1 driver on the back instead of full BSC...

[nerd of nerds]

Hello!

I've read several times (over on diyaudio.com) of people putting another midwoofer on the back of a cabinet instead of putting a full BSC circuit in the crossover.

I was wondering what everyone's opinion of this was...the idea is appealing because it seems like it would help keep the design relatively sensative via frewer XO components...but i'm just not sure...

[Paul Ebert]

I assume this is done bipole. Seems like a good idea to me when the drivers are cheap and one desires to keep sensitivity high. I hope others with more knowledge than me weigh in.

[Jed]

Hello!

I've read several times (over on diyaudio.com) of people putting another midwoofer on the back of a cabinet instead of putting a full BSC circuit in the crossover.

I was wondering what everyone's opinion of this was...the idea is appealing because it seems like it would help keep the design relatively sensative via frewer XO components...but i'm just not sure...

Why not just do a TMM 2.5way? Similar effect, less problems with cancellations with FR and easy to place in a normal room.

[jdybnis]

I agree with Jed a 2.5-way is a better choice. The advantage of putting one driver on the back is that it will sound OK if your crossover isn't exactly right with respect to the baffle. When you design a speaker you've got to get the driver rolloff to match the actual baffle step rolloff. In practice, with the tools we use it isn't hard. You can use programs like the Edge to simulate if all you have is infinite baffle measurements. And if you just measure the drivers on the speaker's actual baffle you don't have to worry about the problem at all.

[nerd of nerds]

Yeah, I kinda figured as much...

I thought a 2.5 way would be essentially the same thing and this was more of something for people using FE103 drivers who are deathly afraid of crossovers ruining their "perfect" .00001watts of tube sound...

[Paul Ebert]

When you design a speaker you've got to get the driver rolloff to match the actual baffle step rolloff.

Josh,

Can you please elaborate on this? Are you saying that one should only design the width to align with the desired roll-off? Or, are you saying that one needs to deal with the roll-off and aligning it with the driver roll-off is an easy way to do so?

BTW, what's the formula for determining the frequency of baffle step roll-off for a certain width? It's one of those things I never seem to be able to remember.

Thanks.

[cjd]

Putting a driver on the back DOES effectively "fix" baffle step loss since you only get forward radiation over the frequencies you're losing on the front-facing drivers assuming both drivers are identically placed on the baffle.

However, this is not ALL it does, which is where you start running into problems. Because you gain rear-radiation of all sorts of frequencies you didn't used to have there.

It might work for people that find the breakup of a 15" driver covering up to 3kHz is pleasant...

Also, baffle dimensions dictate the frequency or frequencies that baffle step loss occurs. It's simple once you wrap your mind around it, but is complex to try to describe. There are, however, some superb write-ups out there on the grand 'ole internet.

C

[jdybnis]

Paul,

I meant the latter and I was referring to a 2.5-way.

As for a formula, I believe the roll-off starts at the frequency where the wavelength is twice the baffle width. But don't use a formula like that. It is not going to be right unless your baffle is a disc with the driver in the middle. On a real speaker the distance to the edge of the baffle varies depending on the shape of the baffle and placement of the driver. That's why you've got to either simulate or measure the driver on the actual baffle.

[Paul Ebert]

OK, now I'm with you. Makes sense (all of it).

Thanks!

[noah katz]

"Why not just do a TMM 2.5way? Similar effect, less problems with cancellations with FR and easy to place in a normal room."

Isn't it a trade between bigger reflection effects at one freq w/both drivers in front vs. smaller more closely spaced effects from one driver on front and one on back?

[Jed]

Isn't it a trade between bigger reflection effects at one freq w/both drivers in front vs. smaller more closely spaced effects from one driver on front and one on back?

I think cjd's comment above pretty much sums it up.

[Landroval]

However, this is not ALL it does, which is where you start running into problems. Because you gain rear-radiation of all sorts of frequencies you didn't used to have there.

It might work for people that find the breakup of a 15" driver covering up to 3kHz is pleasant...

The higher frequencies can always be cut off from the back driver. It's ok as long as it covers the whole baffle step area of frequency responce.

[Paul W]

Even without wall reflections, interference notches are possible when the distance from the listener to the rear driver is one-half wavelength further than the distance to the front driver. Driver size, baffle width, frequency, etc, all play a roll.

Bipoles can be useful, but they need careful consideration...sorta like everything else in speakers