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Old 22nd April 2010, 11:51 PM   #61
get-lit is offline get-lit  United States
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Quote:
Originally Posted by wintermute View Post
And finally I worked out where the 5.5K dip is coming from. It corresponds exactly to the width of the baffle to the start of the chamfer. so I guess that is a good reason for chamfering the baffle, it pushes up the frequency that the baffle diffraction happens at.
Wow, could you please illustrate on a photo where the baffle may be affecting the dip?
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Old 23rd April 2010, 12:20 AM   #62
Loren42 is offline Loren42  United States
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Originally Posted by wintermute View Post
And finally I worked out where the 5.5K dip is coming from. It corresponds exactly to the width of the baffle to the start of the chamfer. so I guess that is a good reason for chamfering the baffle, it pushes up the frequency that the baffle diffraction happens at.
Hmm, are you are confusing baffle step response with edge diffraction.

It sounds like edge diffraction is the source of the problem. Rounding the corners requires the edge radius to be equal to the wavelength (or larger) of the offending wave. That could be several inches (2.6" to be exact) in radius to be effective.

You could try relocating the tweeter so that it is not centered on the baffle. The only other recourse is increasing the radius of the edge or what I did was fold back the baffle at about 30 to lesson the diffraction effect.

Click the image to open in full size.
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Old 23rd April 2010, 03:54 AM   #63
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Hi Get-Lit and Loren. Yes it is edge diffration I was talking about, though I'm going to have to check the measurement again because looking back at it it doesn't sound right.

I think I measured from the centre to the beginning of the chamfer but 62mm seems to be too small as the width from edge to edge (of the chamfer) *should* be about 150mm which would correspond to a freq of surprise surprise my ugly dip point around 2200 Hz....

In my haste to find a reason for the dip at 5.5K (which shows on both the tweeter and the midbass measurements) I found out the wavelength at 5.5K and ran a tape measure over the front of the baffle, and my recollection was that the two matched (but now I'm uncertain).

I'm pretty sure I have measured these woofers not on the baffle and seen the same dip around 2K but I'm now wondering whether I have an unfortunate natural dip at this freq which is re-inforced by my chosen baffle geometry! (which the quick and dirty measurement in post 58 would tend to support...)

Post 6 in this thread has the BDS simulation of how the baffle diffraction should affect the freq response. It is looking scarily accurate for the unchamfered baffle, but Not showing the predicted benefits of the chamfered baffle in post #7.

I'm actually not that concerned about the 5.5K small dip, it is the big one at around 2k that bugs me. But I'm going to get some 6.4mm felt to put on the baffles and see how that affects things. Redesigning the box at this point isn't an option

The chamfer is about 25mm I will do some more measurements again this weekend of both chamfered and non-chamfered baffles now that I have a slightly better measurement setup (the quick and dirty measurements in post 58 definitely showed a difference, but it could have been due to speaker positioning maybe).

I know a lot of people say that small chamfers have no effect, I'm not entirely convinced, the main reason I have only chamfered one at this stage is so I can get conclusive evidence one way or the other

Tony.
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Old 23rd April 2010, 11:02 AM   #64
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by wintermute View Post
Hi Get-Lit and Loren. Yes it is edge diffration I was talking about talking about, though I'm going to have to check the measurement again because looking back at it it doesn't sound right.

I think I measured from the centre to the beginning of the chamfer but 62mm seems to be too small as the width from edge to edge (of the chamfer) *should* be about 150mm which would correspond to a freq of surprise surprise my ugly dip point around 2200 Hz....

In my haste to find a reason for the dip at 5.5K (which shows on both the tweeter and the midbass measurements) I found out the wavelength at 5.5K and ran a tape measure over the front of the baffle, and my recollection was that the two matched (but now I'm uncertain).

