Audiometric masking

1. Audiometric Masking Ozarks Technical Community College

2. Inhale….now, exhale Masking is a very difficult concept to understand You must understand the science behind masking Masking requires memorization of interaural attenuation values and formulae for the amount of masking to use Many testers are nervous that they are not masking effectively/appropriately. When in doubt, ask the patient which ear they are hearing the beeps in! Example: If you are masking in the right ear and trying to obtain the left ear threshold, ask the patient which ear they hear the beeps in. If they say, “right,” then you know you need to increase the masking noise in the right ear in order to prevent that ear from hearing the sound.

3. Why do we mask? To prevent the non-test ear (NTE) from participating in the test (cross-hearing) We want to make sure that when we are testing the right ear, that we are getting the true threshold of the test ear (TE) and that the NTE is not helping out This is necessary because of BONE CONDUCTION! O ur ears are housed in a bony skull, which vibrates BOTH cochleae when sound is present at a certain level

4. Interaural Attenuation Interaural attenuation (IA) is the amount of sound that is attenuated/reduced when crossing from one ear to the other The IA varies: Depending on the frequency From patient-to-patient (skull thickness) Depending on the transducer used (phones, inserts, bone)

5. IA of Air Conduction - Headphones The minimum IA of headphones is 40 dB HL If we present 80 dB to the test ear, 40 dB of that sound is reaching the cochlea of the non-test ear Image from: wikipedia.org

6. IA of Air Conduction - Inserts The minimum IA of inserts is 70 dB HL If we present 80 dB to the test ear, 10 dB of that sound is reaching the cochlea of the non-test ear You are less likely to mask for air-conduction testing when you use inserts  Image adapted from: wikipedia.org

7. IA of Bone Conduction Image from: wikipedia.org The minimum IA of bone conduction is 0 dB HL If we present 30 dB to the test ear, 30 dB of that sound is reaching the cochlea of the non-test ear You must mask for bone-conduction testing in conductive losses and asymmetrical losses 

8. When to M ask-Air Conduction Headphones : when there is a 40 dB or more difference between the air conduction threshold of the test ear and the bone conduction threshold of the non-test ear. Inserts: when there is a 70 dB or more difference between the air conduction threshold of the test ear and the bone conduction threshold of the non-test ear. NOTE: B ecause we begin our audiogram with air conduction testing, we assume that there is no air-bone gap and compare air-to-air to determine the need for masking. Occasionally, you will have to go back and mask some AC thresholds after you have actual BC thresholds to compare to.

9. When to Mask - Bone Conduction W hen there is a 15dB or more difference between the air conduction threshold of the test ear and the bone conduction threshold of the same ear aka: Mask for BC when there is an air-bone gap

10. Types of Masking Noise Puretone Testing The masker is a narrow-band noise (NBN) that is centered around the test frequency Speech Testing The masker is a wide-band noise that consists of sound energy from 300-3000 Hz

11. How Much Noise? Well, you need to “cover up” the NTE….so, the level of the masking noise has to be greater than the patients AC threshold in the NTE, but not so loud that overmasking occurs.

12. Undermasking Not using enough noise in the non-test ear The obtained threshold is really the response of the non-test ear

13. Overmasking Using too much noise The obtained threshold is exaggerated when the masker crosses the head and shifts the threshold of the test ear. Traditionally, we use the plateau method to ensure that we have obtained a true threshold and are not undermasking or overmasking .

14. The Plateau Method Always start with the noise 10 dB above the AC threshold of the NTE With noise in the NTE, present the tone in the test ear at threshold level If the patient hears the tone, increase the masking noise in the NTE by 5 dB If the patient does not hear the tone, increase the tone in the TE until the patient responds After the patient responds to the tone with three consecutive, 5 dB increases of noise in the NTE, you have achieved your plateau, and the patient’s masked threshold can be recorded.

15. Occlusion Effect During masking for bone conduction, the bone oscillator is on the TE mastoid, while the NTE is plugged up with a phone or insert. In this situation, the occlusion of the plugged up, non-test ear will actually increase the likelihood that the NTE will respond to the tone (especially at .25, .5, and 1 kHz). As a result, the patient’s occlusion effect must be added to the AC threshold of the non-test ear at the onset of masking. It’s recommended to add an extra 10 dB at .25 and .5 kHz and an extra 5 dB at 1 kHz. So instead of starting your masking at 10 dB above the NTE threshold at .25 kHz, you would start your masking at 20 dB above the threshold.

16. The Masking Dilemma In bilateral, conductive hearing losses, the amount of masking noise required is often sufficient to result in overmasking and a shifting of the true threshold. In these cases, it is best to record the unmasked thresholds and note below the audiogram, “could not mask-masking dilemma”

17. Pearls of Wisdom When in doubt, mask Using insert earphones will greatly reduce the need for masking due to a higher interaural attenuation Inserts also reduce the risk of overmasking Ask the patient to tell you which ear they hear the beeps in (if they can)