Wavefront Interference
In my prior post on Choosing a Crossover Frequency, I discussed the fundamental role played by a Subwoofer in any good, Home Theater system, and the equally important choice of allowing that Subwoofer to handle the lowest bass frequencies which would otherwise be played through the Main speakers. This task of "steering" bass from the various Main speaker channels to the Subwoofer is the job of the Crossover processor -- part of the "Bass Management" system in your Audio Video Receiver (AVR).
The Crossover does not act like a switch, with bass suddenly cut off to each Main speaker and sent instead to the Sub. Rather, the Crossover rolls into effect over a range of frequencies -- typically one Octave (factor of 2 in frequency). So for example, an 80 Hz Crossover -- the most common choice -- actually rolls into effect from 80 Hz down to 40 Hz. At 80 Hz, all the audio in the Main speaker channel is still coming out of the Main speaker. At 40 Hz, all the audio is being sent to the Subwoofer. And IN BETWEEN, the Main speaker and the Subwoofer SHARE the job of producing the audio output!
So in that critical range of in-between frequencies, a given Main speaker and the Sub are playing the SAME audio at the SAME time. But the Main and Sub are not located at the same spots in your room, and the differing designs of the Main and Sub undoubtedly introduce different delays in how the electrical input signal to each turns into motion of their speaker cones -- creating the audio you hear. The upshot is the audio waves arriving from the Main and Sub may very well NOT reach your ears at precisely the same instant of time! The sound which SHOULD have blended perfectly from the two of them will not do so.
Technically, this timing mismatch is called Phase error, and it can be a real problem! If the two wavefronts are a full 180 degrees out of Phase, for example, they'll CANCEL each other out! That is, the result of improper Phase matching is anemic sounding bass.
In this post, we'll talk about how to get your Subwoofer(s) into proper Phase with your Main speakers.
Before we get to the actual Phase correction process itself, there are preliminary steps to get out of the way.
At bass frequencies, what you hear from any speaker (including the Subwoofer) depends on how that speaker's bass audio output "couples" to the geometry of the room. This is a function both of the positioning of the speaker with respect to the walls, and of your listening position. The Main speakers, of course should be positioned as close as possible to the recommended layout (around your listening position) for your particular surround sound speaker configuration. The Manual for your AVR likely includes diagrams detailing that. Meanwhile, the Subwoofer COULD be placed just about anywhere in the room. The Sub works by pressurizing the entire airspace volume of the room with bass audio, meaning the bass from it appears to come "from everywhere" rather than from the actual location of the Sub itself. However, for most people, there are only a few locations in their room which will be practical for the Sub -- either due to physical constraints or aesthetics.
Ideally you want to pick the location which produces the fullest bass at the main listening position, without undue appearance of cancellation nulls or resonance peaks at different bass frequencies. With a mic and bass measurement charting app for your computer, you can actually test different locations to see which appears to work best. A simpler approach is the "Sub Crawl": Temporarily position the Sub at your main listening position and then get down on your hands and knees and go listen to its bass output at each or your candidate locations. Pick the one that has the most bass and move the Sub over there.
The above discussion has been deliberately hand-wavy, as picking the TRULY BEST location for your Sub(s) could easily be a topic for an entire post of its own. But there's an important reason for mentioning this step first: At these bass frequencies, even INCHES matter in the positioning of the Subwoofer(s) and speakers. A small shift will change how they couple to the room. So if you MOVE the Sub or speakers later on down the road, you should re-check your Subwoofer Phase adjustment as described below.
The next preliminary step is to enter your correct speaker Distances into the listener position settings in your AVR. The AVR uses these to correct the relative timing between the speakers. That is, it delays sending audio to nearer speakers for a bit so their timing aligns with audio from the more distant speakers as the audio from each reaches your ears. As described above, Phase error is just such an error in relative timing, so you'd THINK putting in the correct distances would be all that's needed! However, this doesn't account for the relative audio processing delays INSIDE the speakers and the Sub. So we're not done yet!
TECHNICAL NOTE: If you have more than one Subwoofer, and your AVR only allows you to enter one distance setting for the lot of them, use the distance of the FARTHEST Subwoofer.
