One of the most basic needs for PA applications in public areas, such as restaurants, retail stores, hotels and transportation centers, is the need to adjust volume depending on the amount of noise in the space. As more people enter the space, the underlying background noise naturally increases. The problem is that the varying nature of this background noise makes setting the PA volume problematic. If the system is loud enough to be heard over the crowd at peak times, it may be too loud for other times, when there aren’t as many people and the area is quieter.

Obviously, the entire goal of the PA system is to be heard, meaning it needs to be loud enough to rise above whatever sound is already emanating from the space (e.g., talking). In order for the PA system to be at the correct loudness at any point in time, the PA volume level needs to vary with the loudness of the room. You can certainly address this by manually adjusting the volume as needs change. However, in hotels, shopping centers and other large venues, there is rarely someone around that is monitoring the volume in every area of the facility and able to adjust the PA system accordingly. Even in restaurants and retail stores, manual adjustment is typically performed by a manager who is busy with other things, meaning the volume often only gets adjusted when a customer complains that the music is too loud.

Fortunately, it’s also possible to design the audio system so it automatically adjusts for this ambient variance. It is achieved using an effect called Ambient Noise Compensation (ANC). ANC, which is available in digital signal processors, such as the dbx ZonePRO and BSS Soundweb London Series, works by using a microphone called an ambient mic to “listen” to the space. The system then automatically adjusts the volume according to the noise level. This ensures people in the space will always hear the loudspeaker clearly, regardless of how noisy the space may be. In a pair of posts, I’m going to dive into this advanced and powerful algorithm, starting with getting suitable microphone coverage. Next time, I’ll address how to use the effect, including adjusting settings and looking at some of the important considerations.

Picking the Right Boundary Mic

As noted above, the ANC algorithm adjusts the volume of the “program” audio (the background music, PA announcements, etc. that are going through the sound system) based upon how noisy the room is at any given time. This is very useful, because it provides sophistication and intelligence to the audio system, ensuring the audio is always at the perfect level for the amount of people and activity in the space. However, for the ANC algorithm to know how to adjust the volume, you must first have boundary microphones installed in a location like the ceiling, so the system can pick up the ambient noise. Having proper ambient mic coverage is an important part of ANC, and without it, the algorithm will do more harm than good.

Check microphone polar patterns to ensure proper coverage for your space

The first consideration when looking at ambient microphones is the polar pattern. Depending on the model, boundary microphones can employ either hemispherical or supercardiod/hypercardiod polar patterns, and both formats are useful for different applications. Supercardiod and hypercardiod microphones have very tight pick up patterns designed for applications where sound sources are further away, making them ideal for installations with high ceilings or a lot of reflections. A hemispherical pattern, on the other hand, is essentially “half-omnidirectional.” This is a good choice for rooms with less reflections and lower ceilings, as they take in the broadest range of sound from the space without capturing sound from above a dropped ceiling, where air conditioning, etc. might be louder than it would be in the space itself.

Unfortunately, I can’t give a single recommendation for which microphone would be best for ANC, any more than I could give a recommendation for a single speaker for every application. In fact, the reasons that drive the need for different speaker options are the same ones that drive the need for multiple ambient microphone options; although there are notable differences between microphones and speakers, the underlying technologies are very similar. The upside of this is that much of the design philosophy for selecting PA speakers and ambient microphones is the same as well. So, if you are familiar with selecting coverage patterns for speakers in a PA system, you can apply the same approach for finding the right microphone for ANC.

Covering the Entire Space

Much like when installing speakers for a space, selecting an ambient microphone installation means ensuring you have suitable coverage where people are going to be in the space. It also means reducing or eliminating coverage where there aren’t people. The difference is that instead of being concerned about creating reflections, as you would with a speaker installation, you are instead concerned about picking up reflections and other noise from areas outside the space you are targeting. This is because the volume of those reflections might be louder at the microphone than they are where people are sitting or standing. For ANC to work properly, the microphones should bring in an accurate account of the noise level in the physical area where you are trying to manage volume.

Using zoning with ANC ensures proper volume adjustment for each area of the building

A good example of where this is true is when you are designing audio systems for larger retail and restaurant locations, not to mention large venues, such as hotels and transportation centers. Once the venue gets past a certain size, the coverage area becomes too large for a single microphone to accurately measure the audio in the entire space. If you find that you need more mics in order to have suitable coverage, then you can mix the microphones together and send the mixed signal to the algorithm.

If you have a larger venue, where there are different sound levels in different areas, you can use zoning to adjust the volume in the different spaces. This provides a finer element of control if you need it. In this instance, you would have a mic (or multiple mics) for each zone, with a different ANC effect applied to each zone. These zones would operate independently, but it is important to ensure the audio in each zone feels consistent, especially if you are using background music. If the volume for one area is noticeably louder than another area, it can feel inconsistent and off-putting. Ultimately, trial and error is required to ensure you achieve the proper balance.

Of course, achieving this consistency involves adjusting the ANC settings to meet the needs of the application. In our next post, we’ll look closer at how the ANC algorithm works and how to adjust it. If you’d like to learn even more about ANC and the other processing capabilities available in HARMAN audio products, you can sign up for training from HARMAN Professional University, which offers a number of online courses.

Stay tuned for part two of this post in two weeks.

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