Technical Articles

Bass Management Woes: Trouble on the Slopes

Introduction

It's amazing really.  Any decent speaker manufacturer will take excruciating pains in designing and executing the crossover between tweeter and woofer.  Most go on to devote pages in their literature to tell you about it, offering up trademarked ways to all say the same thing.

Yet when it comes time to apply a crossover between a speaker and a subwoofer, the industry at large has turned a blind eye, leaving us with a mess, patting us on the head, saying, "Oh, it doesn't really matter that much".

There is of course a grave inconsistency here, but before we get into that, let us step back in time for just a moment.

The Old Days

It's the 80s, and Dolby Pro Logic has become a household word.  Basic A/V receivers have no facility for connecting a subwoofer, and even the better, expensive processors provide only a summed mono output, low-passed at 200 Hz if you're lucky.  In that day and age, subwoofers provided their own set of controls which effectively allowed you to adjust them, literally, to fill in where your main speakers left off.  You would adjust the low-pass and the gain as best you could, ending up with, at best, just a fudge.

Then came THX.  Part of their whole shee-bang was the mandate of a dedicated satellite speaker and subwoofer system, complete with a thought-out crossover scheme.  What a novel idea:  Giving the speaker/subwoofer crossover more than a casual glance.

The crossovers of "Bass Management" are exactly that:  crossovers.  The signal sent to the subwoofer does not magically stop at a given frequency, and vice versa for the speaker.  The output of the two cross over each other.  As with the crossover from a tweeter to a woofer, the summation of the two components must be correct if we want any semblance of high fidelity.

THX satellite speakers by definition have a 2nd order roll-off at 80 Hz (-3dB).  The THX processor applies a further 2nd order 80 Hz roll-off to the speaker signal, the sum constituting a 4th order high-pass.  The subwoofer signal gets a 4th order roll-off at the same 80 Hz and Presto!:  A perfect 4th order Linkwitz/Riley crossover with its characteristic freedom from phase shift and low subwoofer detection, thanks to the steep slope.

For all THX's sins, their sub/sat crossover remains today a thing of beauty.

One size does not fit all.

Then came Dolby Digital and mandatory bass management in all receivers and processors.  The trouble is, Dolby never mandated much beyond that.  As a result, to this day, surround sound processors feature odd, mis-matched slope combinations.  I recall an old receiver I once owned which would low-pass the sub with a 3rd order slope and high pass the mains with a 2nd order slope, at, of all frequencies, 90 Hz.  I never did find a speaker which "fit" that particular combination.

For the most part, things have not improved.  While a choice of crossover frequencies is now a ubiquitous option, what we're still seeing is either:

A) The same arbitrary mismatch of slopes we've had for years, or

B) A trend to use THX's crossover slope combination (even in non-THX processors), with total disregard for the fact that unless you have THX speakers, this will be just as arbitrary.

With the exception of THX speakers, and select few other dedicated satellite speakers with similar response, by and large, the loudspeakers on the market all strive to be "full-range", or as full-range as they possibly can be within the confines of their design, size, etc.  We've reviewed a lot of speakers over the years, actively listened to even more, and it's safe to say that most are actually pretty flat down to the mid 40s or lower (give or take).

And Now . . .

Using any full-range speaker with an arbitrary combination of slopes just doesn't work.  Let's take the THX crossover as an example which, as we said, is just as arbitrary as anything else when used with non-THX speakers.  At 40 Hz, the speaker should be down 24 dB if it's to fit into the scheme of things.  But a full-range speaker, having no 80 Hz roll-off of its own, will be down only 12 dB.  This means that in the region of 40 Hz - 60 Hz, the speaker is literally doing too much work.  You can't compensate by lowering the subwoofer level, as that lowers the entire subwoofer signal, essentially robbing you of the juicy 20 Hz - 30 Hz region.

Many of you many already be thinking, "Just lower the crossover frequency to be closer the speaker's actual cutoff."  Yes, no, maybe.  You'd be closer to getting it right, but still would not have a perfect splice:  your speaker would still have to exhibit a 2nd order roll-off at whatever frequency you chose.

Ported speakers will never fit.  In addition, lowering the crossover frequency means you'll be cashing in on all the negative aspects of using an inordinately low crossover frequency.  We have an entire article detailing why you don't want to do that, and why you would not want to set the crossover frequency different for each speaker in your setup.

Further, one cannot successfully "fudge" a 4th order crossover.  One of its features is that the phase shifts are complimentary (in that even though the phase shifts are introduced, both sides of the crossover are 'in phase' with each other).  When you stray from a perfect symmetry with the 4th order crossover, this is no longer true, the slopes don't sum to the same degree, and now we're in an even worse mess.

Maybe one size could fit all (. . . all except THX speakers that is).

So what's the solution?  One solution is the Thiel standalone bass management box (SmartSub Integrator) recently reviewed.

The up front solution lies in getting receiver and processor manufacturers to start giving us a bass management scheme that caters to "full-range" speakers.  Taking the THX Linkwitz/Riley scheme as a good place to start, all they need to do is provide a choice of high-pass:  2nd order for THX speakers (and other true satellites), and 4th order for all others.  With such a crossover, the main speakers need only be reasonably flat to an octave below the chosen frequency, which, in the case of the common 80 Hz crossover, means being flat to 40 Hz.  Virtually all "full-range" speakers - even most of the smaller bookshelf models - qualify.

Yes, it's that simple.

Why haven't receiver and processor manufacturers done this?  It has absolutely nothing to do with cost or processing power or anything like that.  The best excuse we've been given (and it's not a very good one), is that they are afraid of confusing the end user with too many options.

That's a pretty weak argument, as they would only be adding one more set-up item: High-Pass Slope:  2nd order (THX) / 4th order

They can bury it in an "Advanced" menu if they like.  It's not rocket science.

B&K is already doing this in some of their units.  Lexicon as well, though the way they do it is pretty transparent to the user.  At any crossover frequency setting other than THX 80 Hz, their system uses a 4th/4th combination (choose THX 80 Hz and the system naturally uses a 2nd/4th combination for the sake of THX speakers).  Theta also offers crossover slope customization, but the UI is designed for custom installers rather than the consumer.

The ultimate would actually be to have a choice of 2nd, 3rd, or 4th order high-pass so that even odd-ball speakers could be dialed in, but we don't want to press our luck.

Conclusions

In the meantime, you can either buy speakers that fit the crossover in your processor, or in the case of separates, you can add a 2nd order filter between processor and amplifier to give a full-range speaker the roll-off needed to fall in line with the standard THX crossover.  As we mentioned, setting the crossover frequency closer to your speaker's actual response can help, but just be aware of the trade-offs in doing so, and don't allow yourself to think it is 100% correct.

The only other thing to do is bug the living heck out of your receiver or processor manufacturer and convince them to get with the times. This article is a starter.