1	Headphone Noise: Occupational Noise Exposure Assessment for Communication Personnel A. Peretti^a,b, F. Pedrielli^c, M. Baiamonte^a, F. Mauli^d and A. Farina^e ^a Peretti e Associati sas, via Ivrea 1/4, 35100 Padova, Italy, ^b Scuola di Specializzazione in Medicina del Lavoro, University of Padova ^c Imamoter, National Research Council of Italy, via Canal Bianco, Ferrara ^d Gruppo Banco Popolare di Verona e Novara (BPVN), Verona ^e Industrial Engineering Department, University of Parma
2	Background Headphone-sound risk rarely taken into account Large number of workers subjected to this risk Many of them do complain about the problem The risk of hearing loss appears to be directly connected with the occupational profile of these workers, and consequently the employee is responsible of it
3	Law and technical standards In Italy the law which rules the occupational noise control is D.L. n° 277 (1991) This law specifies to make measurement far from the body of the worker, for avoiding modification of the sound field, but in the same position where the ears of the worker happen to be during the normal exposure. This of course is completely unapplicable to the case of sound produced by external or in-the-ear headphones This topic is covered by two preliminary standards:
4	Outline of the method Measurement through the microphones of a dummy head The headphone is connected in parallel with a second unit, being weared by the operator An 1/3 octave spectrum of the equivalent level is measured at each ear for the whole exposure period From the measured spectrum it is possible to derive a correspondent far-field noise spectrum, which would create the same spectrum at the ear of the mannequin. This means to remove the head-related transfer function
5	Spectral correction 1/3 octave spectrum measured through a dummy head Subtraction of the head-related transfer function The A-weighted total value is computed from the modified spectrum This A-weighted total value represents the “normal” noie exposure of subject immerged in a soundfield capable of creating the same effects inside the ears as the sound generated by the headphones
6	The head-related transfer function Diffuse field and free field Values suggested by the ISO 11904-II standard Values declared by the manufacturer (B&K type 4128) The manufacturer’s data where verified in anechoic chamber
7	What HRTF is better? The experimental verification has shown differences even greater than those between the declared data and the ISO values But in practice, applying any of these 6 different curves to the measured spectra, cause limited variations of the estimated far-field equivalent A-weighted noise level These variations where always less than 1 dB(A) for the worst cases, meaning that in practice the usage of any of these HRTF spectra is acceptable.
8	Instruments employed Bruel & Kjaer type 4128 head and torso simulator (equipped with 2 ear simulators) Larson Davis LD-2900 dual-channel, real-time, 1/3 octave spectrum analyzer B&K type 4230 reference sound source Outline ET-1 electronic turntable PC equipped with an Echo Layla sound card and employing the software tools Cool Edit Pro and Aurora.
9	Experimental results 49 workplaces, 96 measurements, total averaging time : 17 h
10	Headphones with limiter The electronic circuit automatically reduces the gain when the sound exceeds a predefined value The response time is less than 10ms After 200ms, a further gain reduction is applied
11	Conclusions Literature does not report cases of deafness due to exposition to headphone sound. Nevertheless, the experimental values reported here show that in some cases the workers are subjected to potentially risky sound pressure levels (L_Aeq > 80 dBA). Consequently, some risk-reductive actions are to be taken: Employment of headphones equipped with active electronic limiter Reduction of the background noise (this makes it possible to select a lower listening volume, still ensuring good intelligibility) Instructing the personnel to proper usage of these electronic devices and to adopt a proper behavior, aimed to the reduction of the exposure to harmful sound levels The methodology developed allows for the periodic evaluation of the risk and for the assessment of the exposure reduction obtained.