Question
Please solve a conflict: must you still account for the occlusion effect when masking if using insert phones, even if there is shallow insertion?
Answer
For all intents and purposes, unmasked thresholds, for both air conduction (AC) and bone conduction (BC) should be obtained prior to masking for actual threshold values. During bone conduction testing, the bone vibrator should be placed on the better ear, and theoretically, supra-aural headphones and insert earphones should be removed to avoid the occlusion effect. Having one ear occluded can improve BC thresholds, which will give a false result. Once unmasked bone conduction thresholds have been obtained, then the surpa-aural headphone or insert earphone should be placed on the non-test ear (NTE) for masking purposes. Masking protocols will not be discussed here in detail. For additional information on masking protocols, please refer to the reading materials in the references section.
If insert earphones are inserted deeply into the ear properly (the foam flush with the outer ear canal), then the values for accounting for the occlusion effect are very small (Turner, 2004, 44). For those of you who follow the "plateau masking method", Yacullo (1996, 128) recommends 10 dB at 250 Hz, and no correction values at any other frequency when using insert earphones. For supra-aural earphones, Yacullo (1996, 63) recommends correction factors of 30 dB for 250 Hz, 20 dB for 500 Hz, and 10 dB for 1000 Hz, and no correction for higher frequencies. The above correction values would be in addition to the 10 dB that is added to the AC threshold of the NTE.
For those of you who follow the "optimized masking method" proposed by Robert G. Turner, when testing BC thresholds, always set the initial masking level at least 30 dB greater than the unmasked BC threshold. This strategy compensates for occlusion effect correction factors up to 20 dB, for frequencies 250-1000 Hz.
Insert earphones must be inserted deeply into the ear canal. Shallow insertion of insert earphones can lead to leaking of the sound out of the ear, thereby elevating your air conduction thresholds because a louder tone is needed to produce a response. Anytime when supra-aural headphone is covering the NTE or an insert is in the NTE, the occlusion effect is a factor and must be accounted for. Masking must be increased to offset the additional sound pressure that is transmitted to the cochlea. The correction values are in addition to the normal amount of masking noise presented to the NTE, and you must account for occlusion effect for frequencies 250-1000 Hz when using supra-aural headphones, and only 250 Hz when using insert earphones.
References
Katz, Jack (ed.). Handbook of Clinical Audiology: 5th Edition. Baltimore, MD: Lippincott, Williams and Wilkins.
Turner, Robert G. (2004). Masking Redux I: An Optimized Masking Method. Journal of the American Academy of Audiology 15: 17-28.
Turner, Robert G. (2004). Masking Redux II: A Recommended Masking Protocol. Journal of the American Academy of Audiology 15: 29-46.
Yacullo, W. (1996). Clinical Masking Procedures. Needham Heights, MA: Allyn and Bacon.
Marni Novick currently works at Pacific Eye and Ear Specialists in West Los Angeles. Marni received her Master's Degree from the University of Iowa in 2000 and completed a Masters' Thesis under direction of Ruth A. Bentler, Ph.D. In 2004, she received her Au.D. from the Pennsylvania College of Optometry, School of Audiology. She has been practicing audiology for 5 years. In her spare time, Marni enjoys spending time with her family and reading books.