Human Exposure to Low Frequency (20 Hz – 20 kHz) Magnetic Fields from Earphones

While much information is available about the biological effects and the exposure levels of Magnetic Fields (MFs) at power frequency (50/60Hz), there is an absence of such data at other low frequencies (up to 100kHz), mainly due to lack of widespread exposure sources in this range. Audio signals at acoustic frequencies (20Hz–20kHz) are usually produced by devices such as earphones, consisting of an electric current-bearing coil, fed by an alternating electric current, thus generating MFs at these frequencies. Given the limited knowledge described above, it is of great importance to assess the exposure from these sources.

In this study, we measured the MFs from several earphones of various manufacturers, sizes and designs. The MFs were measured by an EMDEX-II MF-meter attached to the earphones, which were connected to an MP3 player playing either various types of music or tones at a single frequency and at different sound levels. Substantial effort was dedicated to overcome technical challenges, i.e. calibrating the meter (that demonstrated a significant nonlinear frequency response with fairly poor response at high frequencies), measuring the sound absolute intensities, and more.

An important result was that different earphones generate not only different MF levels, but also very different patterns of frequency response. Out of three earphones that were intensively measured, the MFs near two of them were 1-10mG depending on the sound frequency, while the fields near the third one were 5-35mG (for comparison, the average 24-hour exposure of adolescents in Israel to power frequency MFs was found to be 0.73±0.67mG, MFs above 3-4mG were classified by the International Agency for Research on Cancer as possibly carcinogenic to humans, and the international exposure limits at this frequency are ≥2,000mG). The average MFs measured while playing classical music, hip-hop, jazz and rock-and-roll were 1.7, 4.4, 5.7 and 5.9mG respectively, in correlation with their average measured sound level. However, due to the complex frequency content of the music, these results were not corrected for frequency response, which leave them underestimated. As expected, for any single tone frequency, the MF intensified while increasing the sound level. Finally, we demonstrated that the MFs are not generated by coil vibrations or by the permanent magnet movements, but rather by the current alone.

To summarize, although MFs from earphones are limited to a very small area of the body and seem to be very low relative to international recommendations, they are fairly high when compared to typical levels of exposure form power frequency sources. Moreover, earphones are used in close proximity to the head, for significant lengths of time, and by many users. Further research is necessary to validate and extend the results of this innovative study.