To measure the sound levels generated by the headphones of commercially available portable compact disc players and provide hearing healthcare providers with safety guidelines based on a theoretical noise dose model.
Using a Knowles Electronics Manikin for Acoustical Research and a personal computer, output levels across volume control settings were recorded from headphones driven by a standard signal (white noise) and compared with output levels from music samples of eight different genres. Many commercially available models from different manufacturers were investigated. Several different styles of headphones (insert, supra-aural, vertical, and circumaural) were used to determine if style of headphone influenced output level.
Free-field equivalent sound pressure levels measured at maximum volume control setting ranged from 91 dBA to 121 dBA. Output levels varied across manufacturers and style of headphone, although generally the smaller the headphone, the higher the sound level for a given volume control setting. Specifically, in one manufacturer, insert earphones increased output level 7–9 dB, relative to the output from stock headphones included in the purchase of the CD player. In a few headphone–CD player combinations, peak sound pressure levels exceeded 130 dB SPL.
Based on measured sound pressure levels across systems and the noise dose model recommended by National Institute for Occupational Safety and Health for protecting the occupational worker, a maximum permissible noise dose would typically be reached within 1 hr of listening with the volume control set to 70% of maximum gain using supra-aural headphones. Using headphones that resulted in boosting the output level (e.g., insert earphones used in this study) would significantly decrease the maximum safe volume control setting; this effect was unpredictable from one manufacturer to another. In the interest of providing a straightforward recommendation that should protect the hearing of the majority of consumers, reasonable guidelines would include a recommendation to limit headphone use to 1 hr or less per day if using supra-aural style headphones at a gain control setting of 60% of maximum.
While governmental regulations based on established damage-risk criteria are in place to protect the hearing of people occupationally exposed to toxic levels of noise, guidelines for protection against recreational noise exposure are limited. Previous literature has documented that personal stereo systems (headphones) are capable of delivering potentially toxic levels of sound under certain conditions, but no clinically applicable guidelines are available for the hearing healthcare provider to recommend responsible use. This study sought to measure output levels from a variety of manufacturers of personal stereo systems and several different styles of headphones and calculate the theoretical listening duration and volume control setting that would constitute a hazardous noise dose. Findings indicated variation in output levels among CD player manufacturers and systematic differences in output levels depending on the style of headphone. All CD players studied were capable of delivering sound levels that could result in toxic noise exposure given sufficient listening duration. Guidelines constituting responsible portable CD player use are provided for specific CD players and headphones at various volume control settings.
Sargent College of Health and Rehabilitation Sciences, Boston University (B.J.F.) and Boston University School of Medicine (L.C.C.), Boston, Massachusetts.
Address for correspondence: Brian J. Fligor, Children’s Hospital Boston, 300 Longwood Avenue, Boston, MA 02115.
Received August 14, 2003; accepted June 18, 2004.