# Midrange driver in a TL enclosure



## Jason Schultz (Jul 31, 2007)

I'm toying with the idea of of loading my midrange driver into a short piece of 6inch pipe, stuffed with fiberglass, forming a short Transmission line. I read about this in the Loudspeaker design cook book. The supposed advantages of this type of enclosure are a lack of reflections back to the cone and thus less colouration. I wonder if anyone has tried this before. My enclosure will be 31 cm deep so thats the length of the TL pipe.
mid woofer 5 inch Fs 61.5 hz
Qts .42
vas 8.6 l
xmax 3.5mm one way.

proposed xo high pass is a 12db at 300 hz.
My Question is: will 31 cm be long enough to use a 2nd order high pass filter without driver over excursion or modulating the higher frequencies. and does it need a port at the rear as in the diagramn in the LDCB

Jason


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## Lucky7! (Jan 7, 2008)

I don't tune my rear cavities as my aim is not to affect the LF response for the driver, but as you mentioned to control the rear wave. The ones behind my JBL 2123s will be about 50 cm for a 300Hz HP xover. If it is sufficiently long and well damped, and there are little to no opportunities for reflections back to the rear of the cone, you'll be fine. I've tone it before in a couple of other designs and it didn't seem critical as LF extension was not the reason for doing it.

As for Xmax, simply model it in Unibox and sue the volume of your tapered section as Vb, then check the graphs.


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## BoomieMCT (Dec 11, 2006)

Check out Martin J. King's quarter wave website. He's "the man" when it comes to transmission lines. This should answer a lot of your questions.


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## Jason Schultz (Jul 31, 2007)

Thanks boomie and x. X , From where do i get unibox .is it a free progm?
I will surely checkout Martin kings link this evening. I was hoping somone would know a suitable site for TL's.
as we say in oz Good on yers!


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## BoomieMCT (Dec 11, 2006)

Jason Schultz said:


> Thanks boomie and x. X , From where do i get unibox .is it a free progm?
> I will surely checkout Martin kings link this evening. I was hoping somone would know a suitable site for TL's.
> as we say in oz Good on yers!


You can get Unibox (and other stuff) free from the FRD Consortium.


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## Bill Fitzmaurice (Jun 14, 2008)

Unless you plan on using the vent output there's not much point. Stuffing a TL is suppresses harmonic resonances above the pipe frequency while still passing at and near the 1/4 wavelength fp. Very few midranges would benefit from the vent output attained.


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## Jason Schultz (Jul 31, 2007)

Thanks bill.
I would never consider using vent output as part of my audible FR for a midrange driver. That would be deliberate sabotage of the sound. given that vents produce a change in phasing below and around the resonant frequency and also they change the sound character from whatever the sound source was into a sine wave response. (This is not so noticable in low bass sounds but is audible in upper bass through to treble.)

I was just curious as to why the diagramn in the loudspeaker Design CB. pictured a TL pipe with a port at the rear. I can only summise that the port does not really function as a port! 

The lenghth of pipe I plan on using will be 150mm in diameter and 32 cm long (and stuffed) I'm not sure that this will produce much in the way of pipe resonances. I wish i new how to calculate or predict pipe resonanses 
does any one know of a link for this.

Thanks 
Jason


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## Bill Fitzmaurice (Jun 14, 2008)

Jason Schultz said:


> Thanks bill.
> I would never consider using vent output as part of my audible FR for a midrange driver. That would be deliberate sabotage of the sound. given that vents produce a change in phasing below and around the resonant frequency and also they change the sound character from whatever the sound source was into a sine wave response. (This is not so noticable in low bass sounds but is audible in upper bass through to treble.)


Properly executed vents don't change anything in what matters, which is what you can hear, but I agree that they have no usefulness with a mid driver. 



> I was just curious as to why the diagramn in the loudspeaker Design CB. pictured a TL pipe with a port at the rear. I can only summise that the port does not really function as a port!


 It won't if the duct is sufficiently stuffed to suppress resonances above fp and fp is below the driver passband. 



> The lenghth of pipe I plan on using will be 150mm in diameter and 32 cm long (and stuffed) I'm not sure that this will produce much in the way of pipe resonances. I wish i new how to calculate or predict pipe resonanses


The fp will be around 280 Hz. You'll see it as a resonant peak in an impedance sweep, along with all the harmonic resonance impedance peaks as well. Add stuffing until only the fp peak remains.


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## Jason Schultz (Jul 31, 2007)

Thanks for the info bill. I suppose adding stuffing will also effectively lengthen the pipe, making fp even lower (which would be better i think)
i want to high pass mid driver at 300Hz 12dB/ oct. do myou forsee any problems?


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## Bill Fitzmaurice (Jun 14, 2008)

Jason Schultz said:


> Thanks for the info bill. I suppose adding stuffing will also effectively lengthen the pipe, making fp even lower


It won't. Bailey espoused the notion that the lowering of the speed of sound with stuffing would lower fp, but the actual result is negligable.


> (which would be better i think)
> i want to high pass mid driver at 300Hz 12dB/ oct. do myou forsee any problems?


That's a bit close to fp, especially with that shallow a slope. But you can easily lengthen the pipe without exceeding the enclosure depth, just put a pair of 90 degree elbows midway.


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