midbass drivers that requre no filter?

[johnmark]

I have been fooling around with a 3-way speaker system that will run the midbass driver full range without filter. The midbass is enclosed in a heavy stuffed tunnel with an open back. To this I add a well behaved woofer with only a 1st order lowpass around 200hz, this helps fill in baffle step as well as extend lows. The tweater is brought in with a 2nd order filter at the appropriate point depending on the natural roll-off of the midbass.

My first try was with the following drivers and I was very happy with the results:
Seas T25-001
Audax HM130X0 5" TPX cone with phase plug
Seas P21RF/P 8" poly woofer with pase plug

This sytem was assigned to my den and now I will build 3 more in a similar fashion for the L-C-R of my home theater. For my home theater version, I plan to use 2 woofers and also to bi-amp the woofers. Maybe use a 1st order PLLXO.

Since the Audax is no longer available, I need to find a suitable midbass that sound good run full range and I have identified a few,drivers but have not listended to them, the midbass must cover the normal range of human voice and I found that the upper rolloff should be no lower than around 5k:
Peerless 5" HDS 850488 w/o phase plug
Morel MW-114S
Vifa 7" PL line XG18WH00
Parts Express- 5" Dynavox or HiVi F6

Any help choosing a driver or comments would be greatly appreciated!!

[5th element]

The only problem with filter free loudspeakers is that you tailor the tweeters reponse to the natural roll off of the midrange woofer. This means thats the off axis response is compromised, you will naturally design the response around the on axis response, which means off axis, around the crossover frequency you will get a dip, because the midrange off axis will start to droop.

Usually you decide on an xover frequency which will allow for no dip when you go off axis, or if you do want one a very small dip.

Obviously this is the choice you have to make and using a driver full range is certainly a way to not use a filter on it. But that does not automatically guarantee good sound. You still have to make sure you get good integration atleast on axis, and you get good cancellation when the polarity is reversed. I think this would make the job a great deal tougher and I would opt to use a crossover on the midrange, even if its simple, just to make sure everything integrated well.

The drivers you mention all go through break up in the 4k+ range which is not really what you want to use. I would crossover as far away from this as is possible. Having said that I would use a metal cone and do away with break up entirely in the passband and use a filter to wipe out the peak due to resonance.

Just my two pence worth.

Although out of those drivers I would use the peerless.

[johnmark]

Based on your comments I looked more closely at the manufacturers curves for these drivers.

The Vifa PL18 is flat to 3.5k at 30deg off axis and 6 db down at 5k

THe Morel MW166 is flat to 4k at 30deg off axis and only 3db down at 5k
The MW168 even looks a little better.

The Peerless 5" hds is flat to 3k at 30deg off axis and 7db down at 5k
The Hivi F6 is about the same.

I am still a beginner in crossover design, so bear with me.
If I chose to cross over to the tweater at 5k with a 2nd order LR at 5k using the natural woofer roll-off then in theory both speakers should be 6db down at this point. If the baffle for the midwoofer were tilted back to provide some time alignment (with the tweater pointing straight ahead above the midwoofer) then the response on the design axis of the speaker should be flat horizontally. There might be a bump in response at the crossover freq above the design axis. It seems to me that the off axis power response should not suffer much.

I realize that at 5k the midwoofer is beaming, but does not the better off axis response of the tweeter at the crossover freq fill in off axis?


I am sure that all those full range guys put up with much more severe beaming at high frequencies than what i propose. I think keeping the crossover away from the critical range of human voice is important, particularly for my center channel.

There is nothing I can do to get rid of the breakup modes that these drivers experience in the higher frequencies, but I think that the drivers I am considering don't suffer too much from this.

I guess that if I proceed with this approach and don't like the off-axis response, I can always lower the xover later.

[johnmark]

I was searching on the Morel MW166/168 and found that Eaggleston Works uses one or 2 of these drivers full range in several of their speakers.

[5th element]

Yeah im not saying full range on a mid cannot work, but its quite tough to manage.

If you put a 2nd order LR at 5k on the tweeter, you will get a 2nd order roll off initially which will then change to probably 4th order when the tweeter starts to roll off. The 2nd order of the filter and the 2nd order nature of the tweeter roll off will sum to give 4th order.

If you were to use a tweeter like the scanspeak sd2905/97 (i know this is expensive and im only using it as an example because its flat down to 500hz or so) which is flat down to 500hz, if you were to apply a 2nd order LW with an active crossover you would end up with a 2nd order acoustic roll off. Which is what you want, if it matches the woofers roll off.

