to excessive sound levels, fewer, reduced, or no OAEs can be present. This could be an indication of noise-induced hearing loss before it is seen on an audiogram. In one study, a group of subjects with noise exposure was compared to a group of subjects with normal audiograms and a history of noise exposure, as well as a group of military recruits with no history of noise exposure and a normal audiogram. They found that an increase in severity of the noise-induced hearing loss resulted in OAEs with a smaller range of emissions and reduced amplitude of the emissions. The loss of emissions due to noise exposure was found to occur mostly in higher frequencies, and it was more prominent in the groups that had noise exposure in comparison to the non-exposed group. It was found that OAEs were more sensitive to identifying noise-induced cochlear damage than pure tone audiometry. In conclusion, the study identified OAEs as a method for helping with detection of the early onset of noise-induced hearing loss. It has been found that distortion-product otoacoustic emissions (DPOAE's) have provided the most information for detecting hearing loss in high frequencies when compared to transient-evoked otoacoustic emissions (TEOAE). This is an indication that DPOAE's can help with detecting an early onset of noise-induced hearing loss. A study measuring audiometric thresholds and DPOAEs among individuals in the military showed that there was a decrease in DPOAEs after noise exposure, but did not show a shift in audiometric threshold. This supports OAEs as predicting early signs of noise damage. Biometric importance In 2009, Stephen Beeby of the University of Southampton led research into utilizing otoacoustic emissions for biometric identification. Devices equipped with a microphone could detect these subsonic emissions and potentially identify an individual, thereby providing access to the device, without the need of a traditional password. It is speculated, however, that colds, medication, trimming one's ear hair, or recording and playing back a signal to the microphone could subvert the identification process. Measuring otoacoustic emissions on earphones High-end personalized headphone products (e.g., Nuraphone) are being designed to measure OAEs and determine the listener’s sensitivity to different acoustic frequencies. This is then used to personalize the audio signal for each listener. In 2022, researchers at the University of Washington built a low-cost prototype that can reliably detect otoacoustic emissions using commodity earphones and microphones attached to a smartphone. The low-cost prototype