MTM with Vifa PL18?

[VikingP]

Hi,

I have been fooling around with DIY speaker building for some time now, but all the speakers I've done are nowhere near as complex as most of the designs on this forum.
I recently got 4 Vifa PL18WO0908, and I am now tempted on trying to make a pair of floorstanders that would be the best they can be with the PL18s. I listen to 75% music, and 25% HT, so these will mostly be for music, and therefore they would need enought low end for 2ch listening without the need for a sub. Is that asking too much of these 7"s?
I was thinking that an MTM would be good, but I don't know if it should be ported. From what I understood, it doesn't really have the Xmax for the sealed, and it would fit better in a ported enclosure due to it's EBP.
I was also thinking of setting them up in a D'Appolito geometry, all three drivers on axis with a 3rd order hi-pass and a 2nd order low-pass crossover, but it was mentionned in other threads in this forum that it is better to offset the mids and tweeter, in which case I guess the crossover would be totally different.

So I was hoping to get some help from the in-house experts of this forum on a potential floorstanding MTM design with these drivers, and hope to learn some more about the art of speaker design in the process.

Any help will be greatly appreciated.

Thank you in advance.

Paul M.

[PMazz]

I built the Eros years back and was very impressed with the sound. I tried both versions of the xover and stayed with the parallel version.


SpeakerBuilder.net

Pete

[Paul H]

I don't consider myself one of those in-house experts, but I've been soaking knowledge from them for a while now, and should be able to save them a little typing here and there.

You will likely want to go with a ported box to gain the lower extension if you want to run these full-range. Do some modelling in 'Unibox' - a free excel-based download - to see what lower range - at what sound levels - these drivers can provide and the range of speaker box sizes you will need.

There was a thread going a while back (over a year ago) about ported MTM's that I built. If you haven't seen it already, do an advanced search for "ported" in the title - the thread was started by Thomas W. It contains lots of basic questions and answers about a ported MTM, which you might be headed towards.

The offset driver layouts commonly suggested in this forum achieve two things.
First, by providing different distances from the driver centres to each side of the front baffle, they distribute the fall-off which happens at the baffle edges over a wider range of frequencies. This produces a smoother frequency response.
Second, an offset MTM layout allows the drivers to be packed more closely together, which reduces directional and lobing (sound cancellation between drivers playing the same frequencies) problems. Drivers that are too far apart sound like separate individual drivers - placed more closely together they sound like a single source. This is frequency dependent - the higher the frequency the easier it is for our ears to determine specific directions and the more localized the individual drivers become. Jon M has suggested a rule of thumb for the crossover frequency versus driver spacing of:
Max spacing (inches) = 13768/Crossover freq of the drivers
So, if you wanted to cross your 7" speakers to the tweeter at 1500Hz (random number, not a particular recommendation) you should attempt to keep the centre-centre distance of the two 7" drivers at no more than ~9 1/4". This formula is a rule of thumb, not an absolute. I've played with an adjustable crossover and noted the directional effects, and to my ears 'Jon's rule' is a very good target.

Paul

[sonicbeauty]

well I am using the P17 model from VIFA in my speakers and I am using the sealed box. The reason why I use this is that the bass is fast and driver settles down in very less time than in the proted ones.. But in sealed the lower extension is not much. But as I need highend sounding rather than slow speakers i went to sealed.

Ive practically simulated them i found that sealed enclosure gives a group delay of 3milli secs and a ported for the same driver gives in 14milli secs. I love fastness of my drivers now ..

But use a floorstanding in MTM config with littlebit more internal volume and use sealed you get the best resutls... If you want lowend just go for a sub and match the sensitivities and get the flat freq response curve to get the audiophile sound.

best regards,
Sandeep

[JonMarsh]

Whether a ported alignment sounds slow or fast has a lot to do with the room placment and the design of the alignment. When you make a speaker with an extended bottom end, if you locate it at the "wrong" relationships to the most adjacent floor/wall boundaries, you WILL get a hump in the in room response which sounds "slow".

Several modern speaker design programs allow you to calculate the impact of the most near by boundaries, but I wrote a MatCAD doc in the early 90's which does the same thing. In this case, I examine the impact with the three most immediate boundaries- floor, rear wall, and side wall. This could be extended to cover the impact of the ceiling and other side wall, too, but in general the distances are much greater and the effect seen only at very low freuqencies and lower levels.

"Wrong"


Here, the peaking in the 40-50 Hz region, combined with the dip from boundary nulls in the upper bass, will make the bass sound bloated and slow, though the raw speaker response modelled is quite good.

"Much Better"



This placement uses golden mean ratios to stagger the boundary influeces and an overall distance to tune the room gain to complement the natural roll off of the speaker, giving it the perception of substantial clean extension in the bottom end compared with the raw speaker repsonse.

Also, many ported alignments are not properly designed for in room placement with the lift which does occur from boundaries - in point, the best alignments are those which are not flat anechoicly, but have a gradual taper to the respones from 100Hz down to the Fb, which may be actually tuned below the nominal driver Fs.

No once (in my knowledge) accuses Avalon Acoustic speakers of having "slow" bass, and most of their designs of the last 5+ years are ported, for example.

PMAZZ and others can comment more objectively about the alignments we've recommended for the HiVa M8a and other projects.

That doesn't mean I favor ported in all circumstances or all drivers- for many, the T/S parameters are not designed for that. But for smaller speakers using small midwoofers, tne advantages in reducing cone motion and LF distortion clearly outweigh the the small change in step settling time between a sealed system and a well designed ported one.

And while you're considering step reponse and "settling time", try taking a look at your room characteristics- unless you've put a lot of work into acoustical treatements, the speakers aren't going to be the limiting factor.

Just my 0.02.

~Jon

[tktran]

What an excellent reply.

I concur. In fact, I have found that one should NOT be allowed to comment on bass response without taking into account the room. The room is EVERYTHING. (almost)

Anyone interested in a spreadsheet that is similar to Jon's program is welcome to download BASSANDROOM.XLS from my website.

http://kram0.com/gallery/main.php?g2_view=core:ShowItem&g2_itemId=14

[JonMarsh]

Nice job on the spreadsheet, Thanh! I downloaded it and took a browse!

Certainly recommended for anyone wanting to see the interaction of alignment and room position!

~Jon

[kalin]

hello,

Anyone interested in a spreadsheet that is similar to Jon's program is welcome to download BASSANDROOM.XLS from my website.

excuse me, but i can't find this file.
can you place here direct link, or send me this file by e-mail?
thank you very much in advance.

[ThomasW]

hello,

excuse me, but i can't find this file.
can you place here direct link, or send me this file by e-mail?
thank you very much in advance.

http://kram0.com/gallery/main.php?g2_view=core:DownloadItem&g2_itemId=761

[kalin]

thank you. its great.