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.

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!

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.

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!

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.

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

Why? What problem? 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!

You do raise an interesting point, however, as I'm having trouble
running the numbers as tpo 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?

Four AV15's in an IB will give you 3x the output capability at
Xmax (15 liters vs 3 liters).

Unfortunately, I have to use what's available in the UK, which doesn't
include the Stryke AV15 - and I can't afford a pair of Tumults!

Although the Tumult is the king of displacement I doubt anyone
would use on for IB due to cost. Since an IB lends itself to
multiple drivers it's more economical to use 'lesser' drivers.
Four Tempests will get you the same displacement as two Tumults.

In your particular type of IB, this is true, although for 'in-room'
IBs the Tumult and DPL12 are sonically superior due to their rear
basket aerodynamics.

Seriously, the Tempest is an excellent driver for an IB. Along
with the Dayton 15" drivers (available internationally through
partsexpress) the Tempest was the most popular IB driver until
the AV15 came along. The AV15 has about 50% more displacement
per driver than a Tempest or Dayton and costs slightly less per
liter. For the same output you just need a few more
Tempests...or fewer AV15s.

If you want an AV15 (or multiple ones) I'll order them from
acoustic-visions and ship them to you at my cost (no added fee).
You've been posting here long enough...I trust you

Do you have Thiele Small numbers for the AV15? I might take you up on
that if I can't source them any other way. As you say, displacement is
everything at LF.
--

Stewart Pinkerton | Music is Art - Audio is Engineering