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Assessing Room Correction Products: Dirac And Acourate

Examining the reality of tools designed to counter low frequency acoustic problems with equalization or processing.

The best way to deal with acoustic problems in a home studio or professional control room is with bass traps and other acoustic treatment. All rooms have peaks and deep nulls, and their amount and frequency changes radically with position. Even at very low frequencies, below 80 Hz, the response can vary as much as 12 dB or even more over a span of just a few inches.

Besides a skewed response, all rooms also suffer from modal ringing at the peak frequencies. This causes those frequencies to continue after the source sound stops, then decay over time. Unlike reverb that extends all frequencies more or less evenly, modal ringing extends only those frequencies that are related to the room dimensions.

The additional energy that lingers makes musical notes at (or near) those frequencies even more prominent, exaggerating the peaks that are already elevated. Modal ringing also harms music clarity because multiple bass notes sound at the same time. If you’ve ever played chords in the low register of a piano, you know they sound much less clear than the same chords played above middle C.

When bass traps are added to a room, all three of these problems are reduced. Peaks come down in level, nulls are raised up, and modal ringing decay times are shortened. Nulls are especially problematic because they can be very deep — 30 dB or even more is common. So raising a null by 20 dB, which is typical after adding bass traps, makes a very big improvement. Thin, weak bass suddenly becomes rich and full sounding.

Bass traps also reduces the response variation around the room. But good bass traps are large, and you need quite a few of them for a home size room to sound clear and be accurate enough for professional mixing. Likewise for home theaters and hi-fi listening rooms — the audible benefit of bass traps is undeniable, but the appearance of a dozen or more traps might be unacceptable.

If you build it they will come

It would be great if there was a way to counter low frequency acoustic problems with an equalizer or similar small device. Enter capitalism.

The first product I’m aware of that claimed to correct room acoustic problems was the original Audyssey MultEQ system. This very expensive device claims to flatten the response and reduce ringing over a usably large area of the room.

In a typical home theater that could be front and rear rows having three or four seats each. But basic physics dictates that the more boost or cut you apply to flatten the response, the smaller the physical area that’s improved. An adult human’s ears are about seven inches apart, and EQ can’t even deliver a flat response to both ears at the same time.

Further, not only are other locations improved less, many of them are made worse! If you raise 70 Hz by 10 dB to counter a null at the prime seat, a peak near that frequency elsewhere in the room becomes even larger.

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