Choosing a Crossover Frequency <-- Awesome Bass Begins Here

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Much of the visceral excitement of Home Theater -- whether for movies or music -- comes from the proper rendering of Bass frequencies.  Unfortunately, setting things up to ACHIEVE awesome Bass is complicated -- almost to the point of being a Black Art!

But take heart.   As with many such complicated things, there's always a place to begin!  And with Bass audio, that means understanding why you need to include a Subwoofer in your speaker configuration, and learning how to select a Crossover Frequency to drive it.

There's no set definition of what constitutes Bass frequencies, but for purpose of discussion lets focus on frequencies below 150 Hz.  To put that in context, the low end of male voices is around 100 Hz.  And most of us have -- alas! -- experienced 60 Hz, power line interference hum, at one time or another.  So we know what THAT sounds like.  The bulk of the energy in "big" movie sound effects -- things like explosions -- comes in around 50 Hz.  The lowest frequencies you'll "hear" will be around 30 Hz.  This includes almost all of the lowest Bass notes from musical instruments.  But bass doesn't stop there!  It's just that the frequencies BELOW 30 Hz are more "felt" than "heard".  That thud you feel in your gut from a really deep effects sound, or perhaps from the lowest pedal notes of a pipe organ, are found down here!

The Bass we'll be most concerned with in this discussion is the range below 100 Hz.  One of those sciencey facts you probably have stashed away from school is the sounds we hear are made up of repeating, pressure "waves" traveling through the air to our ears.  From the speed of sound, and the number of cycles per second (Hz) for a given sound, we can calculate the distance spanned by just one such cycle -- its "wavelength".  For 100 Hz audio, that's just a little over 11 feet!  The lower the frequency, the longer the wavelength.

Here's a link to a handy table from JdB Sound Acoustics listing the Sound Wave Lengths for various frequencies of interest -- along with the frequency ranges of voices, pianos, organs, and guitars for comparison.

Now think about that for a moment.  Picture in your mind the dimensions of your Home Theater viewing room:  Front to Back, Side to Side, and Floor to Ceiling.  Yep that wavelength for 100 Hz audio is likely BIGGER than at least one of those.  And by the time you get down to 50 Hz audio (see that table), the wavelength is likely bigger than ALL THREE dimensions of your room.

So, umm, how does it fit?

Well what happens is that the natural reflections of the audio set up what are called "Standing Waves" of sound for each frequency.  A common, pseudo-technical description of Bass audio constrained inside a room like this is that the Bass audio "pressurizes" the entire volume of the listening room.  And one important consequence of that is the Bass appears to be coming "from everywhere", as opposed to coming from the location of any given speaker.

Get up to around 150 Hz, on the other hand, and the wavelength is now under 8 feet -- smaller than the room's dimensions.  And that means you hear it as directional audio.  That is, you can now "localize" the audio, in your mind, as coming from the direction of one of your speakers.

Without getting too technical, I want to make a key point now, which will, I hope, be intuitively obvious to you given the explanation above.  To wit:

You have to huff A LOT of air to "pressurize" an entire listening room at these low, Bass frequencies!

And that means you need both size AND power in the speaker doing the huffing!

Subwoofers are, of course, specialty speakers designed with just this in mind.  Home Theater Subwoofers have cone diameters in the range roughly 10-18 inches, and, equally important, they have power amps built in, dedicated solely to moving that big cone.  (Which also means they need to be plugged in to wall power.)

Subwoofers are also huge -- both in size and weight.  Among other things, this keeps them from hopping around as that cone moves!

Some of the major manufacturers of Subwoofers have helpful tools on their websites you can use to figure out just how big of a Subwoofer you should get from them -- based on the dimensions of your listening room.  Even taking into consideration these tools are designed to be conservative (after all, they'd really like you to pay for a bigger, more expensive model), the size of Subwoofer you'll find recommended for typical listening rooms can be daunting.  But there's an alternative.  You can get two (or more) of a smaller model and position them around your room to work as a set.  Again, this can be done because each of them is trying to pressurize that same room rather than producing "localizable" sound.  So their different locations are not a problem, and can, in fact, produce other benefits!

Now compare that to the normal speakers in your Home Theater.  If they are smaller, bookshelf style speakers there is of course no comparison.  They simply can't move enough air to have a chance of handling Bass properly.

