SPS Post 1: Equalization

Pat Brown: The Bigger Picture on Equalization

In setting up a sound system, there is tuning and optimization to think about. The precision of the design and implementation of each element of a P.A. is critical. Shaping the sound is of utmost importance.

Typically, when using equalization on sound systems, third-octave "continuous" equalizers are used to create a flat frequency response. A signal, usually pink noise, is run through the system, while a measurement microphone is placed "on-axis", at the best spot or critical distance, while a RTA (real-time analyzer) is used to view frequencies that need to be cut or boosted.

The problem with this process is that not everyone in the house will be at that sweet spot, on-axis and in the direct field of the sound sources. Therefore, the author suggests changing the approach to equalization on a sound system.

One suggestion is to take the "common denominator", which means taking measurements at various locations in the house and averaging out the EQ curves. However, with the microphone being placed at different positions, the sound field changes, and the balance between sound arriving directly from the source and from the reverberant field is changed with each measurement. The addition or subtraction of reflections in each measurement must be "excluded" before taking the average. This makes the "spacial average" technique difficult to put into practice, since frequencies, especially the mids-to-highs, will be randomly affected by the position in the house.

Another suggestion Pat Brown has is to consider the fact that sound radiates outwards 3-dimensionally. The "3-dimensional radiation balloon" needs to be studied to avoid issues with off-axis seats in the house. He also suggests using "precision signal delay" or leaving notches or dips at certain frequencies "simply ignored", and therefore dealing with phase or time relationships instead of using EQ filters.

Even though the first thing to do with tuning a system is equalization on the direct field, boosting or cutting a frequency simply based on the measurement taken on-axis in the direct field may cause problems to be aggravated at other off-axis listening positions, thus much consideration needs to be taken.

Even before thinking about using EQ to fix issues, careful, precise measurements have to be made such that the loudspeakers are placed in the right spot for the room.

Pat Brown recommends that technicians look at the data file that already has been created by the CLF (Common Loudspeaker Format) group, which will allow one to calculate the average frequency response over a "range of angles" even before setting up and measuring the pink noise signal. Since the direct field does not really depend on the way the room is, the data found on the site (www.clfgroup.org) can be used to get the "common denominator" curve for the direct field, as mentioned above in his first suggestion.

With the second method of looking at the 3-D balloon, the distance between elements needs to be taken into consideration, and the delay created by that positioning. The ballon needs to be "re-shaped" if there are multiple sound sources.

Pat Brown reminds us that humans tend to pick up on frequencies that are too loud, rather than missing frequencies, and thus suggests that technicians avoid over-boosting frequencies. Also, if boosts are avoided when equalizing based on the on-axis measurement, technicians can be sure that there will not be peaks at off-axis positions that will be further worsened.

In conclusion, technicians must consider off-axis locations, avoid boosting when equalizing, and make planning and precise calculation part of the process. Depending on how well the elements in the system match up with their specifications, fine tuning based on observation and using our ears, on top of all the tools we have, is important. DSPs with settings based on CLF data and auto-RTAs can be used, but in the end, it is for the technician to listen and think to make decision that will give the best possible response to every listener in the house.

I feel that the point here is that though there are practices that have become set standards over the years, engineers and technicians should always think, observe and decide, and not just follow the "normal" way of doing things.