1. Fletcher’s band-widening experiment (1940)
Present a pure tone in the presence of a broadband noise.
Both signal and noise are presented simultaneously.
Signal is kept constant (in frequency).
Bandwidth of the noise is increased, keeping spectrum level constant.
Measure masked signal threshold as a function of the bandwidth of the masker.

2. Frequency (Hz)
Power
Noise spectrum
Signal

3. Concept of the critical bandwidth
Results: At first, the masked threshold of the signal increased. After a certain bandwidth, no more change in the signal threshold.
This bandwidth was termed the critical bandwidth.
Bandwidth of masker
Masked
signal
threshold
Critical bandwidth

4. Explanation of results
Theory: Bandpass ‘internal’ filter centered around signal frequency.
Both signal and the noise passed through this bandpass filter.
Only the bandwidth of the noise that passes through the internal filter has an effect on the signal threshold.
Beyond this point, noise only increases in loudness.

5. Frequency (Hz)
Power
Internal filter
Noise spectrum
Signal