Transmission line subwoofer question

[planet10]

Originally posted by Street
What does BW stand for?

Bandwidth
Black & White
Boris & Winona

...

dave

[Bob Brines]

Street,

Don't look at the impeadance peak, look at the FR peak at cut-off. Now, a problem with the strawman I gave you is that it doesn't take room lift into account. But, it does show you wat is possible, if not exactly practical. You have to wipe your mind clean of all of the rules-of-thumb and old-wives tales about TL's. Reread GM's post. The short, fat pipes IS the way to go.

Bob

[Street]

The designing is comming along pretty good thanks to the information you all have given me. I was wondering if anyone knows of a good resource on room gain. I have found plenty of information on calculating room modes but very little on translating this to room gain. Due to the relativly small size of my room (12'x14'x8') room gain will likely have a significant impact on FR which needs to be compensated for.

-Street

[Street]

I think I'm pretty much decided on my design and I'm looking for a little feedback on what I've come up with. I'm going to use the Lambda SB10 driver with the single faraday ring motor, which can be seen at http://www.lambdacoustics.com/drivers/SB10.html.

My transmission line will have a quasi-cone shape with x-sectional area of 2800 cm^2 at the driver end and 150 cm^2 at the open end. I have decided on using 8 sides made of MDF. The driver will be mounted on the bottom facing downwards and slightly off center. The line length will be 78 inches. I would have made the line length a bit longer but ceilings have a bad habit of getting in the way. This will be stuffed to taste although probably close to 0.7 lbs/ft^3 from modeling.

I'm planning on finishing it with two different types of veneer, which will alternate around the 8 sides. I will likely use walnut and kewazinga for this so although it will be large it should be an attractive piece of furniture.

Oh, I also need to buy an amp to power this. Any suggestions on an amp or diy kit that would match well with this driver?

Let me know what you think.

-Street

[GM]

>I was wondering if anyone knows of a good resource on room gain. I have found plenty of information on calculating room modes but very little on translating this to room gain. Due to the relativly small size of my room (12'x14'x8') room gain will likely have a significant impact on FR which needs to be compensated for.

====

In a perfectly sealed room that's built like a vault, room gain begins at ~the average of all the first axial modes, rising at 12dB/octave with decreasing frequency, so the ideal speaker would be a sealed cab with a complementary curve to yield a flat amplitude response.

So theoretically, your room gain would begin ~at:

~(1130/2)/(12*14*8)^0.33 = ~52.44Hz and ramp up to +12dB/~26.22Hz, etc..

So much for theory. In reality, most closed in HIFI/HT rooms have a choppy looking response that averages 6-9dB down to around 1WL/pi of the longest dim (~30Hz in your room) and rolling off dependent on how lossy the room's construction is. If there's an open doorway, then the room becomes part of a tuned multichamber resonant ckt, making for some interesting room gain measurements. Toss in furniture, etc., and little wonder you couldn't find much info on the subject. Still, it's best to design a room around theory whenever possible.

FWIW, I prefer to design around very little gain since distortion wise it's better to EQ it out than boost it in.

====

>I think I'm pretty much decided on my design and I'm looking for a little feedback on what I've come up with. I'm going to use the Lambda SB10 driver with the single faraday ring motor, which can be seen at http://www.lambdacoustics.com/drivers/SB10.html.

My transmission line will have a quasi-cone shape with x-sectional area of 2800 cm^2 at the driver end and 150 cm^2 at the open end.

> I have decided on using 8 sides made of MDF. The driver will be mounted on the bottom facing downwards and slightly off center. The line length will be 78 inches. I would have made the line length a bit longer but ceilings have a bad habit of getting in the way. This will be stuffed to taste although probably close to 0.7 lbs/ft^3 from modeling.


====

Hmm, a taper ratio of ~18.67:1 in such a long pipe will get you a severely underdamped (ringing) response (poor transient response). 8-10:1 is usually the limit and the pipe's Vb needs to be ~ = or < the T/S max flat alignment for good performance.

====


>Oh, I also need to buy an amp to power this. Any suggestions on an amp or diy kit that would match well with this driver?

Let me know what you think.

====

With such a low efficiency driver, the amp needs to be rated at least its peak transient rating, or 1kW/8ohms for this one. Any of the better pro sound amps from Mackie, Crown, etc., will be fine since HF BW/quality isn't an issue. The lower its damping factor and cutoff frequency the better IMO. My bang/buck fave was the QSC 'USA' series, but I don't keep up with this stuff anymore, so don't know if they're still available.

HTH,

GM

[Street]

Thanks for the info GM.

What is the T/S max flat alignment and if its not a listed spec how do I find it? I searched around but could not find a definition/description for it.

-Street

[GM]

You're welcome! Messrs Thiele and Small (T/S) used electrical filter theory to arrive at a way to design cabs based on their electro-mechanical properties to a much greater degree than previously. They calculate the aneochoically maximally flat response, i.e. F3 and Fb is the same frequency. Any other alignment, whether over/underdamped, F3 will be >Fb. I was referring to the Vb required to make this alignment, and the one that most speaker programs default to. With published specs, the SB10 vented defaults to 5.7ft^3/17hz Fb in BoxPlot.

Different programs yield slightly different numbers due to whether they used the original formulas or which one of the lumped constants versions.

GM