Fig. 7. We observed substantial changes in amplitude at 6.5 kHz as beak gape was varied between 0 and 11 mm. Amplitude increases linearly with beak gape at 6.5 kHz but is plotted on a logarithmic scale (dB). Amplitude at this frequency across all subjects can be approximated as A=–5+32 log(B), where A is amplitude (dB) and B is beak gape (broken line). Amplitude values are plotted relative to those obtained with a closed beak (0 mm).