But even if you have larger, "full range" speakers -- speakers with specifications saying they are good down to 30 Hz, or even lower -- you are not going to get the same quality of Bass out of them as from a well selected Subwoofer.  The problem is, they may be able to produce sound down that low, but they won't be able to do so AT VOLUME!

Indeed, you can pretty much be guaranteed:  Unless your main speakers EACH have to be plugged into wall power, there's pretty much no chance they can handle bass as well as a Subwoofer.  And even for speakers that DO plug in, unless they have large Bass driver elements (cones) and dedicated amplification for those, they are not going to hold their own against a decent Subwoofer.

(If you DO have speakers which plug in to wall power, and have driver elements specifically designed and amplified for Bass (at VOLUME), then what you've got your hands on is a speaker with a Subwoofer built into the same cabinet!  Some speaker makers also sell "satellite" Subwoofers, which are separate units intended to be attached to a single speaker, each -- and thus functioning AS IF a Subwoofer was built into each such single speaker.  Such setups do exist, but they are not what most people end up getting.  In addition -- see below -- you will *STILL* need a separate Subwoofer for proper handling of Low Frequency Effects (LFE) Bass!  So you'll have a Subwoofer built into each such speaker, plus another, stand alone Subwoofer for LFE.)

OK, if you've followed me so far, you should now be convinced you need at least one Subwoofer in your Home Theater setup (maybe MORE than one) -- budget and physical space allowing, of course.

And this should start you thinking, "How do I get the Bass audio into it?"

From my post on Calibration Discs, you'll recall that multi-channel audio tracks frequently (but not always) include a Low Frequency Effects (LFE) channel.  The ".1" in "5.1" or "7.1" tracks, for example.  This is a special channel reserved for carrying LOUD Bass -- definitely the sort of stuff you'd want to send to your Subwoofer!

But in addition, all the REGULAR speaker channels can also carry Bass -- with no limit on how low their frequencies can go.  The Subwoofer should ALSO be handling the lowest frequencies of this audio, as well, so that your regular speakers are not challenged with trying to reproduce it!

"Steering" bass from the regular speaker channels to the Subwoofer is the job of "Crossover" processing.  Crossover is simply the task of taking a single stream of audio and sending higher frequencies to one speaker and lower frequencies to another.

Indeed, your regular speakers already include their own, internal Crossover electronics to split the audio among their various driver elements:  Tweeter, Mid-range, and Woofer for example.

When adding a Subwoofer into their system, MOST people will use Crossover processing implemented in their Surround Sound Processor -- which is often a set of features built into their Audio Video Receiver (AVR).  There ARE other ways of implementing Crossover, but we'll focus on this typical way.

So the job of the Crossover is to "filter" the audio intended for each speaker so that the Bass frequencies are actually sent to the Subwoofer, and the remaining, higher frequencies go to that speaker.  Keep in mind, this has to be done for EACH speaker channel.

The Subwoofer then receives the COMBO of all this Steered Bass -- i.e., from all the speakers -- in addition to the special Bass found in the LFE channel.

In essence the Subwoofer supports the low-end of every speaker (along with handling the special, LFE Bass audio).  You might wonder how this can possibly work, since the Subwoofer -- or even multiple Subwoofers -- are not sitting in the exact same position(s) as any of the speakers.  Indeed some of those speakers are likely on the opposite side of the room!

This goes back to the concept of Bass frequencies "pressurizing" the entire volume of space in the listening room.  The Bass comes "from everywhere" instead of from the location of any speaker.  And so even ONE Subwoofer can handle the Bass from ALL the regular speakers without confusing the audio imaging.

At least, it can do so if you make sure to only send frequencies to the Subwoofer which WORK THAT WAY!

Fortunately, Crossover processors let you specify which Bass frequency range you want steered to the Subwoofer.  Called the Crossover Frequency, this setting sets the dividing line:  Higher frequencies go to the speaker and lower frequencies go the Subwoofer.

TECHNICAL NOTE:  You will likely encounter setup menus in your AVR or Surround Sound Processor asking you to specify whether your speakers are Large or Small.  This rather unfortunate nomenclature has become pretty much industry-standard.  The thing to know is, the choice here has NOTHING TO DO with the physical size of your speakers!  Instead, you are simply specifying whether or not you want Crossover processing to happen.  A Small speaker will be processed for Crossover.  A Large speaker will not -- the full range of frequencies in its audio channel will be sent to that speaker.  As we've just discussed, if you do happen to have a decent Subwoofer, you should be using it to support the low Bass in *ALL* your speakers -- even speakers you have purchased which are marketed as "full range".  And thus you would set these speakers all to Small -- regardless of their actual size!

