Well, I finally tried a fully open baffle setup, and the results have me puzzled. Firstly, I'll say I'm impressed. After a number of hours of listening, and watching King Kong, I notice the following:
* much bigger sweet spot
* midrange is more transparent
* bass is more accurate than I thought my drivers were capable of, most noticeable is that it is subjectively much more "quick" and it makes me aware of how much was added to the sound. In particular I notice this on things like kick drum - very convincing as if it is actually in the room - previously bass sounded exaggerated, but not due to frequency response
Before:
TL mains with Vifa P17 in MTM with two push pull sealed AV12 subs in the corner, EQ'd flat with Ultracurve (including room modes which aren't that bad in this room)
After:
The same drivers placed on 400mm wide x 500 tall main panels with 50 - 150mm wings and subs push pull H frame approx 400mm wide and deep
I notice the tonal balance doesn't sound the same, though the response is calibrated flat. The bass sounds more lean and quick, also the midrange does as well. I put this down to box coloration, and perhaps time domain performance.
Now here's the mystery:
The EQ required to compensate for dipole roll-off is ALMOST non existant!
Here are the settings used by Ultracurve to calibrate it flat. The response in the middle shows the "before", on the top a monopole sub is used and on the bottom is the full OB setup. Note, this is the EQ settings, so the actual response is close to the inverse.
Around 35 Hz there is a room mode peak which is EQ'd out in all of them.
So my question to OB gurus:
How do you explain the absence of EQ required? The main panel should have an fequal around 200 Hz, and similar for the H frame.
Another Q:
When calculating Fequal for a H frame, should D be taken as merely half the depth, or should I add half the baffle width?
* much bigger sweet spot
* midrange is more transparent
* bass is more accurate than I thought my drivers were capable of, most noticeable is that it is subjectively much more "quick" and it makes me aware of how much was added to the sound. In particular I notice this on things like kick drum - very convincing as if it is actually in the room - previously bass sounded exaggerated, but not due to frequency response
Before:
TL mains with Vifa P17 in MTM with two push pull sealed AV12 subs in the corner, EQ'd flat with Ultracurve (including room modes which aren't that bad in this room)
After:
The same drivers placed on 400mm wide x 500 tall main panels with 50 - 150mm wings and subs push pull H frame approx 400mm wide and deep
I notice the tonal balance doesn't sound the same, though the response is calibrated flat. The bass sounds more lean and quick, also the midrange does as well. I put this down to box coloration, and perhaps time domain performance.
Now here's the mystery:
The EQ required to compensate for dipole roll-off is ALMOST non existant!
Here are the settings used by Ultracurve to calibrate it flat. The response in the middle shows the "before", on the top a monopole sub is used and on the bottom is the full OB setup. Note, this is the EQ settings, so the actual response is close to the inverse.
Around 35 Hz there is a room mode peak which is EQ'd out in all of them.
So my question to OB gurus:
How do you explain the absence of EQ required? The main panel should have an fequal around 200 Hz, and similar for the H frame.
Another Q:
When calculating Fequal for a H frame, should D be taken as merely half the depth, or should I add half the baffle width?
Attachments
As you can see, the construction quality and finish is outstanding! ... 5kg weights keep them from falling, and I'm using my existing passive 2.5 way crossovers
Also as you can see, those panels were precisely calculated for target fequal after hundreds of hours of research, simulation and testing ... *coughs* ... well, actually they were what I had lying around ...
Also as you can see, those panels were precisely calculated for target fequal after hundreds of hours of research, simulation and testing ... *coughs* ... well, actually they were what I had lying around ...
Attachments
Hi Paul,
Great to see the MTM VifaP17's up in an open baffle . I have been planning this for a couple of years ! However I keep looking for more info on other implementations and their results.
After reading your post I've decided to go ahead and build one myself. I've had 4 P17's stashed away just for this kind of a design. My tweeters however are Vifa , D27AG , the XT25 ring radiator and a Dayton Planar PT2. Not quite sure what I should use.
What crossover frequency did you use with the P17 ?
I did want to use a narrower baffle but most things I read indicate that a wider baffle is preferable . I have been toying with a partially wooden and partially perspex baffle so that it "looks" smaller than it really is. Lots of designs around with this scheme.
Your system looks unfinished but OK. Certainly not ugly as you said. Some self adhesive wood finish vinyl will give it a nice appearance. The "stand" could be improved of course ...black ?
Cheers.
Great to see the MTM VifaP17's up in an open baffle . I have been planning this for a couple of years ! However I keep looking for more info on other implementations and their results.
After reading your post I've decided to go ahead and build one myself. I've had 4 P17's stashed away just for this kind of a design. My tweeters however are Vifa , D27AG , the XT25 ring radiator and a Dayton Planar PT2. Not quite sure what I should use.
What crossover frequency did you use with the P17 ?
I did want to use a narrower baffle but most things I read indicate that a wider baffle is preferable . I have been toying with a partially wooden and partially perspex baffle so that it "looks" smaller than it really is. Lots of designs around with this scheme.
