Headphone equalization - measurement, design and psychoacoustic evaluation

Unlike loudspeakers, headphones are not designed to have a flat frequency response. Instead they should compensate for the spectral coloration caused by the acoustic transfer path when sound is travelling from a point in space to the ear. For correct headphone reproduction it is essential to control the sound pressure at the listener's ears. In the literature, there is no consensus about the optimal transfer function and equalization of headphones. In this work, several equalization strategies were tested on different commercially available headphones. Headphones were measured on an artificial head and equalization filters were designed in the frequency domain consisting of two parts: The first part of the filter is specific for each headphone and linearizes the frequency response of the headphone at the point of measurement, i.e. at the beginning of the blocked ear-channel. The second part of the filter is generic for all headphones and allows testing of different target responses on different headphones. Target responses from literature and the ISO 11904-1 diffusefield equalization were tested. Further, target responses were designed by expertlisteners by directly comparing headphone and loudspeakers playback. A listening test showed a general preference for the equalization curves adjusted by experts and the diffuse-field equalization. The experiment also verified that the equalization process enhances the perceptual quality irrespective of the headphone model.