Phase problems combining LP and HP xo in Speaker Workshop - diyAudio
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Old 15th May 2006, 02:27 PM   #1
tcpip is offline tcpip  India
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Default Phase problems combining LP and HP xo in Speaker Workshop

Hi all,

I'm back to seek help after being away for a long time. I have been busy trying to learn to design and build my first pair of speakers, using Speaker Workshop.

I'm building an MTM floorstander. The box is built, I've fitted the drivers on the enclosure, and I've just completed taking measurements of the woofers and the tweeter using a Panasonic mic and Speaker Workshop. The SPL curves look good. I have also designed a pretty straightforward xo using SW, and things were going on okay, when I hit a serious roadblock.

I have been getting guidance from my seniors in the game, who have many years experience in speaker design and SW usage. They told me that I should combine the LP and HP xo and see whether SW gives me a flat combined SPL curve. They also said that after I get a flat graph for combining the two curves, I should flip the polarity of one of the drivers and see whether I get a sharp notch at the xo point now. These steps were suggested as very sensible sanity checks before I order the caps and coils to build the thing.

This sanity check fails, and quite spectacularly.

Here is the tweeter's SPL curve, on-axis at 1m:

Click the image to open in full size.

Here is the woofer's curve, measured without touching the mic, in the same session:

Click the image to open in full size.

I created the HP and LP xo's (LR4 at 2KHz) and got this set of SPL curves (shown here with their optimisation targets, to show you how well the actual curves conform to the target curves):

Click the image to open in full size.

Till now, all went well. Then I used the "copy" and "paste" functions of SW and made a copy of the HP xo's frequency response. I used the "Combine" function to combine it with the LP xo's response, keeping +ve polarity to both the drivers. I got the following SPL curve:

Click the image to open in full size.

The jagged portion near the xo point is very disturbing. Note that the SPL at the xo point is not in a peak or trough... it's at the same level as the flat portions of the curve. However, there's a sharp trough on either side of the xo point. I inverted the tweeter's polarity, and got:

Click the image to open in full size.

Note that in this graph, there's a sharp dip at the xo point. These jagged peaks and troughs in the xo region are very upsetting. Why am I not getting a straight, smooth combined graph?

Help!
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Old 15th May 2006, 02:41 PM   #2
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I was following you up to this point:
"Then I used the "copy" and "paste" functions of SW and made a copy of the HP xo's frequency response."

I've never tried that before and don't know if it would work.

What I do is to create a 3rd network, which has all the elements of your hi-pass and low-pass network in it, both connected to one "source". Then I right click on the network, and select "calculate response". That always works for me.
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Old 15th May 2006, 02:44 PM   #3
tcpip is offline tcpip  India
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Default Some more details about this design

You can see some photos of the finished boxes here and here.
  • This speaker uses two Kevlar midbass drivers and one Al-dome tweeter, all unknown outside India, so there's no point in me telling you which drivers they are. These drivers are made by Peerless India.
  • The midbass units have DC resistance of 3.6 Ohms, so I'm connecting them in series. In the graphs above, when you see the woofer's SPL curve, you are actually seeing the output of both these woofers, hooked up in series. I saw no point in measuring each woofer separately for designing and building this speaker.
  • When I say I've taken the SPL readings at 1 metre, it's not exactly one metre. But at least I am certain that the mic didn't move between the tweeter readings and woofer readings.
  • I've not included the right enclosure's readings here, because I'm getting almost exactly the same SPL curves from that enclosure too, and finally hitting the same problem combining the xo curves. Incidentally, when I created the xo's for the right enclosure using its drivers' readings, I got the final optimised circuit to stabilise at about the same set of values as the left enclosure's xo's, +/-10-15%. I guess this can act as a sort of sanity check against careless mistakes.
  • I tried a second method to combine the xo curves. I created a new network object in SW, and copied and pasted the components of first the LP xo, then the HP xo, then hooked the two up in parallel with a single source. Then I did a "calculate response" and looked at the total response of the combined xo. I got exactly the same SPL curve this way that I'd got by running the "combine" operation on the individual SPL curves. (Jbateman, you've preempted me here even as I'm proof-reading my next post. )
  • I have prepared a SW file (a .swd file) with the SPL and impedance data of the four drivers and nothing else. If you want, you can take a look at it here. Try building an HP and an LP xo and see if the combined curve shows up the same jagged shape I'm getting. One of my friends did this already, and he's able to reproduce this sort of ugly curve. He's an old hand at SW; he's never seen this before. (But he's never built MTMs.)
  • My friends and I tried to even create alternate xo's, even at other xo frequencies. We tried at 2.8KHz, 2.5KHz, etc. The same jagged SPL behaviour remains.

Do you think the problems I'm facing could be because I'm trying to build an MTM? Is some sort of comb filtering causing this problem? Just a thought.
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Old 15th May 2006, 02:48 PM   #4
soongsc is offline soongsc  Taiwan
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I don't think you can do direct cut and paste. The FR with XO in there should be combined taking into consideration the phase response.
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Old 15th May 2006, 02:48 PM   #5
tcpip is offline tcpip  India
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Quote:
Originally posted by jbateman
What I do is to create a 3rd network, which has all the elements of your hi-pass and low-pass network in it, both connected to one "source". Then I right click on the network, and select "calculate response". That always works for me.
Done that too, just to ensure that the "combine" operation really does what I thought it does. The two give identical results.
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Old 15th May 2006, 02:50 PM   #6
tcpip is offline tcpip  India
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Quote:
Originally posted by soongsc
I don't think you can do direct cut and paste. The FR with XO in there should be combined taking into consideration the phase response.
Either you are misunderstanding me or I am misunderstanding you. The way I combined the two FR curves of the two xo's does take both SPL and phase into consideration. The "combine" operation of SW does this automatically.
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Old 15th May 2006, 02:51 PM   #7
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by tcpip

Done that too, just to ensure that the "combine" operation really does what I thought it does. The two give identical results.

Have you checked whether you had minimu phase measurement when you did that? I'me asking because the charts don't show phase curves
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Old 15th May 2006, 02:54 PM   #8
tcpip is offline tcpip  India
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Quote:
Originally posted by soongsc
Have you checked whether you had minimu phase measurement when you did that?
I don't know what this means, and I don't know how to check it. (Did I mention that this is my first attempt to design a pair of speakers?) Please can you tell me what I need to do?

And I certainly don't want to sound lazy, but do you think you could take a look at the SWD file I've put up? I'd do whatever you said myself, if I knew what to do.
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Old 15th May 2006, 02:56 PM   #9
tcpip is offline tcpip  India
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I have to go attend to some "work" work now. I'll check the thread again in a few hours. Sorry I've got to go for a bit.
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Old 15th May 2006, 03:00 PM   #10
soongsc is offline soongsc  Taiwan
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Quote:
Originally posted by tcpip

I don't know what this means, and I don't know how to check it. (Did I mention that this is my first attempt to design a pair of speakers?) Please can you tell me what I need to do?
Different systems work slightly differently. But basically what needs to be done is shifting the starting market in the impulse chart just before the rise of the impulse, then do an FFT or frequency response to generate and FR plot. Look at the phase curve to see where it wraps. Initially you need to set the marker so that there is not wrap back or maybe only one. If you can show an impulse chart and the FR/Phase chart, I could probably walk you though a bit.
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