During our summer residency at Théâtre Garonne we have had the occasion to test our front-fills on a ground level stage. These little speakers have a coaxial design with a wide coverage angle of 120°. When they are put on the ground there are important reflections bouncing on the floor that can colour or interfere with the original signal.
The speakers used here are KS-Audio CPD04; small 15cm cubes with a 4.5 inch and a 1 inch speakers. They are self-powered with class-D biamplification (50W + 25W) and a FIR digital crossover. The coverage angle advertised is 120° conical.
Here is a series of measurements with the reference microphone at different distances from the speaker: 1m, 1st row of seats (roughly 2.5m) and 5th row of seats (5m). Some meeasurements were done with fabric placed on the floor in front of the speaker to absorb some of the first reflections.
When the speaker was on a microphone stand we only measure without the fabric with the microphone a 1m.
Here is to start the impulse response at 1m with and without the fabric.
Same with the microphone placed in the 1st row of seats:
And now in the 5th row of seats:
If we look at the measurements changing the distance for a fixed speaker height, with and without the fabric.
Speaker laying on the ground (speaker axis at 10cm off the ground):
Speaker slightly off the ground (axis at 17cm):
Speaker on a microphone stand (axis at 1.20m):
We see the the fabric does indeed absorb some of the first reflections on the floor. Depending on the placement of the speaker in the venue we also get some reflections off the side walls and cieling. We used some fabric on top and on the side of the speaker to track where each reflections in the impulse response bounced.
We can see that the spectrum is more even with the fabric on the floor. The dips also shift depending on the position of the microphone. The extra distance traveled by the reflected wave depends on the angle of incidence.
We can also see that the spectrum changes depending on the height of the speaker for the same reason. Interference is greater when the speaker is a little off the floor than resting on the ground because the first dip is at a lower frequency.
The best option for a ground level stage (not raised) seems to be when the speaker is resting on the ground with some absorbing fabric in front to dampen some of the reflections.