An echo detection circuit comprising: an acoustic sensor ( 104, 201) configured to convert sensed acoustic pressures into a pre-processed analog electrical signal; analog processing circuitry configured to transform the pre-processed analog electrical signal into a post-processed analog electrical signal suitable for providing to an analog-to-digital converter; an analog to digital converter configured to convert the post-processed analog electrical signal into a digital signal; a sine mixer configured to digitally mix the digital signal provided by the analog to digital converter with a sine wave of approximately the same frequency as a carrier wave of a transmitted acoustic signal whose echo is being detected; a cosine mixer configured to digitally mix the digital signal provided by the analog to digital converter with a cosine wave of approximately the same frequency as the carrier wave of the transmitted acoustic signal whose echo is being detected and wherein the frequency of the cosine wave is the same as the frequency of the sine wave; a vector magnitude detection mechanism that detects a magnitude of the vector represented by at least a first derived signal that is derived from a signal received from the sine mixer, and at least a second derived signal that is derived from a signal received from the cosine mixer; a threshold detection circuit that compares the magnitude of the vector with one or more thresholds to determine whether or not an echo is detected, characterized in that the threshold detection circuit compares the magnitude of the vector with one or more thresholds (702, 703, 704) by performing the following acts: an act of determining whether an absolute value of a sine component of the vector is above a sine threshold value; an act of determining whether an absolute value of a cosine component of the vector is above a cosine threshold value; an act of determining whether the value of the division of the sum of the absolute value of the sine component and the absolute value of the cosine component by the square root of two exceeds a diagonal echo threshold, wherein the threshold detection circuit detects an echo if any of the acts of determining result in the corresponding threshold (702, 703, 704) being exceeded. an acoustic sensor ( 104, 201) configured to convert sensed acoustic pressures into a pre-processed analog electrical signal; analog processing circuitry configured to transform the pre-processed analog electrical signal into a post-processed analog electrical signal suitable for providing to an analog-to-digital converter; an analog to digital converter configured to convert the post-processed analog electrical signal into a digital signal; a sine mixer configured to digitally mix the digital signal provided by the analog to digital converter with a sine wave of approximately the same frequency as a carrier wave of a transmitted acoustic signal whose echo is being detected; a cosine mixer configured to digitally mix the digital signal provided by the analog to digital converter with a cosine wave of approximately the same frequency as the carrier wave of the transmitted acoustic signal whose echo is being detected and wherein the frequency of the cosine wave is the same as the frequency of the sine wave; a vector magnitude detection mechanism that detects a magnitude of the vector represented by at least a first derived signal that is derived from a signal received from the sine mixer, and at least a second derived signal that is derived from a signal received from the cosine mixer; a threshold detection circuit that compares the magnitude of the vector with one or more thresholds to determine whether or not an echo is detected, characterized in that the threshold detection circuit compares the magnitude of the vector with one or more thresholds (702, 703, 704) by performing the following acts: an act of determining whether an absolute value of a sine component of the vector is above a sine threshold value; an act of determining whether an absolute value of a cosine component of the vector is above a cosine threshold value; an act of determining whether the value of the division of the sum of the absolute value of the sine component and the absolute value of the cosine component by the square root of two exceeds a diagonal echo threshold, an act of determining whether an absolute value of a sine component of the vector is above a sine threshold value; an act of determining whether an absolute value of a cosine component of the vector is above a cosine threshold value; an act of determining whether the value of the division of the sum of the absolute value of the sine component and the absolute value of the cosine component by the square root of two exceeds a diagonal echo threshold, wherein the threshold detection circuit detects an echo if any of the acts of determining result in the corresponding threshold (702, 703, 704) being exceeded.