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Old 28th June 2020
Gear Head
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I believe what you are measuring is a null (or at least a non-peak amplitude) of a particular frequency.

If you locate at 2 positions in the room relative to the peak and trough of a resonant frequency, the amplitudes, on your SPL meter/ software, will be the same and maximal at both locations- but not opposite in magnitude. The measurement device rectifies the input and averages it over time.

If you are located at a node, the amplitude will be minimal, appearing as a ‘dip’ relative to other positions or frequencies. The important point is that, at a node there is an absence of amplitude/ sound pressure. You need to determine why you have this behaviour at that location.

If it is a result of normal modal resonance, the solution is to re-locate your mix position, not add absorption (note, I’m not talking about the situation of coincident modes, causing increased amplitude, in which case, tuned absorption would be appropriate)

If the issue is a result of reflected bass frequencies and destructive interference, the solution is to apply broadband absorption to the reflecting surface(s).

A reasonable first step is to determine if the problematic frequency is one of your room modes. There are online calculators for this. I would look at axial modes first. But beware, most calculators assume a rectangular room. If you have parallel boundaries though, the math could be valid.

Reflections will cause constructive and destructive interference, meaning that you will see boosting and attenuation of different frequencies at different locations - far from a flat line. So, using broadband absorption is common.

I prefer membrane absorbers (limp mass type), rather than porous absorption.

Reflected bass frequencies are actually a very common problem, and the reason why the rear wall is usually loaded with broadband bass absorption.

Ethan Weiner has a lot of great information about this problem on his Realtraps website.