[SAC]

Originally Posted by DanDan

An absorber would need to be quite deep to have any significant effect at 110Hz.
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This may help you find a complementary boost at 110Hz by changes of speaker position.

An absorber would need to be quite deep to have any significant effect at 110Hz…and a complimentary boost and measure tweak and average????

I dare say this snipe hunt is out of control! ;-)

If said issue is a result of superposition, which it might be, for an initial frequency null of 110 Hz, the frequency of the reflection is NOT 110 Hz requiring an absorber effective at 110 Hz!

This is a function of time!

The result of the direct/reflected signal time delay offset manifests itself in frequency, not because the reflection is ‘at’ a particular frequency! And the propagated energy at that frequency is most likely below the Schroeder critical frequency and will most likely manifest itself as modal behavior, unlike the energy above that frequency which will be specular. For specular energy, the math describing the behavior in terms of the initial null frequency and null frequency interval is pretty simple.


The delay can be expressed as a time in seconds or a distance in feet or meters.
The delay predicts the “initial null frequency” (INF) and the null frequency interval (NFI) (e.g.: the distance in Hz between nulls).

INF = 0.5 (c/D)
D = 0.5 (c/INF)
C = {D(INF)} / 0.5
NFI = c / D
D = c / NFI
T = D / c
D = Tc

Where:
C is the velocity of sound on feet or meters
D is the distance in feet or meters of the delay (beyond the direct signal)
T is the time in seconds


Thus, for an initial null frequency of 110 Hz, and a velocity of sound of 1130 feet/second;
110 Hz = .5 (1130 / D)
D. the distance of the signal delay = 1130 / (110/ .5) = 1130 / 220 = 5.13636 ft = 5.1 feet

The delay in time is = D / c = 5.13636 ft / 1130 ft/sec = .004545 sec = .0045 = 4.5 msec


And the null frequency interval would be:
NFI = c / D
= 1130 / 5.13636 ft
= 220 Hz

Assuming the null is caused by a reflection, you simply need a broadband absorber capable of mitigating a simple reflection delayed by 4.5 msec that travels an additional 5.1 feet further than the direct signal.

An initial ETC would have verified the existence of such a reflection, as well as a simple and quick verification of the precise path (for which a vertically mounted absorber would work just as well as a horizontal absorber laid precariously on the floor!). And it would have provided ALL of this directly without any “high end maths”(sic) and the path quickly confirmed as well without knowing ANY of the additional information gathered in a very roundabout manner.

If indeed the cause is a reflection ! Which the ETC would have also confirmed or denied! And if not, pursuit of a modal solution would most definitely be in order.

Witness the perils of living solely in the frequency domain!