Study Hall

Up Periscope: Using A Probe When You’re Not In The Room

We should always do whatever possible to work with event management to not let this happen, but the reality is that it does – here's an approach to deal with the challenge.

Sometimes we’re forced to use a mix position that’s not out among the audience of the event. In my career, I’ve been stationed behind the stage, to the side, in a balcony, and even in a different room. We should always do whatever possible to work with event management to not let this happen, but the reality is that it does.

If possible, during the event I have another crew member walk around the audience and report back to me if it’s loud enough. If there’s a dedicated A2 on the team, we can go a bit deeper into the analysis, such as, is it intelligible? Too dynamic? Too sibilant or harsh?

I’m referring to situations with an individual talker or singer because most of the time the PA system is designed to be intelligible and have minimal variance while acting as a carrier for the program material. It also doesn’t mean that we don’t have to tweak input settings for a specific person’s voice or instrument.

If I’m on a job where I’m not in the intended coverage zone of the audience during the event, and I don’t have a helper with me, I usually send out a probe to check atmospheric pressure, inert gas levels, radiation levels, barometric pressure, and O2 levels… Just kidding. The “probe” is actually a calibrated measurement microphone capturing sound pressure level (SPL) data.

If possible, I place the mic somewhere in the audience area, preferably on-axis of a PA source. This runs to my interface and I’ll touch on what I’m using for analysis tools later on. First, let’s briefly look at how one mic in an audience plane can clue me in to data over the entire room.

Laying It Out

Almost every system I design follows the “minimum variance” principle. If I can achieve it, I can place the mic strategically for show time and know what’s happening anywhere in the audience.

A recent corporate lecture project (four days of seminars) had me mixing from a side balcony in a square ballroom, with coverage provided by two loudspeakers (Figure 1). The room measured 46.5 x 46.5 feet (14.17 square meters), and by using a loudspeaker placement calculator by Dan Lundberg, I was able to place the loudspeakers so there was only 1.7 dB of overlap onto the walls.

Figure 1.

With the loudspeakers placed 28.31 feet (8.63 meters) apart, the room was covered to a maximum of 46.5 feet (14.17 meters) and the “unity line” that exists halfway down the loudspeaker’s coverage was neatly placed halfway down the room (Figure 2).

Figure 2.

The system was designed to lose a maximum of 6 dB of SPL from the middle of the room to the back. Since it was indoors, it only dropped about 3 dB. If I could place the measurement mic halfway back, on-axis (if possible), I would know that it’s 3 dB down at the back wall.

Each color change in Figure 3 represents a change of 3 dB, so we can see that because of the room gain (presence of walls) there’s only one color change from the mic position to the back wall (at 8 kHz).

Figure 3.

From the mid-point to the front of the room was a little more difficult. First, no one was standing directly in front of the loudspeaker – the first chair at the front table is a few feet away.

To calculate the drop from the first row to the measurement mic’s position, I used a laser measurement device to attain the distance from loudspeaker to the first chair, then from the loudspeaker to the mic position.

Take 20*LOG (Distance 2/Distance 1) and the answer is how much louder it is in dB at the first chair than the mic. Now whatever the SPL meter showing at the mic position allowed me to mentally calculate how loud it was in the front or back of the room.

The Right Level

Who says how loud a system in this situation should be? It’s largely up to you, more specifically your ears. After the system is set up and tuned, go to the audience area and listen for yourself. Have a co-worker talk on the podium or lavalier mic and decide how loud is loud enough. Keep in mind that additional gain may be necessary once the audience arrives since they absorb sound energy.

Once the system is loud enough at the very back then place the measurement mic according to plan and see what the number is. In my example, I was shooting for 67 dB(C) (slow). The peak reading was louder, and in between inhales (during speech), it was quieter. This was just a target for all of the presenters, as there were many, each with a different voice and speaking technique. But in general, I knew that at the back of the room I was shooting for 64 dB and 69 dB (both SPL C) at the front.

In addition to helping with SPL metering, my “periscope” mic has the functionality of real-time dual channel FFT (fast Fourier transform). This means that I can look at level over frequency of the output of the console and what’s different or the same once it goes through the speakers and reaches the measurement mic. This is handy for detecting excessive sibilance that you might not hear when located off-axis and/or for finding the resonance of a particular presenter’s voice and fixing it with EQ. In Figure 4, red is from the console and green is from the mic.

Figure 4.

To be clear, when I made a significant dynamics or EQ change I tried to sneak out to the room and check the change with my ears. This is important because an SPL meter and FFT don’t discern tone or whether the dynamics are natural or not.

When subs are involved, it’s important to also consider phase aligning at your mic’s position if your spot is compromised because the summation of the low end adds variation to the room. More bass might be added to the mix than the mic shows. Another factor to take into consideration is noise floor issues. If someone is making noise right next to the mic, it might throw off the data, so always try to keep the mic in sight.

I’ve had consistent results with these approaches and recommend you give them a try if/when you’re forced to compromise with the location of the mix position.

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