Why I dislike traditional bass management

TLS Guy

TLS Guy

Seriously, I have no life.
Thank you for this post. I have been on about this for years. The current approach to bass management is wrong and bad, and needs to be abandoned. It is totally contrary to my experience and measurements.

My system is built on the lines you suggest.

There is no sub. The mains run full range and the shorter line also runs full range and rolls off second order at 42 Hz. The longer line handles the BSC and the LFE and offloads the two 7" drivers. The BSC rolls off first order transitioning to second order above the start of the BSC frequency. The lower driver handles below 60 Hz and the LFE. This requires three amps. These speakers roll off around 20 Hz second order.

The center channel is fed a full range signal and has an F3 of 44 Hz and rolls off second order and so has significant output to 20 Hz. The BSC is active and totally controllable and set by instruments as is the BSC for the mains.

The surrounds are run full range and are sealed with an F3 of 55 Hz. The rears are biamped lines with a 12 db roll off at 30 Hz. So there is excellent output to 20 Hz. They are run full range.

The bass is absolutely natural and very realistic. It sounds like it should. I have never heard it boom or doing anything ugly. I'm yet to hear another system that comes remotely close to the realism of this one, especially the bass reproduction. For most music I use the Dolby PL IIx. This means that all speakers are active with surrounds and rears playing the ambient content which contains significant bass.

As you suggest the bass is much smoother than in simple 2 channel mode. However this does not work for pop/rock, only for well recorded classical recordings with good ambient content which is almost all of them.

As stated before Audyssey totally ruins the effect. I can be certain that this is due to time/phase misalignment. I only have experience with Audyssey, but Auto DSP programs are highly likely to be quality spoilers. Good systems should have no need of any of that type of treatment.

From what I have heard of other systems, we have gone far down the wrong road of late.
 
gene

gene

Audioholics Master Chief
Administrator
Ok so I've commented a few times on other threads about the notion that bass management in traditional HT products, a hold over from the THX legacy, is flawed. Based on some good science and some bad science, but ultimately focused on simplicity instead of optimal sound.

The current standard approach to bass management uses true crossovers, meaning there is a symmetric high-pass and low-pass. A 4th order LR low pass for the subwoofer and a 2nd order BW high pass for the mains (that when mixed with a sealed enclosure speaker with an 80hz -3db fs, would yield a 4th order LR high pass acoustically). This made sense if the mains were sitting on top of the subwoofer and/or crossed quite a bit higher. However, when we consider that the length of a period at 100hz is 10ms or 11.4 feet, what sense does it make to use symmetric and steep crossovers. The 80hz standard chosen for THX came from the fact that this was 2 standard deviations below the point at which people could not detect the location of a subwoofer. It was thus believed that this was sufficient to ensure that the majority of people could not detect the subwoofer's location.

Let's start by discussing the benefits to the standard approach, it has merit after all.

Advantages:
-It's easy for most people to understand and utilize, setup is simple, and good results are possible
-The high pass filter on the mains can improve their dynamic range, reduce the load on the main amp, and reduce distortion
-It's simple enough that automated setup algorithms can be made to work successfully with fewer rules to follow. My approach would require predictive modeling of the various scenarios to detect the optimal settings, something not possible a decade or two ago.
-I'm sure many of you will come up with many other benefits, as I said, it has its virtues, so those go here.

