Abstract: A method and apparatus for synthesizing an approximation to a sine wave comprising generating a number of pulse width modulated signals, each having a predetermine duty cycle and being the same frequency as each other, from a clock signal. The generated pulse width modulated signals are then combined to produce an approximation to a sine wave having the same frequency as the pulse width modulated signals. The clock signal is provided by an oscillator arranged to produce clock signals over a continuous range of frequencies so that approximations to a sine wave can be produced over a continuous range of frequencies.

Abstract: A method and apparatus for determining the speed of sound in a gas. An acoustic transmitter is arranged to apply an acoustic signal to the interior of a resonator and an acoustic receiver is arranged to detect the amplitude of the acoustic signal in the interior of the resonator. A control means determines the speed of sound of gas within the resonator from the frequency of a detected non-radial resonant mode.

Abstract: Determininig a frequency at which a resonator resonates comprising: driving an acoustic transmitter for applying an acoustic signal to the interior of a resonator by scanning through a first range of frequencies in substantially equidistant steps of a first size; detecting a signal from an acoustic receiver arranged to detect the amplitude of an acoustic signal within the resonator produced by driving the acoustic transmitter over the first range of frequencies; determining a frequency at which a maximum occurs in the detected signal for the first range of frequencies; driving the acoustic transmitter by scanning through a second range of frequencies containing the determined frequency from the first range of frequencies, the second range being smaller than the first range, and scanning through the second range of frequencies in substantially equidistant steps of a second size which is smaller than the steps of the first size; detecting a signal from the acoustic receiver produced by driving the acoustic tran