Enough Headroom

"A good soundman knows how to keep the clip lights flashing," I was told by a young and earnest sound technician. This is certainly the method that many live sound technicians have learned in getting the "most" out of a portable sound system. Unfortunately, the lack of consistency between amplifier clip indicators and common errors in system gain structure will undermine this method. It requires more subtle measures to achieve the best balance between audio quality and maximum acoustical output.

There are many ways to match the components of a sound system to achieve the most efficient acoustical output. The gain structure of a sound system and the matching of amplifier power to loudspeaker drivers is often a significantly different problem for the permanently installed sound system and a touring rig. However, the ubiquity of system processors that offer preset configurations may be able to close the gap between these two design approaches. In touring sound systems, the efficiency of the system includes consideration of trucking weight, equipment dimensions and setup/teardown time. In the permanently installed system, overriding concerns for reliability, consistent performance, and defined versatility may drive the parameters for gain structure and system headroom. The ideal touring system will reach the system clipping point on the peak dynamic of the performance, whether that is once per night or once per chorus will depend on the style of the performance. The ideal installed sound system will never reach the system clipping level, but will engage one or more stages of limiting before the amplifiers hit the rails.

Digital system processors now handle many of the traditional roles of equalizer, crossover and limiter. This has simplified the adjustment of gain structure, as the dynamic component to determining the clip level of an analogue equalizer need not be considered. Adjust the system components to reach their individual clipping points at the same RMS level and the peaks are likely to clip consistently in each component. Within DSP-based components, it may be more difficult to gauge the correct settings of individual gain elements unless accurate metering is provided in each stage of the processor. Some digital processors offer comprehensive metering and even offer gain structure analysis using a probe to move about the virtual signal path. This can be essential when complex signal paths and dynamic processing have been included in the processor setup. Check out the Peavey MediaMatrix® MWare™ Version 3.02 or BSS Soundweb Designer V1.16 to look at the current state-of-the-art in programmable system processors. In the next year, more companies will be offering products that provide the system-in-a-box concept, including the Innovative Electronic Designs (IED) 3200 series and the Yamaha DME32.

If you want to be able to provide high-quality sound reinforcement for any application, then it may be necessary to allow 20 dB of power above the nominal operating level of system. This can be expensive, as the amplifiers must have 100 times the output power than a system that goes into limiting as soon as the level rises above nominal. Trucking an extra 20 dB of amplifiers is certainly not within the budget of most touring systems. Instead, carefully adjusted signal processing can provide the necessary driver protection (nothing kills drivers as fast as sustained amplifier clipping) while having a less obvious impact on the sound quality produced by the system. This must be done with some knowledge of the performance being reinforced. If the sound produced by the stage instruments (without limiting) can be heard by the audience, system limiting will cause the sound to appear to be more distant during dynamic peaks in the performance. The limiters reduce the output from the FOH system but the stage instruments continue to increase in volume, leading to a shift in perspective for some or all of the audience. This may be the exact opposite to the effect the performer was trying to achieve with these dynamics. However, clipping is the worst form of limiting.

A system with sufficient headroom will pass the dynamic range of the performers through to the audience, without shifting the tone or perspective of the sound during the triple forte (really loud) passages. This ideal may be easier to achieve as signal processing for sound reinforcement systems offers greater sophistication. More complex forms of compression, using multiple stages with varying time constants, can allow for less headroom in the power amplifiers with less audible limiting of the peaks. This includes using frequency-sensitive compression and soft-knee thresholds to prevent the onset of amp clipping. Once we can tweak every last dB of sound pressure level from the loudspeakers, then the cost of headroom can be minimized. Now, set your probes to clip and see where your power amps can be set. You may be able to get the same subjective volume without ever seeing a clip light again.

United Entertainment Media Inc.
460 Park Avenue South, 9th Floor
New York, NY, 10016
Ph. 212-378-0400
FAX 212-378-2160

page construction