8 Subwoofers
8Subwoofers
- taken from tech note T N99051701 “setting the subwoofers”, by Craig Connelly
8.1I ntroduction
T he topic of setting subwoofer levels is the subject of numerous articles and even more numerous opinions, but of only a single proposed standard (SMPT E RP 200, and I T U 10-11R/T emp/11-E). T here are many reasons for this. First, the use of the modern digital audio subwoofer is unique in comparison to the other channels and it is recorded with a different monitoring reference. T he purpose of the subwoofer has changed with the evolution of cinema sound, from compensating for the poor low frequency response and bass power capability of older screen speakers, to adding power to low frequency effects even in theatres having full range screen speakers and capable amplifiers. Secondly, different post production facilities and even different engineers have followed their own alignment conventions. Finally, equipment and even meter standards are different when comparing the USA, Europe, and elsewhere. T his wordy article is only an overview and is not meant to comprise a specific calibration process or set of instructions.
8.2Set ting the acoustical reference level on the dub stage.
A signal, generally wide band pink noise, is sent through the dubbing console to each of the monitoring system’s screen speakers at the electrical reference level (also known as Dolby Level). Using a real time third octave analyser, the monitor system is adjusted for the preferred acoustical response at each loudspeaker, given the properties of the loudspeaker, the room, the screen, and the “X-curve” of SMPT E
202M. T he electrical gain of each channel in the monitoring system is then adjusted to give an acoustical reference level in the room from each screen speaker.
8.3Setting the subwoofer acoustical gain using a screen speaker as a reference.
T he channel used for subwoofer or LFE (low frequency effects) in the dubbing theatre is then adjusted as above, except that the SPL is measured differently. T o make this adjustment properly, a multi-channel real time analyser must be used. Analyser bands in the flat-response region of a screen speaker (generally, the centre speaker) are taken as a reference. T hese bands will not individually measure 85dB SPL, but will be somewhat less, depending on the bandwidth of the analyser’s bands, typically about
70 dB SPL for a 1/3-octave analyser as required by SMPT E 202M. T his flat-response region, between the low frequency roll off caused by the loudspeaker cabinet and the high frequency roll off due to the screen and X-curve, is referred to as the in-band (acoustical) response of the monitor speaker. T he subwoofer electrical gain is adjusted such that the analyser channels in its in-band region are 10dB greater than those in the screen speaker’s in-band region. T his is referred to as “10dB of in -band gain” (subwoofer level relative to each screen speaker level).