3 way - or 2 way and sub?

[Ardee]

How is it decided whether a system with three drivers is a 3-way, a 2.5-way or a 2-way plus subwoofer? Is it just the degree of separation of the crossover points, or is there more to it? I read that a 3-way crossover cannot simply be designed by combining two independent 2-way crossovers, but this seems to be exactly what is done when the lower crossover frequency falls below a certain point. Are there any rules?

[Brett]

Do a 3 way. Then add multiple subs.

[Moondog55]

Just guessing ; but if using a separate amplifier with high pass/low pass for the bass and the cut-off frequency is around 80/120 Hertz, then it is a sub-woofer.
Active system

2.5-Way uses a woofer with a first order low-pass as low as affordable to boost the LF and relieve the midrange of some Doppler distortion and lower the impedance peak at resonance
3-way uses a passive XO with at least 3 separate drivers.
Bass (Woofer) Midrange ( Squawker/Talker ? ) High frequency (Tweeter )

[Ardee]

Originally Posted by Brett

Do a 3 way. Then add multiple subs.

So what is the difference between that and a 4 way, then?

[chris661]

A subwoofer is generally defined as a speaker, usually seperate from the stereo cabinets, designed to reproduce from ~100Hz down to as low as possible.
In a 4-way, the bass driver might be asked to go from Fo (lower cutoff) to 300 or 400Hz. A mid/bass would cover there to, say 2kHz, then the upper-mid would do from there to 8kHz, then a super-tweeter (usually ribbon) is used to get somewhere up to 30kHz.

You are correct in thinking that a pair of 3-ways with a pair of subs would be a four-way system. You would be wrong in thinking that is was a 4-way speaker. These are a single cabinet, with four different drivers covering different parts of the frequency range.

The ~.5 way speakers usually have the following (we'll take 2-way for example)
A tweeter, crossed at 3kHz (this is usual, but not necessarily optimal), then one of the mid/basses is brought in at 3kHz. It's electrical input is then left alone. Further down the requency range (ie, where bass rolloff has started) you'll get a second mid/bass, playing the same stuff as the first, but without midrange duties. The idea is extra bass efficiency at rolloff, so you can put off rolloff for another few Hz. The ~.5 is defined where there's a mid/bass driver that has the bass frequencies in common with a driver specified for LF. These drivers are usually the same for aethetic purposes, and it makes it easier to predict what will happen (different LF drivers have different LF signatures, ie, deep, fast, smudgy). If you combine two drivers playing LF, the bass you'll get will be a mixture of characters, which isn't always a good thing - one driver will show up the other's shortcommings.

HTH
Chris

[Moondog55]

Although successful speakers have been built where the .5 woofer was different from the main mid-woofer

[PeteMcK]

a sub is a badly designed woofer designed by the marketing dept for people with too much money...
'Are there any rules?' - only Hoffman's Iron Law...

[Moondog55]

Don't agree at all, just degrees of cost effectiveness.

we can't all have the space for 4-way towers with eighteen inch bass drivers and a subwoofer can add a certain degree of depth to a good 2-way

[richie00boy]

Originally Posted by Ardee

I read that a 3-way crossover cannot simply be designed by combining two independent 2-way crossovers, but this seems to be exactly what is done when the lower crossover frequency falls below a certain point. Are there any rules?

That is correct, if there is more than two octaves between the crossover points then you don't need to worry about the interaction so you can combine independent filters.

[Moondog55]

Originally Posted by richie00boy

That is correct, if there is more than two octaves between the crossover points then you don't need to worry about the interaction so you can combine independent filters.

RichieBoy are you sure that's correct? I always thought when using passive XO that it had to be much wider than that;
3 -> 3.5 octaves or more to avoid interaction, one of the reasons for the ROT 300-3000 band-pass for the midrange.