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

2. Noise spectrum Frequency (Hz) Power 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. 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. 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. Theory: Bandpass ‘internal’ filter centered around signal frequency. Both signal and the noise passed through this bandpass filter. 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. 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. Beyond this point, noise only increases in loudness.

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