Before discussing the specific disadvantages, it's important to look at the limited research conducted here. The one I cite most for this is Earl Geddes, in the early 80's he completed a PhD Dissertation LF's in small rooms. At its core, the paper developed the first complete mathematical model of LF sources in small acoustic spaces along with the inclusion of the effect of boundaries. The models were confirmed through testing, but of course, the advantage of modeling is that we can also test a lot of scenarios that we wouldn't want to test in real life. His conclusions were interesting and cause us to want to draw conclusions in contrast to the traditional bass management approach, as well as the notion of 2-channel full range speakers. First, that there are specific locations that are best for subwoofers, and they are near barriers. Second, that damping on one wall per dimension is all that is needed (but doing both doesn't hurt, it is just not more effective than doubling it on just one side). Third, that bass smoothness was poor with traditional LF sources, such as 2-channel full range speakers or a single subwoofer. In fact, there was a direct correlation to the number of LF sources and bass smoothness. Spreading the LF sources out around the room activated more of the modes in the room causing there to be higher modal density and thus less variation in the peaks and dips. While it didn't necessarily make the response flatter or smoother, it did make it more consistent. We can eq consistent, but we can't eq variable.

Now, this work was theoretical, it did not explicitly test a subwoofer integration method. That didn't happen until 20+ years later with Welti, Toole, etc. at Harman. Their work copied much of the same principles, but in the context of an applicable approach (not that Geddes didn't develop his own, he just didn't publish the subwoofer method itself). Both Geddes and Welti's approaches are more similar than different, which I address in my Geddes Sub video. However, the key difference in Geddes approach, which matters here, is the bass management. Geddes notes that since the more LF sources the better the consistency of the bass, that the mains should be LF sources too. In fact, the modeling even suggested that this was a better scenario than simply adding more subwoofers. Integration and smoothness, as well as consistency, was improved when the mains were operated at full range (that doesn't mean response down to 20hz, just that they didn't have a high pass filter). Geddes also advocates for shallow low pass filters since steep filters don't really benefit us at such low frequencies, but do potentially cause harm in the sense that they increase group delay.

Disadvantages of the traditional approach:
-It neglects the mains as an additional LF source
-It increases group delay
-It can be less spatially robust in both consistency overall and at the integration point
-Speakers have different responses that make the use of asymmetric electrical filters with equal frequency dumb, that only made sense if all speakers were built the way THX indicated.

I think that last point may be one of the most important. Most speakers are ported and most have different port tunings from each other (that is, most speakers aren't tuned to a single common frequency, every speaker is different). Further, the overall response of the speakers varies greatly, so while two speakers could both have a port tuning of 35hz, one may be -6dB at that point and the other +3dB. As such, creating symmetric filters, as the original approach dictated, is impossible. Just like a speaker crossover often uses different slopes and different frequencies between, say, the tweeter and midbass to achieve a flat summed response, so should the subwoofer and mains.

Now remember something else, all speakers have an acoustic highpass. There is no such thing as a speaker with no highpass filter. The enclosure itself (be it ported, sealed, or TL) have a natural highpass. Since the shape of that highpass and its -3dB is different for every speaker, we need flexibility in the tuning of the high pass filters. Both its frequency and slope needs to be adjustable separately from that of the subwoofer low pass. Further, it should be standard on all devices that you simply have no highpass, but that this is unrelated to bass management (directing all bass from all channels to a dedicated bass/LFE channel.

My point with this is not to promote a brand or model, but the notion that bass management should be more flexible. That we hold back the ability to optimize systems with the current approach.

Unfortunately, the misunderstandings around low frequencies in small rooms and the way bass management actually operates leads to so much confusion that I fear there will never be a change.

I am not naive, I don't think most people will want to take the time to study and learn these concepts. Instead, I suspect that our saving grace will be advancements in bass management that will allow more nuanced decision making by the processor. If more LF sources is better, and this improvement is most notable above 50hz, then even fairly robust surround speakers could aid in the improvement of LF seat to seat consistency, smoothness, and bass integration.

Ok what about group delay, what is this business and how serious is it? Well, normally I would say not very important. I mean, the science is thin, but what we have suggests that if the group delay is kept below 1.5 periods, it shouldn't be audible. 100hz period is 10ms, that's a long time for a speaker, it would take a lot to add more than 15ms of groupd delay, right? Well, maybe not. The times are changing, and modern DSP does a lot to screw this up. First, the sub amplifiers now have DSP and DSP itself has overall delay. But a subwoofer is bandwidth limited. What happens when you add a bandwidth limited speaker to the overall system and it has a delay. It becomes group delay.