Now look at the controls on your Sub(s). Some Subs will have NO controls related to Phase or "delay". Others will have a switch offering just two choices -- typically labeled Normal/Inverted or 0/180 degrees. That switch itself may be called Polarity or Phase, but if it only offers two choices we'll refer to it here as a "Polarity" setting. The Rule of Thumb for a Polarity setting is to place it in the Normal (or 0 degrees) choice for a Subwoofer positioned near the front of the room (near the TV screen). For a Subwoofer positioned near the rear of the room use the Inverted (or 180 degrees) choice.
And some Subs will have a true Phase adjustment setting -- typically in the form of a knob which allows you to set a value anywhere in the RANGE from 0 to 360 degrees. This may also be labeled as a "Delay" adjustment. In this post I'll refer to both as the Phase knob. For those Subs, set the Phase adjustment knob to 0 degrees (0 delay) regardless of the placement of the Sub.
The next step is to make sure your Subwoofer(s) are Volume matched to your Main speakers. See my post on Balancing Speaker Volume Trims with an SPL Meter for details. This will make it easier to detect proper Phase correction in the steps below.
TECHNICAL NOTE: If you have more than one Subwoofer, power them one at a time and set the volume knob on each to get the same volume level -- lower than your target level. That is, make sure the Subs are matched to each other in volume. Pick that lower level so the COMBINED output of the Subs hits your target level when all the Subs are powered. So for example, with two Subs, you would trim each individually to produce 72dB SPL. When powered together, that should yield 75dB SPL -- a typical target level.
Lastly, you need to make sure your AVR is configured so you can actually hear its Crossover processing working! That is, you'll need to have your speakers set to "Small" so bass steering actually happens, and you'll need to pick a Crossover frequency. If you've not had time to consider the best choice of Crossover frequency yet, use 80 Hz.
It is also wise to disable any Surround Sound Processing feature in your AVR. That is, you want Stereo content to produce output only on the Left Front, Right Front speakers, and the Subwoofer. And similarly, you don't want 5.1 content producing 7.1 speaker output, or any output going to Height speakers you may have configured.
But don't go so far as to disable ALL audio processing in the AVR (e.g., a "Direct" mode), as that will likely disable the Crossover too!
We are finally at the stage where we can do the Subwoofer Phase adjustment! To do this you'll need appropriate test tones. You can do the adjustment itself by ear, but some folks will find it easier to use an SPL Meter. For instructions on using an SPL Meter properly, see my prior post on Balancing Speaker Volume Trims with an SPL Meter. Note that whether you are checking by ear or with the SPL Meter, you should be doing so at your primary listening position.
There are many possible sources of test tones. Just make sure the tones you are using actually include bass frequencies which will be steered by the Crossover! (So called Pink Noise test tones for the Main speakers typically will not include such bass frequencies.)
Note that you CAN use the Subwoofer test tone built into your AVR for the step below regarding setup for more than one Subwoofer, but you probably can NOT use the built-in test tones in your AVR for, e.g., the Left Front speaker, as that tone likely will not be processed through the AVR's Crossover.
You can double check whether the Crossover is actually functioning when playing your choice of test tone into the Left Front speaker, for example, simply by walking up to the Sub and lightly resting your fingers on the cone of the Sub. You should be able to feel it vibrating -- thus confirming that bass from that Left Front test tone is indeed being steered to the Sub.
To go through a specific example in this post, I'll use a test track from the AIX Audio Calibration, Blu-ray, disc described in my prior post on Calibration Discs.
The first step is specifically for folks who have MORE than one Subwoofer. If you have only one Subwoofer you can skip to the next step.
For folks with more than one Sub, there are actually two parts to the Phase adjustment. First we need to make sure the Subs are in Phase with EACH OTHER. Then we need to adjust the Phase for that combo of Subs with respect to the Main speakers.
For the first part, you can use the Subwoofer test tone in your AVR, if it indeed sends that test tone to ALL your Subs simultaneously. If it only sends the test tone to one of your Subs at a time, you will need to use an external test tone. For example, on the AIX Blu-ray disc just mentioned, go to System Calibration > Speaker Balance and play the 5.1 test track. Let that play until it gets around to the Subwoofer tone, and measure using that tone.
TECHNICAL NOTE: If your player allows, set up an A-B Repeat on that Subwoofer Test tone so it keeps playing. If not, use REWIND on your player to back up as necessary to repeat the Subwoofer tone so you don't have to wait for the other speakers to play through their tones again.
If you have more than TWO Subs you are going to be testing the FARTHEST Sub with one at a time of the nearer Subs. I.e., power the farthest Sub and just one of those nearer Subs at a time. The controls on the farthest Sub will never be changed. Instead, raise the Phase (or Delay) knob on the nearer Sub to produce the maximum bass output at your listening position. This is a little easier if you have someone else twiddle that Phase knob while you do the checking at your listening position.
