Vifa Recession Buster kit Transmission Line

[rhapsodee]

Hi,

I came across this kit on Madisound, seems too good a deal to pass.

I'm thinking of building pair a transmission line mains, with two sets of speakers, ie two tweeters and two woofers in each speaker, for better dynamics and SPL capability. I will get two sets of the kit for a pair of speakers.

Does this sound like a good idea?

Looking at the VAS of the woofer (10.5 litres) and Fs (53 Hz), first order approximation suggest a 1.7 m line length, with maybe a 5:1 taper.

Any comments?

http://www.madisound.com/catalog/pro...oducts_id=8525

Following are the specs of the woofer and tweeter.
Tweeter
Nominal impedance [ohm] 6
Voice coil resistance [ohm] 4.6
Nominal power [W] 100
Short term max power [W] -
Long term max power [W] -
Operating power [W] 4
Sensitivity [dB] 91
Frequency range [kHz] 2.5-35
Free air resonance [Hz] 1500
Voice coil diameter [mm] 25
Voice coil height [mm] 1.6
Air gap height [mm] 2
Voice coil inductance [mH] -
Eff. diaphragm Area [cm²] 7.1
Moving mass [g] 0.3
Magnet weight [g]/[oz] 240/8.5
Force factor [Bl] 3.5
VAS [l] -
Qms -
Qes -
Qts -

Woofer
Nominal impedance [ohm] 8
Voice coil resistance [ohm] 5.55
Nominal power [W] 35
Short term max power [W] 110
Long term max power [W] 110
Operating power [W] 10
Sensitivity [dB] 86
Frequency range [Hz] 55-4000
Free air resonance [Hz] 53
Voice coil diameter [mm] 25
Voice coil height [mm] 10
Air gap height [mm] 4
Voice coil inductance [mH] 0.60
Eff. diaphragm Area [cm²] 80
Moving mass [g] 7.8
Magnet weight [g]/[oz] 240/8.5
Force factor [Bl] 5.2
VAS [l] 10.5
Qms 2.73
Qes 0.53
Qts 0.45

[GM]

Greets!

FYI (may be others here and other websites also):

RB kit : grab it while you can ......

Madisound RB in a BIB?

Dual woofers, yes, if placed well away from any walls since it offsets baffle step loss as a 2.5 way and best if laid out as a bipole (one on rear).

TL loading, don't see why not. You can use MJK's Classic TL Alignment Tables to try different designs and either sim them in MathCad if you have his software or use Hornresp, though the latter won't allow you to sim any stuffing plus its peaks/nulls are somewhat exaggerated, so don't let them scare you away. Minimizing the deep 3rd harmonic dip is by careful offset driver location is important since otherwise excessive stuffing will be required that will damp down most/all of its gain BW.
http://www.quarter-wave.com/TLs/Alignment_Tables.pdf
http://mywebsite.bigpond.com/dmcbean/

That said, 1.7 m seems too long for even a TL (straight) and especially for a TQWT (reverse tapered), though due to the relatively low Vas I agree that a TQWT is preferable except IME a 10:1 taper is the best overall.

Dual tweeters, maybe, though only if you roll one off, i.e. use it to allow a lower XO point. That, or put it on the rear or top and adjusted to only add a bit of ambiance. The only other condition might be if you sit very far away in a large room where the dual sources appear as one, otherwise you may find the HF comb filtering degrades the imaging/sound-staging too much.

Obviously, any of these major changes will require modifications to the XO design for best performance, so the extra cost/effort probably isn't worth it if you just want to make some relatively inexpensive party speakers.

GM

[rhapsodee]

Hi GM,

Thanks for your comments!

Your suggestions are way more involved that what i was planning.

I was just thinking of building them as separate 2 way speakers in the same enclosure, ie each crossover individually connected to its respective tweeter and woofer. There will be 2 'stock' crossovers per speaker, with two sets of binding posts, driven by separate channels of amplifications. Reason being, if i connected the tweeters and woofers in parallel to 1 crossover, then i would need to tweek the crossovers, and it would end up a 4 ohm speaker? Of course, if tweeking the crossovers is not too difficult, then it would be more practical.

Regarding the comb filtering, the Axiom M80 doesn't seem to have any reported issues with its dual tweeter, dual midrange and dual woofers.

I wasn't planning to use a TQWP, how do you tell that the woofers are more suitable for this rather than a traditional T-line?

With regards to minimising 3rd order harmonic dip, you mean offsetting one of the woofers from the end of the T-line?

Will try to get some sims in Hornresp, but at this point, not too sure how to select the dimensions. This would be my first T-line project.

Hope to see more suggestions! Thanks!

[sreten]

Hi,

I can see where you are coming from regarding 2 of everything
per speaker as the parts are only available as a complete kit.

But TBH I cannot reccommend it, it will not be twice as good
and you will lose the coherence of the single MT pair, to get
the 4 driver double tweeter arrangement to work properly
would require redesign of the crossover for integration.
And of the course the cost of the extra amplication, though
theoretically a good 4 ohm amplifier would handle a pair, and
this is what you would need for a redesigned crossover.

