Balancing Speaker Volumes Trims with an SPL Meter

One of the basic steps in setting up any Home Theater system is making sure all of your speakers, and your subwoofer(s), are matched in output volume.  Although you could try doing this by ear, this is really something best done with the aid of an inexpensive measuring device called a Sound Pressure Level (SPL) Meter.

This post discusses how to use your SPL Meter correctly.

The meter I happen to be using at the moment (pictured here) is a Checkmate model CM 140 from Galaxy Audio. But there are many other choices out there -- all reasonably priced -- which all work basically the same way.

An SPL Meter uses an omnidirectional microphone to pick up a wide range of audio frequencies coming in from all directions.  When balancing speaker volumes, you want to measure a test tone which combines just such a wide range of audio frequencies.  Although your Home Theater electronics may include built-in speaker test tones -- for example in your AVR or one of your playback source devices -- it is generally best, if at all possible to use test tones played from a Calibration Disc.  Playing test tones from real content like this insures that any audio processing you have enabled is actually included in generating the speaker output.  For example, if you have Crossover processing enabled, the test tones from a Calibration Disc will be played through that, whereas built-in test tones in your hardware might not do that.

For configurations of up to 7 main speakers (plus subwoofer(s)), I would recommend, for example, the LPCM 5.1 or LPCM 7.1 "Channel ID" test tracks found on the AIX Audio Calibration Blu-ray disc I discussed in my recent post on Calibration Discs.

First make sure you have fresh batteries in your SPL Meter, particularly if you have not used it in a while.  Weak batteries will result in unreliable measurements.

Next look for two settings on the SPL meter.  The "Response" setting should be set to SLOW so that the measurements don't bounce around so much they are difficult to read.  (Even on SLOW, the SPL measurement for your Subwoofer(s) will bounce around.  This is normal.  So you will have to mentally average those to settle on the Subwoofer result.)

The "Weighting" setting -- typically a choice of "A" or "C" -- should be set to "C".  This adjusts how different frequencies are combined to produce the single, SPL result.  C-Weighting is the best choice for setting up Home Theater speakers.

Some SPL Meters will automatically adjust for different audio levels, but others will expect you to pre-select a "Range".  You'll be making measurements around 75dB, so pick a Range which has 75dB in the middle.

When taking measurements, hold the meter at arm's length, such that the mic tip is pointed straight up at the ceiling.  Position the mic tip where your ears would be when seated for normal listening.

Since the meter picks up sound coming from all directions -- including reflections off the walls and ceiling for example -- you want to make sure you don't position the meter where it is blocked from hearing audio in any direction from the portion of the room above the mic tip.  So for example, if your chair has a high seat back, either raise the mic tip to clear that seat back or move the mic, say, about a foot closer to your TV screen to minimize the blocking from the seat back.

Note that you do NOT point the meter at the speaker you are trying to measure, nor do you move the mic to different locations while taking a set of measurements of your speakers.

Typically you will adjust your speaker Volume Trims using settings in your AVR.  To start, set all of those to zero dB.  Now start your test tones playing -- for example, from the AIX Blu-ray disc I just mentioned.

Adjust the Main Volume on your AVR so that the Left Front speaker measures 75dB SPL.  75dB is loud enough that you will get clean measurements without being so loud that it is painful to listen to while taking the measurements.  (It will still be pretty loud.)

TECHNICAL NOTE:  "dB" -- short for Decibels -- is a measure of audio power.  It is a "logarithmic" measurement; which means dB values compare in a way that's somewhat non-intuitive.  A good rule of thumb is that each 6dB increase represents a doubling of perceived Volume.  The audio setups in commercial movie theaters are calibrated such that "Reference Level" sounds will produce 85dB SPL.  (Peak Level sounds can be quite a bit higher.)  85dB is really too loud for Home Theater -- it includes some additional volume to overcome audience noise for example.  The rule of thumb for Home Theater setups is to calibrate to 75dB SPL.  Just as in movie theaters, your Peak Level sounds can easily be 15dB higher.

Now leaving Main Volume set at that same level, let the test tones play for the other speakers as well.  Adjust the Volume Trims, in turn, in your AVR such that every speaker also measures 75dB SPL.  Since you established your Main Volume setting using the Left Front speaker, of course the Volume Trim for the Left Front speaker will end up staying at zero dB.

