In-Ear Headphones on Ear Canal Simulator vs Real Human Ear Geometries: Quantifying the Differences with Simulations

“Measurements of the pressure at the ear Drum Reference Point (DRP) in humans are extremely difficult and bear a high risk of injury to the eardrum and ear canal. In order to circumvent this challenge, ear simulators have been developed to predict a population average of the pressure response at DRP. I.e. the results of the simulator should mimic the acoustic behavior of an average human ear. However, these simulators do not predict the range and variance of the pressure responses at DRP, only their average. Recent research by Olive et al. suggests that there is significant variance in frequency response of over-ear headphones, and they emphasize the need for personalized headphone solutions [1]. We can estimate the pressure response at DRP for in-ear headphones outfitted with microphones in the cavity between headphone driver and eardrum by means of a transfer function G(f) that relates the pressure at DRP with the pressure inside the earbud. |D(f)|=|H(f)|+|G(f)|, Where |H(f)| is the magnitude of pressure response in dB at the microphone inside the earbud, |D(f)| is the magnitude of the pressure response in dB at DRP, and G(f) is the transfer function from pressure at earbud microphone to pressure at DRP. If an accurate (personalized) function for G_p (f) can be found, then we can accurately predict D_p (f), but obtaining an accurate G_p (f) by means of measurement is nearly impossible in live humans, and using an average approximation of G_a (f) does not accurately describe the personalized sound pressure response at DRP. In order to quantify the variance of error between average G_a (f) provided by the simulator and the personalized G_p (f) in a real human ear we used finite element simulations of MRI scans of 10 subjects (20 ears) at five different insert depths each. The MRI scans were obtained from the openly available “IHA database of human geometries including torso, head, and complete outer ears for acoustic research” [2]. From these simulations we obtained a spread of personalized H_p (f) and D_p (F), from which we could calculated 100 different personalized G_p (f). This ensemble of personalized G_p (f) were then compared to the G_a (f) obtained on G.R.A.S HATS simulator outfitted with 711 couplers. The range of errors observed at frequencies below 1 kHz was less than ±1 dB, assuming no leakage in all cases. Between 1 kHz and 4 kHz, the range of errors was within ±5 dB and above 4 kHz the error grows to ±15 dB or more. These results align well with the observations of Olive et al. that personalized solutions are needed in headphones just room equalization is needed for an optimal listening experience with a convention speaker setup. The difference in shapes of the ear canals are large enough to warrant the investigation of personalized equalization, which can deviate from the standard voicing which is often performed on ear canal simulators. “