Typical low frequency noise levels of electric devices in experimental rooms.

Noise levels (means ± SD) were measured by a noise level meter and calculated as an average of five repeated measurements. Noise levels were measured at a distance of approximately 20 cm from the devices shown in Table 1. Background level (mean ± SD) of low frequency noise at 100 Hz was 35.7±2.7 dB SPL in 5 experimental rooms without noise-generating devices shown in Table 1.