How do I align the levels of a 5.1 system (using pink noise as a test signal)?
The main goal in the alignment of a 5.1 system is to achieve that the subwoofer sound output level (within the band limited subwoofer frequencies) is the same as the sound output level compared to the main loudspeaker system (at other frequencies). The LFE output of the mixing desk or decoder should be connected to the LFE input on the subwoofer.
- 1092A and 1094A(discontinued)
For correct reproduction, the LFE input has the same sensitivity as all other signal inputs. This works correctly however the +10dB level change must be implemented inside the mixing desk or surround sound processor (actually it is the bass management part of the processor that does this), as described in FAQ: In a Dolby Digital or DTS 5.1 system should subwoofer level be aligned up or down by 10 dB?
- 7050B, 7060B, 7070A, 7071A, 7073A
For correct reproduction, the LFE input has the same sensitivity as all other signal inputs, unless the LFE +10 dB DIP switch is ON. The user needs to decide whether or not to use this switch depending on the application. The switch is used when there is no +10 dB gain in the LFE channel (e.g. straight out of the mixing console). The switch is NOT used when there is already a +10 dB gain in the LFE channel (e.g. when using a surround sound decoder, external bass management system, etc).
What Reference Level?
To ensure and achieve repeatable results in the finished production, the SMPTE (Society of Motion Pictures and Television) organization has developed standard monitoring levels for cinema post-production work. For film mixing, the SMPTE acoustical reference is 85 dB SPL, read with a sound pressure level meter (SPL) set on C weighted/Slow scale. The electrical reference required to achieve this acoustical output is -20 dBFSrms full bandwidth pink noise. SMPTE RP200 uses an electrical reference level of -18 dBFSrms.
For release of film material on television various standards state that the operating mixing level should be somewhat lower so that low-level dialogues, which are easily heard in a quiet and acoustically well treated control room, are mixed slightly higher. This is to ensure that in the home environment with higher background noise levels the dialogue is clearly heard. However, for music mixes, there are no standardized levels - as for stereo - because the level that the engineer chooses is arbitrary and based on personal taste, as is the level chosen by the end user.
Thus one absolute reference level does not really apply for all 5.1 surround sound applications, so the following alignment procedures do not mention specific values but rather the relative levels between the various speakers.
Calibration of the Level and Frequency Response with an MLS Signal
First, there is no point in setting the channel levels until the monitors have been calibrated for their individual frequency responses. Furthermore, there is no point in calibrating the frequency responses using the acoustic tone controls if there are fundamental acoustical problems in the room. Level setting is the last setting to be made once all the other issues have been resolved.
The acoustical performance of the main channels and the subwoofer should have a flat frequency response for accurate monitoring. This is achieved by doing the following:
- Calibrate the 5 main channel frequency responses using an MLS or similar measuring system with the subwoofer bypassed or disconnected.
- Then connect the GENELEC subwoofer as described in the supplied operating manual and adjust the subwoofer level, bass roll-off and phase controls (relative to the centre channel) so that the measured frequency response of the subwoofer AND near field monitor is extended smoothly down to the LF cutoff of the subwoofer
Alternative Level Calibration Methods
If MLS type equipment is not available for aligning the system then follow the guidelines that can be found in the operating manual for adjusting the frequency response. Remember though that there is still the need to align the system for level. Below are listed two alternative methods but one should note here that the accuracy of these methods depends greatly on the quality and the LF response of the SPL meter.
- Level Calibration using a 1/3 Octave Real Time Analyser, Broadband Pink Noise and an SPL Meter
Connect the Genelec 5.1 system and play broadband pink noise signal (20Hz - 20kHz) through the subwoofer and one of the main channels, for example, the centre channel. Set the level of each band on the RTA Analyser to read the same value, both in the subwoofer bandwidth AND in the main monitor bandwidth. The specific absolute reference level depends on your application area as mentioned earlier and can be checked using the sound pressure level meter. Play pink noise through the whole system and adjust the level of each channel on the mixing desk to give the same acoustical level in the room. The level depends on the level of the signal but if it is set to SMPTE RP200's -18 dBFSrms (-20 dBFSrms for SMPTE) the SPL meter should read 85 dB for cinema/theatre work, 80-85 dB for television and 85-95 dB for music.
- Level Calibration using Filtered Pink Noise and an SPL Meter
You need to have filtered pink noise to calibrate the levels of the subwoofer and the main channels. You can use a copy of the TMH Corporation 'Multichannel Studio Test Tape' that includes the various test signals required. The essential point is that you need two types of test signal:
- Filtered pink noise from 500Hz to 2kHz for the mid band frequencies of the main monitors. This range is well away from the subwoofer's bandwidth and also suffers less from room cancellation effects.
- Filtered pink noise from 20Hz to 80Hz, to calibrate the subwoofer level. Note: If the recorded level of both the filtered pink noises is -18 dBFSrms for SMPTE RP200's (-20 dBFSrms for SMPTE) then an absolute level calibration can be made according to your application area, for example, 83 dB SPL for theatrical work, 78-83 dB SPL for television and, typically, 83-93 dB for music. This level is 2 dB lower than for broadband pink noise as there will be less energy in the room due to the limited bandwidth signal.
- Connect the Genelec 5.1 system then play the 500Hz to 2kHz filtered pink noise to adjust each of the main channels individually. Set the SPL Meter to C-weighting & Slow scale, and note the reading (say it is 83 dB SPL). All 5 channels have to be adjusted to this same level.
- Next play the 20 to 80Hz filtered pink noise through the subwoofer. The correct adjustment should give a reading 3dB lower than the one for the main channel monitors (in our example, 80 dB SPL). The reason for the difference in level reading is that most SPL meters have a built in HP filter. If there is no HP filter in the SPL meter then the reading should be the same as for the main monitors as the bandwidth, and hence energy in the room, of the two signals is the same.