TMM vs TMW?

[camshaft]

Hey guys,
I thought this would already have been answered here, but I didn't really find what I was looking for by searching. What is the reasoning for using two woofers the same size when you could just make one of them a little smaller and one a little larger to have a 3-way system? For example, why go with a tweeter and two 6 or 7 inch woofers, when you could just as well use a 5 inch and 8 inch for a 3 way? I realize making a good crossover would be more of a pain, but are there other issues?

Thanks,
Austin

[youyoung21147]

Two identical woofers often serve as baffle step compensation.

Also, many commercial designs use two small woofers instead of one big because it allows to design a slimmer and taller enclosure that is more aesthetic.

[camshaft]

Ok thanks, but what is baffle step compensation? Sorry, I haven't bought any books yet and I realize the stuff I'm asking is probably already covered in them.

[youyoung21147]

baffle step diffraction is an issue in speaker design. Under a frequency (related to the width of your baffle), the sound not only goes in front of the speaker but also behind. Therefore, you loose some power in the bass region at the listening point because what goes behind is wasted.

To compensate for that, the second midwoofer kicks in at this frequency to add energy in the bass region and the lack of bass disapears.

Off course, not every speaker that has dual midwoofer works like this, but it may be a purpose of the second woofer.

[sdclc126]

"...why go with a tweeter and two 6 or 7 inch woofers, when you could just as well use a 5 inch and 8 inch for a 3 way?"

Maybe try asking the question in reverse: Why go with a tweeter and a 5 inch and 8 inch for a 3 way, when you could just as well use two 6 or 7 inch woofers for a 2 way?"

In this way I think the answer is self-revealing - if you get the same results you might as well go with the simpler/cheaper design.

[5th element]

Originally posted by sdclc126
"...why go with a tweeter and two 6 or 7 inch woofers, when you could just as well use a 5 inch and 8 inch for a 3 way?"

Maybe try asking the question in reverse: Why go with a tweeter and a 5 inch and 8 inch for a 3 way, when you could just as well use two 6 or 7 inch woofers for a 2 way?"

In this way I think the answer is self-revealing - if you get the same results you might as well go with the simpler/cheaper design.

Although 9 times out of 10 I am willing to bet the three way gives superior performance. Although you were not implying it wouldn't. This is of course if all drivers used are of equal quality and the crossover for each is designed correctly. Why?

1) Reduced IMD distortion on the midrange, as it now doesn't have to produce low frequencies.
2) Midrange unit can often be smaller in diameter, reducing any effects from beaming and can result in a smoother off axis performance.
3) Due to smaller diameter, the upper Xover point can probably be shifted higher, pushing the transition away from the 1-2khz region the ear is most sensitive to.
4) Due to the possibility of a higher Xover point, a 19mm tweeter could potentially be used, thus improving off axis performance. (This is only important if the 6.5-7" was not happy crossing over high up, but then you get more issues with beaming.)
5) An 8" bass driver, will most likely give you more extension then a 6.5" mid/bass.

The way I see it is cost and simplicity. A three way has everything going for it, with regards to sound quality. But will be more expensive.

The 2.5 way, only requires one enclosure, two different types of drive unit and a simple xover to bring in the 0.5 way woofer. The xover parts are also likely to be quite "small" in value = cheap.

The three way, will need separate enclosures for the bass and mid driver, and will generally require more components in the Xover then the 2.5 way. The values of the components in the bass to mid xover (the lower you go the bigger they get) will likely be quite large and thus be expensive.

If you have the skills and the money, then go for a three way, it should sound better then a 2.5 way.

[sdclc126]

5th E I actually couldn't agree with you more. It's just that the 2-way, low crossover point seems to be the "current favorite" in the DIY community, and I'm sure it's mostly due to cost/simplicity rather than superior performance. 3-ways seem to be almost a taboo subject these days.

2-ways used to be called bookshelf speakers, and for a reason, and ported or sealed-with-sub don't address the midrange issues you've cited.

There are always variables like room size, listening tastes etc., but budget permitting and a good design I'd go with a 2.5 or 3-way every time.

Let the debate begin!!

[LineSource]

Hi Austin,

Sometimes bigger is necessary to be better.

Google Dunlavy and search for his technical summary of WMTMW speakers using 1st order crossovers and time alignment. The speaker diameters are carefully selected for each crossover band.


Google Rockport Technology Arrakis and you will see the modern update of Dunlavy's philosohy.

[camshaft]

Thanks guys. I'm guessing the "Arrakis" part comes from the fact that that thing looks like you'd find it on Dune...

Also, how important is having 2 enclosures in something like a tweeter, 5in, 8in 3-way? I'm sure I'll be shown a graph telling me how bad it is, but I mean how different does it actually sound?

Austin

[EDDIEMUNSTER]

I am quite surprised that most DIY people stick to two way designs. I would think that the challenge would tempt more to try three drivers.

I am currently weighing a couple of designs and one is a three way. After hearing what has bee said here I will probably build the three way.

Cheers,


Doug