|
|
|
|
|
Calibrating The 5.1 System
by Bobby Owsinski
This article originally appeared in the premier issue of Surround Professional in October 1998.
Although most facilities have long recognized the need for speaker calibration and "room voicing" while running in stereo, the need for some form of alignment now is even more critical with 5 speakers and a sub-woofer to contend with. But, of all the things regarding surround monitor systems, calibration is perhaps one of the most misunderstood, or at the very least, the most controversial.
Talk to the technical directors of several studios and you'll find that each has a seemingly different way of calibrating their system; some calibrating their subwoofers (where all the misconceptions lie) 10dB above reference level of the main system, some 10dB below, others 4dB above. Although this might make calibration seemingly fall into the realm of voodoo and black magic, there's actually more in common with these figures then meets the eye (or ear). It's just the way of looking at basically the same thing that's different. As I describe the most generally accepted method for surround calibration, I'll also explain how all these figures relate to each other as well.
Keep in mind that we're not really talking about room voicing here (that's for another article). We're only concerned about simple level alignment of the surround loudspeakers so your work will translate well to the outside world.
REFERENCE LEVEL
The first thing we have to do is set the reference level for the five main speakers. The type of program that a studio mixes generally determines the reference level, which may or may not be critical. On a film mix, for instance, the level is set at 85dB SPL and rarely changes in order to maintain consistency between reels and because the film will play back in the theater at (hopefully) that same standardized level. It is a standard that never varies.
A music mixer, however, will work at a variety of levels starting at a moderate level (of perhaps 85dB), boosting to 95 for a quick check of the low end, then getting the remaining balances at a quiet 72-75dB. There is no standardized reference level because the level that the mixer chooses is arbitrary and very variable, and it's the same for the listener.
Television, on the other hand, usually uses 79dB SPL as a reference since the quieter level translates better to the home. If TV sound is mixed at a higher level (like 85dB), then the dialog is frequently lost when it's turned back down to a normal listening level. Figure A gives the recommended levels for various program.
| Film Sound | 85dB SPL |
| Television | 79 " " |
| Music | 79 - 82 " |
The above is the reference level for each of the five speakers (so each speaker level is the same), but keep in mind that Dolby recommends that if you are using a nearfield surround system in a small room, you might get better results by calibrating the surround speakers (the rear speakers) 2dB less than the front speakers to compensate for the room.
PLACEMENT FIRST
Before any calibration can take place, it's very important to find the proper place in the room for the subwoofer. ideally, this is where the sub has the smoothest response and is least affected by standing waves. As with most of these measurements, the only real way to get this right is with a spectrum analyzer. It's also very important that the subwoofer is in proper acoustic phase at the crossover point, so that should be checked by switching the polarity (which can be done on most subwoofers) to the position with the most lwo end.
CALIBRATION BASICS
There are two ways to set the reference level of the main speakers, either by using a spectrum analyzer or by sound pressure level meter. A Real Time Analyzer is by far the more accurate of the two. If you are using an RTA, set all bands of the main speakers to read 70dB SPL. When all bands are added together they will total the 85dB reference level.
You can use an SPL meter, but it will give you only an approximation of the true level because it will read the highest peak of the pink noise in any band. If you use an SPL meter, set to C weight on the slow scale. Start by sitting in your normal mixing position, hold the SPL meter at chest level with the microphone facing up at an angle of approximately 45 degrees at the Center speaker. Keep the meter at arm's length to prevent any audio from reflecting against your body. keep the meter aimed at the Center speaker as you take readings for the Left and Right monitors as well. When taking the SPL readings for the Left Surround or Right Surround speakers, keep the meter at the same angle and position as you did for the front speakers, but turn your body 90 degrees from the Center speaker towards the wall closest to the speaker you are measuring.
REMEMBER: Although this may be obvious, it still should be stated for clarity. You're only measuring one speaker at a time! All the others must be muted.
CALIBRATING THE SUBWOOFER WITH A BASS MANAGER
The level of the subwoofer becomes a special case when a Bass Manager (see either the Glossary or Tom Holman's article in this issue for more details) is involved because the sub is now being fed from two sources; from 80Hz down from each of the five main channels, and from the direct LFE source track. This means that you have to take two measurements to compensate for the extra energy that is being directed to the subwoofer from the main system.
Although not absolutely necessary, it's best to use a test tape for this one. First, do the subwoofer channel. Play back the test tape at the -20dB FS standard for the five main channels. This should give you in-band (filtered) pink noise from 20 to 80 Hz. Set the level of each band on the RTA to read 70dB SPL, which is the same level that each band of the main system read (if your main system was calibrated to 85dB SPL as well). When added together, the level of the subwoofer should read about 79dB SPL.
Now set the LFE channel. If using a test tape with an LFE level of -30, set the level of each band on the RTA to read 70dB SPL, which compensates for the 10dB of gain inherent in the LFE channel. The overall SPL level of the subwoofer should read about 79dB SPL.
If using a test tape with an LFE level of -20, set the level of each band on the RTA to read 80dB SPL. The overall SPL level of the subwoofer should read about 89dB SPL.
If you don't have a test tape, it's possible to do this test by feeding in-band (filtered) pink noise to the subwoofer from the console. Where it gets tricky is if you choose to set the LFE channel by backing down the level by 10dB, in which case you must be sure that the level decreases by precisely -10dB or else the LFE level will be incorrect!
