusty Boudreaux"
wrote:

"Stewart Pinkerton" wrote in message
...
Well, unless you have the box on springs, it will also be due to
mechanical coupling. IME, ceilings are not so strongly built as
floors!

Not so. With identical opposing drivers the mechanical forces
cancel. It's not like a box sub trying to walk it's way across
the floor.

That's certainly the logical way to minimise box rattle.

IME it's a working plan. You may see both Nousaine and I referring to a
pair of subwoofers engineered and built by our friend Dave Clark. He went
this route and it just works.

Ultra-long throw 15" drivers such as the Stryke AV15 handle
overdrive quite well. Xmax is 23mm but Xsus is over 30mm. The
suspension is designed to handle overdrive without damage up to
the power rating of the coil. That said, once you hit Xsus
you'll never want to do it again...the aluminum voice coil former
hitting the backplate sounds like a shotgun blast.

Yup - that was my point. You can certainly do this with an IB, and I
doubt that it does the components much good! :-)

Stewart, I've always thought your comments were dead on but I
think you're blowing this way out of proportion. With a typical
IB setup you'll never hit Xsus. Even if you do it won't damage
the driver.

OK, I'll take your word on that, but voice coils slamming into
backplates (as you yourself describe) doesn't sound like a recipe for
longevity!

Agreed.

For example, I can play Telarc's DVD-A version of Tchaikovsky's
1812 Overture at such volume that I seriously worry my double
sash windows are going to come out the their frames. I'm talking
about pegging the radio shack SPL meter at 126dB before getting
scared and backing off the volume (even with earplugs). Not once
have those 7Hz digital cannons hit Xsus. Now I can hook up a
tone generator and hit Xsus with a 3Hz tone but how realistic is
that? The sub will play several times louder than my mains so
it's a moot point anyway. To get to the SPL numbers above I have
to turn off the main amplifers to keep from toasting the
speakers. Because of the IB's large headroom I can EQ the sub
for flat response from 10Hz up and hit reference levels without
running out of bass.

OK, so long as you're happy with it. Since I need 500 watts into 4
ohms to drive a Tempest fully for my target 115dB at 30-100Hz, I see
no harm in cushioning the driver by using a box sized so that max
power just drops below Xmax from 20Hz down. I'm not wasting any power
'cos it's there anyway, and I protect the drivers from any wayward
infrabass accidents.

The approach that Clark used was along similar lines. The boxes are minimal,
but large enough for the maximum rated driver motor force to stimulate
enough cone motion to meet design goals for low bass.

Tom Nousaine's IB has eight TC Sounds 15s (23.4mm Xmax) and he
has been unable to test maximum SPL at 10 Hz with a 5000 watt amp
due to his windows not being up to the task. To quote Tom,
"vinyl double windows twist in frame severely".

Yup, I'm familiar with his weapon of house destruction!

Last time I was there the house was still standing and seemed to be
suffering minimal damage. It seems to be a fairly ordinary modern box with
normal to-code wood frame structure and drywall. A tad flimsy compared to
mine, but if I had my druthers...

drivers we're talking over 20 liters of displacement at Xsus.
For another $700 you can make it 40 liters. An amp with an 8 Hz
double integrated 3rd-order Butterworth high-pass filter will
essentially eliminate hitting Xsus.

Agreed.

Or you can just use the correct size of sealed box and avoid the
problem altogether........

Why? What problem?

Agreed. I've suffered with a subwoofer box the size of a nice freezer chest
in my living room for something like 25 years. In the days when it was
built, 400 wpc was a BIIIIIG amp. Good equalizers were not the everyday
items they are today. Thiel-Small was still fairly new. When I do it all
over again it's gonna be a minimal-sized box. The basement solution is a
non-starter the reinforced-concrete and steel box I live in.

The poor sealed box has the problem due to
lack of displacement. If you're hitting Xsus with an IB then a
typical sealed box would have long since hit it's limit . A
sealed box of Tempests with equivalent displacement requires over
1000 liters for a Butterworth alignment. I'd rather have a small
discrete hole in my ceiling than a massive expensive box in the
room.

You can still have the box in the loft space, and a Linkwitz transform
equaliser will give you any alignment you want. A small box doesn't
have to mean a high Qts in 2003/4. I guess I'm just hedging my bets.
Besides, SWMBO doesn't want a hole in the ceiling!

Slits work and are pretty inconspicious. It's all a matter of hydraulics.

You do raise an interesting point, however, as I'm having trouble
running the numbers as to how small the hole in the ceiling can be
before it causes problems with either port noise or turning the
manifold into a reflex cabinet. Any advice on that mattter from your
own experience?

I see several approaches:

Crib details from a working design.

Forget the port and equalize

Build a scale model and then multiply by a suitable integer