High Performance Home Theater Calibration - page 5

Level and Frequency Response Calibration

We were now about 2 ½ hours into Tony’s course when we finally get to “calibration”. (Editor’s note: And I’ve skipped an awful lot of slides and information along the way also! This isn’t the course; it’s a review of some of the course’s highlights.)

To start our calibration procedure we go back to the test signal found in the receiver’s set-up menu. We calibrate each channel for 75dB SPL. Many newer receivers have an auto calibration feature with the microphone being embedded in the receiver’s remote. Tony has found however that many of the auto-cal, internal pink-noise generation programs, in conjunction with a very inexpensive microphone embedded into the remote can end up giving readings as much as 4dB off! That’s why he recommends using the well known Radio Shack analog SPL meter in conjunction with accurate test discs such as the 5.1 Audio Tool Kit.

Frequency response calibration is done after analysis with a Real Time Analyzer (RTA). The idea is to first optimize to a flat frequency response through speaker placement and room treatment (covered in next review, “Room Acoustics: Acoustic Treatments”). Then equalize using spatial averaging, one third octave for the mains and a minimum of 1/12 octave parametric for the subs.

Spatial and temporal averaging refers to inputting multiple response curves (multiplexing) from all seating positions and averaging the results to yield as flat a curve as possible from all seating positions. Remember, all equalization is always done with all controls on the source unit set to flat. And never EQ a system at the surround decoder input.

A note about equalization; Tony and John Dahl of THX recommend avoiding any equalization of known flat-response speakers above 1KHz. Dr. Toole contends that you should never have to equalize above 700-800Hz at the most. Surround speakers may need equalizing depending on placement. At this point in the discussion Tony laid out a step-by-step procedure for EQ’ing the system using a professional RTA. (If you own or have access to an RTA Tony’s tweaking procedure will make perfect sense but I’ll skip past that part of the course for now.)

Documenting the Room Calibration and Final Listening Tests
It is important to note that data on all the calibration and tuning of the system can be documented and given to the customer upon completion in the form of the checklist which you’ve methodically worked through. Plus you’ll have a set of curves you’ll be able to show off that verify the performance of your room/system calibration procedures.

The last steps now are the listening tests starting with the Soundstage Construction:

Now refer back to the very beginning of this review for Tony’s “Eight Performance Goal’s of Home Theater Audio Systems”. And using test software found on the 5.1 Audio Toolkit DVD, Ovation software’s “Avia” DVD or a disc like Delos’ “Surround Spectacular” check that the system can indeed deliver on each of the goals. You’re done!

Note: Information on much of the hardware and software which Tony Grimani talks about in his classes (and uses everyday himself) can be found on his website at www.pmiltd.com.

For more information on becoming an installation professional or having a home theater built contact CEDIA at www.cedia.org.