The invention describes a method of deriving a set of features of an audio input signal, which method comprises identifying a number of first-order features of the audio input signal, generating a number of correlation values from at least part of the first-order features, and compiling the set of features for the audio input signal using the correlation values. Said apparatus comprises at least one ultrasound transmitter for generating the ultrasound waves, at least one ultrasound receiver for receiving the ultrasound waves and at least one phase detecting device for detecting the phase of the ultrasound wave at the ultrasound receiver relative to the same ultrasound wave with the same frequency at the ultrasound transmitter. A novel imaging apparatus may be employed in a flow cytometer performing this method. Each of these inserts is of the same height as that of the mould wall at the position where it is located. The introducer includes an introducer distal end.