The quality of an auditory stimulus depends on repeatable timing and the reliable presentation of the stimulus with respect to its spectral properties. For MEG research the range of available options is limited as the use of electrically driven systems such as loudspeakers is usually not possible because they generate large artifacts. The standard system to date is therefore a combination of a special electromechanical transducer (a tiny speaker) coupled to sound conducting tubes. these tubes are then fitted to or inserted into the ear using earplugs or -molds. The transducers is either positioned outside the MSR or inside the MSR but far away from the device. Hence, the sound conducting tubes are of considerable length and distort the spectrum of the signal heavily. If spectral properties of the stimulus are of importance, it may therefore be necessary to measure the transfer function of the transducer-tube-earpiece system and correct for the losses using a spectral manipulation of the original stimuli - e.g. by using the equalizer of the sound-card used for stimulus presentation.
Electromechanical transducers and pressure tube systems
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