Room EQ in the Time Domain - DEQ2496

People often say the reason not to use EQ for fixing room issues is that it doesn't fix the ringing of a resonance through time, only the amplitude. This isn't the case. A standard normal EQ (parametric preferably) works in the amplitude and time domains (AKA phase).

You don't need expensive room EQ systems to work in the time domain. When you make a notch on a normal parametric EQ, what this basically does is set up a resonance in an electric circuit. These resonances obey much the same laws of physics as acoustical ones; the resonance occurs both in amplitude and in time.. So when you make a notch on an EQ unit that is equal but opposite in amplitude to the resonant peak in your room, what you get is that the resonance in the EQ circuit and that in the room cancel each other out. This not only gives a flat frequency response, but also the ringing of the resonances though the time domain also are opposite and cancel.

In practice acoustic resonances in room tend to be a bit more complex than the one in the EQ unit, but it still works well and the suggestion that only physical treamtents can aid the decay rate of a room mode is false.

Below is a real world example just taken from my room today. Note the resonance at 40Hz and the improvement of the decay.

This is without EQ.



This is after EQ.



You can also see 3 modes piled up near each other around 60Hz. You could EQ these with a single notch on the centre frequency and a reasonably wide Q. However as you can see they are actually 3 resonances near each other. If you EQ them out with 3 parametric EQs of narrow Q then the time domain will be much improved over just using one general EQ bank. That's why graphic EQ is not ideal for room EQ; you want to identify the individual resonances at play.

 

To expand on this a little, below is a waterfall plot of a normal parametric EQ stuck directly in-line with my measurement system. There is no speakers and room here, just the electrical circuit.

The EQ was set to have a -6dB notch at 100Hz and a +6dB boost at 400Hz. We can see the behaviour of these filters now, and how they are capable of reducing the decay of a room mode beyond the simple 'frequency domain' attenuation. Note that even the negative deductive EQ at 100Hz produces a positive output later in time. This is because of the 'minimum phase' behaviour I mentioned earlier - a set of rules basically that define how this type of filter functions and it is the same behaviour as room resonances (provided you stay in the same listening position).




Clearly, for the 'ringing' of the filter to produce a good opposite match to that of the room resonance the correction will only apply within a certain wavelength of the frequency you are adjusting. Basically that means the higher in frequency you try to correct, the smaller your optimised sweet-spot area will be. For correction up to 200Hz this proves to be an area of about 1m square. This is why such correction will not work well for live music venues or cinemas since people listen from all manner of positions, but ideal for correcting minimum phase errors in fixed systems like loudspeakers or semi-fixed systems like the domestic room/listener combination.