Stage Or Floor Coupling? The basics
The key point to understand regarding coupling is that there are two forms - mechanical coupling and acoustic coupling.
With mechanical coupling the stage or floor is excited by vibrations from the cabinet or by the acoustic output from the speakers / port. When this happens the floor itself (or stage) becomes a resonating surface, a bit like a massive drum skin or giant speaker cone. The acoustic output of the resonating floor is very unpredictable, centred around the modes of resonance of the floor. This almost always results in a 'boomy' onstage sound and unpredictable offstage sound (depending on how it reinforces and cancels the output from the speaker cabinet).
It is therefore wise to avoid mechanical coupling. This can be done by:
- Using a well braced speaker cabinet which vibrates less.
- Isolating the speaker cabinet by reducing the amount of physical contact with the floor (e.g. isolating foam, tilting the cab, raising it onto a beer crate).
- Placing the cab on a more solid part of the stage, for instance the corner.
- If you are on a small hollow stage see if it's possible to place the cab offstage - also make sure the PA subs are not on the stage (any sound engineer with a clue wont have done this).
This is when the direct output and reflected output from the speaker combine to give increased output. Low frequency output from any speaker is omnidirectional, which means your speaker is sending low frequencies not just to the front but also both sides, upwards backwards, downwards, north, south, yonder, to the post office, past the post office and to the local shop! Picture a sphere of acoustic energy surrounding the cabinet with the cabinet in at the centre.
If you place your cab on the floor then the bottom half of that sphere is blocked off and it's energy is reflected by the floor, combining with the upper half of the sphere. This can increase your low frequency output by ~3dB - that's equivalent to going from a 100W amplifier to a 200W amplifier. As you can see, this kind of coupling is a really good thing!
So what happens if you put your cab on a beer crate so you can hear it better and/or to stop some boomy sounding mechanical coupling with the stage - does that mean you lose all of that 6dB of acoustic LF coupling? Fortunately the answer is no - what happens is you still get the 3dB gain but the cut-off frequency goes down, so rather than you getting the gain below about 250Hz you might only get it below 100Hz.