MBM Deployment

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JonFo

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As many of you know by now, I’ve been on a decade-long pursuit for solid mid-bass performance in my rig, as it is required to balance the amazing output of the ESL panels and my equally capable low-end from the Infinite Baffle Sub + the 18” sealed sub at the rear of the room.

To me, mid-bass is the range from ~60Hz to 350Hz and is fundamental to a good sense of ‘dynamics’ and ‘power’ in most modern music. It is also where critical fundamentals and harmonics live for instruments like drums and bass guitar (and acoustic Bass and cellos). This also happens to be a range I find many panel speakers to be deficient in due to several factors, the first being their dipole nature in the upper end of this range, with attendant natural cancellation. In addition, blending a large line-source to a dynamic woofer system is a tough trick to pull off with analog passive crossovers. Additionally, the power curves are different between the panel and woofer, leading to an ‘unbalanced’ sound at high volumes.

A few of these issues were addressed by replacing the passive crossovers in the Monoliths with DSP-based active crossovers where we can also time-align the panel to the woofer, fixing a key challenge in the impulse response. Steeper slopes help protect the panel from resonances caused by overdrive at low-frequencies, and PEQ allows us to fine-tune the response blend across the x-over region.
Since I custom tune the speakers in their final location and room, we also do minor PEQ to compensate for location-based artifacts, like floor bounce.

Further improvements in mid-bass came from replacing the stock woofer with one designed more for the range they cover in my setup. I covered that in this thread regarding their replacement.

Nothing though comes close to the mid-bass capabilities of the line-array of mid-bass drivers I built for the SL3XC center channel, that’s how to really augment this frequency range for a line-source system. Someday I’ll build four more of those for the rest of the ESLs.
But even after all of that, there was still a lack of ‘impact’ during movies and certain musical passages that left me wanting a bit more in this region, which lead to the investigation and deployment of something known as ‘Mid Bass Modules’ or MBM’s as I’ll refer to them from here on.

First, a bit on room acoustics realities. So even with plenty of woofage in the room, there is unfortunately a room-induced resonance in the 60 to 80Hz range (a couple actually) at my main listening position (MLP), and no amount of EQ will truly fix that. So, we need a ‘fix’ that can offset that as well as deliver a more tactile sensation of ‘impact’ in this range.

As a regular AVSforum denizen I read the DIY subforums where several bass-heads on there discuss the benefits of augmenting mid-bass with dedicated MBMs. Most in locations along with the other speakers, but some investigated their use in the nearfield (like as in really close to the MLP) to leverage the particle velocity a ported enclosure can create at these frequencies. This was documented as delivering a high degree of tactile sensation that augmented the ‘chest-thump’ and other motion-borne feedback along with the acoustic output that was not as impacted by the room.

Having a couple of spare 12” woofers (HiVi M12) that could be deployed for this purpose, all I needed was a suitable enclosure, which thankfully Parts Express had (Goldwood E-12SP), and along with some stuffing, and foam, I was able to assemble my MBMs in about 30 minutes each. Oh, and yet more amplification needed, so I bought a QSC GX5 500w / ch stereo amp. This is a class H amp with great specs and perfect for the MBM duty, all it needed was a fan mod to quiet it down.

The amp is driven by an output from my DriveRack 4800 speaker processor where I can adjust delays and EQ to align and optimize the MBMs for integration with the rest of the rig. Adjustments primarily focused on achieving a consistent impulse response so that sharp transients, such as drum hits aligned well with the front speakers and the sub. The feed is the Sub1 output from the pre-amp and has content from the second-order low-pass from the 9 other channels as well as the 120Hz and below LFE channel, so its effective range is 0Hz to 240+ Hz.

Placing the MBMs just behind the MLP and adjoining seat maximizes the tactile impact, and since these speakers are so close, they’re hardly affected by room modes and really put out the sound in their range. Even though the high-pass crossover is a sharp one (48dB/Octave) at 45Hz, the cone of the woofers still moves a good bit. So the MBMs operate from 45Hz through 240+, but mainly 45 to 180Hz.