I'm pretty sure I have measured these woofers not on the baffle and seen the same dip around 2K but I'm now wondering whether I have an unfortunate natural dip at this freq which is re-inforced by my chosen baffle geometry! (which the quick and dirty measurement in post 58 would tend to support...)

Post 6 in this thread has the BDS simulation of how the baffle diffraction should affect the freq response. It is looking scarily accurate for the unchamfered baffle, but Not showing the predicted benefits of the chamfered baffle in post #7.

I'm actually not that concerned about the 5.5K small dip, it is the big one at around 2k that bugs me. But I'm going to get some 6.4mm felt to put on the baffles and see how that affects things. Redesigning the box at this point isn't an option

The chamfer is about 25mm I will do some more measurements again this weekend of both chamfered and non-chamfered baffles now that I have a slightly better measurement setup (the quick and dirty measurements in post 58 definitely showed a difference, but it could have been due to speaker positioning maybe).

I know a lot of people say that small chamfers have no effect, I'm not entirely convinced, the main reason I have only chamfered one at this stage is so I can get conclusive evidence one way or the other

Tony.
1" radius is not likely enough to make a real dent in the problem. You need to be more than 2" and probably closer to 3".
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Old 23rd April 2010, 11:15 AM   #65
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OK My memory from getting out the tape measure to the time I posted was obviously a bit on the flaky side The measurement of 62mm is actually the distance from the centre of the tweeter to the roll surround on the midbas... so I have propagated some completely false information!!! Sorry everyone!

I'll do some measurements tomorrow after putting the legs on the second box so that I can make sure that the only difference between the boxes is the chamfer. Will also be very careful positioning the boxes to make sure all things are equal.

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Old 23rd April 2010, 11:35 AM   #66
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by wintermute View Post
Hi Get-Lit and Loren. Yes it is edge diffration I was talking about talking about, though I'm going to have to check the measurement again because looking back at it it doesn't sound right.

I think I measured from the centre to the beginning of the chamfer but 62mm seems to be too small as the width from edge to edge (of the chamfer) *should* be about 150mm which would correspond to a freq of surprise surprise my ugly dip point around 2200 Hz....

In my haste to find a reason for the dip at 5.5K (which shows on both the tweeter and the midbass measurements) I found out the wavelength at 5.5K and ran a tape measure over the front of the baffle, and my recollection was that the two matched (but now I'm uncertain).

I'm pretty sure I have measured these woofers not on the baffle and seen the same dip around 2K but I'm now wondering whether I have an unfortunate natural dip at this freq which is re-inforced by my chosen baffle geometry! (which the quick and dirty measurement in post 58 would tend to support...)

Post 6 in this thread has the BDS simulation of how the baffle diffraction should affect the freq response. It is looking scarily accurate for the unchamfered baffle, but Not showing the predicted benefits of the chamfered baffle in post #7.

I'm actually not that concerned about the 5.5K small dip, it is the big one at around 2k that bugs me. But I'm going to get some 6.4mm felt to put on the baffles and see how that affects things. Redesigning the box at this point isn't an option

The chamfer is about 25mm I will do some more measurements again this weekend of both chamfered and non-chamfered baffles now that I have a slightly better measurement setup (the quick and dirty measurements in post 58 definitely showed a difference, but it could have been due to speaker positioning maybe).

I know a lot of people say that small chamfers have no effect, I'm not entirely convinced, the main reason I have only chamfered one at this stage is so I can get conclusive evidence one way or the other

Tony.
1" radius is not likely enough to make a real dent in the problem. You need to be more than 2" and probably closer to 3".
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Old 23rd April 2010, 11:42 AM   #67
Loren42 is offline Loren42  United States
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I looked over your drawings in your first posts and saw a baffle width of about 200 mm.

The baffle step loss F3 for that width would be about 575 Hz.

Also, try clamping on some wings or flat extensions onto either side of the cabinet and see if the frequency response changes. You should easily be able to predict baffle step loss and diffraction effects and compare theory with observed data.

