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Can we discuss the BMR Tower?

MrPeabody

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... But since I have your attention, the grill design has proven as problematic as Middle East politics. A one-piece grill is prone to shipping damage and will cover a lot of the veneer. So far I haven't been able to come up with a 2-piece design that looks right. The first draft had an oval upper grill that along with the woofer grill looked like an exclamation point. So I got rid of the oval. It's still kind of klunky. Any suggestions are welcome. As for the "oddly spaced drivers," that's what you get when you optimize the woofer and port locations to maximize the quarter wave contribution, and squeeze the MTM configuration as close together as possible. It's called form following function, or more succinctly, science. :)

I've never been able to decide which I dislike the most: speaker grills, or speakers without grills. You're damned if you do and damned if you don't. They're flimsy and ugly. One of the things I always liked about the large Advent, especially the later ones with the bullnose rounding on all the edges, is the way that the grill is flush with the front surface of the speaker. From a purely aesthetic standpoint I like the way that Kloss did that, using a big piece of hardboard with big holes cut out of it, and covered in a fabric that was neither too course nor too fine. But to do this, he had to recess the woofer, and the tweeter is left without much real protection.

I do a lot more thinking about building speakers than actually building them, and while I've thought of solutions for most of the things that bug me, I've never come with a satisfactory solution for grill design. If the front of the speaker is made of attractive wood, I don't like covering it up. But if you put separate grills over the individual drivers, it ends up looking like the interior of a car, which is so hideous that only Yamaha would actually do it.

One thing I especially like about the BMR, from an aesthetic perspective, is the way the two midranges and the tweeter are flush with the front face of the speaker. To my eye, this is really, really clean, to the point that I don't have any desire to want to cover them up. But if you were to put a grill on the woofer and leave the MTM uncovered, this wouldn't look right.

As for the functional benefits of MTM, this is an area where again there are differences of opinion. While I'm not personally a big fan, I recognize that there are specific benefits that are desirable so long as the downside is avoided. To avoid the downside, the two midrange drivers need to be close enough together such that their vertical separation is not too great in comparison to the shortest wavelength (highest frequency) within their operating range. You alluded to this need in your comments, and it is apparent upon glancing at the speaker that you perfectly understood this and met the concern by using unusually small midrange drivers and placing them as close to the tweeter as possible. Just from looking at it, I expect that you succeeded in avoiding the downside, and I'll note that it isn't possible to do this if the tweeter is placed within a big waveguide. And the advantages are real. The vertical dispersion pattern is symmetric about the horizontal plane. I don't know what sort of crossover you have used, but even when a crossover design endeavors for the radiation from the midrange and the tweeter to be fully coherent, this can never be accomplished to perfection, and the general consequence will be some amount of upward or downward tilt in the main lobe. Moreover, I believe there is an oft-ignored penalty with using a phase-coherent crossover: the tweeter and midrange each have to be at -6 dB at the point where they are equal in SPL, which implies a -3 dB dip in the aggregate power response. This small dip in the power response has small yet real consequences for the frequency response at all listening locations where coherency does not apply, i.e., at locations where the tweeter and midrange are not both the same distance from the listener. The only way to avoid this, so far as I've been able to determine, is to use crossover slopes where the tweeter and midrange are both at -3 dB at the point where they are equal in SPL, which (I think) implies a 90-degree difference in mutual phase, and which would imply a vertically tilted main lobe, avoided by way of the MTM arrangement.

The sense I get when I look at this speaker is that you had an intimate understanding of several design issues that are ignored in most speakers produced by major manufacturers, that you invested a great deal of time and effort in coming up with solutions that work.

Except for the damned grill. :rolleyes:
 

Dennis Murphy

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Re BMR with less expensive drivers: I've been playing around with a few ideas to work on in my hopefully-soon retirement. One of them is a 3-way using the BMR and very inexpensive other drivers. Since the use-case would be for multichannel music and Auro 3d upmixing though a Denon X-4700, the efficiency target is around 90 dB after baffle step (there isn't much power available if you are using all 9 channels of amplification).

