A survey was conducted of acoustic noise levels in magnetic resonance (MR) imaging systems. Static magnetic flux and radio-frequency pulses did not affect the function of shielded sound-pressure detection equipment. Noise levels were measured at bore isocenter during a variety of imaging sequences in six MR imaging systems with magnetic fields of 0.35-1.5 T. Measured noise ranged from 82 to 93 dB on the A-weighted scale and from 84 to 103 dB on the linear scale. Noise levels increased during sequences employing thinner section thickness and shorter repetition and echo times and were independent of field strength. Gradient-coil noise in MR imaging is an annoyance but is well within safety guidelines.

Current versions of the U.S. DOT Federal Highway Administration (FHWA) traffic noise model assume 500 Hz as a representative sound frequency and apply an air absorption attenuation rate of 0.54 dB per 1000 ft for the distance associated with the maximum noise level. According to the American National Standards Institute, the air attenuation rate at 500 Hz is approximately 1 dB per 1000 ft for a wide temperature and relative humidity range. However, the rate of decrease of the composite A-weighted traffic noise level depends on the spectral distribution of car and truck noise, and varies with distance. A more representative air attenuation for traffic noise is obtained by including this effect during integration as source distance varies to obtain the attenuated sound exposure level (SEL) since the traffic noise level is determined from single event SEL and number of events. In many cases, an attenuation due to air absorption of 0.5 to 3.0 dB may be warranted, as compared to less than 0.5 dB assigned by the traffic noise model. These considerations are appropriate only for the ‘‘hard site’’ case since the ‘‘soft site’’ attenuation factor assumed by the model includes extra attenuation due to shielding and air absorption. Air absorption and roadway truncation by terrain shielding may account for most of the traffic noise level attenuation now attributed to ‘‘ground absorption’’ in some cases.

Welcome to the Winter 2013 edition of Physiology News 10 Sixtieth anniversary of the famous Mount Everest ascent 12 Why does Impact Factor still have impact? Meetings & events 16 Forthcoming events Meeting preview: Physiology 2014

The sound or noise levels at the patient position in the bore of several different magnetic resonance imaging (MRI) systems were measured. The measurements were conducted with the microphone/preamplifier component connected to the sound-level meter (SLM) with an extension cable so that the SLM was remote from the MRI scan room. Preliminary tests and analyses were conducted to ascertain that the microphone/preamp functioned normally in the strong magnetic field and rf environment. Subsequently, A-weighted, flat-weighted, and octave band noise levels were measured in the MRI system bore areas during a variety of image scanning sequences. The noise levels were in the range 75–95 dB A-wt., and 85–105 dB flat-wt. Greater noise levels tended to be correlated with greater magnetic field strengths and with more rapid and detailed image scanning sequences.

Integral expressions were formulated and solved for the LWP attributable to point or line sources in terms of source noise level and impacted-area dimensions. Expressions were then derived for relative error in LWP due to uncertainty limits on source noise level and source-to-receiver distance. From these results, expected errors in LWP for variable standard deviations in source noise levels or distances were calculated. In the case of multiple sources, the average expected error in LWP for both equally distributed and normally distributed source levels, and source-to-receiver distances were estimated. This paper summarizes the study results.