A typical Crossover Frequency would be 80 Hz.  And you'll recall from that table linked above, this is a "safe" frequency for Bass steering, since the wavelengths of audio 80 Hz and lower are going to be long enough to trigger that "pressurizing" effect -- i.e.,to be "non-localizable".

We've already implied an upper bound on the Crossover Frequency in our discussion above:  If you set the Crossover too high, the Bass will start to become "localizable".  Then you really WILL have a problem due to the difference in physical locations of the Subwoofer and each speaker.

A good Rule of Thumb is you don't want your Crossover to be higher than 100 Hz.  First, this keeps the Bass safely non-localizable.  But second -- and again refer to that table linked above -- this keeps the low frequency end of male dialog from being sent to the Subwoofer!  Your Subwoofer may be very good, but it probably won't be AS good as your regular speakers in rendering something so precise as human speech!

MARKETING NOTE:  Subwoofer buyers typically have two conflicting demands.  First, they want their Subwoofers to be LOUD.  In particular, they'd like to use a Subwoofer that's physically smaller but still capable of putting out enough output to work well in their listening room.  But Second, they want their Subwoofers to be ACCURATE -- to produce Bass of high quality.  The folks who make Subwoofers will target these demands by marketing different models as Dynamic or Musical!  The Dynamic Subwoofers are capable of much higher output for their size, but they aren't all that accurate.  Since they are smaller, they are also often less expensive.  Such Subwoofers may be perfectly adequate for folks looking mostly to handle Bass effects in action movies -- explosions and such.  However, folks who have invested good money in regular speakers, don't want the low end of those speaker channels compromised by inaccurate Bass reproduction!  They want their steered Bass to be handled just as well as their regular speakers handle the higher frequencies.  Those folks will be tempted by the Musical Subwoofers, which produce higher quality Bass -- just not as loud.  Meaning you may have to step up to a larger, more expensive model to handle the size of your listening room.  Subwoofers also differ in how LOW they can go in frequency.  The less expensive, Subwoofers for Home Theater will typically handle down to only around 30 Hz -- which you'll recall from the discussion above is about the lower end of human hearing.  These will also, typically, be of the Dynamic style.  The more expensive (and usually larger) Subwoofers can extend that low end down to, say 15Hz.  That might not sound like much of a difference below 30Hz, but it REALLY IS a big deal!  Those frequencies below 30 Hz are the ones you FEEL rather than hear!  These Subwoofers may be either Dynamic or Musical in design.  But again, expect to have to go up in size (and price!) even MORE to get a Musical design which also has Bass extension that low.

So, we'll mentally note 100 Hz as the upper limit on setting Crossover Frequency.  What about the lower limit?

To figure that, you must first understand the Crossover doesn't work like a simple switch -- with all the frequencies going to just one speaker or just to the other speaker.  Instead the Crossover rolls into effect GRADUALLY -- over a range of frequencies.  There are technical reasons for this we need not go into here.  Suffice it to say filtering which works like a switch -- a so-called "Brick Wall" filter -- will do damage to the audio.

A typical Crossover implementation will roll into effect at a rate of, "-12dB per octave".  12dB, you'll recall from my discussion of Balancing Speaker Volume Trims with an SPL Meter is about a factor of 4 in perceived volume.  An octave is simply common parlance for a factor of 2 in frequency.  So if you have a Crossover which begins at 80 Hz, and takes effect at a rate of -12dB per octave, that means the audio going to the regular speaker will be reduced four-fold by the time you get down to 40 Hz.  This audio doesn't vanish, of course:  THAT'S the audio that's getting steered to the Subwoofer!  The audio going to the regular speaker drops even more below 40 Hz, but by the time you are -12dB down, you can pretty much ignore the audio coming out of the regular speaker.

That is to say, at 80 Hz and above, the regular speaker is carrying the audio.  At 40 Hz and below, the Subwoofer is carrying the audio.  And in between, they SHARE the job of reproducing the audio!

