Patent ID: 8693708

Claim:
The method for driving a speaker which produces an audible acoustic output which comprises the steps of: generating packets of pulses of like voltage in which said pulses are repetitive at a frequency above audible range and which said packets are repetitive at audible frequency using a power source the voltage of which is subject to variation over a given range of voltage; and changing the pulse width modulation (PWM) ratio of said pulses in each of said packets in response to the voltage of said power source so that said packets provide a constant power output, said pulse width modulation ratio being the ratio of on to off duration of said pulses which corresponds to the duty cycle thereof, and applying said constant power output to said speaker to provide said audible acoustic output to produce a warning signal, in which said PWM ratio to provide said constant power output is at least in accordance with the voltage of the power source and affect of inductive reactance of said speaker on rise time of pulses.