My MTM Project

[Twisted85]

Hi,

This is my second full-DIY speaker project, first with using SoundEasy.

My project is a floorstanding MTM with Peerless 810921 tweeters and Dayton RS180-8 woofers.

I built a single prototype for the measurements and it looks like this:

http://koti.mbnet.fi/twisted-/proto.jpg

The crossover point will be around 2KHz.

Now what I've been wondering is, that should I place the tweeter on the middle like in the picture, or should I move it on the x-axis closer to one of the sides?
The frequency response would be smoother measured from 0 angle, but of course this have it's own effects to horizontal polar plots.

But what do you think? Any suggestions based on your own experiences?

[HiFiNutNut]

If you do a search you will find discussions of the topic.

I take what others (S.L., John K) say and believe there is little value of tweeter offsetting. Offsetting the tweeter may produce a flatter on-axis response, but if you move your head left or right a few inches then the response may not be flat any more.

To tackle the diffraction problem, use slim front baffle with a large edge roundover.

[Jay_WJ]

I have used both drivers in my projects. I'd cross lower than 2.0 kHz with the RS180s. Its midrange is not as clean as some top performers like the ScanSpeak and the Usher. The Peerless 810921 can go lower and its low-end performance is excellent.

[Twisted85]

Ok, I will think about the offset later. (Although I will probably stay with the centered version)

At this point I've measured the responses from the prototype and spent some time with the crossover.

http://koti.mbnet.fi/twisted-/sounde...m_response.gif

http://koti.mbnet.fi/twisted-/soundeasy/mtm_phase.gif

There's few things about the crossover that I'd like to ask.

1) Does the phase response look good enough as it is now? Can this considered a good match or not?

2) Do you think that the bass response at 150-300 will cause problems in the sound? Is the level too high at this area? Should I make a filter to lower this area few dB?

3) I lowered the xo point a bit from the earlier design, do you think it's now good?

4) Reading the older discussions I found out that people have made filters to lower the Dayton woofer's response at 4-10Khz. How necessary is this? I spent quite a lot of time examining different filters to lower it down but I have to say that my result weren't very good as I have no earlier experience using these type of filters.

[Jay_WJ]

Originally posted by Twisted85
Ok, I will think about the offset later. (Although I will probably stay with the centered version)

The tweeter response looks okay in your in-box measurement.

1) Does the phase response look good enough as it is now? Can this considered a good match or not?

They seem to be aligned well.

2) Do you think that the bass response at 150-300 will cause problems in the sound? Is the level too high at this area? Should I make a filter to lower this area few dB?

How did you measure your bass response? Did you merge near field response to the gated far field one?

3) I lowered the xo point a bit from the earlier design, do you think it's now good?

It looks good. It's at about 1.6 kHz, isn't it?

4) Reading the older discussions I found out that people have made filters to lower the Dayton woofer's response at 4-10Khz. How necessary is this? I spent quite a lot of time examining different filters to lower it down but I have to say that my result weren't very good as I have no earlier experience using these type of filters.

In your simple LR4 crossover, the main breakups of the RS180s are suppressed only to 60 dB, which should be audible. Have you seen what people did to suppress the RS180's breakup nodes? Take a look at:
http://www.rjbaudio.com/RS180MTM/rs180-rs28-mtm.html

Also,
http://www.geocities.com/woove99/Spk...RS180_2way.htm

The most effective way to suppress the breakups is using the C-E type filter---combination of a 4th order electrical and a notch filter. But in my experience, a simple bottomless notch filter located between the two main breakups works well when combined with a low crossover frequency. In your simulation, try a 0.012 mH inductor in series with your current 33 uF shunted capacitor in the woofer net to form a notch filter, and see how the response changes. Also, try the combination of 30 uF cap and 0.013 mH inductor.

Would you then post the result for me?

[Twisted85]

Thanks for your reply!

Originally posted by Jay_WJ
How did you measure your bass response? Did you merge near field response to the gated far field one?

I measured the farfield with 5.28ms gate, then made the nearfield measurement and port measurement and added the diffraction to the nearfield data. The Merge point is 350Hz.
What actually made me wonder in this, that when I added the diffraction the lowest frequencys droppep lower level.

I have a screencap from adding the diffraction:

http://koti.mbnet.fi/twisted-/soundeasy/diffraction.gif

In JohnK's SoundEasy guide is a picture of adding the diffraction, and in this pic the response only starts to rise from 100Hz, but in my case the response also lowered after under 70Hz. Is this just because the simulated front panels are different or why does this happen?

It looks good. It's at about 1.6 kHz, isn't it?

Yes.

Would you then post the result for me?

Sure!

http://koti.mbnet.fi/twisted-/sounde...thinductor.gif
http://koti.mbnet.fi/twisted-/sounde...hinductor2.gif

Both has the response without the inductor for reference. It seems to do the job pretty good, but where can I get inductors with so low value? In Finland I only know dealers for Intertechnik and Mundorf components, and couldn't find so small values from their inductors. The lowest I could find was 0.05mH

I also tried to do something to the bass response.

http://koti.mbnet.fi/twisted-/sounde...correction.gif

What are your thoughts about this? Is this necessary? I don't want the speaker to sound boomy as it will be placed in a middle sized room with hard stone walls.

In this box the tuning frequency is ~33-34Hz, I've been thinking to use a bit higher 35-36Hz tuning in the final speaker.

[Jay_WJ]

Actually the bump is located a bit higher than the upper bass region that makes speakers sound "boomy." So, even if the bump is real, not measurement artifact, it may make speakers sound a bit warm rather than boomy. Anyway, I'd worry about this later. You can judge it after you hear the speakers, and if necessary you can make a change at that time.

Yeah, the small inductor should be a problem if you can't buy it. Can you ask your parts dealer to unwind the 0.05 mH one to reach that value? They should have an inductance meter. I think its accuracy is important because it's used in a notch filter.

[Twisted85]

Originally posted by Jay_WJ
Yeah, the small inductor should be a problem if you can't buy it. Can you ask your parts dealer to unwind the 0.05 mH one to reach that value? They should have an inductance meter. I think its accuracy is important because it's used in a notch filter.

Ordering from overseas isn't a problem if the shipping cost is reasonable. The Dayton drivers are also from Partsexpress as those are not sold in here.
Do you know any dealer that would sell those kind of inductors? I haven't been able to find any.

[spock]

Janzen makes a .025 mH inductor. I got mine from partsexpress, but I believe that they are a Europian company.

[Jay_WJ]

Madisound winds/unwinds an inductor to a custom value for a small charge. But if you have anyone who has an inductance meter around you, it's very easy to unwind any smallest one to get the value you want.