Your system looks unfinished but OK. Certainly not ugly as you said. Some self adhesive wood finish vinyl will give it a nice appearance. The "stand" could be improved of course ...black ?
Cheers.
Ashok,
You might want to also have a go at the Vifa P17 tweak which I'm in the process of doing. It involves putting glue rings on the back of the cone and adding a phase plug. I've seen documentation for this and the improved frequency and time domain performance. I haven't had a chance for a proper AB comparison, but there does seem to be some improvement. I wrote about this in another thread I started recently.
I'm using my existing 2.5 way xo which is 2nd order and xo @ 3.5k. Ideally this is a bit high WRT off axis lobing but it's good for distortion and power handling. The 0.5 midbass has a first order LP @ 200 Hz.
You could try a 2.5 way where you have WMTMW. I'd try this where the outer woofers have a first order LP. This way you won't have the off axis lobing problem. I'd go with the ring radiator and experiment with a small waveguide to allow it to go a bit lower and take some strain off as they say it tends to want a 3rd order xo higher up than most would prefer to use.
Regarding baffle width, I did come across some guys who measured different widths compared to Linkwitz Phoenix design. Apparently its a trade off between lowering Fequal and polar response. If I stick with OB and make some decent ones then they won't be as wide.
I've been thinking of putting a small H or U frame below each main panel, or I could put the main panels on stands. I might also experiment with a corner dipole woofer.
Next I want to try:
* U frame woofer
* waveguides on the main panels, similar to Zaph's
You might want to also have a go at the Vifa P17 tweak which I'm in the process of doing. It involves putting glue rings on the back of the cone and adding a phase plug. I've seen documentation for this and the improved frequency and time domain performance. I haven't had a chance for a proper AB comparison, but there does seem to be some improvement. I wrote about this in another thread I started recently.
I'm using my existing 2.5 way xo which is 2nd order and xo @ 3.5k. Ideally this is a bit high WRT off axis lobing but it's good for distortion and power handling. The 0.5 midbass has a first order LP @ 200 Hz.
You could try a 2.5 way where you have WMTMW. I'd try this where the outer woofers have a first order LP. This way you won't have the off axis lobing problem. I'd go with the ring radiator and experiment with a small waveguide to allow it to go a bit lower and take some strain off as they say it tends to want a 3rd order xo higher up than most would prefer to use.
Regarding baffle width, I did come across some guys who measured different widths compared to Linkwitz Phoenix design. Apparently its a trade off between lowering Fequal and polar response. If I stick with OB and make some decent ones then they won't be as wide.
I've been thinking of putting a small H or U frame below each main panel, or I could put the main panels on stands. I might also experiment with a corner dipole woofer.
Next I want to try:
* U frame woofer
* waveguides on the main panels, similar to Zaph's
Looks like you did unintentionally what I did intentionnally in my dipoles ... using the various acoustic rolloffs and gains to compensate each other.
If you read the Linkwitz site you'll find that he uses a shelving highpass from 100 to 200 Hz to compensate for 6 dB gain as you move towards the lower frequencies (floor reflection adds increasingly in-phase to direct sound) . In your case and with an fequal of 200 Hz, leaving the floor gain uncorrected means your dipole rolloff is automatically "compensated" down to 100 Hz.
Below that, you might either have a mixture of room effects, or a high Q woofer, producing additional gain. Even with dipoles, you do get room effects. For instance, I have resonances at 25 Hz and multiples thereof up to 125 Hz in my ordinary sized - but asymmetrical - room. The largest peak (around 100 Hz) is easily +6 dB. So, in your case just one broad +6 dB peak say centered at 60 Hz with -3dB at 40 and 80 Hz may be responsible for giving you an EQ-free zone down to practically 40 Hz ...
Or so the story might go.
If you read the Linkwitz site you'll find that he uses a shelving highpass from 100 to 200 Hz to compensate for 6 dB gain as you move towards the lower frequencies (floor reflection adds increasingly in-phase to direct sound) . In your case and with an fequal of 200 Hz, leaving the floor gain uncorrected means your dipole rolloff is automatically "compensated" down to 100 Hz.
Below that, you might either have a mixture of room effects, or a high Q woofer, producing additional gain. Even with dipoles, you do get room effects. For instance, I have resonances at 25 Hz and multiples thereof up to 125 Hz in my ordinary sized - but asymmetrical - room. The largest peak (around 100 Hz) is easily +6 dB. So, in your case just one broad +6 dB peak say centered at 60 Hz with -3dB at 40 and 80 Hz may be responsible for giving you an EQ-free zone down to practically 40 Hz ...
Or so the story might go.
The speakers are 1.1m from the wall at the front of the tweeter. There is a bench on which the TV sits with draws below and this protrudes about 0.5m.
I have found the max SPL is down a fair amount as the drivers move more for a given input power. Previously SPL was limited by the power from the main amp ~ 40 or 50w. Now, drivers approach their excursion limit about the same time as the amp gets into clipping territory.