OK, enough about rationale and setup, how do they sound? I tell you, it’s such fun to hear concert videos with the same kind of physical sensations as one has at real venues, with pants legs flapping and real chest thump. Action movie soundtracks now have seriously impressive realism, explosions shake the chairs and you feel the hairs on your arms move. This setup has impressive, deep bass, but now with this mid-bass range filled-in from the nearfield woofers, it has power in the harmonics of the sub-bass that were not quite there before, and a tactile feedback beyond the floor shaking, your body now feels the air moving around you.

I’ve been to live Blue Man group concerts multiple times, and have all their DVD-Audio and BluRay discs, and playing those now, they sound a lot closer to the live sound I recall from the events. Everything from the large drum whacks to the richness of the PVC-pipe instruments has an impact that resonates in the body like never before. The chair arms vibrate along with the music in a way that you feel.
Even classical sounds much more realistic. I have a great BluRay concert video with immersive audio (Dolby Atmos) soundtrack and the piano sounds like it’s right there in the room, with all the fullness in lower octaves that makes it more ‘real’.

An interesting side effect is that since the mid-bass is now filled in, I can lower the overall volume setting on the system when playing ‘loud’ and it still sounds loud enough. A good 4 or 5dB lower!

In conclusion, this was very much worth the effort and I’m extremely pleased with the results.

Even though that’s >1,000 words, I’m sure you have questions, so fire away.

Oh, and pics, got to have pics

MBM back view.jpg

And a side view of the two, with the 18" rear sub visible to the left (between rear Sequels):

MBM Side View.jpg

And floor view:

MBM Closeup.jpg
 
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JonFo

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Some measurements from the before and after. Starting with before.

With 2 subs, one Infinite Baffle in front centered between the mains, and one 18" sealed sub in the rear centered between the rears, I have a lot of good low-end. It measures pretty damn good with Audyssey MultEQ XT32 Pro (I have a pro kit for my 8802A).

So here is a spectrogram of the performance at the MLP, notice the room-induced ringing at 60Hz, but otherwise, super smooth low end that goes flat to below 20Hz.

LF-2-Subs-Spectro.jpg

The Waterfall view shows how well resonances are managed in this highly treated room, and the benefits of multiple subs

LF-2-Subs-Waterfall.jpg

And an unsmoothed Frequency response

LF-2-Subs-no-smoothing.jpg

And a frequency response with psycoacoustic smoothing (this what one actually hears)

LF-2-Subs-Psyco.jpg

With results like that, why mess with it? Hey, it's me here ;)
 
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JonFo

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After

Here are the measurements after the MBMs were added. Note that I have yet to re-run an Audyssey correction on the combined set, so the MBMs are purely additive and basically boost the mid-bass by 6dB or so. But they also contribute the tactile feeling. I could boost the same range on the other two subs and the sensation is not at all the same. So the nearfield, ported nature of the MBMs definitely has a role here.

So lets compare the smoothed psycoacoustic response of the before (green) and after (red)

MBMAddition-comp-psyco.jpg

We see the clear boost on the 50 to 120Hz range from the MBMs. This will be better integrated when I re-run the room correction.

And here is the spectrogram with the MBMs operational, and we see it smoothed out some of the issues on the 60 to 100Hz range, while also giving a boost there

LF-2-Subs-2MBMs-spectro.jpg

The waterfall plot still looks good, with resonances still managed, but again, big boost in the 60 to 100Hz range

LF-2-Subs-2MBMs-waterfall.jpg

And finally, the unsmoothed FR graph

LF-2-Subs-2MBMs-no smoothing.jpg
 

JonFo

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Just watched Avengers - Infinity War on UHD and the Atmos soundtrack is spectacular on this system. These MBMs got a real workout with the nearly non-stop action in this movie. The added tactile feel is very nice, adding to the engagement during the movies more dramatic moments.

I mentioned this before, but it's notable that I feel more 'sound' and that the soundtrack is definitely loud-enough , but the volume level is about 4 to 5 dB lower than I used to run it before the MBMs. This mid-bass frequency region is critical to the sensation of loudness, and once you have that covered, the rest of the frequency range does not need to be quite as loud.

And on music, it is also quite nice to hear BluRay's like Steven Wilsons Home Invasion with the same type of authority one would expect at the venue.

