Power compression, and why loudspeakers change sound as they heat
Abstract. Voice-coil temperature rises with drive level, resistance rises with temperature, and sensitivity falls with resistance. The result is a loudspeaker whose tonal balance at show level is not the balance it was tuned to at soundcheck. This note quantifies the mechanism and describes the compensation approach behind DynaControl.
The mechanism
A moving-coil driver converts most of its input power to heat, not sound. Typical conversion efficiency for a professional cone driver is a few percent; the remainder heats the voice coil and, more slowly, the magnet assembly. Copper's resistivity rises by roughly 0.39 percent per kelvin, so a coil that climbs from 20 to 200 degrees Celsius increases its DC resistance by about 70 percent.
Sensitivity follows resistance. With the amplifier holding voltage, the current through a hotter coil falls, force falls with it, and acoustic output falls in proportion. A 70 percent resistance rise costs roughly 2.3 dB of level in the affected band. Because each way of a multi-way system heats at its own rate, the loss is not uniform across the spectrum: the balance between bass, mid and treble shifts with programme content and time.
Why it matters on a show
A system is tuned at soundcheck, at soundcheck levels, with cold coils. Two hours into a performance the low-frequency coils of a hard-driven rig are running far hotter than the compression drivers, and the response the audience hears is no longer the response the engineer approved. The effect is gradual enough that operators compensate by ear, usually by pushing the level further, which heats the coils further.
The DynaControl approach
DynaControl runs a thermal model of each voice coil in the system DSP, driven by the signal actually delivered to each way. From coil geometry, thermal mass and dissipation constants, the model estimates temperature continuously, converts it to a predicted sensitivity change per band, and applies the inverse as slow-acting gain correction. Excursion is modelled in parallel, so the correction never drives a coil past its mechanical limits in pursuit of level.
The intent is narrow and specific: hold the tuned balance as the system heats. The correction consumes amplifier headroom rather than creating output, so DynaControl does not raise the maximum level of a system; it keeps the system sounding like itself as that maximum is approached.