I will use WinISD to show a series of changes to the natural group delay of a single subwoofer progressing from no added filters to a worse case scenario where someone using the fanciest new DSP powered sub has gone to down with REW, attempting to fill in every little dip with boost EQ, added steep LP filters, etc.

No issues:
View attachment 27705

Just a low pass filter:
View attachment 27704

OMG What have you done!
View attachment 27703

Ok the last one isn't as bad as it could be, but for the sake of argument, after lots of filters, we are at or beyond 1 period across the entire bandwidth. While group delay was generally said to be a non-issue, they weren't thinking it could get this bad!

Now the room itself also gets in the way and so what I'm showing here is still better than reality. Modes add a ton of group delay, and I'll show some real-world examples of this. My point with all this, though, is that we should take steps to not introduce excessive amounts of extra group delay. I've now begun to measure peoples systems reporting more than 100ms of group delay, well over 30ms by 100hz! That isn't good.

Here is a measurement of a good system in room. This wavelet shows what we would want to see in a pretty ideal system. The black line reflects group delay, basically.
View attachment 27707

This shows a common setup with subwoofers in an ok arrangement. Not terrible, but not as good as above:
View attachment 27706

And...This shows the OMG what have you done scenario:
View attachment 27708

I think this is showing what happens when DSP runs wild. The solution is to go back to the DSP in the processor, handling the bass management, and provide the needed facilities to improve bass integration. This will hopefully reduce the reliance on DSP later in the chain (whose delay can't be compensated for if it is not common to all subwoofers) and where often far too much "fix" is applied.

When I setup a system and I "roll my own" bass management, by the time I've completed all of the setup work related to crossover, phase, speaker placement, etc., I only need 1-2 eq filters, and only cut filters. However, that is never possible with bass management the way it is.
This is a good post and I agree with most of what you say here. However, Welti was first to prove strategic locations over randomly placed subwoofers is what makes for best seat to seat consistency and modal control. He also demonstrated there is little benefit beyond 4 subs in small rectangular rooms if properly placed and set up with Sound Field Management. That said, we can't always pick the best locations and we don''t always have predictable room shapes, so sometimes allowing the main channels to produce bass can be beneficial

This could be good grounds for an editorial on our site, perhaps a combined effort between us?

In my primary system, I don't use bass management on my main speakers since they have gut wrenching bass at extreme SPL levels down to 18Hz. I struggled for years to find a way to route LFE to them while still running a dedicated subwoofer channel. No AV receivers will route LFE to the mains with the sub channel engaged and only a handful of pre/pros will allow this.

This leaves a couple of options:
1. Turn off sub channel and do bass management locally to each sub that needs it (my current configuration).
2. Run stereo L/R subs and adjust the LPF to max setting (usually 250Hz) while running separate LPF either locally to each sub or using mDSP.

I've done both and #2 usually works out best since you can time align all of the subs using mDSP. Currently I got the integration of my mains with 3 additional subs to +-5dB from 15Hz to 100Hz. I could get this even better using option #2 but I'm a bit lazy right now and just waiting to replace my processor to dive into this again. I attached high resolution measurements of all 6 seats, 2 rows, with no smoothing for all to see at the end of this post.

In the end, the current way bass management is being done is practical and useful for 95% of installs. Experienced installers/end users should however consider combining bass capable mains with the additional powered subs provided that they have the ability to route LFE to the main channels and they have PEQ and time alignment options to manage all subs, including the mains.

