Crossover Design in Vituix

Kerr Forrest

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**Apologies for the double post but I realised I started this same thread in the non-DIY speaker forum, so if any moderators see this I would appreciate it if you could remove the original in the "SPEAKERS AND SUBWOOFERS > Speakers" forum

I am designing a crossover using Vituix for a 2-way bass reflex speaker design and am somewhat unsure on which woofer frequency response I should import and use when designing the crossover.

I have the infinite baffle response provided by the manufacturer as well as a simulated response I made of the woofer while placed in the bass reflex cabinet using Vituix's enclosure simulation tool. My assumption was to use the simulated in-cabinet response since this is obviously closer to how the driver will actually behave, however I am finding a large (+6dB) boost centred around 50Hz in the crossover simulation response (note that the both the Fs of the woofer and the port tuning is 30Hz). This way not be related to using the in-cabinet woofer frequency response, but it got me wondering this is interfering with the simulation; I am still playing about the the crossover design so it could be something in there which is causing the issue.

If anyone can advise me on whether to use the infinite baffle woofer response or continue with my bass reflex simulated response when designing a crossover in Vituix I would be very grateful.

Cheers,
Kerr.
 

JLM1948

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Unless you've opted for a way too low x-over frequency, the LF hump due to the funky combo of speaker resonant frequency and box tuning should not interfere with the midrange response. It is quite usual to use the open-baffle response as the basis for the x-over calculation.
Anyway, a better approach should use a response including the effects of baffle diffraction.
Unless you intend to apply electronic correction, I would recommend changing the tuning of the box.
 

Kerr Forrest

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Unless you've opted for a way too low x-over frequency, the LF hump due to the funky combo of speaker resonant frequency and box tuning should not interfere with the midrange response. It is quite usual to use the open-baffle response as the basis for the x-over calculation.
Anyway, a better approach should use a response including the effects of baffle diffraction.
Unless you intend to apply electronic correction, I would recommend changing the tuning of the box.
Hi JLM, thanks for your reply.

My crossover frequency is pretty low at 1kHz; chosen because my woofer (Beyma 8BR40/N) starts to roll off around that point. I am using a Peerless XT25BG60-04 tweeter since it can take over from 1kHz, as its resonant frequency is very low for a tweeter at 589Hz. I should clarify though that the midrange is looking fine in the crossover simulation, the only real issue is the hump around 50Hz which returns to normal by about 100Hz.

Could you explain why the open baffle response is normally used for crossover simulations? I would have thought it makes sense to take the cabinet effects into consideration when designing the crossover.

I hadn't considered baffle diffraction yet, I'll make sure to include that, thanks.

As far as the tuning frequency goes I think it's about right as it is, the low frequency hump that's appeared in the crossover simulation wasn't there in my bass reflex cabinet simulations. The port tuning is pretty low at 30Hz but the internal cabinet volume is approx 105L which seems to match up ok both in WinISD and Vituix. This is why I was originally thinking it was the use of the in-cabinet response during the crossover simulation that caused this bass hump.
 

JLM1948

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The open baffle response is generally good enough for designing the x-over, and is usually less computationally intensive. The midrange response is usually undistinguishable from that of a vented box.
However, the fact that the LF hump appears only in the x-over design phase is strange.
It is known that increasing the source impedance tends to exacerbate the resonant effects; have you checked that the x-over has a low enough output Z?
 

Kerr Forrest

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Apologies for the late reply JLM, thanks again for your thoughts.

Of course, I totally forgot that the enclosure won't actually impact the crossover region.

I've been doing a bit more reading on crossovers and realised that I was using a series resistor for attenuation without the necessary parallel resistor to form an L pad, meaning the impedance was way off, so you were spot on for what was causing the resonance bump.

I was wondering if you could give me your opinion on my design, as I'm somewhat unsure on whether I can achieve what I'm hoping for.

So, my plan has been to use an 8 ohm woofer with a sensitivity of 91 dB (1W @1m) with a 4 ohm tweeter whose sensitivity is 88.6 dB (1W @1m). My thinking was that because the tweeter has half the impedance (nominally, anyway) of the woofer then it should receive double the power that the woofer does when they are both connected to the amp in parallel, bringing its sensitivity up by about 3dB to a similar level of the woofer (assuming that the +3dB of power leads to +3dB of SPL, I am unsure if this is a direct relationship). I am now concerned that this will cause issues to the amplifier by having the two different impedance loads in parallel (I should point out the amp I am using can handle 4ohm loads though), any thoughts on if this idea will cause issues?

In my crossover simulations so far I have also found that the woofer still requires significant attenuation to match SPL levels with the tweeter, suggesting my theory is wrong. The issue this raises is that I have read attenuation of the woofer is to be avoided because it will absorb a lot of power and impact the amplifier damping factor.

I imagine the simplest solution here would be to change one of the drivers, however, I would very much like to avoid this, as my design project is for university work and I have already put a lot of time into driver selection and my deadline is fast approaching. If you can offer any suggestions on how I could solves the mismatches in impedance and sensitivity within the crossover design I would very much appreciate it.

Cheers,
Kerr.
 

JLM1948

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The sensitivity figures suggest that the lack of sensitivity of the tweeter would be more or less compensated by its lower impedance. With the same voltage, the tweeter receives twice the power of the woofer, which compensates the 2.4dB difference in sensitivity.
However I very much doubt the pertinence of these specs, since almost universally, tweeters need attenuation relative to woofer. The thing is it's not so muchthe global sensitivity that must be matched, but also the sensitivity at the cross-over point.
Have you actually checked the individual drivers response?
Regarding the load presented to the amp, you should noit worry, since the drivers are not strictly in parallels. a well-designed x-over distributes power in a way that should avoid issues. However checking the input impedance of the complete speaker is necessary. Often, x-over design softwares propose solutions for linearizing response that tax the amplifier.
 
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