Back Air Chamber - La Scala vs. FH1

[JohnA]

I picked up another FH1 to build into a center channel to match the
other 2 Peavey/Altec systems I built (FH1s and Altec 511Bs and 902-8Bs)

The
OEM woofer in the FH1s absorbs a lot of power but is pretty nasty
sounding with audible THD even at comfortable sound levels. I'd
rather use K-33s in them all. I measured the back air chamber
volume of the FH1s and got 3538 cu. in. (57.98 liters).

Using Andy Barr's sketches and verified measurements I got 3973 cu. in. (65.11 liters) for my La Scalas.

Do
I have an error? Is the difference in volumes enough to be
significant? If my numbers are accurate, the FH1s will have less
lower bass than La Scalas would with K-33s.

There's little
practical way to enlarge the FH1's back air chamber, so would stuffing
it with polyfil be enough to make it behave as though it were as large
as a la Scala's back air chamber?

I'm considering Bob Crite's cast frame woofer for it's lower Fs and 150
watt capacity. Will it be more suitable than late model
K-33-Es?

My brother has my "Loudspeaker Design Cookbook", or I wouldn't ask.

[Guest " "]

Can you vent the cab with a port?

Maybe on the access hatch or some other hidden spot.

There are a few post on the LaScala port mod.....maybe that concept could be applicable.

Don't know what your after in the long run...k-43's would give 200 watts, k-45's 400 watts.

[jwc]

John.

From what I have calculated, the La scala back chamber is 4283.875 cu inches not including the volume displacement of the K33. After volume displacement of K33 (about 7 liters), the chamber volume is 3546.089 cu in.

This could be a little off. But this is the value I have used to calculate potential projects similar to a scala with a K33.

Hope this helps.

jc

[colterphoto1]

loose stuffing with polyfill in an enclosed chamber can increase the apparent volume of the space by something like 25-30%. Might be just the thing for your project.

I'd sure give it a try before getting out the Sawzall.

[JohnA]

Thanks, Michael and all!

The 25 to 30% is the figure I
didn't have. If I can get that much, I'll have to be careful not
to put too much stuffing in.

The original Peav-tec
speakers were intended for my amusement and ocassional use at
church. We're building a new church building and it will have a
100 seat side auditorium. So, since that room needs a sound
system and I've been angling for a theater room for "date/movie night",
This looks like a match made in heaven. (Couldn't help it.)
We already have a pair of KP-480s and a pair of HIPs, so all we need is
another Peav-tec for a center channel..

The OEM woofers
are almost unsuitable for anything but outdoor brute force. K-33s
are known to work pretty well in FH1s, but freddyi posted a response
curve showing falling bass response below 100 Hz, or so.
This was the first clue. I want less distortion and deeper bass
from the front array. While an obvious choice, K-43s are pretty
rare and expensive. Indoors, a standard K-33-E should be plenty
good.

[Guest " "]

"While an obvious choice, K-43s are pretty rare and expensive"

If you need k-43's, I have 2  pairs, maybe 3 pairs.

[colterphoto1]

I don't know about that John, we still use K43's in lots of cinema gear, like the MWM. I just bought 4 brand spanking new ones. They are a cast aluminum basket now, with vented pole piece, but otherwise are the same.

[jwc]

"While an obvious choice, K-43s are pretty rare and expensive"

If you need k-43's, I have 2 pairs, maybe 3 pairs.

SF

Let me know of those K43's before you get rid of them.

jc

[JohnA]

PM sent.

[DrWho]

The volume of the rear chamber is selected for the reactance annulling

of the horn. It is dependant on both the driver and the horn. If you've

got the T/S params of the drivers in question and can measure the

dimensions of the horn, then you might try modelling things up in

Hornresp which is free:

http://www.users.bigpond.com/dmcbean/

You might be surprised by how things model up. The program can also

simulate rear chamber lining as well as porting (but only porting to

the outside, not into the horn).

Anyways, since the FH1 isn't identical to the LaScala, you may not

necessarily need to change the rear chamber volume when you drop a K-33

into the cabinet. Also, the K-43 doesn't dig as low as the K-33 in the lascala. I

think some might get misled by the increased midbass efficiency.

[JohnA]

Mike,

I've heard it said, here by a Klipsch employee,
the K-43 doesn't go as low as a K-33 in a K-horn, but not in a La
Scala, though that is commonly repeated.

