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Old 20th April 2004, 04:53 AM   #11
GM is offline GM  United States
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>Thanks again.

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You're welcome!

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>The extension of the sub is actually not that great, it's about -3db at 50hz and -10db at 25hz according to the measurements. But you may well be right regarding the room gain.

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What are you measuring with? These numbers don't jive with an in-room response implied by the sims. Even though room gain won't begin in ernest until ~565/20 = ~28.25Hz, a half space aneochoic sim I did using MJK's worksheet and Lspcad Pro shows more, so should be pretty strong when up against a wall, and expecially if corner loaded.

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>Would EQ be worthwhile and at what cost?!

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Of course! It will allow you to flatten the rising response and tame any strong room spikes as well as shape the rest of the BW. I don't keep up with that stuff anymore, but I see the Behringer brand pop up in various threads from subs to active XOs, so check them out. I think this is the most bang for the buck http://www.behringer.com/DSP1124P/index.cfm?lang=ENG but browse around on the various forums to be sure.

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>GM, the designer did say that the tuning was underdamped ( I don't understand the implication I'm afraid), but felt that it would suit the room gain.

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It means that it 'rings' due to peaking at Fb, i.e. distortion, and can never 'suit' room gain if good/superior SQ is a performance goal. Sounds like car audio boombox design philosophy to me. Oh well, some folks like it, but it's not HIFI.

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>I sadly cannot lengthen the port. It is 12" already and, being rear facing, nestles quite close to the drive unit as it is.

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You can always build a new cab.

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>The Win ISD data shows a sealed cabinet of the same size gives the same output at 20hz, but slightly less between 80hz and 30hz. I have to admit I don't fully understand the implications of this!

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This driver is 'stiff', IOW it has a low compliance, so the cab can't give it much gain. It's the price you pay for getting big bad drivers to play low in small cabs.

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>But the group delay is significantly less. Having said that the 1500AL was designed for a vented enclosure. So what information do you guys need to allow me to make the next move?

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Well, stuffing and vent damping is going in the right direction to reduce/eliminate the ringing, and using EQ to tonally balance (flatten) the in-room FR may be sufficient.

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>I know that JBL line the cabinets with 1" fibreglass, but this was difficult to source. Some people tell me that it sounds better.

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I'm surprised, I can buy Owens-Corning 1" acoustic fg at my local Home Depot and Lowes. Anyway, in the BW of interest here, Dacron polyfil, R19 fg insulation, Miraflex R25, all work just fine. Stuff up to 1.5lbs/ft^3.

GM
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Old 20th April 2004, 09:09 AM   #12
sreten is offline sreten  United Kingdom
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To detune the port try lining it with foam.

You need a rectangular peice 3 times the diameter x length.

1/2" thick is good for a 3" port. 5/8" for a 4 inch port.

If you like the effect then ideally you should
reoptimise port diameter and length.

IMO an "underdamped" alignment is not good in most real rooms.

sreten.
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Old 20th April 2004, 09:19 AM   #13
Merlin is offline Merlin  United Kingdom
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Sorry I was mistaken. This is from the original design brief

Quote:
I am using 110 L - 120 L.
The simulations show this as an over damped alignment, not optimally flat,
but I have had much better results with larger enclosures. I have the
system tuned to 27 Hz using a 100 mm vent with generous radii on either end
to reduce turbulence noise. It gives a response that looks very much like a
banana. There is a gentle curved roll off below 100 Hz and no sign of a
knee in the curve. This curve shape has much better group delay
characteristics and self equalizes due to room gain very nicely..
So it appears to be overdamped.

Thanks for the in depth reply GM. So the concensus would appear to be to add more damping rather than remove it. Would this still hold true given that it is in fact an over damped alignment?
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Old 20th April 2004, 03:16 PM   #14
GM is offline GM  United States
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He's right that a larger, overdamped alignment is a good performer in a typical HIFI sub app, but using both published and the measured specs of one unit, I get underdamped and severely underdamped alignments, so I don't know what program/specs he used to arrive at his results.

Stuffing lowers any peaking a little, as well as damping the vent's harmonics. Damping the vent swings the cab alignment towards aperiodic (overdamped).

Anyway, we're responding based on what precious little info you've given us and your measurements is at odds with my experience. The bottom line is what do your ears tell you when you add damping?

GM
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Old 20th April 2004, 03:55 PM   #15
sreten is offline sreten  United Kingdom
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Plots are for one driver data from :

http://www.audioheritage.org/images/...0al-spec06.jpg

Indicates a box of 115 litres tuned to
27Hz is a classic overdamped alignment.

sreten.

edit :
Just add for serious low bass you should convert to 6th order.

A 27Hz 2nd order high pass filter with Q adjustable from 1 to 3
will allow optimum alignment to your room gain and give you a
- 3dB point basically of your choosing, down to below 30Hz.
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File Type: gif 1500al.gif (59.7 KB, 270 views)
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Old 21st April 2004, 08:47 PM   #16
Merlin is offline Merlin  United Kingdom
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I'd just like to say thanks to all for the advice.

I have detuned the port with a roll of Dacron and things have improved.

Quote:
A 27Hz 2nd order high pass filter with Q adjustable from 1 to 3
will allow optimum alignment to your room gain and give you a
- 3dB point basically of your choosing, down to below 30Hz.
That sounds exactly what I need! It brings me onto the next part really. I am running a fairly decent system and am currently performing crossover duties with a cheap AV receiver that is acting as a bit of a bottleneck. I'd like a quality module that would allow high pass and low pass filters along with some phase adjustment if possible and level matching. I know of the Behringer kit but I am looking to do this without digital processing (I listen to a lot of vinyl) and would like to preserve the quality of the signal. The main amps are Sim Audio P5/W5.

Are there any companies that specialise in this sort of unit of can you point me to some useful reference material?

Thanks again

Michael
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Old 21st April 2004, 09:13 PM   #17
sreten is offline sreten  United Kingdom
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Quote:
Originally posted by Merlin
I'd just like to say thanks to all for the advice.

I have detuned the port with a roll of Dacron and things have improved.

That sounds exactly what I need! It brings me onto the next part really. I am running a fairly decent system and am currently performing crossover duties with a cheap AV receiver that is acting as a bit of a bottleneck. I'd like a quality module that would allow high pass and low pass filters along with some phase adjustment if possible and level matching. I know of the Behringer kit but I am looking to do this without digital processing (I listen to a lot of vinyl) and would like to preserve the quality of the signal. The main amps are Sim Audio P5/W5.

Are there any companies that specialise in this sort of unit of can you point me to some useful reference material?

Thanks again

Michael
Its not likely at all to be exactly what you need if "detuning" the
port is an improvement. A roll of Dacron is good for killing a port
not detuning it, you'll have a near sealed low Q alignment.

Sealed Q will be 0.5, critically damped, good for room integration.

Given the above I can't recommend going 6th order,
just seal off the port or properly detune to 22Hz.

12" length indicates 12cm diameter but this is discounting the
flares at either end, the designers 10cm sounds reasonable.

A foam insert 30cm x 30cm x 1cm seems reasonable,
adjust length to suit the flares.

sreten.
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