TECHNICAL NOTE:  Depending on your particular hardware, positive dB values of Volume Trim may risk "clipping" the audio to that speaker.  This would be pretty unusual for Volume Trims found in an AVR (because AVR designs allow enough output headroom for their Trims), but may be more likely if you are adjusting Volume Trims for the Analog audio outputs of one of your playback source devices.  To avoid any possible problems, in such case, there's a simple solution.  After having set the Volume Trims as above, go back and lower ALL the trims by the same amount such that the largest Trim is now zero dB and all the other speaker Trims are negative dB values.  If you lower all the Trims by -5dB for example, simply make a mental note that the Main Volume setting you have been using during this would need to be raised an additional +5dB to get back to your "calibrated" level of 75dB for Reference Level audio signals.

Your Subwoofer(s) need special attention in all this.  First, your Subwoofer(s) will have their own Volume knobs.  So before doing the SPL measurements for all your speakers you should start by getting the Subwoofer Volume knob into the right ballpark.  To do that, find the Main Volume setting that produces 75dB SPL from your Left Front speaker.  Then check to be sure the Volume Trim setting for your Subwoofer is currently at zero dB.  Then play the test tone for the Subwoofer and adjust the Volume knob on the Subwoofer to get close to 75dB.  You'll correct for any minor error in that when you do the real set of SPL measurements and select the correct Volume Trim setting for the Subwoofer.

Second, you may have more than one Subwoofer.  In that case you want to make sure that each of your Subwoofers is producing the same output level, so they are contributing equally to your bass audio.  To do THAT, simply unplug all but one Subwoofer and adjust the Volume knob for that Subwoofer as described above.  Repeat with each of the other Subwoofers, powered one at a time.

Now, the output from your individual Subwoofers add together to produce the combined Sub level.  That means if you adjust two Subwoofers to each produce 75dB SPL on their own, their combined output will be HIGHER than 75dB.

So when setting the individual Subwoofer Volume knobs as just described, target a LOWER SPL level for one Subwoofer at a time.  For example, the rule of thumb with two Subwoofers is to set each to 72dB SPL.  When played together, you will get 75dB SPL.

Be sure you remember to restore power to all your Subwoofers before you do your final SPL adjustments.  You want all the Subwoofers to be playing simultaneously when you set the Subwoofer Volume Trim in your AVR.

Finally, if you read my recent post on Calibration Discs, you'll remember that the audio going to the Subwoofer must have "Sub Boost" added to be properly matched in level to your main speakers.  This is due to the standard attenuation of Subwoofer output for LFE content and also for bass steered from the main speakers due to Crossover processing.  The required Sub Boost might be applied by one device, such as your AVR, or by a combination of devices, such as partially in the AVR and partially by raising the Volume knob on the Subwoofer a bit more.

It's very easy to get confused about how to do this properly if you try to think through what's really going on.  But in reality it is actually extremely SIMPLE!   If the SPL you measure for the Subwoofer test tone is THE SAME as the SPL you measure for each of your main speakers then you are done!  The correct Sub Boost -- however it was accomplished -- MUST already be in place.

And this is another reason why you want to do these measurements using real content -- such as from that AIX Audio Calibration Blu-ray -- instead of using built-in test tones.  Because the real audio, from a test track like that, will pass through ALL of the adjustments established in your various pieces of gear.

So again, the procedure for setting up your Subwoofers goes as follows:

  1. Adjust the Volume knob on each Subwoofer individually to make sure all your Subs are producing the same level of output.
  2. When doing that, target a Volume knob setting on your individual Subs so that their COMBINED output is 75dB SPL.  This combined result will be refined when you set the combined, Subwoofer Volume Trim in the next step.
  3. Adjust Volume Trims so that ALL of your speakers in turn (including the combined set of Subwoofers measured as a single number) produce the SAME SPL reading -- typically targeting 75dB SPL for Home Theater setups.

There is an additional complexity for Subwoofer setup that I'll mention here, but put off for discussion in another post:  Your Subwoofers must be set to be in proper "Phase" with your main speakers.

This is particularly important when you have Crossover processing enabled.  (Again, see my recent post on Calibration Discs for discussion of that.)  Crossover processing means that for bass frequencies your main speakers and your Subwoofer(s) are going to be playing the SAME content at exactly the SAME time.  It's their combined output of that bass which produces the desired result.  The identically-timed audio waves coming from your main speakers and your Subwoofer(s) will interact as they traverse the room to your ears.  If your Subwoofer(s) are not in proper Phase relationship to the main speakers this interaction of the separate audio waves can produce Cancellation!  The result is that your bass audio sounds anemic

Indeed if you have more than one Subwoofer, you must also insure the Subwoofers are in proper Phase relationship with each other!  Again, I'll put this off to a future post.