2.5 way efficiency

[michael]

When two drivers are used over the same frequency range their efficiency goes up 3db, but in a 2.5 way 2 drivers are being used for low end but 1 is for mids, does this mean that the speaker would have 88db from 400Hz up but have 91db 400Hz and below... does this mean padding is needed? please explain this to me becuase i really dont get 2.5ways.

[BobEllis]

the output does not rise below the .5 cutoff frequency. That is the point of 2.5 ways. The .5 compensates for the "baffle step" - the drop in response when the speaker transitions from 2 pi to 4 pi radiation.

You may end up padding the .5 if your room setup balances out the baffle step loss, but if your speakers are away from teh walls it is unlikely to be much padding, if any.

For planning purposes, treat your system sensitivity as a single woofer, minus a bit for losses in the crossover.

Did I get this one right, Sreten?

[sreten]

Originally posted by michael
When two drivers are used over the same frequency range their efficiency goes up 3db, but in a 2.5 way 2 drivers are being used for low end but 1 is for mids, does this mean that the speaker would have 88db from 400Hz up but have 91db 400Hz and below... does this mean padding is needed? please explain this to me becuase i really dont get 2.5ways.

When two drivers are used in paralell output goes up 6dB but
efficiency only goes up 3dB as 3dB more current is being drawn.

You need to understand Baffle Step Compensation. In simple
2 ways that only account for crossover, or fullrange drivers,
output rises 6db due to as said to the change from driving a
spherical field to a hemispherical field.
The transition frequency range is dependent on the baffle width.

Full baffle step compensation can be achieved by rolling in the
extra bass unit at the transition frequency with a 1st order filter,
the expected fall of 6dB output is cancelled by the baffle step rise.

The only disadvantage of doing a 2.5 way speaker like
this is you can only have full baffle step compensation.

If its done in the crossover network with an EQ circuit
you can have partial step compensation or none at all,
for use near walls and near corners respectively.

Lots of bad designs have no BSC at all.

http://sound.westhost.com/bafflestep.htm

sreten.

[planet10]

Originally posted by sreten
When two drivers are used in paralell output goes up 6dB but
efficiency only goes up 3dB as 3dB more current is being drawn.

This is kind of a confusing way of saying it. And not completely accurate.

By adding a 2nd driver you double the sensitivity (ie 3dB).

The potentially different output power of the amplifier into half (wired in parallel) or double (series) the impedance also affects output.

A "perfect" solid state amp (not completely attainable in practice) would have double the power output into half the impedance giving a gain of 3 dB -- conversly it would put out half the power into double the impedance and would put out 3 dB less power.

So you get 3 dB with the 2nd driver & any other is dependent on your amp. An article on Baffle Step is in the Wiki.

dave

[sreten]

This is kind of a confusing way of saying it. And not completely accurate.

Sorry but it is entirely accurate.
I only put it this way because the original post said :

When two drivers are used over the same frequency range their efficiency goes up 3db,

Which is true, but not the relevant issue / parameter.

By adding a 2nd driver you double the sensitivity (ie 3dB).

The potentially different output power of the amplifier into half (wired in parallel) or double (series) the impedance also affects output.

Two drivers in parallel :
voltage sensitivity (dB/V) goes up 6dB.
Impedance is halved.
Efficiency is doubled.

Two drivers in series :
voltage sensitivity (dB/V) stays the same.
Impedance is doubled.
Efficiency is doubled.

Sorry but the amplifier does not affect the above at all.

Note that when matching drivers, e.g. tweeter to midrange,
with different impedances, its the voltage sensitivity that
counts. dB/W doesn't. dB at 2.83V does (1W 8R).

sreten.

[planet10]

Originally posted by sreten
Sorry but the amplifier does not affect the above at all.

Sure it does... unless you plan on not listening to your speakers.

dave

[michael]

in my limited opnion i never use the lower impedance as a sign of increased efficiency on the speakers side, just look at the speakers efficiency and how much power the amp can deliver at the speakers impedance, not unrealistically double it...

[wintermute]

The other "side effect" of using a lower impeadance driver is often increased distortion (from the amp). At least that is what I have been led to believe

Tony.

[sreten]

If you compare a MTM d'Appolito configuration (2 way),
to a 2.5 way of doing it (MTM or TMM) the amplifier
issues you are all bringing up are just not relevant.

Adding a extra driver to a MT is a completely different issue.

Don't shoot the messenger ..........

sreten.

[Bricolo]

what will be the impedance of a 2.5 way loudspeaker? (with 8 ohm drivers)

Like a MTM, 4 Ohm?