I talk about all of this in the following article:
https://www.audioholics.com/home-theater-calibration/bass-optimization-for-home-theater

bass-both-rows.JPG
 
ryanosaur

ryanosaur

Audioholic Overlord
I'm still learning this stuff, and considering I'm tied to my SR6012 for a little bit, I have to make due. ;) That said, I've already learned a few things here on this thread, and as I re-read the various bass managemnt articles to help better internalize this knowledge I came across an announcement that I haven't seen mentioned before: Dirac is unveiling a new bass management module at CES this year.
I know several here have opined about which room correction does a better job, with Dirac perhaps getting the more positive comments. Maybe this will be the grail? ;) (OK, I'm not that naive.)
Any advance knowledge about this from those better AV-Educated than I?
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
I'm still learning this stuff, and considering I'm tied to my SR6012 for a little bit, I have to make due. ;) That said, I've already learned a few things here on this thread, and as I re-read the various bass managemnt articles to help better internalize this knowledge I came across an announcement that I haven't seen mentioned before: Dirac is unveiling a new bass management module at CES this year.
I know several here have opined about which room correction does a better job, with Dirac perhaps getting the more positive comments. Maybe this will be the grail? ;) (OK, I'm not that naive.)
Any advance knowledge about this from those better AV-Educated than I?
I’m probably as in the know as anyone can be that doesn’t work for Dirac as I have a relationship with them. I’ve been beta testing all of their products and they have kindly let me have a copy of their Live software to use so that I can help educate people on it and use it in comparison against others.

I actually plan an editorial piece on this with Gene so I won’t say too much. Just know I’m working on a two part series that will go into the science of in-room measured response and it’s relationship to perceived sound. Then the second part will look at specific room correction on the market and put to the test how good a job it does getting it right.

Dirac is a more robust solution and it’s technically very good, but no matter how robust, it can never evade the laws of physics. That is, it still can’t fully disentangle the speaker from the room and a flawed speaker can’t be fixed if the problems are caused by driver problems, crossover problems, or dispersion.

As for bass management. I know what it can do and a little of how it works. They lifted the ambargo in most of the details so i assume most people know what it will be about. It’s probably going to be the best automatic bass management on the market. I say that simply because at a software level it will be highly flexible and able to make smarter decisions on the speakers.

However, I am still awaiting more details. I’ve been promised a product to beta test it once available but that won’t be for a while. They also may release it to the public before they release it to me, that happened with the NAD and Lexicon products. Those were released with serious software issues and I ended up testing them at dealers before I got samples.

The biggest limitation on the Dirac software will be the hardware. Don’t expect it to turn a $500 reciever into a $4000 processor. It can handle essentially an unlimited number of bass channels but most receivers and processors will only have one or two. That means it’s multi-sub optimization won’t be effectively utilized.

As for it’s coolest feature, it uses a very good predictive modeling approach to establishing the settings. How this works I don’t know, those are the details I hope to get after CES. My understanding is that it takes some information on your room and with the measurements makes predictions of what would happen under various settings. It then comes up with an optimum solution that minimzes both spatial and spectral variance.

My understanding is that it will allow anyone with enough subs in the right locations to be able to achieve a Geddes or Welti multi-sub setup with minimal fuss.

I would guess that the flexibility of the filters and such will actually be product dependent. So high end processors will be very flexible and low end ones will probably not.
 
everettT

everettT

Audioholic Spartan
@Steve81 has stated what I believe is the most important fact of integration, being able to match crossover slopes, high and low. That is the key to integration IMHO
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
This is a good post and I agree with most of what you say here. However, Welti was first to prove strategic locations over randomly placed subwoofers is what makes for best seat to seat consistency and modal control. He also demonstrated there is little benefit beyond 4 subs in small rooms if properly placed and set up with Sound Field Management. That said, we can't always pic the best locations so sometimes allowing the main channels to produce bass can be beneficial

This could be good grounds for an editorial on our site, perhaps a combined effort between us?

In my primary system, I don't use bass management on my main speakers since they have gut wrenching bass at extreme SPL levels down to 18Hz. I struggled for years to find a way to route LFE to them while still running a dedicated subwoofer channel. No AV receivers will route LFE to the mains with the sub channel engaged and only a handful of pre/pros will allow this.

