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Passive woofer crossover peaking issues
Passive woofer crossover peaking issues
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Old 27th June 2020, 12:41 PM   #11
wintermute is offline wintermute  Australia
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Passive woofer crossover peaking issues
Hi Simon, I did a little modeling with a completely different driver, but got similar results to what you are showing. It looks like to tame it you need a resonant peak filter at least on the upper impedance peak.

For the test files I had it needed a substantial filter to tame it (resonant peak at 57Hz). approx 17mH - 450uF - 8 ohms .

I suspect if you want to get this to work you are going to need to so something similar. The above was calculated by speaker workshop for the zma and frd I had and it was spot on, and completely got rid of the peaking.

Tony.
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Old 27th June 2020, 12:41 PM   #12
AllenB is offline AllenB  Australia
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Passive woofer crossover peaking issues
Simon, would you post your data files, maybe others could take a shot at it. I know I've done this before but I don't want to guess whether it fits into the constraints in this case.
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Old 27th June 2020, 01:10 PM   #13
DBMandrake is online now DBMandrake  Scotland
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Quote:
Originally Posted by AllenB View Post
Simon, would you post your data files, maybe others could take a shot at it. I know I've done this before but I don't want to guess whether it fits into the constraints in this case.
Sure, no problem.

I've attached a zip of the frequency response (VCad .txt format) and impedance (zma format) in a zip file.

A couple of notes about these files - the impedance curve is a simulation from VCad's enclosure simulator for the proposed box tuning since I don't have a box yet to measure an impedance curve from...

Likewise the frequency response is a splice between manufacturers published response (>500Hz) and the simulated low frequency response in VCad's enclosure simulator, including baffle diffraction for the proposed baffle and driver location, also calculated in VCad. Hence it's a simulated farfield response rising 6dB from bass to midrange.

Of course when I get an actual prototype cabinet built I will be switching over to working from real measurements, but this gives me a rough approximation to work with in the meantime to help me see whether I'm going in the right direction or not and what challenges I might face - with the impedance peak issue being something it has helped me discover already.
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File Type: zip Visaton W300S.zip (13.8 KB, 6 views)
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Old 27th June 2020, 01:17 PM   #14
DBMandrake is online now DBMandrake  Scotland
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Quote:
Originally Posted by wintermute View Post
Hi Simon, I did a little modeling with a completely different driver, but got similar results to what you are showing. It looks like to tame it you need a resonant peak filter at least on the upper impedance peak.
See the frequency response and impedance curves attached in my previous post if you want to try modelling with the same data I'm using.
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For the test files I had it needed a substantial filter to tame it (resonant peak at 57Hz). approx 17mH - 450uF - 8 ohms .
Ouch. That's pretty much what I expected - infeasibly high values. Not only is that capacitor value huge (I'm already sweating about the size of the ~150uF cap I might need for the low pass filter if I go 4th order) that 17mH inductor would have to carry a lot of current at bass frequencies where the notch is most active, so could be prone to saturation if it was cored, and if it wasn't it too would be huge.
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Old 27th June 2020, 01:43 PM   #15
wintermute is offline wintermute  Australia
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Passive woofer crossover peaking issues
It's even worse with your zma.. attached as proof it can work but 46mH is not a practical size... There may be another way to deal with it though...

edit: added second pic without the resonant peak filter for comparison.

Tony.
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File Type: png poc.png (41.9 KB, 77 views)
File Type: png poc_no_rpf.PNG (41.0 KB, 76 views)
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Last edited by wintermute; 27th June 2020 at 01:46 PM. Reason: add without the resonant peak filter
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Old 27th June 2020, 02:20 PM   #16
Zvu is offline Zvu  Serbia
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I'm notching the hell out of those bumps in the bass region. I'm buying wire and wind my own coils. Copper wire costs 10euros/kg so that's cheap. For testing i've reused burned (that's why the plastic former is a bit deformed) regular EI transformer core. I wound 54mH with 0.6mm wire by hand on transformer core by mistake. I had to unwind three layers to get to 16mH that i needed. Dip the plastic former in laquer and voila. PS i don't have large hands.

IMG_20200602_203841.jpg IMG_20200627_161020.jpg

For caps use two 100V polarized elcos connected back to back like this --|(--)|-- it's a notch filter at 80Hz anyways. Notch such as this is working for few years now in Kef R300's i reworked and with a guy who is not being gentle to them (he listens his music with lots of bass very loud). Works like a charm.
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Last edited by Zvu; 27th June 2020 at 02:37 PM.
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Old 27th June 2020, 03:27 PM   #17
hifijim is offline hifijim  United States
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Originally Posted by giralfino View Post

I'm not sure that is not entirely true. The midrange fidelity can be ruined if the driver effectively plays the bass, but if the bass is suppressed by the acoustical filter (the HP filter provided by the box), the driver plays only the midrange, so you won't have doppler effect and IM distortion.

Ralf
I agree with Ralf here... Yes it is true that in the ideal world you would want to remove the bass frequency voltage and current from the midrange voice coil, it is far more important to remove the bass frequency cone displacement. And your small sealed box is providing a 12 dB/octave filter all by itself.

Another thing to consider: a + 4dB peak at 80 Hz is actually pretty mild. In a real room with real modes, you will have larger peaks and valleys just due to normal room response modes. In the USA, most homes are built with 8 ft ceilings, and this translates into a 1/2 wavelength frequency of 70 - 72 Hz. We typically have 10 dB "problems" at this frequency, which is usually a cancellation but can sometimes be a peak. I am guessing you are in the UK, and I don't know what kind of room you are in, but I am sure it will have various peaks and dips between 20 Hz and 200 Hz.

The best way to handle bass frequency room response problems is with an electrical solution, such as DSP... and if you are using DSP anyway, your small 4 dB peak at 80 Hz can easily be handled.

Without room-response DSP, I doubt you will even be aware of a 4 dB peak at 80 Hz. I bet it will sound quite marvelous as is...
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Old 27th June 2020, 10:13 PM   #18
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Simmed the WS300 in Unibox (120L, 25Hz), it shows a peak at 82 Hz, but only 0.6dB. It disappears with increased damping / box size.
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Old 27th June 2020, 11:21 PM   #19
DBMandrake is online now DBMandrake  Scotland
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Quote:
Originally Posted by PeteMcK View Post
Simmed the WS300 in Unibox (120L, 25Hz), it shows a peak at 82 Hz, but only 0.6dB. It disappears with increased damping / box size.
Are you refering to just a straight box sim (driver driven directly, no crossover) or box sim + 4th order L/R low pass at 250Hz ?

If it's just a straight driver/box sim then yes, there is almost no peaking with a smooth gradual low frequency roll off. The peaking I'm having trouble with discussed in this thread is introduced by the low pass filter and isn't present with the driver by itself.

Or if your sim did in fact include a passive low pass filter as well can you share the circuit and values you used ?
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Last edited by DBMandrake; 27th June 2020 at 11:24 PM.
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Old 28th June 2020, 03:03 AM   #20
AllenB is offline AllenB  Australia
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Passive woofer crossover peaking issues
I seem to have come up with three observations. Firstly, the inductor resistance seems to be a useful tool both in terms of the resonance and the sensitivity.

Secondly, in this example I'm using a 10 ohm parallel resistor and the impedance still shows as being above 5 ohms. In fact reducing this resistance will in some circumstances at some frequencies actually increase the impedance as a result of reducing the phase angle of the filter load, if you look close enough.

The other thing, if I were to do this again is to reduce the target sensitivity to the level at 110Hz, and let the 70Hz peak ride because not only is it a place where such a peak can be tolerated, it is also workable with the room.
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