Headroom in the LFE channelIf the LFE channel is aligned at the same level as the main channels then a loud explosion effect could easily overload the tape (or hard disk) recording machine. If this were the case then this would defeat the object of having a special channel for loud sound effects. To overcome this problem the LFE channel is nominally recorded to tape 10 dB lower than the main channels so that there is 10 dB of extra level (headroom) available on the tape. Note that this increase in headroom is at expense of 10 dB of signal-to-noise ratio but as the LFE channel is eventually band limited in the encoder it is a price worth paying.
So how is this achieved in practice in the studio?The film industry has been using the following channel alignment technique for many years:
The monitor output of the LFE channel from the mixing desk includes a +10 dB level change before the input to the studio monitoring system. Note that there are NO level changes on any of the outputs to the tape or hard disk recorder. The effect is that the LFE channel output sent to tape is naturally mixed at a nominal level 10 dB lower when compared to the main channels. In other words, the engineer hears an even balance in the studio on all the channels but records the LFE channel at a nominal -10 dB compared to the main channels on the tape recorder. This gives 10 dB headroom on the LFE channel compared to the main channels and so allows for the big bangs and explosions commonly found in action movie productions to still stay below the 0 dBFS on the recorder without distortion.
How is the sound realigned in the home because the LFE channel level is now 10 dB down compared to the main channels?All Dolby Digital and DTS consumers and professionals 5.1 processors automatically add 10 dB to the LFE channel to restore the level balance between the main channels and the LFE channel as was originally heard in the studio.
So how do I align my monitors in the studio or the home?The subwoofer sound output level (at subwoofer frequencies) must be the same as the sound output level compared to the main loudspeaker system (at other frequencies). The LFE output of the processor should be connected to the LFE input of the subwoofer. For correct reproduction, this LFE input has the same sensitivity as all other signal inputs. This works correctly because the +10 dB level change is always implemented inside the mixing desk or surround sound processor.
The acoustical performance of the main channels should have a flat frequency response for accurate monitoring. This is achieved by calibrating the main channel frequency responses as described in the FAQ: “How do I align the levels of a 5.1 system (using pink noise as a test signal)?”
Some medium format mixing consoles and many smaller consoles do not have the facility to apply the +10 dB gain to the LFE. To overcome this limitation the 7050B, 7060B, 7070A, 7071A and 7073A have a +10 dB LFE gain DIP switch that can be used to give the LFE channel the +10 dB gain that is required for correct reproduction. However, be sure not to have two +10 dB gains in the LFE channel, for example, when listening to a decoder output which already has the gain stage automatically applied.
The block diagram below shows the whole production chain: