PV_Cutoff				zero bins with interpolation

PV_Cutoff(buffer,  wipe)

Clears bins above or below a cutoff point.  Interpolate.
buffer - fft buffer.
wipe - can range between -1 and +1.
if wipe == 0 then there is no effect.
if  wipe > 0 then it acts like a high pass filter, clearing bins from the bottom up. 
if  wipe < 0 then it acts like a low pass filter, clearing bins from the top down.

s.boot;

b = Buffer.alloc(s,2048,1);

(
SynthDef("help-brick", { arg out=0, bufnum=0;
	var in, chain;
	in = {WhiteNoise.ar(0.2)}.dup;
	chain = FFT(bufnum, in);
	chain = PV_Cutoff(chain, SinOsc.kr(0.1)); 
	Out.ar(out, IFFT(chain).dup);
}).play(s,[\out, 0, \bufnum, b.bufnum]);
)