Pitch Estimation and Voiced–Unvoiced Classification of Speech - Digital Speech: Coding for Low Bit Rate Communication Systems - page 177

Figure 6.24 Speech waveform and its zero-crossing rate with a possible voicing
threshold of 60 (shown by the dashed line)
crossing rate. The simple logic shown below can be used to compute the
zero-crossing rate:
count=0;
for(i=1;i<N;i++)
{
if((data[i] x data[i-1]) < 0.0)
count = count + 1
}
Zc=count
A speech waveform and its corresponding zero-crossing rate is shown in
Figure 6.24. The zero-crossing rate also depends on the pitch of the signal (if
voiced). For example, the zero-crossing rate of voiced female speech (with
a short pitch period) is higher than that of voiced male speech (with a long
pitch period). A small pitch weighting can be used to weight the decision
threshold.