A common request I get from engineers is to help solve the problem of bass signal phase cancellation in a live venue. This problem is caused by the bass amp (stage volume) mixing with the bass signal in the sound system, which, depending on where you are sitting, will either cancel out or be amplified.
Because of the enormous power required to generate bass, the bass amp on stage will often be three to six times greater than the power employed by the electric guitar. For example, the Ampeg SVT is 300 watts, while a Marshall 50-watt half stack can easily keep up. Once powerful bass energy is generated, it travels further and of course sends out a wider dispersion pattern, particularly at lower frequencies.
Because the two sound sources are positioned differently with respect to the FOH mix position, they arrive at a different time and therefore become out of phase. This not only happens at the fundamentals, but also at the harmonics, which of course send information to the brain such as slap, tone, and localization. As the bass level coming off the stage increases, the resulting effect (called comb-filtering) makes mixing the sound all the more difficult. Unfortunately, when the stage volume exceeds the sound engineer’s ability to control the signal, the bass is often turned off (muted) in the FOH mix and bass definition is lost for most listeners.
As it is impossible to “solve” all of the phase problems in a room (due to reflections off walls and ceilings and seating position), the intent is to at least provide the mix position with the best sound possible so that the end mix is balanced. This is where a phase correction tool comes in.
A typical stage set-up is as follows: the bass connects to a DI box, which feeds the PA. The bass signal then goes to the stage amp. As sound travelling from the DI to FOH will travel at speeds approaching light, it will arrive, be mixed, and sent to the PA almost instantaneously. This signal will reach the sound engineer’s mix position based on how far he or she is stationed away from the PA system. Sound travels through the air at 1,130 ft. per second. Let’s assume the mix position is 50 ft. away from the PA. This means that the sound will arrive in about 40 milliseconds.
The sound from the bass amp will also be generated almost instantaneously, but since the bass amp is positioned at the back of the stage, well behind the loudspeakers, the bass signal will arrive 10 or 15 milliseconds later. By phase-adjusting the signal going into the PA system, we can time-align the two signals so that it sounds better; they will be in phase. Bass is like the foundation of a house. Fix the bass and all of a sudden, everything gets easier to manage.
Peter Janis is the President of Radial Engineering, the Port Coquitlam, BC-based manufacturer of music and audio equipment, including the Radial Phazer, offering variable phase control. Visit www.radialeng.com for more information.