And THAT means you want to know that your regular speakers are CAPABLE of producing good audio all the way down to 40 Hz!

So if you have "full range" speakers -- rated down to 30 Hz for example -- you do NOT want to set the Crossover at 30 Hz.  Why?  Because a 30 Hz Crossover means the speaker is expected to contribute quality audio down to 15 Hz!

So the LOWER limit for the Crossover Frequency would be TWICE the bottom end of your regular speakers.  If they are rated down to 30 Hz, the Crossover Frequency should be no lower than 60 Hz.  If they are rated down to 50 Hz, the Crossover Frequency should be no lower than 100 Hz,

If you think about that, some problems should immediately jump to mind!

What if my regular speakers don't go low enough?  What if my SUBWOOFER doesn't go HIGH enough?  (Or low enough for that matter!)

If you have bookshelf speakers. they may be rated only down to 80 Hz.  That says the Crossover should be no lower than 160 Hz!  We've already talked about ONE problem with that.  If there's any male dialog in those speaker channels, a 160 Hz Crossover will steer it to the Subwoofer.  AND the audio from the Subwoofer will start to be localizable!

But what's more, it's also possible your Subwoofer is not even CAPABLE of reproducing frequencies as high as 160 Hz!  Subwoofer designs are optimized for Bass, and that means they are not going to do a good job handling higher frequencies of audio.  In addition to its low frequency specification, your Subwoofer likely also comes with a high frequency specification.  And that might be only, say, 120 Hz!

The result is what's called a "Hole" in the Crossover.  The Crossover is steering higher frequencies away from the regular speaker, but the Subwoofer is not able to reproduce those frequencies.  So the combined output from the regular speaker and the Subwoofer is lower than it should be at those in-between frequencies.

If these are the speakers you have to work with, then, of course, this is a problem you will have to live with.  The usual workaround is to set a lower, compromise Crossover -- one that asks the regular speakers to go a little lower than they can handle, but also gets closer to the high frequency limit of the Subwoofer.  So, perhaps a 120Hz Crossover, which will ask the regular speakers (spec'd down to only 80Hz in this example) to try to go down to 60 Hz, but will also do a better job of handling the frequencies between 120 and 160 Hz which the Subwoofer can't handle.

Or, of course, you could upgrade to better speakers!

At the low frequency end, suppose you have speakers rated down to 30 Hz.  So you set your Crossover at 60 Hz.  But what if your Subwoofer is ALSO rated down to only 30 Hz?  I.e., one of those less expensive Home Theater "Dynamic" Subwoofers I described above?

In that case the Crossover range itself (60 to 30 Hz) is handled, but the Subwoofer can not go below that.  Indeed, Crossover processing systems (and better Subwoofer designs) include protection to keep from sending TOO LOW frequency audio to the Subwoofer.  Why?  Because the Subwoofer can be physically damaged if it is forced to reproduce frequencies below what it can handle -- typically due to the cone being asked to travel too far, called "bottoming out".

If you try to push things too far -- say setting the Crossover at 50Hz in this example, trying to take a little more advantage of the "down to 30 Hz" goodness of your regular speakers -- you may bypass the low-end protection in the Crossover and send 25 Hz audio (or even lower) to that Subwoofer!

So now we have a range of possible Crossover Frequency candidates.  From 100 Hz at the high end down to twice the low frequency limit of the main speakers at the low end.

And we know we have to compare those candidates against what our gear can actually handle.  So for example, are the main speakers really good down to 50 Hz or lower?  And what's the top end (and low end) the Subwoofer can handle?

You'll see, by the way, that the 80 Hz Crossover Frequency I described as "typical" towards the top of this discussion is right about in the middle of the likely range.  THAT'S the reason it gets touted as a good starting point for your Crossover setting -- until you have time to think things through more carefully.

So suppose you think through all this and discover you actually DO have a range of candidates to choose between?  For discussion, let's say, perhaps, 60 Hz at the low end up to 100 Hz at the high end.

NOW what?  Which Crossover Frequency do you pick?

And THIS is where we start to run into the REAL complications of Bass Management!

The PROBLEM derives from those Bass audio Standing Waves I described up top.  These form in the room, at the various Bass frequencies, because the wavelengths of those Bass frequencies are bigger than the dimensions of the room.  This is the "pressurization" of the room I've alluded to several times  And the different dimensions of the room --its height, width, and depth -- result in DIFFERENT Standing Waves.

