phase compensation due to driver offsets

[DcibeL]

Hello,
I am nearing the completion of my next speaker project, a tower design consisting of the Peerless 8" HDS (830884) and the Peerless HDS tweeter (810921). Since I lack the tools and experience to make my own measurements, I am relying solely on manufacturer data and the measurements of third parties. The resulting frequency response and phase response is a simulated in box response produced using the excellent FRD tools. The last step of this design is of course, the crossover.

What I've come up with for a crossover is a acoustic 4th order linkwitz-riley at 1600Hz. The drivers are going to be on a flat baffle, with the tweeter inset into the baffle, so I estimated that the tweeters acoustic center is about 35mm in front of the driver.

Here's where the trouble sets in. I have come up with a very good response using a small amount of parts, however the phase response doesn't line up unless I assume the drivers are in the same place. So what can be done to adjust the phase response without affecting the frequency response?

I've attached the relevant data if anyone wants to look at my crossover in Passive Crossover Designer. Just enter 0.035 in the "Driver Z offset" box to see how the response changes when taking driver offset into account. However if you don't have experience with this spreadsheet, I still think we can discuss what can be done about changing phase response without affecting frequency response. I'll post the project in full detail once I've decided that the design is complete.

Thanks!

[Ron E]

I mostly don't use Excel anymore (Open Office is free and why jump on the Microsoft upgrade train) so I can't answer your question. I don't see Jeff B here very often, but he does frequent the parts express techtalk forum. Post mentioning PCD and there are a lot of users there that can help.

You can make allpass delay circuits, but I think they are usually a waste of components. Sometimes you can use a higher order crossover on the tweeter to delay it a bit. Best might be to use an optimizer like Speaker Workshop instead of the PCD - although the effort is larger, it is nice to be able to draw an arbitrary circuit.

[Jay_WJ]

If the woofer and tweeter's rolloffs are symmetric LR4, you can't make phase aligned. That's why people use assymmetric rolloffs to achieve phase alignment on a flat baffle.

Can you email me your frd and zma files to me? Not raw files but the ones with baffle and box effects added. Then I'll show you how you can do this. It will take only a few components to obtain a good FR and phase tracking for these drivers.

My email is at my website.

Edit: I found you already attached them. I'll get back to you soon.

[Jay_WJ]

DcibeL,

Just quickly, there's a problem in your zma files: no electrical phase extracted from the impedance. I'll do this for my PCD sim.

[DcibeL]

Thanks for your replies. Sorry I am still learning crossover design, and was not aware that the asymmetrical roll off was an accepted solution to achieve better phase alignment.

The frequency response plots include the phase information, I wasn't aware that I needed phase information to be included in the impedance plot as well to have accurate results. I'll have to look up how to do this tomorrow, and play with the crossover some more.

I had started my crossover design with Speaker Workshop, but I really dislike the interface. I found it difficult to quickly see the effects of a change in parts. This is where Passive Crossover Designer came in much use.

[Jay_WJ]

This is what I came up with quickly. Certainly not the best I can do. But I hope it can serve you as some starting point.

Don't use full 6 dB baffle step compensation. 4 to 5 dB BSC will give better midrange clarity. Anyway, you can start with 6 dB BSC and can later unwind the primary coil to reduce it when voicing the speaker.

[DcibeL]

Thanks Jay_WJ, you're assistance is greatly appreciated. From your work I have concluded that if I leave the tweeter crossover as it is, but move the crossover on the woofer up to 1800Hz the phase response improves greatly and the resulting summed frequency response is also still very good. However, ideally I would like to keep the woofer crossover at 1600Hz to keep the cone breakup as low as possible (6dB lower in this case), so I tried as well to keep the woofer crossover where it was and move the tweeter crossover point. Moving the tweeter crossover up to 1800Hz also gave excellent phase characteristics, but there was not a dip in the frequency response so it could not be used.

Playing with the values some more I think I have come up with a very good combination. The response is normalized around 85dB, which is 4dB down from the woofers 89dB sensitivity, so that's how I came up with the amount of Baffle Step Compensation.

I think I will be very pleased with the results of this speaker system. Next step is order the parts and make the CAD drawing for my cabinet builder. All the parts will come from Solen, except the big 3mH inductors will have to be iron core so I'll have to import them from Madisound since Solen only carries air core inductors.

I've attached my final crossover result using the part values and DC resistances of what is obtainable for me.

I can't wait to build these speakers

[Jay_WJ]

I just checked your xo sim. I found you set driver geometry parameters incorrectly. You need to set the (vertical and Z) offsets of ONly the woofer, NOt those of tweeter since the simulated mic is at the tweeter axis. Leave tweeter offsets all zero.

This means you'll need to redesign your xo.

[DcibeL]

Crap, I was editing you're file and I saw you didn't enter offsets for the tweeter, so I entered them in but didn't notice you put them in for the woofer offset. Back to the drawing board

If what you say is correct, then shouldn't the offsets entered for the woofer be negative values? Since the woofer will be below the tweeter and the acoustic center will be behind the tweeter's?

[MarkMcK]

This is not my project. But what I know and my experience with systems suggests that something is wrong here. After you build it, test it and look at the impulse response. If you go with a fourth order slope at either 1600 or 1800 Hz I believe you will find that the tweeter is still leading the highest frequency from the woofer by a considerable amount (unless you measure from way off-axis).

The key variables in a lagging acoustic center for a woofer is the high end cut-off frequency and the slope. The lower the frequency and the higher the slope the more apparent lag in acoustic center for the frequencies in the crossover region.

Mark