function gong(vol,frq,dur)

% gong: sounds gong
% by John Gooden - The Netherlands
% 2007
% 
% call gong
% call gong(vol)
% call gong(vol,frq)
% call gong(vol,frq,dur)
%
% input arguments (optional, if 0 then default taken)
% vol = volume (default = 1)
% frq = base frequency (default = 440 Hz)
% dur = duration (default = 1 s)

fb  = 440;
td  = 1;
vl  = 1;
if nargin>=1
    if vol>0 vl = vol; end
end
if nargin>=2
    if frq>0 fb = frq; end
end
if nargin>=3
    if dur>0 td = dur; end
end
    
t   =[0:8192*td]'/8192;
env = exp(-5*t/td);
f   = fb;
vol = 0.3*vl;
tpft = 2*pi*f*t;
sl  = sin(tpft)+0.1*sin(2*tpft)+0.3*sin(3*tpft);
sl  = vol*sl;
sr  = [sl(100:end);sl(1:99)];
vl  = cos(20*t);
vr  = 1-vl;
y(:,1) = vl.*env.*sl;
y(:,2) = vr.*env.*sr;

sound(y)