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Old 6th June 2008, 12:39 AM   #1
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Default I need some advice.............

I am in the process of designing a quarter wavelength loudspeaker based around the Fostex FE-166E.

The peculiarity of this design is that the low frequency cutoff will be at about 100Hz, so I'm not really concerned with getting the lows "right."

My questions are:

Can truncating the low frequency response of this driver through filtering- yield benefits in the mid/high frequency response??

If a transmission-line theory is based upon exploiting a drivers resonant peak, then what happens if you filter-out the resonant frequency??

Please correct me if I'm wrong:

Because music does not have every frequency occurring at the same time (unless you like listening to white noise) I assume that this design will perform exactly the same as if the missing frequencies were present.
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Old 6th June 2008, 04:03 AM   #2
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Hi cadaverdog, your project sounds interesting. May I ask what made you choose QW as the design? Just asking out of curiosity!
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Old 6th June 2008, 01:10 PM   #3
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Default Sure................

I am doing an engineering senior design project- and one of the specifications is that it be an original idea.

What I think makes this original is that this will be the first (that I know of) double transmission-line loudspeaker; Meaning, there will be a t-line mid/high section and a t-line low freq. section within the same enclosure.

I cannot change the specification from quarter-wave to something else as that was the entire point of last semester: Defining the goal of the design.

Anyway, there will be built into the cabinet an integrated amplifier using a pair of Hypex UCD-180 amplifiers, a phase control, level controls (for the individual sections)- and a master volume control. All you have to do is plug in a CD player and go.

The second reason I chose quarter-wave is because I prefer the sound of this enclosure to any other type.

I was requesting suggestions because I figured that it might save me a little research.

I have a lot of expensive equipment that I can access from the universities' inventory if I need it, so I will build and test if I have to.
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Old 6th June 2008, 10:10 PM   #4
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You plan on tuning the TL to a frequency below 100hz, and then run a highpass filter so the driver only sees from 100hz and up ?

I don't see what the point of this is, as the purpose of the TL is for bass response/tuning.

Am I missing something ?


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Old 7th June 2008, 12:02 AM   #5
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I was wondering that myself.
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Old 7th June 2008, 12:47 AM   #6
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Default Re: Sure................

Well, not sure how original it is per se since what you propose (if I understand you correctly) for the mid/high will be an aperiodic alignment if done properly, so the 1/4 WL part is moot and its analogue can be found in some '50s era two way systems. I imagine it's been mated to a TL (or more likely a TQWT) somewhere along the line also due to their popularity, though no consumer product past or present to cite comes to mind at the moment.

Anyway, just to make sure we're 'on the same page', I'm interpreting your goal is to use a TL as an acoustic series notch filter to ~completely damp the mid/HF driver's resonant peak so that no electrical high pass is required, or that if one is used, that a textbook solution will work as predicted.

WRT to 1/4 WL theory, what you want is a max flat impedance alignment which the casual DIYer can now easily design using Rick Shultz's (aka 'Exolinear') Alpha TL math. Unfortunately, I know of no program that will accurately sim such a short pipe due to its typically high stuffing ratio, so getting it right and/or 'forcing' a 166E to fit the app may prove a formidable undertaking, but have no doubt you'll be pleased with the results once dialed in.

Regardless, anytime you relieve a wide BW driver of high excursion it's sonically beneficial.

GM
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Old 7th June 2008, 03:14 AM   #7
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Default Here goes..............

Please correct my thinking if I'm wrong:

I thought that the purpose of a t-line is to eliminate the backwave of the driver for all frequencies except the tuning frequency.

Isn't a t-line is essentially an infinite baffle at all frequencies except the tuning frequency (which is why resonance is chosen being that there's an excees of energy at this point?)

Even if used in a mid/high frequency application, isn't eliminating the driver's backwave desirable; even if the resonant frequency is never seen??

If not a t-line, what would be a suitable enclosure for the mid/high section??

Thanks for your time
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Old 7th June 2008, 03:18 AM   #8
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Default Another thing...................

I've noticed that there is somewhat of a midrange "dip" in the frequency response of the FE-166E- so could I simply tune the driver to a frequency that will augment the midrange??
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Old 7th June 2008, 11:33 AM   #9
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Default Re: Here goes..............

Quote:
Originally posted by cadaverdog
Please correct my thinking if I'm wrong:

I thought that the purpose of a t-line is to eliminate the backwave of the driver for all frequencies except the tuning frequency.

Isn't a t-line is essentially an infinite baffle at all frequencies except the tuning frequency (which is why resonance is chosen being that there's an excees of energy at this point?)

Even if used in a mid/high frequency application, isn't eliminating the driver's backwave desirable; even if the resonant frequency is never seen??

If not a t-line, what would be a suitable enclosure for the mid/high section??

Thanks for your time
Looking at your queries in turn:

1\ not really. The problem is that TL is used by a host of people to mean completely different things -sometimes even exact opposites. By strict definition, a pure TL is a max-flat impedance design of the type outlined by GM above. There are other types of line which seem to have come under the catch-all definition -hybrid semi-resonant lines (like Bailey's, Augspurger's etc), and very lightly damped, highly resonant QW lines. Depends what's wanted. In the latter two catagories, the cabinet is used to support the LF regions to varying extents, only slightly in the former catagory, and to a much larger extent in the latter.

2\ Yes. Although heavily damping other types of cab. will probably get you ~'close enough' to render it of questionable utility.

3\ Heavily damped aperiodic or sealed should do the job fine.

It's worth observing though that B&W did a sealed midrange 1/2 wavelength line damping the full BW of said mid. driver in their Nautilus cabinet, although IIRC it was closer to Olney's 1937 Acoustic Labyrinth (or PMC's current range of commercial cabinets, for that matter) with damped walls rather than the fibreous stuffing Bailey introduced with the TL moniker in 1965.

BTW -the 167 might be a better choice for you than the 166.


Quote:
Originally posted by cadaverdog
I've noticed that there is somewhat of a midrange "dip" in the frequency response of the FE-166E- so could I simply tune the driver to a frequency that will augment the midrange??
Not really -it'd be the hell of a small enclosure, and you really don't want to hear midrange informartion coming from two different places. Especially if they are physically widely separated. You'll get destructive interference (lobing / phase issues), plus some other problems.

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Old 7th June 2008, 01:35 PM   #10
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Default OK

When you say "aperiodic", do you mean a line that's not necessarily 1/4-wave??

If I were to do a sealed midrange, is there any specification for the volume of the enclosure??

Thanks
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