This leaves a couple of options:
1. Turn off sub channel and do bass management locally to each sub that needs it (my current configuration).
2. Run stereo L/R subs and adjust the LPF to max setting (usually 250Hz) while running separate LPF either locally to each sub or using mDSP.

I've done both and #2 usually works out best since you can time align all of the subs using mDSP. Currently I got the integration of my mains with 3 additional subs to +-5dB from 15Hz to 100Hz. I could get this even better using option #2 but I'm a bit lazy right now and just waiting to replace my processor to dive into this again. I attached high resolution measurements of all 6 seats, 2 rows, with no smoothing for all to see at the end of this post.

In the end, the current way bass management is being done is practical and useful for 95% of installs. Experienced installers/end users should however consider combining bass capable mains with the additional powered subs provided that they have the ability to route LFE to the main channels and they have PEQ and time alignment options to manage all subs, including the mains.

I talk about all of this in the following article:
https://www.audioholics.com/home-theater-calibration/bass-optimization-for-home-theater

View attachment 27718
Well we already have a lot planned together but I’m happy to work on this with you as well.

Those are excellent measured results. I had to double check the scaling just to see, that’s a 60dB range that looks like that people! Impressive.

Ps I think the proof of concept video I sent you was the project file instead of the video file. I’ll fix it when I get home if that is the case.
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
@Steve81 has stated what I believe is the most important fact of integration, being able to match crossover slopes, high and low. That is the key to integration IMHO
What do you specifically mean by this? That the crossovers should have the same slope for high and low pass?
 
everettT

everettT

Audioholic Spartan
What do you specifically mean by this? That the crossovers should have the same slope for high and low pass?
Matching ported or sealed subs that are not the same as the mains with most AVRs. With no option to match the roll off is less than optimal. Same with the LPF. It's not cut n dry with RC. Depending on your gear, it's far from optimal to have a sub roll off at 24bd per.. and vice versa controlling the mains. I'm sure you understand these conflicts as your way smarter than I at this. If I didnt have professional help with some of it, I'd be missing out..
 
Last edited:
Irvrobinson

Irvrobinson

Audioholic Spartan
Matching ported or sealed subs that are not the same as the mains with most AVRs. With no option to match the roll off is less than optimal. Same with the LPF. It's not cut n dry with RC. Depending on your gear, it's far from optimal to have a sub roll off at 24bd per.. and vice versa controlling the mains. I'm sure you understand these conflicts as your way smarter than I at this. If I didnt have professional help with some of it, I'd be missing out..
Perhaps it is a result of my mains/sub strategy with manual equalization, but my mains are ported and sub is sealed, and the match seems seamless. Are you saying the AVR automated equalization software screws up mains and subs with different cabinet designs?
 
gene

gene

Audioholics Master Chief
Administrator
Matching ported or sealed subs that are not the same as the mains with most AVRs. With no option to match the roll off is less than optimal. Same with the LPF. It's not cut n dry with RC. Depending on your gear, it's far from optimal to have a sub roll off at 24bd per.. and vice versa controlling the mains. I'm sure you understand these conflicts as your way smarter than I at this. If I didnt have professional help with some of it, I'd be missing out..
I've got a combo of sealed and ported subs in my system. The key to getting them to all play nice below the tuning of the ported subs is to adjust the HPF slopes and sometimes Fc of the subs. I did this on my 2 Velodyne DD15+ subs and you can see how well the integration is with my ported speakers in the measurement of my prior post.
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
Matching ported or sealed subs that are not the same as the mains with most AVRs. With no option to match the roll off is less than optimal. Same with the LPF. It's not cut n dry with RC. Depending on your gear, it's far from optimal to have a sub roll off at 24bd per.. and vice versa controlling the mains. I'm sure you understand these conflicts as your way smarter than I at this. If I didnt have professional help with some of it, I'd be missing out..
I see.