Last night I listened to an album I though was never that good, I found the bass a bit boomy. It's John Butler Trio, they have a fair amount of acoustic instruments, esp acoustic bass. Well, now in open baffle mode, it actually sounds very good, much more natural, and I actually like this recording!
I have found the max SPL is down a fair amount as the drivers move more for a given input power. Previously SPL was limited by the power from the main amp ~ 40 or 50w. Now, drivers approach their excursion limit about the same time as the amp gets into clipping territory.
Last night I listened to an album I though was never that good, I found the bass a bit boomy. It's John Butler Trio, they have a fair amount of acoustic instruments, esp acoustic bass. Well, now in open baffle mode, it actually sounds very good, much more natural, and I actually like this recording!
Konnichiwa,
Same as I did before. The basic "milkmaid" calculations presented variously are sufficietly abstract to be useless in reality. Floor reflection/image boost, room gain and so on give much more LF output than these "milkmaid" calculations predict.
Linkwitz actually does cover this in a hidden corner of his site, where he first equalises the dipole rolloff (6db/8ve) and then equalises out seperately the various other boosts, cancelling in effect a large amount of the EQ he applied in the first place.
Sayonara
paulspencer said:
Same as I did before. The basic "milkmaid" calculations presented variously are sufficietly abstract to be useless in reality. Floor reflection/image boost, room gain and so on give much more LF output than these "milkmaid" calculations predict.
Linkwitz actually does cover this in a hidden corner of his site, where he first equalises the dipole rolloff (6db/8ve) and then equalises out seperately the various other boosts, cancelling in effect a large amount of the EQ he applied in the first place.
Sayonara
Thanks for the responses, obviously there is more to this. I would like to know EXACTLY what is going on, but for now I will accept that there are unaccounted for factors boosting already.
I'd like to try a few things hopefully next weekend:
* moving the baffle forward so it forms a U frame; I'm a bit curious to experiment with this and see what happens as you transition from H to U
* adding a waveguide to the tweeter then EQ out the gain
I'd also like to get my measurement setup finished (mic preamp and figure out speaker workshop properly). Then I can test outside and find out what is going on without the room.
What surprises me with this system is how good it sounds with drivers I didn't think were really up to integrating that well. I really thought I'd need a woofer inbetween the mids and sub to get decent sound without making the sub go higher than it will do well, or making the mids work too hard.
I plan to keep this going for a few weeks, get used to them on both movies and a variety of music, then switch back and see how I feel about each.
I'd like to try a few things hopefully next weekend:
* moving the baffle forward so it forms a U frame; I'm a bit curious to experiment with this and see what happens as you transition from H to U
* adding a waveguide to the tweeter then EQ out the gain
I'd also like to get my measurement setup finished (mic preamp and figure out speaker workshop properly). Then I can test outside and find out what is going on without the room.
What surprises me with this system is how good it sounds with drivers I didn't think were really up to integrating that well. I really thought I'd need a woofer inbetween the mids and sub to get decent sound without making the sub go higher than it will do well, or making the mids work too hard.
I plan to keep this going for a few weeks, get used to them on both movies and a variety of music, then switch back and see how I feel about each.
The reason I asked about the distances to the rear, was to determine if the back wave is being diffused or absorbed. If so this will create a less than ideal dipole response by ideal I mean a perfect figure 8 response and the "normal" low end cancellations/rolloff. You are not alone in this, someonelse put a woofer in a room at an angle and did not get the usual rolloff.
Im not saying this is the reason, Im merely speculating based on thought experiments that have been loosely confirmed by two people.
To test it either remove the table/shelving
and retest the FR or move the speakers to a different wall/room, if you care anyways.I have built 2 OB speakers, and found that the required EQ was less than indicated by a simple 6 db per octave.
However, since TL has posted this information on several threads, I was trying to keep my baffle width @ about 20" to minimize my EQ.
TL has a calculator on his web site that better predicts this than anything else I have seen.
My test OB have usable base below 40, with only 3 db of EQ.
My Version 2 OB needs 6 db of EQ, but the drivers are much lower Q.
Doug
However, since TL has posted this information on several threads, I was trying to keep my baffle width @ about 20" to minimize my EQ.
TL has a calculator on his web site that better predicts this than anything else I have seen.
My test OB have usable base below 40, with only 3 db of EQ.
My Version 2 OB needs 6 db of EQ, but the drivers are much lower Q.
Doug
one thing that I think most here probably didn't get to is:
what was your mic?
I've found that the behringer mic and the deq isn't entirely too accurate down low. The DEQ does not even recommend measuring below 80hz. That might explains the flatness you're seeing.
How did you measure your speakers?
What's strange to me is that even using the same crossovers. You're getting very different response in the midrange and highs with the MTMs going from TL to dipole. Were they all measured the same way?
what was your mic?
I've found that the behringer mic and the deq isn't entirely too accurate down low. The DEQ does not even recommend measuring below 80hz. That might explains the flatness you're seeing.
How did you measure your speakers?
What's strange to me is that even using the same crossovers. You're getting very different response in the midrange and highs with the MTMs going from TL to dipole. Were they all measured the same way?
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