The Correlation between Distortion Audibility and Listener Preference in Headphones

It is well-known that the frequency response of loudspeakers and headphones has a dramatic impact on sound quality and listener preference, but what role does distortion have on perceived sound quality? To answer this question, we selected 5 popular headphones with varying degrees of distortion and equalized them all to the same frequency response. Trained listeners compared them subjectively using music as the test signal, and the distortion of each headphone was measured objectively using a well-known commercial audio test system. The correlation between subjective listener preference and objective distortion measurement is discussed. Presenter Bio: Steve Temme is founder and President of Listen, Inc. a manufacturer of software-based measurement systems for testing electro-acoustic and audio electronic products. Prior to founding Listen, Steve worked for many years as an acoustic test and measurement applications engineer at Bruel & Kjaer, and also as a loudspeaker design engineer at Apogee Acoustics. He has authored numerous papers on acoustic testing and lectured extensively throughout the world. Additional Information: There has been much research published on how a loudspeaker’s linear performance, e.g. frequency, time and directional responses, affects perceived sound quality. However, there is little research published on how nonlinear distortion affects perceived sound quality. In recent years, the increasing availability and affordability of high quality headphones and personal digital music players e.g. MP3 players, has brought high quality music playback to the masses. The transducer performance is critical to listener enjoyment, and Dr. Olive and others have presented research on what they believe the target frequency response of the headphone should be for optimum sound quality. In this paper, we determine what level and what kind of distortion is audible and how it affects the perceived sound quality. We objectively measure and subjectively rate five different pairs of good quality over-the-ear headphones with varying levels of distortion for their perceived sound quality. First, each headphone is equalized to the same target frequency response. Several different kinds of distortion metrics including harmonic, intermodulation, and noncoherent distortion, are measured for each headphone. A listening test is then conducted where the five headphones are rated by eight trained listeners based on preference and distortion using four short musical excerpts. The program material was selected for wide dynamic and frequency ranges to excite mechanisms in the headphone transducers that would cause distortion. The different headphones were presented virtually to listeners via binaural recordings of the headphones reproduced through a calibrated low-distortion reference headphone, Stax SR-009. This virtual headphone test method minimized headphone leakage effects, and removed the influence of non-auditory biases (brand, price, visual appearance, comfort, etc.) from listeners’ judgment of sound quality. We examine correlations between subjective and objective ratings of distortion as we have done previously in an attempt to develop an objective metric for measuring distortion audibility in headphones and other loudspeakers. This could possibly be extended to other types of audio devices such as amplifiers.