Michail shows the effect of test-induced heating on a voice coil
After welcoming everyone, Section Chair Graeme Huon introduced Michail who started his presentation with a brief history of his career at Plessey Rola (pre-1975), including and his involvement with the 1977 Australian Standard for Sound System Equipment (AS1127, Part 5 — Loudspeakers), before establishing Lorantz Audio Services in 1976.
Michail spoke at length about his experiences testing the power handling capabilities of his loudspeaker products using the AES2 standard.
He cited examples using his 12inch and 18inch bass-mid drivers. He spent some time describing the test methodology, pointing out the importance of selecting the appropriate high-pass filter cutoff point to take into account the cone excursion characteristics of the device and ensure it isn't driven past it's safe excursion point.
He pointed out that his 12inch version tests to 400 watts, resulting in an 800 watt program power rating (as per convention), commenting that 800W is just over one horsepower, and drawing analogies to the torque developed by a 1HP motor, or the heating developed by an 800W electric radiator element. His point was that this is a power level not to be trifled with.
He showed examples of the overheating effect of running the 2 hour AES2 test on voice coils of his product, and spoke at length about the techniques for reducing the temperature rise via forced convection and heatsinking. He warned that, as commercial loudspeaker systems become more higher-powered, yet more lightweight, driven by a range of factors, that the risks of overheating increase potentially creating fire hazards.
He also spoke of the mechanical factors that can limit the power handling capabilities of a particular design. He covered issues like voice coil damage through bottoming and cone/surround damage from excessive excursion.
He indicated some topics that the AES test does not address, such as the distortion level, stress fatigue, the acoustic load, the end user (source content — speech or type of music), end use (HiFi vs Professional use), and safety considerations.
Michail went on to show some high stress use cases where the speaker should be derated such as where box design or placement increase the heat rise, or where there is a non-symmetric acoustic load.
He also covered the dangers associated with driver failures in passive crossover situations, where a failure can result in dangerous overheating of crossover inductors and covered some protection strategies to cover these cases.
He concluded his presentation by stating that the AES power test is a demanding test testing the speaker's build integrity and thermal capabilities, however the end use, customer safety and fidelity acceptance also need to be considered.
