Sound System Commissioning - Say What?
One of the most overlooked parts of a new sound system’s installation is the tuning. It requires the perfect blend of science and art to ensure that all parts of the system are working harmoniously with the acoustics and architecture, to sound great to the people who will be in the space. Since we can’t actually see what the sound is doing in space, we rely on other tools including microphones, analysis software and most importantly our ears to understand what is happening in the room. Using this mix of data and an experienced ear we can adjust the speakers and processing and fine-tune the installation to the space. It would be simple to assume that the sound system is finished when the components are all wired and turned on, but the system is not really integrated for the space until it’s tuned.
The first step is getting the numbers. Once a system is installed, the AV contractor is asked to submit final Site Acceptance Testing reports. These test reports demonstrate that the system’s components are built and functioning correctly, and that the system is assembled correctly on site but measuring what can be objectively measured - testing signal flow, gain staging, the polarity of every cable and component, and the impedance of every speaker load at the output of the amplifier.
During an AV commissioning visit, we will verify these tests on site with acoustic or electronic testing procedures. Then we fire up the sound system, and listen to it by playing music, and measure it with test signals.
When sound designers discuss the color of sound, we are typically referring to the way the loudspeaker reproduces each frequency band in its range. The characteristic sound of the speaker might be tubby, boxy, edgy, honky, sibilant, or natural… and while manufacturers that design loudspeakers often have built-in (either digital or analog circuitry) means of creating or correcting a characteristic “color,” each space has different acoustics, and end user preferences that just can’t be accounted for in the box. It’s critical to listen to how the system works in the space, and then know what to do about it. This is how we can add the art in tuning the sound system.
Leading up to the actual site visit, it’s important to identify the desired tonal character of the system by discussing it with the owner. Do you want the system to be punchy or natural; warm or bright? Sound designers sometimes call this the “house curve,” which really means the personality or color of this particular system. Do you want your lead vocals to cut through? Do you want to be able to feel a punchy bass guitar?
Tuning a sound system typically involves using reference microphones, which are omnidirectional (equally sensitive in every direction), and have a “flat” frequency response. Unlike a microphone a performer or presenter might use for amplification or recording, these reference microphones are designed to capture every frequency of sound at an equal amplitude. We use pink noise played through each loudspeaker to measure its frequency, phase, and impulse response in the space. Pink noise is a static-like sound, that uses equal energy for each octave band of sound between 20Hz and 20kHz, which makes it ideal for a computer measurement.
Since every speaker is designed to have a specific coverage pattern and frequency response, measuring the speaker when it is installed in the room validates and verifies our calculations in the prediction/modeling software. Then, with digital processing, we can adjust EQ (frequency adjustments in Hz, dB, and bandwidth) and delay (time adjustments in milliseconds) to adjust the frequency response and time alignment of the sound system, so that it behaves the way it needs to. This may involve actually adjusting the speakers physically in the space, moving them up or down, or adjusting their angles. Sometimes the best thing to do to resolve an undesirable reflection of sound is adjust the position or angle of the speaker itself.
The analysis software used to do this is called an FFT (Fast Fourier Transform) Analyzer. Unlike an RTA (real time analyzer), it looks at a reference signal (we use pink noise and uncompressed music) and it compares the signal to the sound that’s coming out of the speakers. The graphical output should be a flat horizontal line, and any variation from that line tells us what is happening in the space.
When we commission AV systems, we go through this full process with every speaker and speaker system, in every room. Even when we are not fully commissioning the AV system, during our final punch visit, we’ll often bring an analyzer and microphone to verify the contractor’s EQ and delay settings.
We typically start by listening to the speaker’s response “on axis,” meaning head-on. This is the best way to know what the speaker is doing without worrying about the impact of the room. If the speaker sounds right on its own, we’ll walk the coverage pattern of the speaker to ensure that the audience gets a uniform experience. Where one speaker leaves off, we typically expect another speaker to pick up, again providing even coverage and the same experience for every guest or audience member. For each measurement with microphones and noise, we always validate the data with our ears by listening to music, and if there’s a concerning measurement, we want to make sure that our ears are confirming the problem before making a correction. Depending on the microphone position, something as simple as an errant reflection can yield concerning data. Moving the microphone by a foot or two, and confirming the data with our ears, reveals the full picture of how sound is behaving in the space.
At the end of the process, confirming the adjustments with our ears is critical. Sound designers that rely too much on chasing small deviations in the numbers and don’t rely enough on their ears can find themselves with a system that sounds unenergetic, dull, or veiled but perfect on paper. Since sound is invisible, this measurement equipment is the only tool we have to verify that the sound is behaving in the space the way that we expect it to, and our trained ears are what make sure that the system has the sparkle, punch, warmth or the brightness that is desired by the Owner and end user.
During tunings, we’ve found critical errors, such as: speakers or other components that are wired backwards (which reverses polarity and can cancel out the sound waves in the space), bad electrical connections or cold solders in wall plates, distortion in the sound system caused by improper gain staging, improper signal separation causing interference, wireless microphone dropouts caused by multipath interference, and more. While none of these mistakes are dangerous, they do impede the performance of the sound system, and the intelligibility of a performer or presenter. This is the importance of having a second set of trained ears listen to a system – just because it’s plugged in and it turns on doesn’t mean it’s working well.
Ultimately, it’s important to know that sound systems can undergo this testing and verification at any point in their lifetime. Tuning is an easy way to make an underperforming sound system sound much better, and because good sound is subjective, it’s important to have a consultant that you trust have the final eyes and ears on the system. If your sound system could use some fine-tuning, let’s take a look (and listen.)
Want to learn more? Contact Matt Eckstein, CTS, Technology Systems Designer.