Various Approaches
While this is slightly off-topic, methods do exist to reduce distortion once you understand how and why it’s being generated. With a LF loudspeaker, the enclosure’s tuning can be optimized for a specific task if it’s possible to alter the port area and port length.
If an application doesn’t need 25 Hz reproduction but demands a lot of power at 40 Hz, re-tuning the enclosure by changing the ports may reduce the distortion considerably at 40 Hz, and the transient response will probably improve too, as a byproduct.
Another approach is to use more PA than you might normally think is needed. The Grateful Dead were big on this concept, sometimes putting stadium-sized loudspeakers into small theatres. The reason this works is because most modern loudspeakers exhibit respectable distortion figures at low to moderate operating levels, but not necessarily when they’re being pushed hard.
Therefore, if the PA isn’t being operated at the upper end of its power output capability, then the sonic quality will likely be clearer and more natural, providing (of course) that the additional loudspeakers are not causing destructive acoustical interference among one another.
Yet another approach is to introduce protective limiters that are set to engage before the loudspeaker’s distortion products enter the dreaded non-linear zone. The use of an analyzer will greatly assist in determining where to set the limiter threshold and ratio, but time constants are a different matter.
Unless you have advanced test and measurement skills – and plenty of time to use them – it’s likely that you can arrive at perfectly usable attack and decay settings by ear. Make sure to use music that has similar instrumentation and dynamic content as the program material of the actual show.
Among the myriad of functions that modern audio analyzers can perform, one that stands out is the waterfall plot. Waterfall plots have their roots in mathematics, long before analyzers existed. They expand on the familiar X-Y Cartesian plot by including a third axis that reveals a new dimension to the information being viewed.
Waterfall plots can be used to display many things, but one of exceptional value is the ability to view the frequency response of a loudspeaker, while at the same time viewing its acceleration and deceleration characteristics, also referred to as transient response (Figure 4).
It’s fascinating to see how some segments of the audible spectrum keep decaying from kinetic energy, while others have already stopped. These measurements are helpful in evaluating a new loudspeaker system or working with an existing one to improve its dampening characteristics.
Note The Differences
While a number of suppliers have introduced value-priced measurement microphones during the past few years, they’re usually intended for use with a loudspeaker controller and are suited only for measuring an approximation of frequency response. It’s important to understand that there’s a significant difference between a mic that’s been designed for a quick frequency response measurement and one that’s painstakingly crafted for flat and linear frequency response, phase response, impulse response, and distortion measurements at a wide range of sound pressure levels.
The long-standing leaders in the field, such as Bruel & Kjaer, have spent decades fine-tuning the efficacy of their products. For truly accurate information when measuring a loudspeaker system, particularly as it applies to distortion and impulse response, a highly accurate measurement mic that’s mated with a compatible mic pre-amp must be used.
There’s a lot more to discuss when it comes to measuring and understanding audio devices; a serious treatment would fill a sizeable book. Today a plethora of analysis products exist that offer varying degrees of usefulness, accuracy, and efficiency for the vast range of tasks that sound system designers and engineers need to accomplish.
Those with the luxury of time and budget will want a complex and versatile analysis system, while others – i.e., a touring club engineer with little time to run a sound check before catching dinner on the fly – will likely want a less costly, portable solution.
Over the course of more than four decades, Ken DeLoria has tuned hundreds of sound systems, and as the founder of Apogee Sound, he developed the TEC Award-winning AE-9 loudspeaker.
