The use of acoustical test fixtures for the measurement of hearing protector attenuation. Part I: Review of previous work and the design of an improved test fixture

Abstract

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.

The use of acoustical test fixtures for the measurement of hearing protector attenuation. Part II: Modeling the external ear, simulating bone conduction, and comparing test fixture and real-ear data

“…Nevertheless, there will be a high sensitivity of the insertion loss to any changes in the radiation impedance ZR at midfrequencies. This is a plausible explanation for some of the curious effects on objective and semi-obje.ctive insertion losses and on REAT which have been reported in the literature (see Schroeter, 1986). (Attenuation is mostly measured at octave-band center frequencies.…”

“…• What is the reason for the sensitivity of the model at about 2 kHz? (Also see as yet unexplained effects noted to exist at about 2 kHz in Schroeter, 1986.) Assuming small errors, Fig.…”

“…In this paper, we show that the sensitivity of the insertion loss due to manipulations of the external ear also exists for the radiation impedance. This may increase our understanding of yet unexplained effects around 2 kHz (see Schroeter, 1986). Section I summarizes part of the work reported in German by Schroeter (1983a). In Sec.…”

“…The aim is to substitute this subjective method by an objective method for as many applications as possible--either by employing a head simulator or by measuring the SPL in the ear canal of a person (see Schroeter, 1986). In principle, both objective methods measure the insertion loss of an HPD for air-conducted sound.…”

“…An earlier paper (Schroeter, 1986) reviewed efforts to use acoustical test fixtures (ATFs) for objectively measuring the attenuation of hearing protector devices (HPDs).…”