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Old 3rd February 2008, 06:01 PM   #1
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Default 3-way with tilted front baffle

Hi,

A while ago I made a discussion about similiar project, but back then I was thinking about OB design. After finding out more about ob speakers and realizing how hard it is to get it work, I decided to postpone that idea and go with sealed/reflex boxes for this project.

The drivers I bought for this project:

ScanSpeak D2010
Seas MCA12RC
Peerless 830868

And this is what I've been planning:

Layout

Or at least this is the latest layout as I've been trying to tweak it for few weeks now. I've tried many different tilt angles and now it's 15 degrees.

At the moment the crossover has lowpass for the woofer, bandpass for the midrange and a lpad and highpass for the tweeter. All 2nd order electrically. The midrange polarity is inverted.

This is how the responses look like:

http://koti.mbnet.fi/twisted-/3way/f...sponses_01.gif

I made the measurements by myself in a "infinite" baffle (and added near field measurements), then added baffle diffraction to the midrange and woofer. After that I substracted -6d from the woofer and midrange so the levels matched with the tweeter. The woofer will be placed in a reflex box but as the box doesn't exist, the response is lacking the port output. The midrange will be placed in a closed box.
I didn't mount the woofer flange to the measurement baffle, as I thought it would be best for the relative acoustic offsets.

Now the thing I've been using most of my time is to get the phase responses to match at xo points.

This is how the responses looks like without the relative differences in the tilted layout (just for a comparison):

Phase responses
Red is tweeter, green is midrange and pink is woofer.

And here is the system phase responses with relative offsets included:
System phase responses

XO points are 450Hz and 3250Hz.

And here are my questions :

- The 15degree tilt sounds quite a lot to me, is this a reasonable amount of tilt? Has anyone of you made anything similiar and how did it turn out?

- I've been using 300cm for the mic distance in the system frequency simulations as I though this would be pretty close to the real listening distance. Is this the right way to do it? Changing the mic distance obviously has an effect to the relative phase responses.

- The bass output at ~150hz is about 2dB above the midrange level and I've been wondering if it's going to make the sound too boomy. If the sound turns out unsatisfactory, can I low down the woofer with series resistor just like a tweeter or is there any kind of problem with this (or alternatively using l-pad)?

- How does the phase responses look like to you? Whatever I've tried to do, the responses at 450Hz xo point hasn't been significantly improved. Is this good enough?

-Have I done the layout in Z direction correctly? The tweeter is 300cm away from the mic, but the midrange and woofer is more closer. Now I tried to put the woofer to value 0, the midrange to 10,30cm and tweeter to 13,7cm. The relationships are equal to this , but now all of the drivers are more than 300cm away from the mic and the phases look like this:

System Phase responses 2
Again, red is tweeter, green is midrange and pink is woofer.

Which is the correct way? Can any of the drivers be closer to the mic than the defined mic distance?

And here's two questions that aren't exactly related to this particular project, but if someone wants to clarify these things to me it would be appreciated.

- There are lots of commercial 3-way designs where the bass driver is located to the side of the cabinet like this. How the xo point phase can be matched in this kind of situation and how low the xo point has to be this to work properly? I'm guessing the flaw of this design is that the woofer is quite a lot more far away from the listening point than the mid/tweeter.

- Does the location of the reflex port have any real effect to the driver/port output combined response? I am guessing there is somekind of phase difference between these two outputs? Does it make a difference how far away from the driver the port output is? (for example in the front or the back of the baffle)
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Old 5th February 2008, 01:45 AM   #2
blue934 is offline blue934  Canada
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what software did you use to model the baffle layout?
david
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Old 5th February 2008, 10:01 AM   #3
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SoundEasy, just like everything else.
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Old 9th February 2008, 05:46 PM   #4
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Any help with this project?
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Old 4th March 2008, 04:44 PM   #5
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Comments and help would be appreciated, the project is still under designing
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Old 4th March 2008, 09:21 PM   #6
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I believe the lower the crossover frequency, phase alignment becomes less important. Whether your phase mis-alignment being out at 450Hz by approx 55 degrees is "good enough" I'm not sure. Reading John Kreskovsky's phase paper might help understand.

