Bass Optimization for Home Theater with Multi-Sub + mDSP

For the past decade I've been on a quest to achieve what seemed to be the impossible in my theater room: state-of-the-art bass reproduction for EVERY seat, not just the sweet spots.  We've written numerous articles on the benefits of multiple subwoofers and how when properly placed (and set up) one can smooth out the bass response across the entire listening area through the process of room modes manipulation which some regard as averaging, but in reality it changes their magnitude and timing so that there is less destructive interference (the major cause of seat-to-seat variations). The result is much increased efficiency and a more uniform sound field within the room.

Before digging into the details of this article, we suggest watching our YouTube video discussion that gives you a basic breakdown of this complex topic along calibration techniques with illustrations to help you on your path towards bass nirvana.

Home Theater Bass Optimization with Multi-Sub + mDSP Youtube Video Discussion

In rooms of predictable shapes and dimensions, there are clear guidelines regarding where to place the subs to achieve optimal bass response. But what about asymmetric rooms like mine?  Well, your guess is as good as mine and thankfully this time I was armed with a little black box from a company called miniDSP to help take out some of the guesswork.

Does Pushing the Red Button Solve All Problems?

Auto Room-EQ to the Rescue?

Some may wonder, "Why not just use auto-room EQ to solve my dilemma?"  The problem with auto-EQ is they rarely work as advertised.  Some don't even do low frequency correction for the subwoofer channel while others don't quite integrate multi-subs very well, as we've seen in the past.

Frequency Response vs. SPL - Front Row
Yellow: all 4 subs; Blue: all 4 subs + Audyssey

This image was borrowed from our Multi-Subwoofer Calibration Article and demonstrates what happens when auto-EQ systems try to equalize each sub individually rather than first individually setting level and delay for each sub and then measuring the combined output of all subs simultaneously to apply global equalization.  Audyssey has since then updated their algorithms, which helps considerably but we've found some auto-EQ systems do worse or nothing at all for the subwoofer channel.  It's really a crap shoot, which is fine if you're a gambling man but we prefer to make our own luck. 

Editorial Note about the Limitations of EQ
It's important to note that GLOBAL EQ alone cannot improve seat-to-seat consistency regardless of what any of the Auto-EQ companies claim.  You need something far more sophisticated like Harman's proprietary SFM algorithm or a painstakingly manual process of EQ, delay, phase, and crossover adjustability, (like the approach we've taken with mDSP.)  Seat-to-seat consistency is best first optimized through positional EQ (seat and sub placement), the proper deployment and setup of multi subs or low frequency bass traps or a combo of both.  By itself, EQ is useful to flatten peaks and sometimes fill in nulls (only if they are small), but beyond that it can't manipulate room modes like positional EQ of multi-sub, SFM, or bass trapping does.

Bass Traps

Others may say, "Why not just add a bunch of bass traps to fix your bass woes?"  Bass trapping can be an effective solution for low frequency problems, but to get good results down to the sub 20Hz region, you will need a lot of them and they are quite bulky and unsightly.  In addition, anytime you add passive absorption to the room, it turns mechanical energy into heat and thus reduces overall system efficiency.  This means you lose overall headroom since you have to turn the bass levels up higher to compensate.

Please note I am not advocating against bass traps. Bass traps can still be a very useful tool as supplementation for reproducing music, especially in asymmetric rooms.  Bass traps are often necessary when producing music using instruments in a recording studio environment.  In such instances you simply have no alternative if you want better bass.  However in reproduction of music in a home theater environment utilizing active subwoofers this is NOT the case. 

Editorial Note about Bass Traps:
Bass Traps are acoustic energy absorbers designed to damp low frequency sound energy by reducing low frequency resonances in a room in attempts to flatten out the overall response.  Since low frequencies are long wavelengths (ie 20Hz represents a 57ft wavelength), bass traps must also be quite large to be effective down in the bass region.   For more information, read our article:  Bass Trapping Ideas for Non-Ideal Spaces

Multi-Sub + DSP

Multi-subs + DSP or to borrow a term from Harman - Sound Field Management (SFM) is the better solution for reproduction of music in small room acoustics.  Multi-subs allow the end user to deploy slightly smaller subs than one super sub and achieving the benefits of modal manipulation for reduction of standing waves while also increasing low frequency coupling.  So instead of buying a hulking 15" ported sub, you can now consider dual 12" models or perhaps quad 10" models.  While you don't achieve +6dB at all frequencies each time you double up on your subs and co-locate them, you do achieve almost this amount of gain at the very low frequencies where the wavelengths are physically much larger than the distance between the subs (ie. 30Hz = 38ft).  Once you fix the seat-to-seat consistency issues utilizing multi-sub, you can then take advantage of equalization of a device like the mDSP 2x4 to fine tune and flatten out any excessive peaks.

