keith: Where do you position measurement mic(s) for system equalization?

John: It depends on the wavelengths being measured. The main idea is to avoid reflected energy, this means relatively close mic’ing most of the time, and there is no one position that accurately displays the entire audio spectrum except in an anechoic chamber. That's why they are used.

Tucci: John, could you discuss the benefits and consequences of a ground plane measurement vs. a regular mic on a stand/floor bounce/comb filter situation.

John: If you put the measurement mic on a stand, even when widowing, comb filtering will contaminate the measurement and you will have a tough time deciding what is non-equalizable (comb filter) and what can be fixed. With a ground plane measurement the strongest contaminant of the measurement has been removed. Now in RTA days, this was OK because the power response was the dominant character, but with TEF, SIMM, SMAART you can't tolerate you want to avoid these comb filters whenever possible.

Tucci: A ground plane measurement could still fall in a null area, correct?

John: If a reflection is very close in level and time to the direct sound, as with a hard surface near the speaker, yes it could. Or are you speaking about room modes?

Tucci: I refer to the nulls present in the room. A ground plane measurement won’t erase that from the measurement, will it? Just the floor bounce would disappear.

John: That's correct. I have been recently using a technique invented by Don Keele in 1974 to do low-frequency equalization. It is the only way to combat room modes without taking the subs out to the parking lot.

keith: Can you contrast low-frequency equalization indoors and outdoors? Specifically, what are the factors at play?

John: Outdoors, you can use a ground plane measurement to EQ subs. Know that you would still have to account for fractional-space loading, which is always present in some form with long wavelengths. Indoors, room modes will dominate and you really can't use a mic at any distance with confidence. This is why many have the misconception that low frequency horns need big rooms to generate the wave. The fact is that the room modes always null the bass at the dance floor and sum where the glasses hang above the bar :>).

Tucci: So what specifically is the technique from Don Keele?