The Phase knob adjustment on the nearer Sub which produces the maximum bass output as measured at your primary listening position achieves the correct Phase match between that nearer Sub and your farthest Sub.
Now power down that nearer Sub and power up one of the other nearer Subs and repeat -- adjusting each in turn to be in correct Phase with the farthest Sub. Once they are each in correct Phase with the farthest Sub they are ALSO in correct Phase with each other!
If you have only one Subwoofer, or after completing the step above about getting your set of Subwoofers in proper Phase with each other, the last step is to get your Sub(s) in proper Phase with your Main speakers.
For that we COULD use the Phase controls on the Sub(s), but there's a better way! To understand WHY it is better I need to digress a moment.
I mentioned above simply setting the correct distances for your speakers and Sub was not enough to insure correct Phase adjustment for the Subs. The REASON is the audio processing in modern Digital Subs takes some time. 5 to 10 milliseconds (thousandths of a second) is typical. Thus the audio coming out of the Subwoofer is later than the AVR expects based on your distance settings!
The Phase adjustment on the Sub works by adding ADDITIONAL delay. Add enough additional delay and the output audio wavefront cycles around and gets back in proper Phase alignment with the input electrical signal (or more properly, with the audio you are trying to match from the Main speakers). However, even though the Phase is correct, the audio is now even LATER -- for example, a full frequency cycle later.
There's no way to reverse this. The audio processing time inside the Sub is what it is, and you can not make the audio come out of the Sub any faster. You can just delay it further.
But suppose you could delay ALL THE OTHER SPEAKERS instead? By adding the right amount of delay to ALL the other speakers, you can get them back in proper Phase alignment with the Sub(s) while ALSO keeping both the Mains and the Sub(s) on the SAME frequency cycle of the audio!
Well, it turns out there's a simple way to do just this! To wit: Tell the AVR the Subwoofer is further away than it really is! The way speaker distance compensation works in AVRs is the nearer speakers get delayed to time align with the farther speakers. If you tell the AVR the Sub is farther away, it will compensate by delaying ALL the other speakers.
I mentioned above typical audio processing time in modern, Digital Subs is about 5 to 10 milliseconds. Well it turns out the speed of sound is such sound travels just over 1 foot in 1 millisecond. (More accurately, 1.1164 feet/millisecond, but 1 to 1 is close enough for discussion.)
So that says if we tell the AVR the Sub is several feet further away than it really is, the AVR will add the necessary delay to the Mains to get things back in proper Phase *AND* Timing. "Several feet" here is likely going to be something in the range 0 to 16 feet beyond the actual physical distance of the Subwoofer. Predicting the correct setting is tricky due to details in the processing in the Sub, and also in the Mains, but we don't need to predict. We can just test it and FIND the right distance.
To do that with the AIX Blu-ray disc, go to System Calibration > Speaker Balance, and select the 2.0 LPCM test track. This sends a test tone to just Left Front and then just Right Front. If your player offers A-B Repeat you can use that to isolate just the Left Front tone so it plays over and over again. Otherwise, the Chapter Back button on your player's remote will restart that test track -- playing the Left Front tone again.
That test tone to Left Front will have its lowest bass frequencies steered to the Sub(s) via the Crossover. Check by ear or with your SPL Meter as you raise the distance setting of the Subwoofer in the AVR. For example if the correct Subwoofer distance is 11 feet, you would check 11 through 27 feet looking for the distance setting which yields the maximum bass at your primary listening position.
TECHNICAL NOTE 1: If your AVR allows you to enter SEPARATE distances for multiple Subs then raise all of them the same amount at each step. So add 1 foot to each, then 2 feet, then, etc. This way you will retain the correct Phase relationship BETWEEN those Subs which you set above.
TECHNICAL NOTE 2: If you are using an automated Room Correction or Room EQ feature in your AVR with includes setting speaker distances as part of its processing, you may discover that the distance it selects for your Subwoofer is larger than its actual distance. This simply means your automated system has already attempted to provide the Phase adjustment just described. Nevertheless, it wouldn't hurt to to validate that Sub distance choice with your own, manual test after the fact.
And that's it! With Subwoofer Phase correctly adjusted, you are now using the Crossover processing in your AVR to maximum advantage!
--Bob