This may be either a bit academic or obvious to most of you.
I didn't want to hijack the other thread, but I am thinking about a comment in the Flat Omnis thread.
Flat omnis? Quote:
Originally Posted by mljung  The sound was not "blown up" as I find easily happens because of low+lo-mid build up from traditional pressure omnis. |
OK, I have seen occasional references to this phenomenon, and I have experienced it myself. But I don't completely understand exactly why this happens from a physical acoustics standpoint.
Case in point. I recently did a recording of a solo piano (NY Steinway D) in a decent hall, pair of MK2H, 2m out, high enough to avoid lid reflections, spacing 36cm. I had terrible problems with low-mid buildup.
But why exactly? I know that 36cm is 1/4 wavelength of 238Hz, which is roughly the center frequency of the problem area. It seems to consist of some odd phasiness, not just boosted frequency response. This is consistent with a 90-degree phase shift at 1/4 wavelength.
Now, to experiment I tried changing the spacing. Adding just 4cm changed the buildup frequency noticably. But even at 50cm is was still rather noticable. I didn't have time to compare larger spacings.
I have to conclude that this is a direct result of sidewall reflections. But this is a large hall. The sidewalls are at least 10m away. I experimented with using choir shell panels to divert sidewall reflections, but didn't have enough time to prove anything.
I can't believe it is simply related to the low frequency response of omnis compared to cardis. Most cardiods don't have much of any noticable frequency drop at 238Hz.
Does anyone have a handle on exactly why this happens?
Nathan