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Old 9th July 2008, 12:20 PM   #11
AJinFLA is offline AJinFLA  United States
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Calvin,

Can you post the vertical off axis FR's in 15 degree increments through to 60 degrees?
Which of course would be the horizontal when this MTM is placed on its side as a center channel. Thanks.

cheers,

AJ
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Old 9th July 2008, 12:35 PM   #12
Jay_WJ is offline Jay_WJ  United States
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Quote:
Originally posted by AJinFLA
Calvin,

Can you post the vertical off axis FR's in 15 degree increments through to 60 degrees?
Which of course would be the horizontal when this MTM is placed on its side as a center channel. Thanks.

cheers,

AJ
I wouldn't need to ask this design too much of a vertical off axis dispersion. We can more or less predict it based on the info he gave. According to measurements, the crossover must be acoustic LR2 with tweeter polarity inverted. Its vertical center lobe should be much narrower than a LR4 design with a lower xover point. Not much of a sweet spot vertically, but I wouldn't mind it if that fits the design goal---listening at tweeter axis when placed vertically. Not suitable for horizontal use for CC.
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Old 9th July 2008, 12:54 PM   #13
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Default diy horn loading

Calvin

How easy would it be for a diyer to horn load the XT25 (or other tweeter) "properly"?

Thanks
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Old 9th July 2008, 01:32 PM   #14
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how can you predict horn loaded tweeter's off axis response? unless you are usung horn simulation software.
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Old 9th July 2008, 01:40 PM   #15
Jay_WJ is offline Jay_WJ  United States
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Quote:
Originally posted by MisterTwister
how can you predict horn loaded tweeter's off axis response? unless you are usung horn simulation software.
I didn't mean to predict the tweeter's off-axis high frequency rolloff. I meant the crossover's vertical polar response around the crossover frequency. Oh, I forgot to say about the midwoofers' lobing errors. With 2.5 kHz LR2, the 6.5" midwoofers in MTM are more prone to comb filtering when the listening height is off axis vertically. But again, if this fits the design goal, I wouldn't mind. This must not have been designed for horizontal use as CC whereas my MTM design with 1.6 kHz LR4 can be used as a hortizontal CC.
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Old 9th July 2008, 08:32 PM   #16
Calvin is offline Calvin  Germany
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Hi,

I don´t have vertical response curves any more, but the sweet spot is small, vertically as well as horizontally. The horn has a rather high directivity. So its best to sit right on the tweeter´s axis.
But I found this one. Its the pure tweeter with horn measured from 0°-45° in 5° steps.
Click the image to open in full size.

The crossover for the bass consists of a series inductor and a RC parallel network for impedance correction.
The tweeter features a LC-network (12dB) and a parallel notch on its Fs. There´s a RCL-notch at the input connectors of the crossover to linearize impedance (with one of my computer crashes the crossover files have vanisched too ) so I don´t know the exakt values at the moment, but could look them up if there´s deeper interest.

The horn is made from aluminium.
I measured the shape of the tweeters frontplatte, calculated the shape using the tractrix-formula and went to a shop, payed some bucks a few days later and it fitted perfectly and works well ;-)
Click the image to open in full size.
Click the image to open in full size.
So its rather a matter of the capabilities of a professional shop. I wouldn´t recommend to do it yourself unless You have the right machines and knowledge to do so. At the back face of the horn there is kind of small ´blade´ of ~42mm diameter. This blade fits the shape of the tweeter´s frontplate very precisely. It centers the horn on the tweeter and the transition from membrane into the hornm is very smooth.
Click the image to open in full size.
The tweeter is simply screwed to the horn and the horn is glued with silicone into the box front from inside.

jauu
Calvin
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Old 9th July 2008, 08:51 PM   #17
Jay_WJ is offline Jay_WJ  United States
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Quote:
Originally posted by Calvin

I don´t have vertical response curves any more, but the sweet spot is small, vertically as well as horizontally. The horn has a rather high directivity. So its best to sit right on the tweeter´s axis.
The horn loaded tweeter's high frequency rolloff shouldn't be much of an issue. In general, a wave guided tweeter has a better HF off-axis response within a certain degree of listening window.

A more important off-axis characteristic of your design should be its vertical lobing patterns around the crossover frequency. Generally, acoustic LR2 crossovers have narrower center lobes over a wider frequency range than LR4 crossovers. Also, higher crossover frequency and shallow LP rolloff required by LR2 crossovers make this MTM design more prone to the midwoofers' lobing errors. But I wouldn't necessarily see this vertically narrow sweet spot as a weakness. Unless it is used as a horizontal center channel speaker, this type of design gives its unique power response which may be preferred by some people.
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Old 10th July 2008, 09:49 AM   #18
Calvin is offline Calvin  Germany
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Hi,

the initial point to use a horn simply was, that the two 17WN225 are slightly more efficient than the XT25. So I needed a device that would raise the efficiency somehow, or I´d used a lossy LP-filter for the woofers. Since it was intended to use the box with amps as low as 10W in wattage, I decided to save on efficiency and use the horn. As I was not very familiar with wave guides and only found some with quite large outer diameter I opted for the easier and smaller Traktrix-horn. Maybe I was just lucky but the resultant freq-response allowed for a simple crossover design with a (electrically) raised crossover-freq that has several advantages I ´ve mentioned before. Experiments with steeper filter curves (towards LR4) and lower CO-freq always gave inferior sonic results (to my taste), though in some cases directivity was broader. But anybody who wants to build this box can try his own.
Here´s a pic of the CO:
Click the image to open in full size.

