Acoustics Experiment 1 – Choir Foldback

I’ve known our main building to be tricky from a mixing/acoustics perspective, even if the usual ‘reverb tail’ people associate with difficult acoustics is actually quite short at just under 2 seconds (based on measurements taken by a Bose dealer some years ago).

I’ve long thought that this building is unusual in that there’s really not much sound absorption or diffusion going on between the ‘stage’/chancel area and the congregation/audience areas.

Our sound mix position is now at the back of the church tucked in just in front and to the side of the main doors, so it’s about 15m from the action on the chancel. I’ve noted that singers sound better from then mix position when they’re running without monitors, so I did some quick and dirty measurements with pink-noise generated by the desk and my iPhone to see what was going on.

I set up the choir monitors in their usual position relative to the choir, and took these measurements:

Mid-field (front row of choir, as heard by choir member)
Sound mix position

Now, the boxes are small Ramsa single-driver boxes mounted on mic stands (without booms) to get them up off the floor and closer to the ears that need to hear them. They have some EQ and HPF applied to roll off from 150Hz down (4-inch drivers tend to struggle with low frequencies at reasonable volume!) and to gently shape their sound so that they punch above the ambient stage-noise without causing feedback issues. Typical output levels would be expected to be between 60 and 85dB – they top out at around 95dB (@1m in their intended location) whatever their specs might suggest.

What the measurements show is that their on-axis response is far from linear, though in use they’re just about ‘good enough’ for what we need. Their off-axis response is much reduced at higher frequencies. Over the 15m distance between them (turned away from the mix position towards the choir) and our mix position, we’re only losing around 7dB overall sound pressure level (SPL, A weighted). That seems rather less than I would like. Of that loss, most of it is from 3KHz upwards – but we lose much less at lower frequencies, which i believe explains the boxiness they add to the vocal sound when driven alongside our main sound system.

It’s worth noting too that the RTA app has picked up a lot of energy below 100Hz, which I was neither hearing nor expecting from such small boxes set up the way they were. I’d suggest this inaccuracy is built into the design of the measuring device and the software algorithms, as well as possible noise due to hand-held operation.

Maybe my theory is out of whack, but above 100Hz or at higher SPL’s the measurements certainly tally with my experience.

Anyone else done similar measurements of their systems/buildings that i could compare this to?

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