Appendix III
. Evaluating Noise Exposure of Employees Wearing Sound-Generating Headsets
Employees at Risk
Noise overexposure in the workplace can occur where employees wear a communications headset as part of their employment. Clerical personnel, aircraft pilots and other cockpit personnel, air traffic controllers, emergency personnel, reservation clerks, receptionists, and telephone operators are just a few examples of the more than three million workers who can be exposed to high noise levels via communications headsets. For a person wearing a sound-generating headset, the sound/noise exists predominantly between the ear drum and the headset. Because of the amplification properties of the human ear, the sound that exists inside the ear while wearing a headset is quite different from ambient levels.
Probe microphones and similar devices allow sound levels to be measured inside the ear. Most people, however, find that inserting a probe microphone into their ear canal is uncomfortable and object to wearing a probe for an eight hour workday. In addition, there is a possibility of damaging the ear drum, which means that the person inserting the probe requires careful training. For these reasons, probe microphones should not be used for compliance purposes.
Methodology
A method of monitoring employee exposure without invading the ear canal has been developed. This sampling method evaluates the noise dose that an employee receives during the actual workday while wearing an insert type headset, a monaural or binaural muff, or a monaural or binaural foam headset. The technique measures the electrical signal input to the worker's headset using a conventional Bruel & KjaerTM noise dosimeter and a set of filters (audio equalizer). The filters take into account the electrical-to-acoustical conversion efficiency of the headset and the differences between the sound levels occurring in the ear canal and the free field. The method is safe, easy to operate, and convenient for field use. Current analysis indicates that the accuracy of the system is ±1.5 dB.
Prior to sampling, the actual headset or an identical model of the headset that is worn by the employee must be sent to the OSHA Salt Lake Technical Center to calibrate the equipment. The headset is placed on a head and ear simulator, which simulates the acoustic response of a median human ear. White noise is input to the headset and the transfer function for the headset is determined at all frequencies over the range of human hearing.
A digitally programmable one-third octave filter set (audio equalizer) is preprogrammed with the transfer functions obtained in the earlier laboratory headset evaluation. Employee monitoring can then be conducted in the workplace. The input signal to the employee's headset is split, with one branch going into an audio equalizer attached to a Bruel & KjaerTM dosimeter, and the second branch going into the employee's headset. After the sampling period (which is normally an eight hour workday), the employee's noise dose (in percent exposure) is recorded from the dosimeter. Any citations for employee overexposure must be issued in accordance with procedures outlined in the OSHA Field Inspection Reference Manual.
Contact the OSHA Salt Lake Technical Center at (801) 233-4900 for more information.
Acoustic Limited Devices
Laboratory evaluations have determined that headsets can be categorized in two basic groups:
* Those without any form of electronic limiting device.
* Those with some form of limiting device built into the headset.
Most modern telecommunication headsets use sophisticated limiting circuits, while headsets for special applications (for example, WalkmanTM head-sets) do not. Headsets that contain acoustic limiting devices that are functioning as they were designed have been shown in both laboratory and field tests to provide sufficient protection to maintain employee noise exposures below OSHA permissible noise levels. However, headsets without limiting devices have, in some work environments, caused employee noise exposures to exceed the levels permitted by OSHA.