Drivers: Techtonic TEBM46C20N, Peerless FSL-0615R02 vented 15 liters (Denovo cabinet) f3 78 Hz, Peerless DX25TG59-04 (or BC25TG15-04 if it goes low enough with a resonance comp circuit). I plan on experimenting with the following: 1) a crossover loosely based on a Duelund where the mid level is about 4dB below the woofer and tweeter but sums flat. 2) with and without a shallow modeling clay waveguide on the tweeter. 3) With and without dual chamber reflex. I do not know how much BMR "flavor" the mids would have but the -6dB directivity should be very good. Will need a sub or two. Unfortunately, this will be a lot of work and will have to wait for my retirement.
I would be very surprised if you can get 90 dB in the midrange even with lots of help from the woofer and tweeter.
I've never been able to decide which I dislike the most: speaker grills, or speakers without grills. You're damned if you do and damned if you don't. They're flimsy and ugly. One of the things I always liked about the large Advent, especially the later ones with the bullnose rounding on all the edges, is the way that the grill is flush with the front surface of the speaker. From a purely aesthetic standpoint I like the way that Kloss did that, using a big piece of hardboard with big holes cut out of it, and covered in a fabric that was neither too course nor too fine. But to do this, he had to recess the woofer, and the tweeter is left without much real protection.

I do a lot more thinking about building speakers than actually building them, and while I've thought of solutions for most of the things that bug me, I've never come with a satisfactory solution for grill design. If the front of the speaker is made of attractive wood, I don't like covering it up. But if you put separate grills over the individual drivers, it ends up looking like the interior of a car, which is so hideous that only Yamaha would actually do it.

One thing I especially like about the BMR, from an aesthetic perspective, is the way the two midranges and the tweeter are flush with the front face of the speaker. To my eye, this is really, really clean, to the point that I don't have any desire to want to cover them up. But if you were to put a grill on the woofer and leave the MTM uncovered, this wouldn't look right.

As for the functional benefits of MTM, this is an area where again there are differences of opinion. While I'm not personally a big fan, I recognize that there are specific benefits that are desirable so long as the downside is avoided. To avoid the downside, the two midrange drivers need to be close enough together such that their vertical separation is not too great in comparison to the shortest wavelength (highest frequency) within their operating range. You alluded to this need in your comments, and it is apparent upon glancing at the speaker that you perfectly understood this and met the concern by using unusually small midrange drivers and placing them as close to the tweeter as possible. Just from looking at it, I expect that you succeeded in avoiding the downside, and I'll note that it isn't possible to do this if the tweeter is placed within a big waveguide. And the advantages are real. The vertical dispersion pattern is symmetric about the horizontal plane. I don't know what sort of crossover you have used, but even when a crossover design endeavors for the radiation from the midrange and the tweeter to be fully coherent, this can never be accomplished to perfection, and the general consequence will be some amount of upward or downward tilt in the main lobe. Moreover, I believe there is an oft-ignored penalty with using a phase-coherent crossover: the tweeter and midrange each have to be at -6 dB at the point where they are equal in SPL, which implies a -3 dB dip in the aggregate power response. This small dip in the power response has small yet real consequences for the frequency response at all listening locations where coherency does not apply, i.e., at locations where the tweeter and midrange are not both the same distance from the listener. The only way to avoid this, so far as I've been able to determine, is to use crossover slopes where the tweeter and midrange are both at -3 dB at the point where they are equal in SPL, which (I think) implies a 90-degree difference in mutual phase, and which would imply a vertically tilted main lobe, avoided by way of the MTM arrangement.

The sense I get when I look at this speaker is that you had an intimate understanding of several design issues that are ignored in most speakers produced by major manufacturers, that you invested a great deal of time and effort in coming up with solutions that work.

Except for the damned grill. :rolleyes:

Thanks for your comments. I've tried to conform to best practice with the MTM, except the crossover slopes aren't what Joe specified originally. Even he gave up on that and used 4th order for the Thor kit he did for Seas and Madisound. The RAAL needs steep slopes to avoid high THD at the bottom end. I'm trying to work up enough energy to get my tower downstairs and measure the vertical response. I don't seem to have saved any of those files.
 

JustJones

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But if you put separate grills over the individual drivers, it ends up looking like the interior of a car, which is so hideous that only Yamaha would actually do it.