I suggest you just bite the bullet and accept it will be ~
1/2 the volume for an MT and build a slimline speaker.

TBH if taking the amplifier side seriously perhaps a better
but of course more expensive speaker might be better.

Perhaps buy two pairs and use an HT amplifier in 4 channel ?
Serious front speakers and somewhat simpler vented rears ?

I'm not quite following GM, a 10:1 tapered TQWT, what
makes this different to a 10:1 tapered TL ? the position
of the driver ? 10:1 taper seems excessive...
I'll go through MJK's tables again .....

/sreten.

Another option is to buy two pairs and modify one pair into
a 2.5 way leaving a pair of spare crossovers and tweeters.
The 2.5 way would not be a near wall design .....

[rhapsodee]

So each speaker, having two separate crossovers with their own inputs, driven by separate channels of amp with the same signal is not a good idea?

Cause i have a pair of stereo power amps that i can easily use for this.
No tweaking of crossovers needed.

I don't mind some lack of coherence, my primary music is heavy metal, where the ability to play loud and clean is very important, hence this idea.

Will also probably be using a sub, if it makes a difference to the suggested tuning of the TL.

Two woofers would double the effective VAS right? As far as the simulations are concerned...

[GM]

Greets!

You're welcome!

Not familiar with the M80, so can't comment, but mine are based on mine and a host of others experience WRT the sound quality (SQ) of such speaker systems and the conditions where/how it can be made acceptable. Of course if they sound good to you, then that's all that matters.

If a driver requires a large, long vent to tune it to near/at Fs, then it's better suited for a TQWT (reverse taper) since a TL's (zero taper) cross sectional area (CSA) will be too small acoustically to allow the driver to 'breathe' which is typically due to too small a Vas. Of course if you want to severely over-damp it as was typical for whatever reason in bygone days, then an acoustically small pipe such as Sd or frame size = CSA is desirable.

You would offset both woofers, so if in tandem the offset would be the mean between the two, ergo best to butt them right up against each other.

GM

[GM]

Originally posted by sreten

I'm not quite following GM, a 10:1 tapered TQWT, what
makes this different to a 10:1 tapered TL ? the position
of the driver ? 10:1 taper seems excessive...
I'll go through MJK's tables again .....

There's no such thing as a tapered TL AFAIK. I mean I've never seen a tapered electrical transmission line, so see no reason why there should be an acoustic one and why we have 'TQWT' and 'horn' to define them.

WRT why I recommend 10:1, it's nothing scientific for sure, merely that when I was teaching myself by experimentation how TLs, horns, etc. worked in the hope of developing some decent design routines since I couldn't do the higher math required otherwise; in my ignorance I tried 10:1 since the pioneers of audio had determined that a 10:1 compression ratio (CR) was the practical limit for a max gain BW horn driver. It worked so well once I figured out how to calc its path-length 'close enough' that I've never bothered to try and 'fix what ain't broke' hence the only ratio I ever recommend unless I'm trying to hit some specific dims and can't make it work without resorting to some weird folding scheme.

GM

[GM]

Originally posted by rhapsodee
So each speaker, having two separate crossovers with their own inputs, driven by separate channels of amp with the same signal is not a good idea?

I don't mind some lack of coherence, my primary music is heavy metal, where the ability to play loud and clean is very important, hence this idea.

Will also probably be using a sub, if it makes a difference to the suggested tuning of the TL.

Two woofers would double the effective VAS right? As far as the simulations are concerned...

Not unless the app meets one or more of the exceptions I listed.

How can it sound 'clean' without good coherence?

Where you tune them depends on where you want to XO the sub.

Correct.

GM

[planet10]

Originally posted by GM
There's no such thing as a tapered TL AFAIK. I mean I've never seen a tapered electrical transmission line, so see no reason why there should be an acoustic one and why we have 'TQWT' and 'horn' to define them.

Historically, the nomencalture has been:

TL = any QW pipe where the open end is the same or smaller than the closed end. A narrower definition also adds that the line is to be damped until it is non-resonant (or aperiodic). This is an unfortunate addition to the definition, one would assume from Bailey's seminal article, since one of Bailey's claims was additional bass reinforcement, which implies the line was resonant. The most famous historical TLs were all tapered.

TQWT is a synonym for a Voigt pipe where the open end is larger than the closed end. This could also be called a chamberless or tapped horn.

Personally i use TL to refer to the entire broad category of QW pipes.

dave

[planet10]

Originally posted by rhapsodee
So each speaker, having two separate crossovers with their own inputs, driven by separate channels of amp with the same signal is not a good idea?

The idea is perfectly fine... a way to use the kits at hand without having to redesign the XO. The 2 terminals could be paralleled and a 4 ohm capable amp used or you can use 4 identical amplification channels.

I would thou, only consider using them in a bipole arrangement... somethig like the TLb...

http://t-linespeakers.org/projects/tlB/index.html



It doesn't have to be a TL, but the bipole arrangement skirts most of the issues with doubling up 2 sets of midbass+tweeter.

dave