Editorial Note about Harman's Sound Field Management (SFM)
Sound Field Management (SFM) is a reference to a very specific Harman proprietary system to reduce seat-to-seat variations. Part of it is the usage of multiple subs but there is also a proprietary algorithm developed by Todd Welti and Allan Devantier, under the investigation of Dr. Floyd Toole, that further reduces seat-to-seat variation, which is part of their PC-based “ARCOS” characterization, EQ and processing solution. During the characterization phase, each subwoofer is measured independently, and each is equalized, and otherwise adjusted independently only for the purpose of minimizing seat-to-seat variations. Only after that is completed does the system calculate the coefficients for global EQ, which is really only possible once one has reduced the seat-to-seat variations in response.

The Procedure

The first step to achieving excellent bass is proper placement of your subs and seats.  In symmetric rooms you can follow our placement guidelines.  However, in asymmetric rooms, you're going to need to make a small investment in a measurement system.  REW is a very good acoustical measurement program and its absolutely FREE.  Download it!  Get yourself a calibrated microphone that is good down to below 20Hz and a mating preamp/USB DAC and you will be on your way.

A quick and dirty guideline to proper multi-sub setup:

1. Properly position all of your subwoofers for best performance.  Also consider seating locations and how you can move them to help achieve this goal. This is known as positional EQ.
2.  Get all of your subs playing the same bass signal (mono LFE + summed bass from all channels set to small in your receiver's bass management).
3.  Route the subwoofer outputs of your AV receiver into a device like mDSP 2x4 to manipulate level, delay, PEQ function, etc.
4.  Using your measurement system, be sure to adjust delay, level, phase, crossover points, etc to get all of your subs playing well together by minimizing acoustical cancellations and getting the smoothest frequency response possible for all of your seats. Start initially with the two most important seats, then expand outward to all of your seats as you hone in the response.
Note: Measure all subs + main L/R speakers in mono from 10Hz to 200Hz for each seat.
5.  Once you've got all of your subs properly integrated, use EQ to flatten peaks and boost sparingly (never more than +6dB) to fix dips.  If boosting doesn't fix a dip, don't do it! 
Note: Since this is a manual process, I initially applied a small amount of EQ to each sub individually to optimize the response as best as possible. Then, I applied global EQ to really fine tune and flatten the overall response for ALL seats.

Getting All of Your Subs To Integrate at Very Low Frequencies
If you're using subwoofers with different bass alignments (ie. ported vs sealed) and/or bandwidth and output capabilities, you may need employ a HPF to one or more of the subs via mDSP to achieve proper integration of the summed response at very low frequencies where the subs begin to naturally roll off.  In my case, I had a mix and match between ported (Status 8T speakers) and sealed (Velodyne DD-15+ and RBH SI-1010 subs) and was able to tailor the very low frequency roll-off response to achieve synergistic coupling of ALL subwoofers at and below the system -3dB point.

Here is a graph of all six of my seated positions BEFORE adding mDSP to the equation.

Bass Measurements Across all Six Seated Positions Before mDSP

While the bass response of the front three seats is flat and even across the entire bass region, the back seats have excessive boost below 40Hz and bad suck outs in the 40-70Hz range, particularly in the back right seat which I deemed the "mother-in-law" seat.  As bad as this may look, the results are actually quite good and difficult to achieve with a system employing only one subwoofer.  The response measures roughly 15Hz to 100Hz +/-15dB or up to a 30dB swing depending on frequency or seated position.  Certainly we can do better.