CALIBRATING THE SUBWOOFER WITH NO BASS MANAGER
If you're not using a Bass Manager then you should be! Why have a subwoofer if you can't take full advantage of it? A Bass Manager allows your main system response to extend down to 25Hz or so and enables you to hear some things (like rumble, thumps, glitches, extended bass content) that you might miss without it. Remember, the people at home are unsing bass management and they might be hearing the things you can't as a result,
OK, so you're either philosophically opposed or you just don't have a Bass Manager. No problem. The subwoofer level should be set exactly like the LFE channel (because that's the only signal it's getting) described above.
THE SHORTCUT METHOD FOR SUBWOOFER CALIBRATION
If you don't have an RTA or proper test disc or tape available, it's still possible to get the LFE channel of the subwoofer in the ballpark with an SPL meter. You can use even an inexpensive Radio Shack meter, but keep in mind that the low frequency response of these units varies quite a bit from meter to meter.
First, make sure that the meter is set for a C weighting on the SLOW scale. Then, simply feed some in-band (filtered so it only has a 20 to 80Hz bandwidth) pink noise to the subwoofer and calibrate for +4dB SPL ABOVE the reference level. This means that if you've selected 85dB SPL as your reference, the sub level at the mixers position will read about 89dB.
Depending on the variables of the noise source, the SPL meter, and subwoofer placement, you still might have to adjust this a dB either way (either +3 or +5 over your reference level), but you at least have a relatively close approximation as to where your sub level should be.
HOW ALL THESE FIGURE RELATE
Now you might be wondering; how do all these figures relate to one another? How can setting the subwoofer at -10 or +10 or +4dB SPL even remotely be the same? First of all, the extra 10dB of gain on the LFE channel to the subwoofer is actually the culprit, causing different people to send different signal levels to it for measurement. But really the difference is in the way people read essentially the same thing on the RTA.
If someone says they set the subwoofer at -10dB below the reference level, what they mean is that they are compensating for extra 10dB of gain in the LFE channel by sending it a signal that is 10dB lower. Each band of the RTA from 20 to 80Hz will read 70dB, which is the same as each band of the main system.
If someone says they set the subwoofer at +10dB above the reference level, what they mean is that they are not compensating for extra 10dB of gain in the LFE channel and sending it a signal that is the same as the five main channels (-20dB FS if from a test tape). Each band of the RTA from 20 to 80Hz will read 80dB SPL, or 10dB above what each band of the main system read. When all the energy from those bands add up, it will be approximately 89dB SPL (maybe a bit more or less depending upon standing waves). This is also why some engineers say they set their subwoofer at +4dB SPL above the main reference level.
Essentially, all of these readings are the same, with any differences mainly attributed to standing waves and meter variations. Figure B shows the comparisons.
|
Source |
Signal Level |
RTA (per band) | SPL Meter |
|
Main System |
-20dB FS |
70dB SPL |
85dB SPL approx. |
|
Subwoofer or LFE |
-20dB FS |
80dB SPL |
89dB SPL " |
|
" |
-30dB FS |
70dB SPL |
79dB SPL " |
NOTES: 1) Source signal readings the in-band noise derived from a test tape.
2) SPL meter readings are approximate due to meter variations and room standing waves.
QUICK STEPS TO SURROUND SYSTEM CALIBRATION
- Using a Real Time Analyzer - Sub Woofer With No Bass Management
1. Using in-band pink noise, set the reference level for each of the 5 main channels to 85dB SPL (or other reference level in Figure A).
2. For the LFE channel if the LFE level on the test tape is at -30, set the level of each band on the RTA to read 70dB SPL (subtract the appropriate amount if using another reference level). The overall SPL should be approximately 79dB if you're using an 85dB SPL reference.
2A. For the LFE channel if the LFE level on the test tape is at -20, on the LFE channel, set the level of each band on the RTA to read 80dB SPL (subtract the appropriate amount if using another reference level). The overall SPL should be approximately 89dB if you're using an 85dB SPL reference.
Using a Real Time Analyzer - Sub Woofer With Bass Management
1. Using in-band pink noise, set the reference level for each of the 5 main channels to 85dB SPL (or other reference level in Figure A).
2. For the subwoofer channel, set the level of each band on the RTA to read 70dB SPL. The overall SPL should be approximately 79dB if you're using an 85dB SPL reference.
3. For the LFE channel if the LFE level on the test tape is at -30, set the level of each band on the RTA to read 70dB SPL (subtract the appropriate amount if using another reference level). The overall SPL should be approximately 79dB if you're using an 85dB SPL reference.
3A. For the LFE channel if the LFE level on the test tape is at -20, set the level of each band on the RTA to read 80dB SPL (subtract the appropriate amount if using another reference level). The overall SPL should be approximately 89dB if you're using an 85dB SPL reference.
Using Only An SPL Meter
1. Using in-band pink noise, set the reference level for each of the 5 main channels to 85dB SPL (or other reference level in Figure A).
2. Set the level of the LFE channel of the subwoofer to 4dB above the chosen reference level (89dB if the reference level is 85dB).
3. Adjust the subwoofer up or down a dB or so to adjust for possible standing waves.
