Amp-Stage Coupling Experience

[billfitzmaurice]

Sorry. About 10 feet.

That places your speakers about 8 feet from the ceiling. That's a quarter-wavelength at 31 Hz, below where cancellation is an issue. But...

the "wall" behind the stage was a garage-door style opening which was open the whole time.

That's where your problems came from. With no wall close behind you lost 6dB of boundary reinforcement, and that's the equivalent of cutting your available power by 75%.

 

[Bob Lee (QSC)]

Sorry. About 10 feet.

You might've also been experiencing the phenomena of room modes. A room 25 feet wide would tend to exhibit an axial resonant mode at about 22.6 Hz and its integer multiples--45.2 Hz, 90 Hz, etc. A 10 ft distance between floor and ceiling would cause a mode at about 56.5 Hz and its harmonics. When there is a resonant mode, there will be locations in the room where the SPL of the affected frequency is abnormally high due to the sound waves reinforcing each other, and there will also be spots where the waves cancel, causing possibly large dips in SPL. Here's a pretty good explanation: http://en.wikipedia.org/wiki/Resonant_room_modes

This phenomenon comes up sometimes in recording control rooms, where the producer might be sitting behind the engineer and asks for more bottom end, while the engineer is wondering, "Is he nuts? The bass is killing me!"

 

[greenboy]

You might've also been experiencing the phenomena of room modes. {...}

This phenomenon comes up sometimes in recording control rooms, where the producer might be sitting behind the engineer and asks for more bottom end, while the engineer is wondering, "Is he nuts? The bass is killing me!"

Actually it comes up everywhere and all the time. That's why as a bassist I don't waste time tweaking on stage for "trouble frequencies" - I can just move over three feet and it's a whole different thing anyway, and the guitarist might be hearing a whole different thing anyway! Or I can move my rig slightly. About all I ever do is vary my bass shelving at the start of a gig to fit the genral requirements of the stage mix.

And a s a soundman, I hate to see bassists wasting a bunch of time tweaking and looking frustrated. Just get your rig and EQ skills together, understand the difference between mechanical stage coupling and acoustic coupling, and either understand rig placement or leave it to me. That's just for stage sound anyway.

For the house, anyone who isn't actually in it can't appreciate or understand what's actually out in it, and that's a different story. There are all kind of room mode interactions there too, and all a soundman can really do is go for a good average response at his location and using educated guess for the dancefloor. In the case of dance clubs, you mix for the dance floor really - and even then, if you were dancing you could be hearing different bass stuff going on just by dancing a few feet this way or that, depending on subwoofer placement, how close to stage bass rig, how close to walls, etc.