Third Update: More Noise

Since the last post I have performed several more experiments at increasing dB level and increasing length of exposure to the noise.  Most recently I have tested 4 birds with a noise event of broadband noise at a dB level between 115 and 120dB and 4 birds with a noise event of a pure tone of 3kHz at a dB level of 120dB both for a length of 2 hours.

These experiments yielded results that suggest mild hearing loss and recovery in about a week’s time.  There does not appear to be a difference between birds that were exposed to the Pure Tone Noise versus birds exposed to the Broadband Noise.  This is a small sample size, however I believe it to be accurate.

I recently read a paper that analyzed species differences in hearing loss at a severe level of hearing loss, comparing zebra finch, canaries, budgerigars and quail.  This paper found that exposure to a pure tone (~3kHz) at 112dB for 12 hours produced no immediate measurable hair cell damage in zebra finch and canaries.  However exposure to 120dB broadband noise for 24 hours produced a significant amount of hair cell damage and a significant threshold shift of 50dB.  Impressively, the zebra finch returned with 10 dB of normal threshold in one month.

[Threshold is the measured decibel when a sound is heard half the time.  So an increase of 50dB in the threshold means that instead of hearing a particular sound at 5dB half the time, these birds cannot hear that same sound until it is played at 55dB.  This is an extremely large difference because the decibel system is logarithmic. An increase in 10dB is equivalent to a doubling of the intensity of the sound.]

This paper suggests to me that I am changing the noise event in a direction that is beneficial to my purpose of damaging zebra finch hearing and that these birds are extremely resilient.  It will take more noise than I expected originally to damage their hearing.

[As a reminder, my intention for damaging zebra finch hearing is to be able to test methods of protecting their ability to hear from noise damage.  In order to assure that the healing and prevention of hearing loss is due to the treatment of the coming experiment, I must be able to consistently damage all individual zebra finch’s hearing.]