Question. My square-shaped home theatre room is only 12 x 12 x 9 ft., with the AV components and the TV in one corner, and two settees in a "V" arrangement in the diagonally opposite corner. My question is: What volume levels (in SPL terms) should I subject my friends to when watching DVD movies? Using the AVIA test DVD, I did the calibration at 84 dB SPL; the sub is set at 88 dB. –Bryan
Answer. For your friends, I would initially set playback levels to average about 75 to 80 dB SPL (Sound Pressure Level) at the seating area, with occasional peaks allowed to 85 dB SPL (C-weighted). That will still leave room for increasing the center-channel loudness level by a few dB for dialog clarity if needed. If someone says they can't hear the dialog, just raise the center channel volume by 3 to 5 dB rather than increasing the loudness of the entire system. (In this discussion, I'm assuming you measured SPL levels using the Radio Shack Sound Level Meter, model 33-4050, set to the "C" weighting scale, in the averaging mode, and at your seating area. The Radio Shack meter is accurate to within 2 dB of expensive professional Bruell and Kjaer SPL meters.)
Individual tolerance of loudness levels varies quite a bit from one person to the next, and is also age-related, but many years of careful testing of large numbers of people with normal hearing have yielded a generally agreed upon set of subjective standards that quantify loudness levels for the majority of listeners. For example, the 84-dB SPL calibration level that you used with the Avia test DVD is termed "quite loud" by most listeners.
THX's certification program for movie theaters typically sets cinema playback levels to average around 85 dB SPL in the middle area of the cinema. In non-THX movie theaters, sound levels may vary quite dramatically from one cinema to the next. Some of the most complete research I collected on movie theater sound levels was conducted in 2003 by a consumer TV show, "Marketplace," broadcast by CBC television in Canada. They commissioned a Toronto engineering firm to measure sound levels in five different movie theaters, with different films, including a THX theater. All were measured using the A-weighting scale, which gives less emphasis to deep bass. The results are interesting:
The THX promo clip measured 85 dB SPL, while average movie sound levels were between 70 dB and 78 dB–comfortable and not too loud. Coming attractions previews, however, were louder, peaking at 90 dB SPL, a lot louder than the actual movie presentations.
Several of the blockbuster movies, The Lord of the Rings: The Twin Towers, and Harry Potter and the Chamber of Secrets peaked at levels of 95 dB and 93 dB, respectively. Those peak levels correspond to "very loud" according to accepted subjective testing, but average levels for the two movies were considerably lower.
Back to your home theater room: Raising playback levels to peak at 95 dB SPL, a level that most listeners term "very loud," is risky unless you're well acquainted with your friends' listening preferences.
Certain DVDs may call for different levels. A DVD music concert of amplified rock music, cleanly recorded, can certainly be enjoyed on a good system at levels up to 100 dB, but only if your friends like electric rock at those levels—and lots do. A movie like War of the Worlds is thrilling played loud. Some concert material in real life peaks at 100 dB SPL or higher, so you can certainly duplicate the levels experienced with live music if you choose to do so… and if your system, speakers and amplifiers are up to it. But be careful: tolerance levels vary by 3 to 6 dB or more between individuals, and since your guests can't move farther back to reduce the subjective volume, watch for any signs of duress or discomfort on their faces, and note their ages. After age 40, everyone gets a bit of age-related hearing loss. What that means is that some sounds have to be a bit louder before you can hear them. .. but at the same time, the loudest sound you can tolerate decreases as well.
In a home theater session, guests are often reluctant to complain if everyone else seems to be comfortable with a given playback volume – always ask your guests if the playback volume is too loud or if they are comfortable with it. – A.L.