The analogy between the decay of the electrical energy in a capacitor shunted by a resistor and the decay of sound energy in a room with sound absorbing walls is the basis of this bridge. The sound pressure is converted into a corresponding electrical voltage by a microphone and amplifier. This voltage is measured by a linear type v.t. voltmeter. The voltage across the resistor shunting the capacitor is measured by another similar v.t. voltmeter. The room and capacitor are “charged” until steady state conditions are established, the capacitor charging voltage and the sound source are then cut off simultaneously. The bridge is balanced by adjustment of the variable discharge resistor, until the two voltages are equal during the decay period. The value of “t” in the “time constant” of the electrical circuit is then the actual reverberaone time.