Your filtering idea, could probably be made to work. As an alternative, you might consider the use of a Schmidt trigger, to "debounce" the signal. It is extremely simple. You would adjust the lower threshold to be large enough to avoid false triggers.
Even with this approach there will be a tradeoff between false alarms, and the delay to detect them. With a large threshold and a slowly descending signal there will be a some samples of delay between the true zero crossing and when the lower threshold is crossed, and the trigger event occurs.
If you aren't familiar with Schmidt triggers you can Google them, and find lots of references.
Here's a rough example:
% Parameters
aNoise = 0.1; % noise amplitude
f = 1; % example signal frequency [Hz]
a = 3; % example signal amplitude
numCycles = 10; % number of signal cycles
fsmpl = 100; % sampling frequency [Hz]
xLower = 0.3; % Lower Schmidt Threshold
xUpper = 0; % Upper Schmidt Threshold
% Generate noisy signal
T = 1/f; % signal period
Tsmpl = 1/fsmpl;
t = 0:Tsmpl:numCycles*T;
x = a*sin(2*pi/T*t) + aNoise*randn(size(t));
% Loop through signal values one sample at a time to simulate real time
% application
val = zeros(size(t));
soundOn = zeros(size(t));
prevVal = 0;
for k = 1:numel(x)
% Pass signal through Schmidt Trigger
val(k) = schmidt(x(k),xLower,xUpper);
% Check if this was a zero crossing (schmidt changed state)
if (val(k) - prevVal) > 0
soundOn(k) = 1;
end
prevVal = val(k);
end
% Plot results
plot(t,x,t,val,t,soundOn,'o')
grid
legend('signal','schmidtState','soundOn')
function val = schmidt(x,xLower,xUpper)
persistent isNeg % trigger state
% initialize
if isempty(isNeg)
isNeg = false;
end
if x <= xLower && ~isNeg
isNeg = true;
end
if x > xUpper && isNeg
isNeg = false;
end
% Compute output signal, as zero or 1 numerical value
val = double(isNeg);
end
