Since return loss figures may be highly dependent on frequency another method for specifying voiceband return loss performance is often used . For audible echoes, the frequencies between about 560 to 2000 hertz are considered in one (multi-frequency) return loss measurement. Echo return loss, or ERL as it is known, is measured by passing a white noise source through a passband shaping filter and them through a comparison bridge circuit with the circuit under test in one of its arms. The residual signal remaining after the bridge is nulled is indicated on a meter (Figure 2, e₂) as echo return loss.