Manual Labor

I purchased an SPL meter to level the speakers in my 5.1 surround system. I read an article in Home Theater magazine about how to use this device, but I'm still not sure how to do it. For example, I'm not sure where to put the dial—do I start at 80 or 120?

John Krause

You don't say what brand of SPL meter you bought, so I'm going to assume it's a RadioShack analog meter as pictured above, which is one of the most common consumer SPL meters.

First of all, most modern A/V receivers and preamp/processors now offer an automatic-setup function (Audyssey, Yamaha YPAO, Pioneer MCACC, etc.), which uses a microphone included with the unit. If you have an AVR or pre/pro with this feature, there's no need to perform a manual setup, though it's a good idea to check the automatic results with your SPL meter to make sure they're correct. If your AVR or pre/pro doesn't have auto setup, you'll have to do it manually.

Let's start with a few fundamentals. The "reference level" for theaters is defined by the movie industry to be an average sound pressure level of 85 decibels, C-weighted (dBC), which means that setting your AVR's level control to 0 results in an average sound pressure level of 85dBC. (Don't worry about what "C-weighted" means; it doesn't matter for the purpose of this discussion.) The important word here is average. When you calibrate to this average level, setting your AVR's volume control to 0 can result in a peak level of 105dB SPL on many films and even higher in the LFE channel. This is louder than most people can tolerate, so you normally set the volume lower than 0 when listening to music or movies.

Because 85dBC test tones can be quite loud, Dolby decided that a reasonable test-tone level is 75dBC, which is what most AVRs use these days. Thus, you need to set the SPL meter's Range control to 70 or 80. Also, set the Weighting control to C and the Response control to Slow. (If you set it to Fast, the needle will jump around and make it difficult to read a value on the meter.)

For example, say you set the Range control to 80—when the meter reads 0, it is measuring a sound pressure level of 80dBC, and when it reads -5, it is measuring 75dBC. If you set the range control to 70, a reading of +5 means it's measuring 75dBC. (BTW, if you set the Range control to 120, a reading of 0 on the meter indicates a sound pressure level of 120dBC, which will damage your hearing very quickly! But not to worry—most consumer audio systems can't play that loud anyway.)

When you put an AVR into its manual-setup mode, it will typically disable the main volume control and play test tones—actually, test noises—from each speaker at what it thinks is 75dBC. Mount the SPL meter on a tripod and place it in your listening position at seated ear height. Point the meter straight up and note the level reading as each speaker plays the test signal. Use the individual channel-level controls in the AVR's setup menu to adjust the level for each channel in turn so they all read 75dBC on the meter.

Some AVRs do not disable their master volume control for manual setup. In this case, set the center-channel level to 0 in the setup menu and adjust the master volume until the center channel reads 75dBC, making note of that setting. Then adjust the other channels' individual controls to 75dBC.

In addition to setting the individual channel levels, you must also set the distance from each speaker to your listening position. Get a tape measure, determine each distance, and enter that info into the AVR's setup menu.

Then there's equalization, but that's not practical without some sophisticated equipment—or an auto-setup function. If the automatic EQ sounds good to you, leave it enabled; if not, you can turn it off.

If you have an audio/video question for me, please send it to scott.wilkinson@sorc.com.

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COMMENTS
Gerald Steele's picture

Scott, I have used the exact SPL meter that you have used as an example for many years now. Your advise on pointing the meter straight up is contradictory to Radio Shacks instructions to which I quote; " For the most accurate readings and the best response, point the meters microphone toward the sound source". When taking a SPL reading you should also position your body perpendicular to the meter for the best results. Sincerely, Gerald Steele

Scott Wilkinson's picture

Well, maybe the mic is not as omnidirectional as I thought. If so, I agree with the instructions...point the mic at each speaker as the test signal cycles around the room.

Scott Soloway's picture

The Radio Shack meter, having a sealed microphone, is indeed omnidirectional. Of course, nothing is perfect and the larger the diaphragm and the bigger the body the more directional a mic will be at higher frequencies. The Radio Shack unit's response at 90 degrees off axis is about 3dB down above 5kHz. This is of some significance for making frequency response measurements but is of no importance for level setting. So straight up is fine.

Kenny Kraly Jr.'s picture

Hello Scott I listen to you on the tech guy radio show with Leo Laporte and read your articles at home theater and ultimate AV Magaziens. My question how come the print quallity when movies are restored for blu-ray sometimes the prints are not as good? Does that have to do with the film elements themselfs? Why are some films harder to restore for blu-ray? Are movies shot from the 50's, 60's 70's and 80's harder to restore in high-def than films of today?

ender21's picture

Kenny, hopefully Scott doesn't mind my diarrhea of the mouth, but the reasons are so many as to only be touched upon. Probably the biggest reason - Money. For some catalog titles the budget just isn't there to restore an older movie as well as possible. If the original negative still exists then a new interpositive can be made of much higher quality than 10-20 years ago (Baraka is an example). But some companies don't. They use old interpositives that are beat up, have steadiness problems, were made dry instead of wet-gate, etc. Some companies pride themselves so much that they'll use the original camera negative directly to digital (like Bladerunner's VFX elements.. the live action portion was interpositive). Then, what was the quality of the original negative in the first place? Was it shot in 16mm? Was it shot on a poor camera stock? With poor lenses? Like I said, sorry to go off on a tangent... hopefully it made sense!

Scott Wilkinson's picture

What Ender21 said...and I don't mind at all! I completely agree that the reasons are so many that it would require a long article to discuss them all in depth.

Oscar Worthy, DDS's picture

With regards to calibration of channel levels---my understanding is that in setting the levels at 75dbC that the THX/studio reference level is established at 0db on my processor. I have two questions: With this set at 75db, but I guess studio mastered at 85db, is the 0db at home delivering the studio reference? Also, i have a Meridian surround processor, and it has a default 105 peak level limiter that I have disabled to 120db---is there any reason to use the limiter? Thank you for your time in considering this.

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