Since the tweeter loads the horn just up to 15kHz the efficiency above 15kHz equals the pure tweeters efficiency. This means that the box´s freq-response drops above 15kHz slightly. Since there is nearly no music signal to be reproduced here any more I prefer a slightly tilted response in this freq-range, because I often like to listen at loud levels and such a freq-response is much more listenable to at very high levels of SPL. The box played at the Munich fair in 2004 (sidenote: one reporter of a German HiFi-magazine told me that he liked the box very much, because it was one of very few that could reproduce ´handclapping´ realistically) and on several DIY workshops. Just few listeners noticed the drop as long as they sat on-axis. But a lot of listeners were atonished to hear the XT25 sound so dynamic and lively on one hand and silky smooth on the other hand. Especially owners of high wattage amps low efficiency speakers were astonished about the dynamic range that a 20W SE-tubeamp playing through the box allowed for.

Anyway, the design goals for this speaker were:
- small rooms down to 10m²
- larger rooms >30m² with the support of a sub
- placing close to the wall (there is not much place in small rooms )
- short listening distances (see abobe)
- compactness (see above)
- high efficiency -->SE-Tube amp as drive

Especially the first goal asked for a rather narrow directivity. This lead to the choice of the XT25 and the rather ´big´ 17WN225.
Goals no. 2 and 6 lead to the use of a second 17WN225 and the BR loading.
Goals no. 3 and 5 lead to a design with a elevated Fb of ~55Hz-60Hz. Placed close to the wall the lower freqs are ´amplified´ and the box reaches down linearly to ~40Hz. Bass sounds precise and clean, while most other boxes with lower Fb and intended to be positioned free standing sound boomy.
Goal no4 lead to the symmetric MTM-design.
In larger rooms and in a free stading position the box reaches linearly down to ~60Hz. This makes the integration of a subwoofer very easy, especially so when the satellites are not to be filtered (fullrange) and the sub is just run in parallel.
This is a pic of a setup in a studio of ~45m² floor space.
Click the image to open in full size.
The Calvini driven by a KR Antares320 (SE-Tube 22W) supported by a dipole sub driven with BMS power amps and Pre.

jauu
Calvin

ps. Oh before I forget...... even with a nice and well manufactured cabinet and first class crossover parts You can stay well below the 1.000$ budgetary limit
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Old 10th July 2008, 12:29 PM   #19
sreten is offline sreten  United Kingdom
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Hi,

TBH I think Calvins design should go in a different thread.

I do not think it is "just right" .....

It is very suitable in some respects for this particular application,
but not in all respects IMO. It is not suitable as a center or IMO
afguably for HT L+R either due to dispersion / sweet spot issues.

Why do people always want to spend a notional budget and get
the "best / most expensive" drivers they can within that budget ?

The base level is $120 or $150 per enclosure, which is on grief
avoidance well worth it unless your a woodworking genius.
However they are not the last word on high performance cabinets,
(but very good for a basic MDF box, especially the curved version),
you can start another debate elsewhere on how to improve them
and at what point / with what drivers / loudspeaker design do
they become a limiting factor (also if they actually do ........).

You can ignore Jay_WJ at your peril, and pointlessly spend more
on something less satisfactory with more expensive / flashy drivers.

http://www.seas.no/index.php?option=...d=24&Itemid=42

Is flashy / expensive for what you get but is not an optimised center.

The enduring popularity of MTM's for centers is not beyond me, but
they are fundamentally unsuitable for the job, but given that Zaphs
and Jay_WJs designs do minimise the problems of this application.

FWIW the Eros MK II does appear to be suitable (low crossover
frequency) and should work well in 1 cuft sealed enclosures for
your intended application.

However the driver choice is not based on extensive distortion
testing, more on word of mouth that the PL18 is very good, e.g.
see http://www.humblehomemadehifi.com/Tempo.html .
However TG has lifted this directly from the Eros write up .....
The tweeter meanwhile is expensive and therefore must be good ....

Zaphs tests indicate the 810921 is a lot better bet than the
9500 for crossing over as low as possible, this is wanted here.

They also indicate the best bass/mid drivers are like that because
of the way they are built, AFAIK the PL18 does not have any of
the advanced features of the 8945P, and is likely similar to the
more prosaic Seas paper drivers.
Anything in common with the Scanspeak 18W8545K as claimed
by the "reputation" ? (The PL18 is a 18W8545K on the cheap),
technically no. In fact what is claimed in this "reputation" is
more accurately true of the Usher 8945P.

/sreten.

edit : just add IMO you'd be better off with 8945P/810921 MT's.
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Old 11th July 2008, 01:54 PM   #20
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at HTGuide, there are a couple of options with no/reduced baffle step using the Dayton RS drivers.

The designers have top notch reputations, the design process is extensively documented and there are many people on the site who have built the projects. Many of these have centres too.

http://www.htguide.com/forum/forumdisplay.php4?f=39.

The drivers in Jay's project sure are tempting. Have you looked at Zaph's reviews or these? How exciting is it to have a tweeter that competes with the best for a measly $80? I love the looks of that Usher driver too. If we didn't have a baby on the way I'd be all over these.

Well, have fun.

David
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