This paper investigates two main features of the human head which influence the measured attenuation of circumaural and intraaural hearing protection devices (HPDs): the external ear and the different pathways of bone conduction. A theoretical model for the external ear shows that its influence on the insertion loss of HPDs, on the sensitivity level of headphones or earphones, and on the insertion gain of hearing aids, all can be described by one equation. While it is not necessary to simulate the eardrum impedance in order to measure the insertion loss of earmuffs and the sensitivity level of headphones with acoustical test fixtures (ATFs), the required accuracy of an ear simulator is more stringent when the same measurements are performed on intraaural devices. For the evaluation of HPDs, bone conduction plays an important role. We have developed a model to estimate HPD-dependent bone conduction effects. The model includes two bone conduction sources: one in the external ear and one in the middle ear. The model explains, for example, the occlusion effect of HPDs and the masking error at low frequencies due to physiological noise that arises when real-ear attenuation at threshold (REAT) measurements are made. Consequently, objectively measured insertion loss can now be used to predict REAT with improved accuracy. ATF and REAT data are compared using nine earmuffs and nine earplugs. In the majority of cases, the two sets of data agree well. Discrepancies are discussed.

This paper gives a comprehensive progress report on the development of objective methods for measuring the attenuation of hearing protection devices (HPD's), and focuses on the use of acoustic test fixtures (ATF's), i.e., artificial heads. While there are many publications on ATF's for the evaluation of circumaural HPD's (earmuffs), only one serious attempt to construct an ATF for the evaluation of intra-aural HPD's (earplugs) could be found. Consequently, no ATF for testing earplugs has been standardized so far, while two standardized ATF's currently exist for testing earmuffs [see ANSI S3.19-1974 (1975) and ISO/DIS 6290 (1983)]. Both ATF's are suited, however, only for production testing and are not designed for HPD-type testing. It is believed that both ATF's do not provide sufficiently high accuracy for HPD-type testing. A new ATF with appropriate circumaural and intra-aural flesh simulations was constructed, including a suitable ear simulator and a cast of an average pinna. Objectives for design and construction of the new ATF are discussed. The effect of using artificial flesh on the insertion loss of earmuffs (max. 5 dB at 125 and/or 250 Hz) and the effect of using a pinna (max. 12 dB lower insertion loss at 2 kHz) were evaluated.

While acoustical test fixtures (ATFs) for production testing of circumaural hearing protection devices (HPDs) have been shown to be practical (see related ISO- and ASA-activities, for example), no ATF for intraaural HPDs has been standardized so far, and we are still far away from using artificial heads in type testing of HPDs. Additionally, an ideal ATF suitable for type testing of HPDs should also be eligible for measuring headphones of all kinds. This paper reviews the progress achieved in constructing ATFs by various researchers. Special emphasis is placed on the authors' own work, parts of which were presented at the 103rd and 106th meeting of the ASA: We attacked the elaborate goal of constructing an artificial head with appropriate circumaural and intra-aural flesh simulations, which included a suitable ear simulator and a cast of an average pinna. Recently we have refined the method of taking bone conduction into account by including the vibration characteristics of circumaural HPDs and by modeling the outer- and middle-ear component of bone conduction. We can now accurately predict the masking error experienced by the subjective REAT method at low frequencies due to physiological noise. Consequently, objectively measured attenuation data may be converted to predict REAT data or vice versa. The artificial head was tested on 11 circumaural and nine intraaural HPDs.

An artificial head was developed, suitable for the measurement of earmuffs as well as earplugs. Three major factors determine it's performance: (a) The mechanical characteristics of the artificial flesh at the area of contact between the protector and the head. The mechanical impedances of 100 subjects at four points around the pinna and the shear impedances of the ear canal walls were measured. Layers of polyurethane rubber were finally designed which fit the average of the impedances. (b) Bone conduction: We suppose that a set of standard curves of bone/air conduction level differences will prove feasible to give a conservative value of attenuation by simply “adding” it to a measured insertion loss of a protector. (c) Influence of the pinna, ear canal and eardrum: It can be shown that any error in insertion loss caused by a wrong eardrum impedance can be corrected afterwards. However, it seems to be more practical using an appropriate “ear simulator.” A method to determine the needed accuracy of the coupler is given. Finally construction details and sample results compared to subjectively measured data are shown. [Work supported by Bundesanstalt fuer Arbeitsschutz und Unfallforschung, Dortmund, FRG.]

The Zwislocki- and the IEC711- ear simulators are the widely accepted standard devices whenever a mechanically stable representation of the human ear impedance is needed, e.g., for earphone calibration. Though the design of these simulators is rather complex [see Burkhard, J. Audio Eng. Soc. 25, 1008–1015 (1977) for some related problems], they were not intended to reveal “correct” results above 10 kHz. Recently measurement techniques for obtaining ear impedances data up to 20 kHz have become available [Joswig, Acoust. Soc. Am. Suppl. 1 69, S14 (1981) and Hudde, Acoust. Soc. Am. 73, 24–31, 242–247 (1983)]. To construct an ear simulator for high frequencies we studied the effect of closed-end stubs of small diameter to replace the lumped-parameter resonators used so far. These stubs do not require any special damping material. The geometrical tolerances are high. An a priori error estimation is possible during computer simulation. Predicted and measured results agree well enough to make a posteriori fine tuning redundant.