Bass Measurements Across all Six Seated Positions with mDSP

And so we have.  Admittedly I cheated a bit.  I added a 5th sub behind the mother-in-law seat knowing that I had better controls in place with the recently added mDSP product.  This sub was purposely put there to neutralize the back seating area dip in the 50-70Hz range but it did much more than that.  I was able to reduce the seat-to-seat consistency from +/-15dB to +/-5dB.  In most situations, an end user can get pretty close to this type of response with just two or four subs depending on room symmetry and employment of delay and precise equalization compensation. 

RBH SI-1010i In-Ceiling Subwoofer placed behind 2nd Row Seating

In addition, mDSP enabled me to flatten the peaks below 40Hz for better management of the dominate room modes.  We have leveled the playing field.  Also notice the 80Hz dip in the front row was significantly reduced with the 5th sub added.  Without mDSP, I wouldn't have been able to do this.  The independent adjustment of delay and crossover slope in the mDSP device afforded me this luxury.  We now have a bass response from 15Hz to 100Hz +/-8dB across all six seats and this is done with NO added passive bass trapping!

For comparative purposes I wanted to show the difference at the primary listening position between running just my front speakers bi-amped vs having the other subwoofers and SFM engaged.

Front Speakers (Red trace) vs All Subs + mDSP (Blue Trace) @ Primary Listening Position

Subterranean Bass

I thought it prudent to note that these frequency graphs extend down to 10Hz.  Most in-room measurement graphs we've seen published in journals and manufacturer websites stop at 20Hz with the bass already down 3dB or more.  While some folks de-emphasize the importance of sub frequency bass below 20Hz, I personally feel its importance is underestimated. The reality is, it takes multiple high excursion subs and lots of power to dig this deep.  But, if your subwoofer system is capable, you will reap the rewards in LFE-content-driven movies and some multi-channel music too.  The scene in Dark Knight Rises, you can actually feel your spinal column shake when the Batwing takes off in our reference room.  It's eerie but quite intoxicating and in my opinion the true hallmark of an extreme Bassaholic.

How was I able to achieve this? 

• I added a 5th sub to deal with mother-in-law seat (necessary in my asymmetric room). For ideal room shapes, typically 2 or 4 subs are sufficient.
• * I have each sub play the same signal (LFE + summed mono bass for speakers set small).
• Using Superposition – I added one sub at a time and made sure they integrated, then I got the best summed response of all subs blended with the main speakers.
• After the subs were integrated with each other to yield the best possible summed output, I worked on flattening their combined response using multi-band PEQ via mDSP.

Abbreviated Equipment List (related to bass section)

• Processor:  Denon AVP-A1HDCI Preamp/Processor
• Amplifiers: Denon POA-A1HDCI 10CH amp / 2x Emotiva XPR-1 mono-blocks, RBH SA-500 sub amp
• Speakers: Status 8T and 8C (front LCR) bi-amped bass modules connected to sub L/R channels, RBH 66-SE/R (side surrounds), RBH SI-61/R (back surrounds)
• Subwoofers: 2x Velodyne DD-15+, 1x RBH SI-1010 in-ceiling subwoofer
• Bass Management: mDSP 2x4 balanced ($135 with power supply)
  

*Editorial Note about my Bass Management Configuration:
In my particular case, my main speakers are bi-amped with their bass portions used as separate powered subs configured as stereo(L/F + summed bass from "small" channels and LFE).  I run the subs for my front speakers out to 250Hz for optimal integration with the mid/tweet satellite section while the other powered subs in my setup get crossed over at the typical 80Hz 24dB/octave.  THX recommended setting.  This works extremely well for my situation but we typically recommend sending mono summed bass to all subs crossed over at 80Hz.

Time Domain Analysis and Waterfall Graphs

There are some that insist that frequency response analysis alone is NOT a good indicator of great bass response.  Unfortunately for them, the science is NOT on their side.

As the folks at Harman have proven in the past via various white papers and AES contributions regarding the benefits of multi-sub, loudspeaker transducers, including woofers and their enclosures, are minimum-phase devices/systems, and therefore respond to accurately matched parametric equalization based on anechoic data.  In small rooms, the individual resonances behave as minimum-phase systems to the extent that one can attenuate prominent peaks in a high-resolution room curve and thereby both attenuate the peak and dampen the ringing. Again matched parametric EQ is needed.  Above the room transition frequency (roughly 300Hz), EQ becomes less predictable since it’s difficult to separate the speaker and room contributions.  At low frequencies (< 300Hz), the room dominates the response which means if you fix the frequency response, then the time domain response is also corrected since they are related.  Smooth bass response means there will also NOT be any excessive ringing within the bandwidth of concern.  In other words, decay times are directly correlated to frequency response in the subwoofer frequency bands.

