I get dozens of phone calls a month from integrators around the globe looking for help in setting up complex outdoor audio systems, often with DSP-equipped amplifiers like the NEAR XL series amps. I’ve observed that while most integrators have a good grasp of how to use frequency equalization (EQ), and hi/lo-pass filters (crossover networks), very few have a firm notion of what Limiting and Compression are, how they are different, and how to set them up to maximize system reliability, sound quality and client satisfaction. Let’s explore the basics.
In high fidelity indoor music systems, particularly those with large speakers, and quality-audio conscious clients, you probably shouldn't use limiting or compression. Music lovers and audiophiles cherish having the widest possible dynamic range. But in distributed audio systems, and any system in a noisy environment, especially outdoors, limiting and compressing dynamic range can really save your bacon.
What Exactly is Dynamic Range?
In simple terms dynamic range is the volume range between the softest and loudest sounds. A symphonic orchestra has a measured dynamic range of 85dB. That means the loudest sounds would be 85dB louder than the softest sounds you can perceive. Even though high-resolution digital recordings can capture a greater range than 85dB, most recordings limit the dynamic range, so bear with me and let’s use 85dB for illustration purposes (Fig. 1).
Before starting making any adjustments described below, it is important to set the system volume to a typical listening level, as limiting, compression and volume are all interactive. If you do your audible calibrations of any setting at non-typical listening levels, you won't get the same results in real-life use.
The name tells you a lot about what’s going on here. A Limiter limits the maximum power output (volume) of an audio signal. If you have a 200 Watt/channel amplifier driving a speaker that can only safely handle 50 watts, the potential exists to deliver too much power to the speaker, causing it to distort and possibly break. In the hands of responsible adults, that is rarely a concern. But a volume control in irresponsible hands (adult or not) can be disastrous and lead to costly service calls.
The limiter settings determine to what extent, how quickly (or slowly), and how long, the limiter circuit limits the output of the amplifier.
THRESHOLD limits the amount of voltage that the amplifier section will deliver (Fig. 2). The lower the setting value, the lower the volume level where the limiter will engage. Using our example of the 50W speaker and the 200W amplifier, let’s aim for limiting amplifier output to 100W (Fig. 3). Why 100 and not 50? Because you always need reserve power to handle brief musical peaks. Choking off power prematurely can lead to poor sound quality.