niedersteiner writes "This content is brought to you by Audiocourses dot com
As an engineer of live sound, I have often heard performers say, "Can you give me more treble?" but to do so would ruin the mix. Going on stage to listen for myself, I understand their request. There is definitely a thickness in the stage sound.
The challenge is to reduce the on stage bass but not the bass in the mix. Let's analyze the physics involved here to understand what is happening and how to resolve it.
Most portable systems use a split stack as FOH mains. With this setup the bass tends to be strongest between these two stacks. Also PA loudspeakers by design are as directional as practicable, usually with the use of waveguides (horns) or line array variants of some kind. However bass frequencies, because of their long wavelengths, are omni directional. The only high-end the performers hear is what is reverberated from the room and the slight diffraction from the cabinet edges (monitor field notwithstanding). Outdoors, where the reverberant field is virtually non-existent, this effect is even greater.
Since most of my monitors already have their low end rolled off, the natural tendency is to turn them up. However this is not my preference. There is a better solution.
Instead I use a technique which reduces the on-stage bass from split stack mains and also increases bass to the listeners. This octave is usually centered between 80 and 100 Hz.
Acoustically speaking, two in-phase sources, placed 1/2 wavelength from each other will create a null in their line of orientation. In the case of 80 to 100 Hz, 1/2 wavelength is roughly 6½ feet or about 1.9 meters. Assuming the mains are full range, placing a secondary sub this distance from each main loudspeaker on a line towards (or away from) the musicians will create a null along this line. It will also increase the bass perpendicular to the line. Visualize a figure 8 pattern with the line between the subs bisecting the “8”.
Limiting the bandwidth of the secondary sub to somewhat below 150 Hz will prevent the reinforcement of full wavelength frequencies. This can be done with a spare equalizer after the crossover, enabling tailoring of the roll off slope.
By selectively placing the split stack FOH mains and band limited subs in relation to each other and orienting them correctly in relation to the musicians, on-stage bass can be reduced while increasing bass to the listeners. This process can be fine tuned by controlling these subs independently of the mains.
Note: Copyright © 2003 Anton Niedersteiner. All rights reserved.
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