Estimating pressure at the eardrum for binaural reproduction

The head-related transfer function (HRTF) characterizes the spectral transformation of sound on the path from a point sound source in free field to the eardrum. When using HRTFs in binaural reproduction, individual measurements should be favored, since non-individual HRTFs tend to cause coloration and errors in localization of virtual sound sources. HRTF measurements are commonly made at the entrances of the ear canals, which are blocked with earplugs. However, depending on the type of headphone used, this method may cause timbral coloration in reproduction, which is caused by, e.g., the difference in the acoustic coupling of the ear canal to the headphone in comparison to free air. An easy method of measuring HRTFs and headphone transfer functions (HpTFs), i.e., the pressure at the eardrum rather than at the blocked ear canal entrance would solve the aforementioned problem. In addition, HRTFs with the eardrum as point of reference can be used in binaural reproduction with insert headphones without the need to do additional estimations of the transfer function from the blocked ear canal entrance to the eardrum. Probe microphone measurements of the pressure at the eardrum are generally not considered reliable above 4 kHz, and they also involve the risk of damaging the eardrums. Hence, a non-invasive method of measuring or estimating the pressure at the eardrum is needed. In this work, a miniature-sized acoustic pressure-velocity sensor is used to measure energy density at the ear canal entrance, which in turn is used to estimate the sound pressure at the eardrum. The reliability of the estimation method is verified through measurements with simulators and human subjects. In addition, HRTF filters are designed using the estimated HRTFs and HpTFs, and are used in a listening test. The result of the listening test shows that the estimation method presented can reduce coloration in binaural reproduction. In conclusion, the method presented facilitates the obtaining of individual HRTFs and HpTFs at the eardrum using non-invasive measurements. An important procedure in binaural reproduction is the equalization of headphones using measured HpTFs. For insert headphones, however, neither the blocked ear canal nor the probe microphone measurement methods are applicable, since the inserts themselves block the ear canal entrance and since the transducer ports of the inserts are inside the ear canals. This study develops an alternative method of obtaining HpTFs of inserts using measurements with in-ear microphones, computational modeling, and electro-acoustic source models of the inserts. The success of this approach is verified through measurements at the eardrums of simulators and human subjects as well as through a listening test. The method facilitates individual in-situ equalization of inserts for binaural reproduction.