Notch filter at x'over freq. of woofer?

[grantnsw]

Hi, may I ask you learned folk another question please?
Could a parallel notch filter be placed at, or very near the x/over freq.
in a 2way system using a LR-2 network?

Misguidedly and naiivly I bought some 8" drivers cheap 'on special' a few years
ago. It was only recently that I discovered the response graph, and, aarrgghh,
(pls see attachment) they have a huge albeit 'balanced' peak at 2Khz.
So hence my question above. There is no way I would contemplate (another)
'cheap' 3way design and this is what they may only be acceptable for?

Or, is it just possible that they might be ok in a 2way?
Many thanks in anticipation, Grant (...newbie)
(unfortunately, no impedance graph or 'Le' available to Zobel; assume
'Le~~3mH?)

[Tenson]

You may find that though there is a peak on-axis, when measured off-axis the response is pretty flat. If the driver starts beaming around there, then a peak can help keep the off-axis flat.

I see no reason why you could not use a notch filter in the Xover range though as long as you take into account the impedance (or put it before the Xover, maybe not a good idea).

Alternatively you could just crossover lower down. Have a look at the TangBand 871S, it could crossover at about 800Hz 2nd order and still give very decent SPL and its pretty damn good sounding too, though not ultra high-end.

[mashaffer]

Assuming the graph is correct it looks like about 6dB/8va. You could use a 1st order low pass at about 1kHz. That would make it flat out to 2K and give you a second order rolloff above 2K. Shouldn't be too hard to find a tweeter that can work down to 2K with a fairly sharp filter huh?

Just might be a simpler solution.

Alternately it looks like it could make a usable sub in a 2 to 3 cubic foot ported enclosure if the TS are right.

mike

[infinia]

Hi
Peaking most likely the result of cone material/geometry issues. Thinking if you can use crossovers developed for a metal cone type midbass along with a lowish Fs tweeter would be good for a 2 way. (RS180 crossover designs?)
3rd order elliptical lowpass would be an elegant solution to an otherwise 2nd order with a notch.

<edit>Peaking at this freq. for a 8 in woofer would indicate even much more wonky off axis responses, up to 20-30dB variation at 2-3 KHz. So a high order crossover is needed here.

[grantnsw]

Brilliant!!
Great ideas I never even contemplated. You gentlemen have
tremendous insight into response curve issues. Thank you
very much indeed!
Now how to Zobel them? Would, a guesstimate, of 3mH voicecoil
inductance be a 'ball-park' starting figure? I could plug the Re
and that into the Lalena calculator.

[infinia]

Hi
Yes you def. need to zobel for a high order LPF. Just measure the freq. above Fs where the impedace is approx 2xRe. Then place the zobel network so the impedance is taken back to Re from there on out.

[grantnsw]

Hi Tenson, thank you, unfortunately no data on off-axis, but I understand! Crossing lower seems the way to go.

Thanks Mike. I like this idea, corroborates Tenson, so 1st order on woofer at 1Khz and second order(?) on tweeter at 2Khz (have tweets that should do this based on Fs).

Hi Infinia, I will have to 'look up' 3rd elliptical LP filters.

thanks to all for your valuable insights and best regards .. Grant

[tinitus]

regarding big coil at 1khz - you can "adjust" it by placing a big resistor across it - it will delay crossover point

[grantnsw]

Thanks Tinitus, but I'm sorry to say I don't understand, I only have
the 'cookbook' and an old 'Weems' and this is new to me.
Could you enlighten me please? thanks sincerely, Grant

[sreten]

Hi,

Hmmm........ some good advice, some not so good.

The response curve of that driver may not be too bad, it is typical
of a certain type of driver specifically optimised for a simple c/o.

The idea is that a baffle width is chosen where baffle step is pretty
much all over by 1khz. The series inductor for the bass/mid unit is
chosen to implement baffle step. consequently above baffle step
the rising response is compensated and you end up with an ~
2nd order acoustic roll-off where the drivers response stops rising.

So the driver is designed to have a single series inductor.
Full zobelling is not the intention though partial zobelling could
be used to fine tune response, phase or impedance characteristics.

Also note fine tuning needs to take into account the diffraction
ripple, simply put ideally you need to measure and adjust.

The driver categorically does not need a metal cone type crossover.

The drivers parameters also indicate it is a poor choice for a reflexed
alignment. It shoud be used sealed 25 to 35 Litres. If bass extension
is the goal then TL or MLTL or MLTWQT is probably the best option.

/sreten.