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Old 20th April 2005, 06:50 PM   #1
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Smile Cheap fullrange project

Hi,

As the title says, I'm planing to build a pair of cheap fullrangers for really not-so-critical listening (background music, radio, TV, computer etc).

This cheap fullrange driver i'm interested in is a four inch driver made by "Right", and the model "FR100-8". Of course, the expected quality for this driver is propably very low, but so is the price too. I could buy these from my work (electronics repair&service shop) at price of about 5 euros a piece. So getting a couple of these just for testing and tweaking wouldn't really hurt my wallet.

I modelled the driver in MJK's "TL Offset Driver.mcd". As you can see, there are some dips and peaks in the SPL response.

Because I don't have good knowledge of transmission line design, I wonder if it could be improved? Perhaps the Qts&Qes values are too high, or TL parameters I'm using are not good?

The advertised specs:
Qts: 0,75
Qes: 1,00
Qms: 2,966
Fs: 107 Hz
Vas: 2,84 liters

TL parameters:
Line length: 35 inch
Driver position ratio: 9 inch
S0: 6 x Sd
SL: 1,75 x Sd
Density: 0,2 lb/ft

Please post comments, feedback is welcome!
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Old 20th April 2005, 10:30 PM   #2
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With an Fs of 107Hz a 35" line is going to be too long. For example, the Tangband W3-871S with an advertised Fs of 110 Hz works best in ~16" line. Tuning a cabinet below the driver Fs is not beneficial unless you plan on adding heavy EQ. Also, there are some numbers missing such as port radius, length and position and it's going to be difficult getting meaningful models without the driver BL and Re measurements. Overall, I don't think this is a good candidate for a TL but as you pointed out, the price was low enough to take a chance.
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Old 21st April 2005, 04:05 PM   #3
kneadle is offline kneadle  United States
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Timn8ter,

This is a very puzzling subject for me. I have modeled, using the MJK worksheets, the TB 881s to have a flat FR far below its advertised (and measured) Fs of 111Hz. What should I beware of before I construct the prototype?

Or, why should I expect to use heavy EQ when my predictors demonstrate flat FR?

I'm pursuing this as a personal scientific experiment, but it would be nice to add some more data to my simulations, if you have something to offer in that regard.

Thanks,
Dave
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Old 21st April 2005, 04:29 PM   #4
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To be brief I'll just quote Rick Schultz from his 08/03 AudioXpress article.

"TL myth assumes you can lower system response by building a longer pipe; pushing pipe frequency lower must extend bass performance. This myth started 65 years agoin when Voigt set "length just under one quarter of the lowest frequency at which efficient working is desired." He does not relate quarter-wave pipe frequency to driver resonance. Instead, he related it to whatever frequency "is desired."
Speaker builders have assumed just building a longer pipe will extend response. In fact, building a lower frequency pipe just quiets it. Longer TLs perform much worse than shorter TLs. The lower limit for pipe frequency should equal Fs. I recommend going no lower, any longer."
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Old 21st April 2005, 08:15 PM   #5
kneadle is offline kneadle  United States
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I'm still evaluating the article, so the following comments are a little premature.

I'm still unclear what you meant by "adding heavy EQ." Were you referring to an external EQ unit, passive or active, or were you referring to stuffing as an EQ method?

Oh, and I guess when we're talking about "TL," there's a current problem of referent. What I've modeled, and am about to build, is a variation of the MJK ML-TQWT. I consider that a TL in the broader sense. That's my referent.

Thanks,
Dave
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Old 21st April 2005, 09:26 PM   #6
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When I said EQ I was thinking Linkwitz Transform.
I think Martin's worksheet allows for lower driver resonance due to the air mass inside the enclosure loading the cone, however, lower Fs by mass loading changes the Q and thereby the output of the driver, so, are you really gaining anything? My personal experience is that keeping Fb equal to or above Fs is going to yield the best results.
In reference to a 35" long pipe for a driver with a Fs of 107Hz.....well...
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Old 21st April 2005, 09:30 PM   #7
kneadle is offline kneadle  United States
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Quote:
Originally posted by Timn8ter
When I said EQ I was thinking Linkwitz Transform.

In reference to a 35" long pipe for a driver with a Fs of 107Hz.....well...
Ok. Now I see what you're talking about.

And...agreed.


Dave
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Old 21st April 2005, 09:42 PM   #8
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I think you underestimate the low end performance of a TL that is sized according to the theoretical calculations.

Of course, you'll have two drivers, build both and let us know what you find out. Maybe you'll find something new.

:)ensen.
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Old 22nd April 2005, 12:28 AM   #9
GM is offline GM  United States
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Greets!

What are all the specs you are using? Do you know Re, Le, Sd? The IB response (dashed line) is totally whacked for the specs you published so your sim is useless.
Quote:
Longer TLs perform much worse than shorter TLs. The lower limit for pipe frequency should equal Fs.
I disagree! Longer ones are inherently better damped, and with less stuffing density, a 'good thing' when it's a FR driver. That said, 35" is way too long for a reverse tapered pipe since this driver really shouldn't be tuned <~59.3Hz according to T/S. Just running the numbers using:

Sd = 45.6064 cm^2 (assumes 3" effective dia.)
Re = 8 ohms
Le = 0.6 mH
BL = 3.512 N/A
pipe Vb = ~0.71 ft^3 per T/S

I get:

L = 25.25"
eta = 9.42"
SO = 90.48"^2 (~12.8*Sd)
SL = 6.05"^2 (~0.856*Sd)
density = 0.25

Quote:
I think you underestimate the low end performance of a TL that is sized according to the theoretical calculations.
I think you're right. WRT gain, pipe Vb must increase with decreasing Fp (length). When it comes to loading high Q drivers and/or loading a driver below Fs, Vb (air mass) is the acoustic lever required to help a 'weak' driver load the terminus. Voigt understood this. If you don't provide it then RS's statement is at least semi-correct. It's all a matter of what the performance goals/acceptable trade-offs are.

GM
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Old 22nd April 2005, 12:35 AM   #10
kneadle is offline kneadle  United States
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Hey GM,

Just a practical question: how do you build enclosures which taper from 90.48" sq. to 6.05" sq? Am I really that far behind in my woodworking skills?

Please tell me that you round up or down...

Dave
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