Experiments conducted by Woodworth (1938) tested the duplex theory by using a solid sphere to model the shape of the head and measuring the ITDs as a function of azimuth for different frequencies. The model used had a distance between the 2 ears of approximately 22–23 cm. Initial measurements found that there was a maximum time delay of approximately 660 μs when the sound source was placed at directly 90° azimuth to one ear. This time delay correlates to the wavelength of a sound input with a frequency of 1500 Hz. The results concluded that when a sound played had a frequency less than 1500 Hz the wavelength is greater than this maximum time delay between the ears. Therefore, there is a phase difference between the sound waves entering the ears providing acoustic localisation cues. With a sound input with a frequency closer to 1500 Hz the wavelength of the sound wave is similar to the natural time delay. Therefore, due to the size of the head and the distance between the ears there is a reduced phase difference so localisations errors start to be made. When a high frequency sound input is used with a frequency greater than 1500 Hz, the wavelength is shorter than the distance between the 2 ears, a head shadow is produced and ILD provide cues for the localisation of this sound.