Buchardt does on their A700.
 

sajgre

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Nothing technically valuable from my side... But I have to comment that they look great and please don't come near them with leather!! Those are hideous!!
Congrats on what appears to be amazing speaker!
 

Zvu

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I am very confused by that large hump at 600Hz which is persistent from 0 to 90 degrees.

This speaker is missing one notch filter there.
 

Dennis Murphy

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I am very confused by that large hump at 600Hz which is persistent from 0 to 90 degrees.

This speaker is missing one notch filter there.
That apparently is a port resonance. I didn't see it on my measurements, which included a nearfield of the port output. The port is 3" wide with larger flares on either end, and frankly I'm surprised to see it. I haven't had a chance to check it out since I got the towers back. I guess it's possible that it's only on one of the towers and it wasn't the one I measured nearfield. In any event, it's really so noticeable mainly because everything else is so smooth.
 

Dennis Murphy

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That was an interesting read, is there enough there to post a score. Just out of curiosity.
James wasn't able to do a full set of Spins because he measures outside on a tower and he wasn't able to get the mic high enough to do all of the verticals.
 

Zvu

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That apparently is a port resonance. I didn't see it on my measurements, which included a nearfield of the port output. The port is 3" wide with larger flares on either end, and frankly I'm surprised to see it. I haven't had a chance to check it out since I got the towers back. I guess it's possible that it's only on one of the towers and it wasn't the one I measured nearfield. In any event, it's really so noticeable mainly because everything else is so smooth.

Everything else is flat (as it should be in 2021 made loudspeaker) but 3dB isn't a small peak.

Port resonance usually causes dip on axis so that seems unlikely to me. I'd check that more thoroughly if i had the speakers on me.
 

R Swerdlow

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I am very confused by that large hump at 600Hz which is persistent from 0 to 90 degrees.

This speaker is missing one notch filter there.
The reviewer, James Larson, mentioned this hump:
In this graph, we get a wider view of the slight bump in the response at 600Hz which is a port resonance. It is likely too narrow in bandwidth and too low in level to be audible and exists only as a small blemish of academic interest in an otherwise magnificent showing by the BMR Tower.
I think it looks like it stands about 2-3 db above the nearby sound, not such a large hump. If it is a port resonance, it may be due to limitations in the cabinet dimensions that prevent using a wider port.

Would a notch filter lower that port resonance without also hurting bass response?
 

Zvu

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Only way for James Larson to be certain that is port resonance would be if he stuffed the port that day and did the sweep.

About 3dB being small peak, add EQ +3dB at 600Hz with similar Q on one loudspeaker and do the A/B test with complex music.
 
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hardisj

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Port resonance usually causes dip on axis so that seems unlikely to me. I'd check that more thoroughly if i had the speakers on me.

Agreed. With the port being on the front, you should have a dip if the port is the culprit. Rear ports result in a peak. Usually not from the port itself but from the port leaking internal resonance.

Also surprised to not see it show up in the impedance data if it is indeed a resonance. Meanwhile, there is a resonance that shows up in the impedance at ~220Hz in the impedance data.

Just offering some input based on what I’ve experienced in measurements. Not picking on you @Dennis Murphy
 
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amper42

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Larson's review of the BMR Tower is up at Audioholics.

Nice work, @Dennis Murphy - the polars are simply outstanding.
I would love to see the BMR Tower measured by @amirm. The shipping carton method looks a lot like the Revel F328Be minus the aluminum case. Looks like an awesome speaker. I look forward to trying it.

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Zvu

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^Or at Erin's.

Erin doesn't shy away from heavy loudspeakers :)
 

hardisj

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^Or at Erin's.

Erin doesn't shy away from heavy loudspeakers :)

At one time Dennis and I discussed me helping with prototype build measurements but that was over a year ago (before I even had the NFS) and I don’t recall why it didn’t go through (either I got too busy or Dennis decided to do it himself or have someone else do it). At this point, I’m not sure if Dennis really wants to or sees the need. I’ll leave that up to him, though.

They do look like really nice speakers for sure! And I’m loving that scanspeak woofer. :)
 
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