And those different Standing Waves *INTERACT*!

Where the different Standing Waves happen to match up you get "Resonance Peaks" -- a boost in level of that Bass frequency.  Where they are exactly opposite of matching up you get "Cancellation Nulls" -- a loss of level of that Bass frequency.

And these effects are not small!  If you measure Bass volume using a microphone shifted across the width of a typical sofa, you can easily get variations of as much as 12dB for Bass frequencies so unfortunate as to be strongly impacted this way!  The impact will vary both by Bass frequency and listening position.

This effect is called the Bass Room Response of the room.  Again, it is NOT an issue with the audio output quality of any of the speakers, or the Subwoofer.  It is what the room is DOING to that output!

And the challenge of Bass Management lies in taming these problematic, Bass Room Response issues.

If you think about it, the physics of a given speaker cone generating Standing Waves in the room is largely a matter of geometry.  We've already talked about the importance of room dimensions vs. the wavelengths of the various Bass frequencies for example.  But there's ALSO the issue of how that speaker "couples" to the room.  That is, how its physical location in the room amplifies or attenuates various Standing Waves.

Positioning the speaker closer to a wall enhances reflections off that wall.  Positioning it closer to a corner does this even more so.  If the width and depth of the room are different, positions close to the rear wall (near the TV screen) will couple differently than positions close to a side wall.

And at these Bass frequencies, even INCHES make a significant difference in how a Bass speaker couples to the room!

I mentioned up top using multiple Subwoofers (perhaps in lieu of one single, larger Subwoofer) could have additional advantages beyond simple convenience.  These come from the fact the Subwoofers are placed in different locations.  I.e., they couple to the room differently and produce different Standing Waves.  And the natural averaging effect of all this can be a big help in reducing Resonance Peaks and Cancellation Nulls.

Now, I'm not going to try to go into the various methods of taming Room Response problems in THIS post.  Instead, I want to focus on the contribution a good choice of Crossover Frequency can make in this effort!

Why?  Because your Subwoofer(s) and your regular speakers are, themselves, located in different spots in the room.  And thus the Bass frequencies they share in producing across that Crossover octave -- from your chosen Crossover Frequency down to 1/2 that frequency -- couple to the room DIFFERENTLY from each of them.

And raising or lowering the Crossover Frequency changes how MUCH Bass is coming out of the Subwoofer vs. the regular speakers across those shared frequencies.

So, if you DO have a range of Crossover Frequency candidates (after thinking through the upper and lower limit considerations described above), one of the best ways to choose between them is to pick the Crossover Frequency which MINIMIZES the inherent, Bass Room Response issues in your listening room!

You might do this using a tool to measure Bass levels at the different frequencies, or you might do it solely to taste as you try playing different types of content.

Or, instead of investing in audio measuring gear or trusting solely to your own taste in how the audio should sound, you could use a test track such as found on the "AIX Audio Calibration", Blu-ray, disc I described in my post on Calibration Discs.  Its Subwoofer Crossover test track sweeps a test tone up and down across the Bass frequencies.  This test tone goes to just the Front speakers.  At the high frequency end of that sweep, the audio will come entirely from those speakers.  At the low frequency end it will come entirely from the Subwoofer (due to the Crossover processing).  In between, there's a MIX of audio from both the Subwoofer and the Front speakers.  If everything is working RIGHT that sweep tone will appear to have constant Volume from end to end across the frequencies (except for the very lowest Bass frequencies which will drop off because they can not be heard).

TECHNICAL NOTE:  There's preliminary setup you need to do prior to trying this test, of course.  For example, you need to make sure your speakers and Subwoofer are matched for volume.  See my post on Balancing Speaker Volume Trims with an SPL Meter.

This is a TOUGH test, and you are unlikely to get a perfect result without going into more effort in configuring your Bass Management and dealing with Room Response issues.  But it is also a sensitive enough test you may very well hear significant differences in the amount of Volume variation which happens across that frequency sweep for the different Crossover Frequencies!  The idea, of course, is to pick the Crossover Frequency -- from within your range of candidate frequencies -- which produces the smoothest tone sweep (the least variation in Volume).

I'll have more to say about Bass Management and Room Response in future posts, but as I said up top:  The road to awesome Bass begins here!