BTW- I now run with the MBM's a few dB's down from the measurements posted above. When new, it was cool to run them a bit 'hot', but day-to-day, a slightly lower level blends better, and is more comfortable.
 

greg2putt

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Your search for audio perfection is extraordinary. I applaud your application of science - facts, data and charts in your quest for that heavenly sound. Thanks again for sharing your journey.


Sent from my iPad using Tapatalk
 

tsmooth

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How would one, in a more combined and less dedicated space, go about similar (well, not even close) results without the ability to place a sub right behind the main listening position? More subs in more locations?
 

timm

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Just watched Avengers - Infinity War on UHD and the Atmos soundtrack is spectacular on this system. These MBMs got a real workout with the nearly non-stop action in this movie. The added tactile feel is very nice, adding to the engagement during the movies more dramatic moments.

I mentioned this before, but it's notable that I feel more 'sound' and that the soundtrack is definitely loud-enough , but the volume level is about 4 to 5 dB lower than I used to run it before the MBMs. This mid-bass frequency region is critical to the sensation of loudness, and once you have that covered, the rest of the frequency range does not need to be quite as loud.

And on music, it is also quite nice to hear BluRay's like Steven Wilsons Home Invasion with the same type of authority one would expect at the venue.

BTW- I now run with the MBM's a few dB's down from the measurements posted above. When new, it was cool to run them a bit 'hot', but day-to-day, a slightly lower level blends better, and is more comfortable.
Jonathan- this is sick! :). I love the subs right behind the seats. I've lost track. How many subs do you have now
 

JonFo

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Your search for audio perfection is extraordinary. I applaud your application of science - facts, data and charts in your quest for that heavenly sound. Thanks again for sharing your journey.
Thanks Greg, the quest for audio nirvana seems to be never-ending, but the trip is fun, and I enjoy the challenge and sharing the facts and results.

It took one hour to fine-tune the delay settings for the MBMs, going back and forth between the Group Delay metrics and frequency response to ensure I had the optimum blend between the main subs / speakers and the MBMs. Turns out a few ms can make a noticeable difference.
 

JonFo

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How would one, in a more combined and less dedicated space, go about similar (well, not even close) results without the ability to place a sub right behind the main listening position? More subs in more locations?
More subs in ideal locations is a great way to both smooth out the in-room response (the main benefit), but you also gain increased headroom and lower distortion in the mid-bass region.
But most subs are crossed over at 80Hz or lower, so actually contribute very little to the critical region covered by a dedicated MBM.

If space or visual challenge is the problem, there are actually units I think could do well as under-sofa MBMs (or on their sides behind a sofa/seat. Stuff such as this Dayton, which is a vented sub with a tune that is a bit lower than ideal, but likely workable as an MBM
https://www.parts-express.com/dayto...0-watt-low-profile-powered-subwoofer--300-639

The biggest challenge in deploying an MBM is the need for a good DSP crossover and all the attendant measurement and setup required to time-align and EQ the MBM for best integration. Just paralleling a box like that to the sub-out of the preamp is NOT going to yield great results.

One needs to take the sub out, feed it to something like a miniDSP 2x4 HD crossover, then define one output with just minor EQ going to your current sub, and maybe one or more outputs for additional subs, and time-align those to the main (front/center). Then define an MBM channel and set up a high-pass filter with sharp slope (24 or ideally 48dB/octave) at 45Hz. Then add the appropriate amount of time-delay to get the Group Delay graph in REW to be as flat / even as possible.

That's why you likely don't see too many MBMs in system pics / lists ;)
 

JonFo

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Jonathan- this is sick! :). I love the subs right behind the seats. I've lost track. How many subs do you have now
Thanks, here is the sub count:

Home Theater

1 4x15" Infinite Baffle sub Sunfire amp
1 18" DIY sealed sub, 9 CuFt box 2500w Crown amp
2 12" MBMs on QSC 500/wpch amp
Total 7 drivers in 4 boxes

Media Room
4 ML Dynamo 300

Living room
1 SVS Cylinder 500w amp
 
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JonFo

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I happened to pull out the top row of seating for a bit and grabbed some pics of the MBMs from angles that are normally obscured by the seating.

IMG_0171.jpg


And another shot from the front of the room

MBM_Front.jpg
 
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