It is very easy to get to chasing your tail when testing speakers. You need to approach the problem with a paranoid attitude and question every measurement (is this really valid). I recommend testing the same thing multiple ways to see if the tests agree.

It is easy to get errors because there is so many things to think about. Also, the room where you are doing the measurements can make life hell. Gating only does so much.
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Old 23rd April 2010, 12:38 PM   #68
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Thanks Loren I've done measurements in all sorts of places but they always seem to have the same massive dip at around 2Khz, this just doesn't seem to change regardless of how I mount the speaker in what room, or position of mic etc. I can try adding some width to see how it affects the measurements. Also I can try going out on the balcony, have just been lazy as dragging the computer out there is no small task Probably the other thing I should do is re-read my "testing loudspeakers" by Joe D'appolito... I remember thinking ahhh to a few things when I read it the first time, but it was a while ago, and I have forgotten.

I've attached a comparison of what BDS (baffle diffraction simulator) says the response will be like compared to actual, the baffle step prediction from the sim follows pretty closely all things considered, so I don't think there is anything strange going on there. It is what happens after 1.6K that bugs me, certainly isn't something the sim thinks should happen with my baffle geometry.

Black is the actual measured response at 1M. Ignore the big drop at 200Hz that was a problem with me turning on calibration in holm impulse and it screwing up the phase.
Blue is the sim with chamfer, and red is the sim without chamfer (minimal difference, but for the tweeter the chamfer seems to make a bit more of a difference).

I've also attached a somewhat interesting plot from the morel isreal sites mw144 datasheet. Note the original datasheet I saw for the MW144 was very different to this, but it has some interesting similarities to what I have measured. Strange that the dip seems to be at 1K for them but 2K for me but other than that looks rather similar to me...

Tony.
Attached Images
File Type: png mw144_sim_real.png (10.1 KB, 281 views)
File Type: jpg mw144_from_datasheet.jpg (36.0 KB, 276 views)
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Last edited by wintermute; 23rd April 2010 at 12:55 PM.
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Old 23rd April 2010, 01:11 PM   #69
Loren42 is offline Loren42  United States
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Quote:
Originally Posted by wintermute View Post
Thanks Loren I've done measurements in all sorts of places but they always seem to have the same massive dip at around 2Khz, this just doesn't seem to change regardless of how I mount the speaker in what room, or position of mic etc. I can try adding some width to see how it affects the measurements. Also I can try going out on the balcony, have just been lazy as dragging the computer out there is no small task Probably the other thing I should do is re-read my "testing loudspeakers" by Joe D'appolito... I remember thinking ahhh to a few things when I read it the first time, but it was a while ago, and I have forgotten.

I've attached a comparison of what BDS (baffle diffraction simulator) says the response will be like compared to actual, the baffle step prediction from the sim follows pretty closely all things considered, so I don't think there is anything strange going on there. It is what happens after 1.6K that bugs me, certainly isn't something the sim thinks should happen with my baffle geometry.

Black is the actual measured response at 1M. Ignore the big drop at 200Hz that was a problem with me turning on calibration in holm impulse and it screwing up the phase.
Blue is the sim with chamfer, and red is the sim without chamfer (minimal difference, but for the tweeter the chamfer seems to make a bit more of a difference).

I've also attached a somewhat interesting plot from the morel isreal sites mw144 datasheet. Note the original datasheet I saw for the MW144 was very different to this, but it has some interesting similarities to what I have measured. Strange that the dip seems to be at 1K for them but 2K for me but other than that looks rather similar to me...

Tony.
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Old 23rd April 2010, 01:13 PM   #70
Loren42 is offline Loren42  United States
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Also, have you taken on-axis and off-axis plots of the drivers individually without crossovers?

The two woofers should be connected together, but having those plots will tell you is there is a driver or phase issue.

What about far field response of about 2 meters?

Last, have you tried near field response?
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