The FH1 is very
nearly a deep Belle Klipsch and so should actually go deeper, if only a
little. I am aware of the relationship between the horn and back
air chamber, if only in a superficial way. Civil Engineering
doesn't fool with sound and resonances much. Since the Peavey
1504-4 woofer is not up to par, I want to make sure I select the best
woofer of the ones available. Thus, all the questions.

I sure wish I'd popped for that P.E. Woofer Tester when it was in stock months ago.

[Guest " "]

[Guest " "]

K-43 spec

 

TSPs

Fs 36 Hz

Re 3.4 Ohms

Qts 0.314

Qms 5.314

Qes 0.333

Vas 198.474 L

Sd 0.089 m^2

Xmax 0.29 in

BL 15.94

 

It is a 400 Watt woofer with a Sensitivity of 98.5

@ 2.83V/1M. Freq response is 45 to 3,500 Hz and has a 3 in voice coil. 

[Guest " "]

LaScala  is a 53hz hi pass horn (some say 70hz).

below are the ts specs for both drivers.

JC posted the dog house volume in another thread.

If someone wants to model and provide a frequency response compare...it show be informative to the modeler.

[jwc]

Guys...I'm not a pro here but have been crunching these kinda numbers quite a bit. Take these results with that in mind.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Using the T/S parameters to calculate the Vb (chamber volume) of the La Scala won't get you to the real world utilization. PWK has some secrets in there....the Vb used in his designs will come up smaller than what would be calculated with T/S parameters of a driver. The Khorn is the same way.

I was assuming the La scala was a 70Hz horn. The Vb for a 70Hz horn using a modern K33 is about 89.6 liters or 5467.727 cu. in. It gets funny with the k33 or Bob Crites drivers because they have such a low Qt and a large VAS. This ends up making the Vb a large value. Typical Pro drivers now have a low Qt but also have a low Vas.

John.

If you are interested in using a driver that "fits" the Fc and the chamber volume of the FH1 (which I know nothing about), let me know. I have calculated or can calculate quite a few 15" drivers so that I can use their info on the fly.

SF, where did you get that K43 specs. You must know somebody.[] Don't answer that.

jc

[Marvel]

The LaScala horn mouth area is only good to about 125 Hz. To quote someone else on here, "below that is is a big woofer in a sealed box."

This is why the bass mod is such a cool idea.

Bruce

[jwc]

Yep, the final flare is 125hz. But I have heard several times that the Scala is a 70hz horn. The intitial flares are close to that. This is the opposite of the approach of the jubilee which has about 95hz flare initialy and then gets to 38-40Hz on final flares.

So....for a 125Hz horn with a K33......Vb calculated.......... not sure this is usable...3062 cu in.

jc

[JohnA]

Here's what I found on Peavey's site:

Model#	Re	Fs	QTS	QMS	QES	VAS	No	Xmax	VD
1504-4DT	3.5	31.3	.29	6.2	.32	385	3.6%	+/-3.0	240

http://www.peavey.com/support/technotes/misc/blackwidow.cfm

I got this from Eminence many years ago:

My 1981 La Scalas have these woofers.

Model#	Re	Fs	QTS	QMS	QES	VAS	No	Xmax	VD
15162/K-33	3.39	34.46	.390	7.39	.410?	301.66 l	2.91%	8.2mm

SPL = 96.6 dB; BL=11.88 Tm;

Am I correct in thinking the old K-33 is a pretty good match?

Here are freddyi's graphs with the 1504-4: It matches the Peavey published curve pretty well above 200 Hz.

http://forums.klipsch.com/forums/thread/848463.aspx

I'd really like to bring the bottom up. I'll get
some architectural reinforcement from being indoors, but the graph
looks lame below 150 Hz. What measurements do I need to get
for Hornresp?

[DrWho]

Since you have the T/S params for the drivers, all you need to do is

measure the cross-sectional area for every set of expansion in the

horn. Basically, you need to know the starting area, length of the

expansion, and then the final area. You combine all these segments

together to predict the total system response. You could be really lazy

and just measure the throat, mouth, and length of the total horn, but

it won't be as reliable since there are often variable flare rates

involved. Once you have the horn designed, you can just click on "Add"

and it will copy all of the settings over where you can change out the

drivers and compare responses on the fly. If you need help with the program let me know - it took me a while to figure it out and even longer to know my way around. The help file explains a lot, but can be ambiguous much of the time too.

[BEC]

John,

The T/S specs you show above would not be for a K-33E from 1981.  The FS of 34 hz indicates the round magnet version of K-33E made after 1985.  Before 1985 the K-33E had an FS of around 28 hz.

Bob Crites