Overall I would say your phase tracking is bad. The mid/tweeter is out by 120 degrees. Inverting the tweeter will make this 60 degrees - but still bad.

What is the off-axis performance like? specifically what is the upper rolloff like on the midrange? You might want to bring the tweeter / mid XO down a bit if the MCA12 is beaming (ie. more than 3dB down) at 3250Hz

You mentioned you are using a 300cm (ie. 3 metre) mic distance? Is this correct? If so - then that's great if you can gate out room relections as you'll be able to get good point summation of the drivers (being a 3 way - you need more distance between baffle and mic) and you'll be able to measure the lower xo point farfield for xo simulation.

What is your system impedance like? Do you have any nasty phase angles in the bass region and low impedance troughs?

You might need to post your xo schematic and impedance curves so we can be of further help.

You might need to go for asymetric slopes. (ie. 3rd electrical on the midrange HP to better integrate with the woofer and poss. same on the tweeter HP to integrate better with the mid).

David.
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Old 4th March 2008, 09:28 PM   #7
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One more comment - side firing woofers are only a good idea for producing omnidirectional bass (ie. below 100Hz). Anything above this you'll get severe rolloff and poor summing with the midrange. With your drivers, side-firing is not an option.

If you want a cabinet with a leaner profile - try a trapezoidal or angled front baffle (ie. non-parallel sides). However this would be harder to cut / build since you are going for a sloped baffle anyway.

David.
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Old 5th March 2008, 12:44 AM   #8
tinitus is offline tinitus  Europe
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Personally I use series resistors on ALL parallel components, tricky to get right, but it works
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Old 5th March 2008, 01:10 PM   #9
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Quote:
Originally posted by Dave Bullet
I believe the lower the crossover frequency, phase alignment becomes less important. Whether your phase mis-alignment being out at 450Hz by approx 55 degrees is "good enough" I'm not sure. Reading John Kreskovsky's phase paper might help understand.

Overall I would say your phase tracking is bad. The mid/tweeter is out by 120 degrees. Inverting the tweeter will make this 60 degrees - but still bad.
I believe you misunderstood my first post. This is how the simulated phase responses look at 300cm, red is tweeter, green is midrange and pink is woofer.
At the mid/tweeter XO point (approx 3,2KHz) the red and green lines are aligned, at mid/woofer XO point which is around 450Hz, there is about 15 degree difference in green/pink curves.

Here is the other simulation I mentioned, the relationships to each other are same as in the first one, but now the mic is set up in the SoundEasy so, that the bass is exactly 300cm away from the mic, and the other two drivers are more far away. In This first simulation the tweeter is set to 300cm which again equals to the defined mic distance, and the mid/bass drivers are closer than 300cm. I don't know which one is the correct way to make the simulations.


Quote:
You mentioned you are using a 300cm (ie. 3 metre) mic distance? Is this correct? If so - then that's great if you can gate out room relections as you'll be able to get good point summation of the drivers (being a 3 way - you need more distance between baffle and mic) and you'll be able to measure the lower xo point farfield for xo simulation.
I only use 300cm distance for the mic in the SoundEasy simulations, as I thought this would represent more closer to the real listening distance. As I am simulating a tiltel front baffle, moving the mic closer or more far away affects to the distance relationships to each driver, and that way will give different phase curves. Of course when the speaker is ready, the listening distance/height will vary, but I though it would be a good idea to design it so, that the optimal listening point would be 3 meters away from the speaker, and at the height of the tweeter.

I measured all the drivers in this test baffle at 1m, just to get the phase differences or acoustic center differences and infinite baffle frequency responses for the SoundEasy simulations.

Measuring the final speaker at 3m would be a problem, but I could make the measurements from 1m and make the simulations temporarily at 1m in SoundEasy, and if I get a good match I know all the simulations are made correctly, right?
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Old 5th March 2008, 01:37 PM   #10
tinitus is offline tinitus  Europe
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I believe that best compromise of listening height is midrange level, which means that tilt angle will be a little less
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