So our filters are centered at exactly the same frequencies as before, but because we have set their bandwidths to be extremely narrow, we’re now only cutting the feedback points, and cutting very little of the sounds above and below.
Below are the results again, side by side, for comparison.
This improves GBF nicely and has little effect on the sound quality. This is a dramatic improvement over using medium or broad filters.
This technique requires practice. Cutting feedback frequencies with very exacting, narrow filters is a bit tedious at first, but is well worth the effort and gets easier with experience.
This practice of using parametric EQ to increase GBF is not specific to the choir example—it applies to most sound reinforcement miking applications. If there is also a need for artistic, shaping EQ adjustments, the EQ setting then becomes a careful and complex compromise—a balance of taste and required GBF.
Any microphone in a sound reinforcement application will feedback if its gain is raised high enough. We can use proper microphone selection, directivity and placement to maximize gain-before-feedback. In some cases, we still desire more.
Once the physical arrangement is optimized, narrow EQ filters (essentially “notch filters”), achievable with parametric equalizers, can be very effective in further increasing gain-before-feedback without significantly degrading the sound quality.
If the mixing console has fully parametric EQ on its channel strips, great! Just practice. If it does not, external parametric equalization may be inserted.
Very narrow cut filters must be centered exactly on the feedback frequency, which requires care to find. A broader EQ filter is not as challenging to center, and as long as it is “close,” it will help reduce feedback.
But it cuts a wider range of frequencies than is actually needed for feedback control, at the cost of reduced sound quality.
Ear training, practice and the use of live visual tools such as an RTA or FFT display can improve one’s ability to quickly identify and cut a specific feedback frequency with an appropriately narrow EQ filter.
For more worship audio tips and techniques visit Sennheiser.com.