Date of Award


Document Type


Degree Name

Master of Science in Electrical Engineering


Department of Electrical and Computer Engineering

First Advisor

Steven K. Rogers, PhD


This thesis improves the audio display for multiple Morse communications. Factors considered to improve the audio display are frequency of source, volume level of source, and methods of unmasking. The best frequency and volume level of a Morse source is 500 Hz at 70 dB sound pressure level (spi). Two types of masking are researched: frequency masking and expectation driven masking. Experiments showed by amplifying high pitched sources the effects of frequency masking are minimized. Other methods to compensate for frequency masking are 3-D sound and the placement of a source out of phase between the ears. Morse code recognition at 500 Hz is greatest when presented at the NO Sr condition. Greatest unmasking for broadband signals occurs at 3-D locations (between 600 and 900) where the largest ITD (interaural time difference) exists. This thesis theorizes and confirms that greatest unmasking of a source tone in 3-D sound corresponds to the spatial location that gives an ITD equal to a 1800 phase shift for that tone. NASA/Ames Research Center has demonstrated that 3-D sound improves the performance of communication personnel who are required to monitor multiple speech communications. This thesis supports that result and further provides 3-D cues for simultaneous Morse sources. Research focuses on improving accuracy and reducing fatigue rather than increasing intelligibility. Fatigue is measured by subject’s choice of which presentation option is easier to copy. The criteria for improving cues are minimal fatigue and the highest copy accuracy. The presentation options are 2-channel diotic (all sources in each ear) , monaural (each channel contains a unique source, information of a source is presented to only one ear) , 3-D angles of 0°, 10°, 32°, 45°, 58°, 69°, and 82°.

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