by Pat Brown
Pat Brown show how he uses tall microphone stands for system tuning.
One of the most valuable tools for sound system tuning is a tall microphone stand. This allows you to get the measurement microphone away from the seating plane. Why?
The Floor-Bounce Effect – FBE
The first reflection to show up on the measured impulse response of a loudspeaker is usually the reflection from the floor. This produces comb filtering in the measured response that is position-dependent over the listening plane (Fig. 1). See my article from 2011 for some measurements.
Figure 1 – The Floor-Bounce Effect
1. Since the source-to-listener distance is different at every seat, the time offset between the direct sound and floor reflection will be different at every seat. This makes the impulse response and resultant frequency response magnitude different at every seat. See the problem? If you include the floor reflection in the response, you will be applying way too many EQ filters. I dubbed this “The Floor-Bounce Effect – FBE” a number of years ago.
2. We often hear that “The system needs to be re-equalized during the event when the audience is present.” See the previous bullet point. Since an audience is absorptive and acoustically diffuse, when one shows up it is like adding acoustic treatment to the floor plane, essentially taking it out of the equation. If your system tuning did not include the floor plane to start with, you won’t have to compensate the tuning when the audience shows up.
3. Intuition suggests that the measurement mic should be at ear height. But putting it there introduces all sorts of non-equalizable artifacts into the measured response. By putting the mic well above the floor plane, you are equalizing the sound that will eventually strike the audience while it is still in a free field. A big part of the system tuning process is to “Only observe what you can correct” when making measurements.
4. Loudspeaker corrective equalization, and especially the generation of FIR filters, require that the anechoic (no room reflections) response of the loudspeaker be measured. This in turn requires a large reflection-free zone around the loudspeaker and microphone. You will never get there with the mic on a stand at ear-height.
Solutions to the FBE Problem
There are two ways to eliminate the FBE. These include the ground plane technique, or the use of a tall microphone stand for making measurements (Fig. 2)
Figure 2 – FBE solutions
If the room has a smooth, hard floor, place the mic in a ground plane configuration (Fig. 3). This removes the FBE and adds 6 dB of gain to the measured response.
Figure 3 – The ground plane technique. The microphone is on the floor near center court. The loudspeakers are lined up and ready for checkout.
If the floor plane is flat with carpet, place a reflective surface below the microphone. On a recent project we setup a station to test 72 line array boxes before they were flown. A piece of masonite in a large aisle, along with a multi-time windowed measurement gave us an essentially anechoic response at all but the lowest octaves (Fig. 4).
ground plane technique.
If the floor doesn’t meet the criteria for ground plane measurements, then get the microphone away from it. This works when there are seats, stairs or whatever on the floor plane that make it an irregular surface. This is my “go to” method and I always take a tall microphone stand along on system tuning projects.
Here’s a run down on the tall mic stands that I keep in my equipment pool. They are pictured below. These are actually lighting stands, so you will need thread adapters for your microphone clip.
Tall Microphone Stands
Figure 5 shows the model numbers and lengths of the stands. See Below.
A – Manfrotto MA5001B with Manfrotto MA122B extension. This one travels well, since it breaks down. Total height is about 10 ft. John Murray turned me on to this unit at the OptEQ workshop.
B – Manfrotto MA1004BAC. This 12 ft stand can be purchased in a 3-pack with carry bag. They snap together for transit.
C – Manfrotto MA269HDBU with extension. This stand is 24 ft, plus another 6 ft from the extension that I made from an old stand. It fits perfectly in an Elmo projection screen bag for transport.
D – Brenda holding the 6 foot extension.
E – T-Lock™ Stealth TeleScoping Pole with mic stand thread adapter. This aluminum pole extends to 38 ft. It is perfect for doing box-by-box testing of installed loudspeaker systems, such as line arrays. The same company makes carbon fiber poles that extend up to 100 ft.
The use of a tall microphone stand allows you to make measurements suitable for adjusting equalizers. It eliminates a major artifact that is seat-specific, and therefore not subject to equalization. By limiting what you observe on the analyzer to what you can correct, you will get better equalization results in a fraction of the site time. pb
Figure 5 – Tall mic stand line-up