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-   -   A roast requested on my TQWT design (http://www.diyaudio.com/forums/multi-way/30961-roast-requested-my-tqwt-design.html)

Christof 26th March 2004 01:36 AM

A roast requested on my TQWT design
 
2 Attachment(s)
I started off thinking I was going to whip out a quick clone of the Cain and Cain Abby TQWT's w/ FE166E drivers to use with my new SET gear but as always the plan got twisted a bit. Please take a look at the picture and let me know what you think about....

1. Using multiple vent holes vs. a single vent
2. Curved front baffle, I will use 2" spikes to hold the ends out forming an arc.
3. The design in general


note* the overall length of the arc baffle will be 10" shorter than the enclosure, the picture is just a mock-up baffle.

link to more pics: http://www.msnusers.com/Dymaxionsubdesign/shoebox.msnw


Thx, C.

MJK 26th March 2004 01:49 AM

Hi Christof,

Quote:

1. Using multiple vent holes vs. a single vent
Why?

Quote:

2. Curved front baffle, I will use 2" spikes to hold the ends out forming an arc.
Why again? Not trying to be a wise guy, just not sure what you are trying to accomplish with these changes.

Quote:

3. The design in general
Have you run any MathCad simulations of the geometry and the FE-166E? I think that the FE-167E might be a better choice for this type of enclosure. My guess is that without some form of BSC circuitry the bass is going to be very light weight.

Christof 26th March 2004 01:58 AM

Martin

As a sculptor and furniture designer I cannot leave a design alone. The internal volume and dimensions of these TQWT's are an exact copy of Terry Cains Abby speakers, that a friend let me use for a template. the FE166E is what comes in the Abby and I figure it was already a proven speaker design. They also offer the 165E for near field.

There is not explination behind the use of 3 holes vs. one, it just sort of happened.

If your still following this, is there any disadvantage to using multiple vent holes? I think Moth audio does this if I remember right?

Thx...c

leadbelly 26th March 2004 02:51 AM

Well, FWIW:

I don't know if you are arriving at cabinet dimensions by cloning another design or by available modelling tools such as Martin's work. If the unknown is what the effect of a non-constant taper will have on the response, you can always REALLY give yourself a challenge and also curve one of the sides so that the cross-sectional area does have a constant taper. Or something easier is to build an internal shell that matches the base design, and then laminate material on to that to make whatever shape you like.

Rudolf 26th March 2004 08:42 AM

Christof,
I like your design giving the speaker front some visual "tension".

Since your front panel is almost flat around the driver, I believe it would not do any harm acousticwise. But you should keep an ear on possible panel resonances, if the panel arc is only suspended by some spikes at its end.

Distributing the vent diameter to 3 holes does require some end correction. I have seen explanations for this, but only in german. So better someone else should lead you to an appropriate website.
Take care to not obstruct the free airflow from the vents (as you already have done).

MJK 26th March 2004 11:11 AM

Quote:

If your still following this, is there any disadvantage to using multiple vent holes? I think Moth audio does this if I remember right?
I am not sure, I will have to think about that question.

GM 26th March 2004 02:09 PM

As Rudolf notes, the baffle must be stiff or massive enough to push its eigenmodes either above, or below the passband, and since it's easier to damp out the acoustically weaker mids/HF than the lower frequencies, stiffness through bracing is the Hot Ticket.

Since the radius is acoustically small WRT its passband, it's effect will be minor and swamped by the room's effect on the BW, so not an issue IMO, though cavity resonances between the baffle/cab may require you to at least line the back side of the baffle with Deflex or similar.

Using multiple vents makes the alignment ~aperiodic in nature, though normally they would be all the same size. When one has a much larger CSA than the others, it dominates, so to average them out requires some damping material stretched across them, which is what Moth appears to do. Tuning is then accomplished by measuring the FR with different densites till you get the desired roll off with the speaker at the desired in-room speaker/listening position.

GM

sreten 26th March 2004 06:05 PM

Quote:

Originally posted by Christof

There is not explination behind the use of 3 holes vs. one, it just sort of happened.

If your still following this, is there any disadvantage to using multiple vent holes?
I think Moth audio does this if I remember right?

Multiple ports, if all of the same length act as one port of the
same CSA, though effective length will be different to one big
port. No real advantage / disavantage IMO.

The Moth port is a conceit.

:) sreten.

Christof 29th March 2004 02:27 PM

Bump for more info....

I'm still unsure of the overall effect that baffle shape has on a speaker. In this case I have a 12" flat spot for mounting so my baffle is essentially 12" x 9" and then the curve starts. What would be the difference if I made the baffle curve the other way but still kept the 12" x 9" flat spot? |( vs. |)

thx..

GM 29th March 2004 05:57 PM

'Sound is round', so visualize a series of different diameter soap bubbles emanating from the driver. Ones that are larger than the driver 'feels' the baffle as an extension of the effective cone area (Sd) as it spreads into a hemispherical shape. Above 9", or a frequency of ~13560/pi/9 = ~479.6Hz, the baffle's curvature begins to affect the loading, and at 12"/~359.7Hz the lower (larger) frequencies begin to 'fall off' the baffle, i.e. the FR rolls off at some rate up to 6dB/octave. This is why baffle step compensation (BSC) is required if a speaker is well away from boundaries, though normally only 3-4dB/octave is required in a typical HIFI or HT app..

Now if the baffle's top/bottom forward curvature/length is such that it mirrors the width's roll off slope, Then acoustically they balance out and theoretically no BSC is required. Obviously, a lessor curvature/length will have a proportionately less impact. Conversely, if the curvature is towards the rear, then it will increase the rate of roll off Vs a flat or forward curvature baffle. It will also have less ripple in the FR than the forward curvature, so from a technical POV it's the preferred curvature orientation of the two. Maybe this will help: http://www.trueaudio.com/st_diff1.htm

At the minor curvature rate and baffle frequency you mentioned, I doubt the offset in either direction would be audible, and any deviation will be masked/~compensated for by the BSC ckt. and otherwise swamped by the room's effects, so not an issue IMO.

GM


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