Step 3 – Tuning the System: Balancing the System Crossover Band Levels
** Note: on many types/models of systems the crossover level settings may not be adjustable or even accessible to the user – if this is the case skip this step.
To avoid the excessive use of EQ filters which, when overused, can add unnatural artifacts to the sound, the first step should always be to balance the level relationship between the low, mid and high bands of the system using the crossover levels. To avoid losing overall through level one of the three bands should stay set at 0 dB and the other two bands balanced against that band. Set these band levels to get the raw response as close as possible to an even balance between 100 Hz and 8-10 kHz. Obviously there may still be some bumps in the response as this is still the raw response of the system but balancing the band levels first will dramatically reduce the number of EQ filters required to get the system tuned correctly.
When in doubt about band level settings when there are dips in the response always set the particular band level higher with the broad dips being closer to the reference line as it is better to use the added EQ filters later to pull down the peaks rather than add EQ boosts to fix dips in the response.
Step 4 – Tuning the System: Adding EQ Filters
Once the band levels have been adjusted carefully for the best low, mid and high balanced response from 100 Hz on up it may be necessary to add some EQ filters to correct for the remaining anomalies in the system response. Only ever correct for the largest, most obvious response anomalies! What we are looking for with our tuning setup is that the average response of the system is within approximately +/- 6 dB of the ideal “flat” translated response through the system. A system that is over tuned to be absolutely “ruler flat” just doesn’t sound musical and starts to sound far too clinical and unnatural. Due to the many measurement variables in a large venue, attempting to get a system tuned to “ruler flat” is an unrealistic goal. Moving the measurement microphone 20 feet to the left or right or forward and back will very quickly demonstrate that response variables in some venues can be quite dramatic so looking for a realistic “average” response is the only practical goal!
Use broad parametric EQ filters very sparingly to correct all the most obvious response anomalies through the full range of the system. When using parametric filters it is always better (and more musical!) to use one broad Q filter centered at the peak of the area to be corrected rather than many narrow ones. Small very narrow peaks and dips in the raw system response need to be treated very carefully as they may be very false readings due to floor bounce or cancellations etc. If in doubt about a narrow response anomaly leave it alone but try moving the reference microphone around to confirm if it truly is a real response problem. For the high frequency area of the system it is not a good idea to attempt to boost the high end of a system above 8 kHz – let it roll off at it’s own natural rate.
For a rock music style show the frequency range below 125 Hz (no higher!) can be allowed to step up from the 0 dB point forming a low frequency shelf in the response of approximately +4 to +6 dB. Any major large, wide peaks in the response in this low frequency shelf region need to be adjusted very carefully as due to reflections etc. the FFT measurement reading may be very inaccurate at these low frequencies. It is often better to voice the system or use some known music reference material to adjust this area. This low frequency step up in level must never be allowed to go higher than 100 to 125 Hz as it will create a lack of low mid clarity due to the low frequency overtones falling into the critical 100 Hz to 500 Hz musical instrument frequency summation zone*. (See Explanation Note Below). Using this method is the key to maintaining tight, controlled low mid frequency definition within the mix.
Step 5 – Tuning the System: Final “Voicing” of the System
Once the previous steps have been completed turn off the Pink Noise and, using a well recorded “known” music track, play this track into the mains (only!) of the system at a reasonable level. Looking carefully at the magnitude display on the FFT screen you should see a reasonably flat response above 100 Hz indicating that what is coming out of the console is being reproduced on the system. It may be necessary to slightly modify some of the EQ filters you have added due to the fact that you are now seeing the response of the full L+R main system but again the goal is not “ruler flat” but a good average flat response.
FFT software, including Smaart, is a great tool for tuning sound systems but as it relies on a measurement microphone placed at one position within the listening area for each measurement it does not “hear” the full acoustic picture of the system in the venue nearly as well as the human ear does. So as a double check and to find out any extra anomalies between the system interacting with the venue acoustics use a favorite microphone wired into a completely “flat” (i.e., no EQ or high-pass filter!) input channel on the mixing console, and actually “voice” the system that we have just adjusted. Good ol’ “Check 1, 2s” into the system after making all the previous adjustments will soon let you hear what a system that accurately translates what is coming out of the mixing console should sound like.
For some users new to this tuning method there will be a learning curve! It may all sound a little different to what you may have been used to hearing previously especially if you are not used to hearing really “flat” sound as heard on reference monitors in studio mixing rooms. For this method to work well try not to change your previous settings between 100 Hz to 10 kHz too much but focus on the area below 100 Hz and with the board mic running into the full main system (L+R only – everything else and the subs still muted!) adjust the EQ filters to remove any overtones, room interactions etc. In many cases just a solid medium Q parametric EQ cut at 100 to 125 Hz can make a huge difference to the entire overtone, interaction, definition and tightness of a sound system in a large venue.
Make as small as possible adjustments based on what you are hearing when voicing the system and be sure to check that now the main left and right sides of the system sound really close in tonality by panning back and forth between the left and right of the mains system.
Once you’re happy with the “mains” of the system, it is time to first align the sub bass system(s) to the them and then start repeating these steps for all the other support sections of the system.
*Note: The “Musical Instrument Frequency Summation Zone” is the range of frequencies where most musical instruments naturally put out their strongest volume without any added amplification. Looking at a graph of the frequency range of musical instruments (available in many of the sound engineering handbooks), it can be readily seen that the area where most musical instruments frequency ranges overlap is the range from 100 to 500 Hz. When we create a “mix” of all these various instruments (i.e. overlap them) we get more summation of sound within this frequency range. For this reason and the additional problem that a large sound system will naturally “sum” within a large reverberant room in this SAME frequency range, when tuning a large sound system it is vital we are extremely careful how we treat this low mid area (100 Hz up to 500 Hz) of the overall system frequency response.