This demonstration of the masking of a single 2000-Hz tone by spectrally flat noise of different bandwidths is based on the experiments of Fletcher (1940). First, we use broadband noise and then noise with bandwidths of 100, 250, and 10 Hz. In order to determine the level of the tone that can just be jheard in the presence of the noise, in each case, we presnt the 2000-Hz tone in 10 decreasing steps of 5 decibels each. Since the critical bandwidth at 2000 Hz is about 280 Hz, you would expect to hear more steps in the 2000-Hz tone staircase when the noise bandwidht is reduced below this value. Since the spectrum level of the noise is kept constant, its intensity (and its subbjective loudness) will decreas markedly as the bandwidth is decrease.
DEMONSTRATION
You will hear a 2000-Hz tone in 10 decreasing steps of 5 dB. Count how many steps you can hear.
Now the signal is masked with broadband noise.
2000-Hz
Tone + Broadband Noise
Next, the noise has a bandwidth of 1000 Hz.
2000-Hz Tone
+ 1000-Hz Wide Band of Noise
Next, noise with a bandwidth of 250-Hz is used.
2000-Hz Tone + 250-Hz Wide Band of Noise
Finally, the bandwidth is reduced to only 10 Hz.
2000-Hz Tone + 10-Hz Wide Band of Noise