I think when you are dealing with speakers that are close together this could be an issue (but keep in mind that plenty of companies make speakers with ported midrange, midbass, and woofers all co-locates that measure fine). However when you are talking about periods of ten feet or more and physical spatial variance that is that or more, I don’t think it really matters. The response of all the LF sources, including ports, tend to be random phase across measurement points in a room. So while a port is out of phase with the woofer, it doesn’t seem that way once your 15 feet away in a room and the port output has bounced off every wall 3 times.

In other words, I actually am not so concerned. I wouldn’t personally have steves concerns.

If anything, I think there is an argument to be made that all of the mains should be sealed. They have a shallow roll-off that provides a good natural roll-off to integrate with the subs. Ports are always non-linear and never seem to behave like you want or expect. Avoiding them is best if possible. It’s just not possible if you need that bass efficiency.

That’s just my opinion though.
 
gene

gene

Audioholics Master Chief
Administrator
Well we already have a lot planned together but I’m happy to work on this with you as well.

Those are excellent measured results. I had to double check the scaling just to see, that’s a 60dB range that looks like that people! Impressive.

Ps I think the proof of concept video I sent you was the project file instead of the video file. I’ll fix it when I get home if that is the case.
Thanks. I'm just catching up on email now and will check.
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
I've got a combo of sealed and ported subs in my system. The key to getting them to all play nice below the tuning of the ported subs is to adjust the HPF slopes and sometimes Fc of the subs. I did this on my 2 Velodyne DD15+ subs and you can see how well the integration is with my ported speakers in the measurement of my prior post.
I do too and I haven’t had any integration issues.
 
Steve81

Steve81

Audioholics Five-0
I wouldn’t personally have steves concerns.
Hey don't bring me into this, I just said I appreciated my pre/pro's flexibility on setting high and low pass filters for the mains / subs. :p

As far as mixing and matching ported & sealed subs (or full range mains I suppose) goes, all I can say is that it's not recommended by guys like Ed Mullen @ SVS and Mark Seaton among others. However, as always, YMMV.
 
Matthew J Poes

Matthew J Poes

Audioholic Chief
Staff member
Hey don't bring me into this, I just said I appreciated my pre/pro's flexibility on setting high and low pass filters for the mains / subs. :p

As far as mixing and matching ported & sealed subs (or full range mains I suppose) goes, all I can say is that it's not recommended by guys like Ed Mullen @ SVS and Mark Seaton among others. However, as always, YMMV.
Mark and I know each other, I’ll set him straight ;)

I suspect they don’t recommend it because it adds variables to contend with which can cause things to go wrong. But I don’t think it should be treated as an absolute.
 
Steve81

Steve81

Audioholics Five-0
I suspect they don’t recommend it because it adds variables to contend with which can cause things to go wrong. But I don’t think it should be treated as an absolute.
Agreed. If you've already got the hardware, it's not like you're out anything giving it a shot and seeing what happens.
 
lovinthehd

lovinthehd

Audioholic Jedi
I see.

I think when you are dealing with speakers that are close together this could be an issue (but keep in mind that plenty of companies make speakers with ported midrange, midbass, and woofers all co-locates that measure fine). However when you are talking about periods of ten feet or more and physical spatial variance that is that or more, I don’t think it really matters. The response of all the LF sources, including ports, tend to be random phase across measurement points in a room. So while a port is out of phase with the woofer, it doesn’t seem that way once your 15 feet away in a room and the port output has bounced off every wall 3 times.

In other words, I actually am not so concerned. I wouldn’t personally have steves concerns.

If anything, I think there is an argument to be made that all of the mains should be sealed. They have a shallow roll-off that provides a good natural roll-off to integrate with the subs. Ports are always non-linear and never seem to behave like you want or expect. Avoiding them is best if possible. It’s just not possible if you need that bass efficiency.

That’s just my opinion though.
What do you think of Barry Ober's whitepaper on sub setup?
 

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