Note: Transition frequency – the frequency at which standing waves dominate the loudspeaker response in small room acoustics.

Time domain behavior is often represented in waterfall graphs, but such graphs are easily manipulated as can be seen in CH 13 of "Sound Reproduction: Loudspeakers and Rooms" by Dr. Floyd Toole.  Floyd discusses how waterfall graphs can be manipulated to appear bad by jacking up the frequency response resolution at the expense of the time domain resolution, while the latter is what we are actually most concerned about in such graphs.

To illustrate ringing in a waterfall graph, here is a measurement at my primary listening seat with just one subwoofer playing with NO EQ.

Waterfall Graph: Primary Seat, 1 sub, no EQ

Here is the corresponding frequency response measurement.

Frequency Response Graph: Primary Seat, 1 sub, no EQ

Notice the prominent peak in the 45-65Hz region.  There is some ringing going on here.  But let's see what happens at the exact same seating position with multi-sub + DSP using the exact same settings in my REW measurement program to generate the corresponding waterfall plot below.

Waterfall Graph: Primary Seat, 5 sub, EQ

Frequency Response Graph: Primary Seat, 5 sub, EQ

Notice how the response is much smoother now.  The ringing has been damped which corresponds to the much flatter frequency response behavior.  The science works!

But what about the other seats?  How do we know the time domain behavior is good there too?  Remember that we fixed the frequency response for ALL seats so it's good at those seats too but the proof is in the pudding so let's show a couple of examples.

Waterfall Graph: Front Right Seat, 5 sub, EQ

Waterfall Graph: Back Center Seat, 5 sub, EQ

Once again we see well-damped time domain behavior for the various seating locations, all of which correspond to their smooth frequency response measurements.

The Bottom Line

Claims that there is only one way to skin a cat, or in this case, fix your bass, are unfounded and frankly intellectually dishonest or naive.  Bass trapping is one method, multi-sub + DSP is another, and a combo of both can also be an effective solution.  Be leery of anyone who tells you otherwise.  The science has proven the benefits of multi-sub with SFM (or in our case manual delay and EQ compensation via mDSP) as a viable and often preferred solution to improving bass response for reproduction systems in small rooms.  When fine-tuning bass in your system, careful examination and improvement of frequency response behavior across all of your seats is a key metric of achieving great bass.

The End Result

Bass nirvana was finally achieved in the Audioholics Showcase theater room system through multi-sub plus equalization through the mDSP device (review forthcoming).  I now have smooth, even, tight, extended bass for EVERY SEAT, not just a couple of preferred seats. Bass level and tonality sounds similar no matter what seat you chose and this is easily demonstrated using the Diana Krall - Love Scenes DTS CD, which is a very bass-intensive recording.  The mother-in-law seat I often talk about in my room has been terminated once and for all.  Using EQ, I could still flatten the 20-30Hz region a bit more, but quite frankly, I like the sound of that region slightly elevated since it adds to the tactile feel in scenes like when the Batwing takes off in Dark Knight Rises.  Another take away from this is if you flatten out the bass and fix your seat-to-seat consistency, research shows that a boost of up to 5dB in the bass region is perfectly acceptable and often preferred by home theater enthusiasts.

As good as the bass is in my system, it doesn't mean I have completed my journey.  Anyone who knows me appreciates that I am not one to rest on my laurels.  I still plan on investigating the benefits of employing modest amounts of low frequency corner bass trapping with some Gik Acoustics products to see if that further improves the bass in my system.  With eight 10" and two 15" bass drivers, and over 7 kwatts of power behind them, I certainly have enough power and piston area for the subwoofer channel to sacrifice a little efficiency if I can further fine tune the bass sound in my room.  Would you expect anything less from a true Bassaholic?

Acknowledgements:

Many thanks to Dr. Floyd Toole, Kevin Voecks and Todd Welti of Harman for their contributions and peer review of this article.