Where the problem arises here is with the varying impedance of the tweeter, applying a textbook crossover to a tweeter will probably not give you the desired roll of you are looking for, especially when you approach resonance. Having said that the higher you cross the less of a problem this will be because you are further away from the resonance.

Time alignment is a funny issue and is one that is mis understood.

You have a tweeter and a woofer in a box both flush mounted.

You apply filters to them and optomise them both to have 2nd order LR roll off at 3000hz, so if measure individually will both have what looks like perfect 2nd order roll off.

Naturally you would assume they would sum well and when the phase is reversed, you would get good cancellation. This is what you may think may happen, but it doesnt. You apply time alignment via an active circuit of say (plucks figure out of the air) 100us, and hey you get a good flat requency response and when the polarity is reversed you get a whopping great big suck out, great! Now 100us = 3.4cm, so instead of putting 100us of delay on the tweeter you could move it back by 3.4cm to accomplish the same thing.

Now you think Ill try a 4th order LR at 3000hz instead so you do the same thing and think well I needed 3.4 cm last time lets leave the tweeter where it is and it should work. WRONG, this time you need to delay the tweeter by say 50us, or 1.7cm.

I made all those numbers up so dont take them literally.

The delay required to bring two drivers into phase with one another is dependant on the type of crossover used and the frequency the crossover is set at. Most speakers use drivers that are mounted flush on the baffle which means the tweeter is always firing ahead of the woofer, but this is not a problem because you can almost always find a crossover that will bring the drivers into phase with one another and then antiphase when the polarity is reversed.

Also the acoustic centre of the driver changes with respect to frequency. So you might be time aligned at 3000hz, but you are not at 10000hz.

[johnmark]

....I'll invest in the proper measurement gear and design software.
Till now, I have been playing around with various drivers and a few different commercial multi channnel 4th order LR crossovers. They do not allow response shaping per se, but I have been thinking about buying an Audio Control EQX which is a combo adjustable xover and built in equalizer. It is actualy built for the auto sound folks but I spoke with the manufacturer here in Seattle and they say it works good for home systems also. It has a lot of features and good specs for the price of $250

I know most serious DIY guys would pooh pooh this approach, but I figure if I have taken out the passive crossover components I have not really taken a step backwards. It seems that a lot of the serious DIY guys eventually move on to custom active xovers.

The 2.5 way system I started off descibing at the top of this thread is the first passive crossover system that I have built myself and I think I got lucky cause it sounds better than my expensive B&W 603-S2 which are also a 2.5 design. Using drivers with smoothe roll offs is key, and they are not the most revealling drivers, but they help take the edge off of some poorer recordings (most classic rock) .

I hope to take some pictures and measurements soon.

[5th element]

I think the eventual use of active crossovers is no surprise. In the DIY arena you tend to work your way through and finally want to arrive at the top. Which in most cases for a loudspeaker is using active crossoevers. I for one have a three way active system, I have always wanted an active system but until I built my own amplifiers this was not a forseeable thing because high quality power amps are not cheap. Building them on the other hand is.

[rabbitz]

Have a look at the Vifa P13WH-00-08 in it's own sealed enclosure as it could work with a nice roll off at each end. Great midrange to boot as well.

[johnmark]

Really I found that just hooking up an unboxed speaker is a good way to get a first impression of a drivers sound. I set the speaker at ear level on a table setting it on a foam pad with a pillow behind it.
I then run it up until it sound over driven, then back it off and just listen to some music and then to some dialog to see what it sound like on it's own. Obviously this is just for midwoofers. I was surprised to hear some expensive drivers with this test. I had bought some closeout xyz drivers and when driven hard they exhibited a lot of noise, buzzing and vibrations that seemed to come from within the motor structure. Others just tend to slowly distort.
Thats how I approved the drivers for my first project mentioned at start of thread.
My choice is to look for the driver that can best reproduce the overall spectrum of sound in this simple manner, then add the woofer and tweeter. I don't think I belong over on the full-range forum yet ?

I think I'll buy a few of the Morel MW168 or 144-S, the Peerless HDS and maybe the Vifa PL18 and P13WH.
It would be interesting to compare these type of drivers to a more rigid cone like one of the newer Seas aluminum. I would at least have to add a notch filter to listen like this I think?

I wonder if one supplier carries all these drivers and would let me return the unused drivers?