Instrumentation for transient evoked otoacoustic emissions elicited by maximum length sequences

A system for recording Transient Evoked Otoacoustic Emissions (TEOAE) for neonatal screening, utilizing Maximum Length Sequences (MLS) for the generation of stimulus signals, is described. The main advantage of the system is its ability to obtain responses within a very short period of time, compared to conventional OAE systems. The actual recording time for the MLS system is reduced by a factor of between 10 and 20 for comparable signal-to-noise ratios. Stimulus levels range from 50 to 90 dB peak equivalent SPL and available stimulus rates are 31, 62, 125, 250, 500, 1000 and 2000/sec. A dynamic noise rejection algorithm is performed during data collection, and, moreover, all sweeps (both accepted and rejected) are stored in memory so that the signal-to-noise ratio can be optimized by post-processing immediately after data collection. As a result, robust response can be achieved even if the subject is noisy during the test. A new semi-non-linear recording technique is described, combining the benefits from the conventional linear and non-linear recording methods. Examples of adult and neonatal responses under different conditions are provided to demonstrate the value of the recording principle.