I'm sure there's a simple explanation but I can't discover it. Consider two frequencies, a and b. a is higher than b. I can create a wave with both frequencies. But when I create a wave with frequency decreasing from a to b, the frequency decreases to even slower than frequency b. Consider the following MATLAB code.
timeVals = 0:0.001:1; sourceFreq = 8; freqPulse = linspace(1,.5,length(timeVals)); sourceActivity1 = cos(2 * pi * (sourceFreq .* freqPulse .* timeVals)); freqPulse = linspace(.5,.5,length(timeVals)); sourceActivity2 = cos(2 * pi * (sourceFreq .* freqPulse .* timeVals)); plot(sourceActivity1); hold on; plot(sourceActivity2)
Now, compare the two sourceActivities. The end frequency of sourceActivity1 is even slower than 4. And the frequency at the end of the wave is slower than sourceActivity2. This can be determined using visual inspection alone, or with spectrograms.