## 1 Introduction  
Aural cartilage gives form to the pinna and the exterior half of the external auditory canal. When a transducer is pressed against the pinna, the cartilage acts as a diaphragm and produces sound in the external auditory canal [1, 2]. The generated sounds and propagated vibrations that arise when a transducer is applied to the ear lobe (adipose tissue) and temporal bone are weaker than those at the tragus, scaphoid fossa, and concha because these latter structures contain cartilage [3, 4]. As a result, this sound pathway is termed _cartilage conduction_[2, 3, 4, 5, 6, 7, 8, 9]. Hearing devices based on cartilage conduction (e.g., hearing aids [10] and earphones [11]) are currently being marketed in Japan. Although cartilage conduction and bone conduction are both realized by touching a transducer to the user's body, the vibrations that propagate through the skull bone when cartilage is stimulated are sufficiently small to disregard the contribution of hearing perception [6]. Although cartilage-conduction-based devices are widely used, there are no standardized procedures or indicators for measuring the output volumes of such devices. Because the transmission pathway of cartilage conduction differs from those of air and bone conduction, the calibration methods of the International Organization for Standardization (air conduction [12] and bone conduction [13]) do not apply to cartilage-conduction-based devices. For example, for bone conduction, an artificial mastoid (Type 4130; Bruel & Kjaer, Naerum, Denmark) is used to simulate the mechanical impedance of the human mastoid [14]. The output of an artificial mastoid can be calibrated by pressing a transducer onto the mastoid sensor with a force of 1 N. However, an artificial mastoid is unfit for monitoring sound in the external auditory canal because it does not include a pinna simulator. To measure air conduction, the sound pressure at the ear drum can be simulated using a pinna simulator [15] embedded in a head-and-torso simulator (HATS) [16]. However, the output from the HATS is different from that obtained in the human external auditory canal by inserting a probe microphone in it, even if the same transducer is used to stimulate the ear at the same position. For instance, when a ring-shaped transducer was placed on the entrance of the external auditory canal, the sound pressure levels (SPLs) in the canal disagreed with those obtained using the HATS, especially in the low-frequency range, i.e., below 1.5 kHz (see Figure 1a) [17]. However, we found that a soft polyurethane pipe attached to a skull bone model could be used to simulate the aural cartilage of the external auditory canal and that this arrangement was able to produce an SPL that was consistent with data from human ears at this low-frequency range (Figure 1b). Polyurethane resin was used to mimic the elasticity of human skin (human skin gel, Exseal Corporation, Mino, Japan). In contrast, the pinna simulator in the HATS (Type 4128; Bruel & Kjaer, Naerum, Denmark) is embedded as a unit in a silicon rubber base (width \(\times\) height \(\times\) thickness: 50 \(\times\) 60 \(\times\) 10 mm\({}^{3}\)), and the hardness (Shore 00 35 or Shore A5 from durometer measurements) is lower than that of human aural cartilage. The hardness of the pinna simulator in the HATS is likely to be an important variable in accurately simulating the cartilage conduction sound. Therefore, our research group examined the generated cartilage-conduction sound in five ear simulators, each made of silicon rubber with a different hardness [18]. The ear simulators included the pinna and external auditory canal, and the cartilage-conduction sounds were measured via a probe microphone (type 4182; Bruel & Kjaer, Naerum, Denmark) inserted into the canal. Our results indicated that the spectral characteristics of the cartilage-conduction sounds recorded from the ear simulators approached those from human ears when the hardness of the simulators coincided with that of human aural cartilage (Shore A10 to A20).