Returns the time-reversed low-pass synthesis waveform of the filter, which is the low-pass filter. This is the time-reversed impulse response of the low-pass synthesis filter. It is used to calculate the L2-norm of the synthesis basis functions for a particular subband (also called energy weight).

       <p>The returned array may not be modified (i.e. a reference to the
       internal array may be returned by the implementation of this
       method).</p>