Sound comparison of seven TMS coils at matched stimulation strength.

Background: Accurate data on the sound emitted by transcranial magnetic stimulation (TMS) coils is lacking.

Methods: We recorded the sound waveforms of seven coils with high bandwidth. We estimated the neural stimulation strength by measuring the induced electric field and applying a strength-duration model to account for different waveforms.

Results: Across coils, at maximum stimulator output and 25 cm distance, the sound pressure level (SPL) was 98-125 dB(Z) per pulse and 76-98 dB(A) for a 20 Hz pulse train. At 5 cm distance, these values were estimated to increase to 112-139 dB(Z) and 90-112 dB(A), respectively.

Conclusions: The coils' airborne sound can exceed some exposure limits for TMS subjects and, in some cases, for operators. These findings are consistent with the current TMS safety guidelines that recommend the use of hearing protection.