2.5 way design with different size woofers?

[johnmark]

Most of the projects I've seen use 2 identical mid woofers for a 2.5 way design, but is this really necessary or ideal? Since the .5 driver is only asked to reproduce up to 1 k or so cleanly, wouldn't it be better to use a larger driver for this purpose. Say a 4.5" midwoofer with a 6.5 "- 8" for the helper woofer. I realize that the smaller driver would still need to be capable of playing the full bass range, so it would need to be chosen wisely and have enough xmax or it could be partially mitigated by placing it in a smaller sealed box from the larger woofer. And the 2 drivers should have a similar sonic character ie. both paper or both metal, etc..

I have a few of the CSS FR125 and a few Audax 8" paper woofs that I intend to try. Was wondering if others have gone this route before?

[Bose(o)]

You're absolutely right and with a little bit of searching you will find a couple DIY projects that would agree with your hypothesis.

I remember at least two or three projects that I found through searching google that were made within a year or two from today.

[Cal Weldon]

If I may play devils advocate here:

Do not do this.

Two different drivers producing the same frequencies will sound different. Even if the T/S parameters are identical, they aren't going to sound the same. I don't think you will ever be happy with them.

To overcome this, you can try crossing over very low from the 6.5 to the 4.5. Around 100 to 150 Hz and keep the drivers in separate boxes. Or, if you've experimented with OB, you might consider going that route for the 4.5 but you'll need a pretty big baffle for 150 Hz. You would have to up the XO frequency in that case.

[David Gatti]

That what I did.
10" in 40 litres + 7" in 10 litres simply because I wanted wider bandwidth & a smaller box and was prepared to sacrifice a little low frequency power output. You have to do your simulations to work out how much effective baffle step you're getting. Expect around 3dB greater low frequency output compared to the 6dB you'd expect with 2 identical larger woofers.

[Zaph]

I've seen it done on a few DIY projects, and even on some commercial products. That said, I have to agree with Cal but for different reasons.

The issue concerning me is that the smaller woofer will always run out of Xmax first. Maximum system output is still determined by the smaller woofer, essentially throwing out the benefits of the larger woofer. There are some other issues, but that seems like the heavy hitter to me. If I wanted to make good used of some drivers on hand, I'd do a three way rather than a mismatched 2.5 way.

Regards,
John

[David Gatti]

Actually if you're using appropriate drivers & volumes (sealed) you can design for them to run out of excursion at similar levels.

[sreten]

Hi,

I think this is one of those questions where the answer is in the details.

For someone who doesn't know what they are doing changing a 2.5 way design
to a bigger lower bass unit will probably end up with a non-ideal solution.

However if you know exactly what you are doing, using separate box
volumes etc, then its a perfectly valid approach to speaker design.

A further option is to use sealed loading for the
upper driver and reflex loading for the lower driver.

/sreten.

[speaker]

Originally posted by johnmark
Most of the projects I've seen use 2 identical mid woofers for a 2.5 way design, but is this really necessary or ideal?

My understanding of a 2.5-way design is to increase output in the bass region by halving the impedance where the 2 drivers are in parallel and doubling up the cone area. You would essentially be throwing that away.

The advantage I can see however would be when contrasted to a 3-way design using a similar sized smaller driver or midrange. I believe your proposal would have have greater power handling than the 3-way mid as it would be able to cool itself more effectively by its longer excrusion.

My own experience with 3-ways over the years is that the mid is usually the element to fail first under high power use. In the bandpass of its crossover it has very little excursion and is the voice-coil is cooled almost exclusively by radiation into the magnetic structure. With longer excursion you'd have greater convection.

Anyways, that's how I see it. Your idea is interesting though and I hope you pursue it.

[johnmark]

I decided to first try open baffle based on some positive comments about the Creative Sound FR125. I also went overboard and chose a 10" audax as the .5 driver. The FR125 is crossed over to a Seas 27tdfc using a first order series crossover. The crossover point is between 5-6k. On the .5 woofer i used a 5.5 coil and impedance comp.
Since I wanted to keep the baffle around 16" wide, I added some short wings. Still I could tell the bass faded out so I am running it thru a Rane SSE 22 equalizer/ crossover and applying some mid/upper bass boost. I then cross it over to seperate stereo subwoofer around 125hz.
The sub if a sort of suedo-H frame/U frame using two 10" TC2+ per side and the Rane also provides some EQ for the sub.
Within it's limitations the Rane SSE 22 is a nice unit as it contains EQ and a 4th order lowpass for stereo subs. It also allows EQ for the upper units. It's shortcoming is that it has no high pass for the upper units, but i have inserted a 2nd order passive between it and my amp.

The whole thing sounds suprisingly cohesive and I like the lack of boomy bass that I had before when I ran a couple of NHT1259 in closed boxes. I am undecided on the sound of the open baffle.

Not really the 2.5 I had in mind when I started, but I had the drivers and thought I would give the open baffle/dipole bass a try a try. Building the cabinets was easy compared to closed boxes.

I will try to post a few pictures. When I built the open baffle i only had some mdf to build with, so I decided to cover the baffle with several layers of wool felt that I laminated on with contact cement.
I think this really worked to the benefit of having a very dead baffle.
The subwoofer cabs are OSB, but since they are an open box I didn't think this would be an issue.