Sarah’s father quickly turns on some music, which is louder than the faint murmur of the conversation in the other room. He has masked the “crossover”.

Sarah complains that she doesn’t like that song and changes it, alas to one that is quieter. Dad can hear bits of the conversation, so he asks Sarah to turn up the volume.

Not unexpectedly, the radio may be heard in the room where grandmother and mother are talking by phone. If the radio is lower than the conversation, it’s not a problem.

A favorite song comes on and Sarah turns up the volume considerably and stops her cookie making to dance to the music. The grandmother complains “Can you turn down the radio you have in the background? It’s louder than you are and I’m having trouble understanding you.” Problematically, Sarah’s tunes have crossed from the kitchen back to the room where the all-important conversation about gift choices is taking place.

But what if the mother’s cell phone was lousy and/or the grandmother’s phone had low volume and the only way for them to converse is if the mother speaks very loudly? The mother’s loud voice reaches the kitchen, so the father has to have the radio that loud to keep Sarah from hearing. This is a no-win situation. The radio is interfering with the phone conversation, so Grandmother can’t hear Sarah’s mother, but the mother needs to speak loudly.

Sarah’s mother wouldn’t have much luck at keeping the phone conversation secret if a glass bead curtain separated the two rooms – the decorative curtain provides no sound attenuation. (Sarah’s mother would have known that other sound needs to mask her conversation right from the start.) The hollow-core interior doors in most homes provide pretty minimal sound attenuation; if the door is solid wood, the attenuation would be better.

Let’s imagine that Sarah’s parents are "new-age" types who believe that family should not be separated by solid doors. And since it’s raining outside, the mother is not inclined to go outside to make the phone call. Mom will be easily heard, any music that is in the kitchen will readily cross back to Mom and interfere with the Grandmother’s hearing about what gifts Sarah would like.

Solution: have Sarah wear earphones to listen to the music while baking the cookies. Now, for this analogy to work, the earphones have to be the foam type that sit on top of the ear and don’t block sound out – they just present the music directly to Sarah, which allows masking. Sarah’s wearing of earphones has the advantage of less chance of cross-back. Imagine how loud the music presented via earphones would have to be to be heard by Grandmother.

This part of the analogy is a bit weak, but let’s try it. Could there ever be a time when the mother’s voice (through the glass curtain) is more audible in the kitchen than over the phone? Sure, how about if phone Grandmother was using had insufficient volume? Well, that doesn’t make for a good analogy for bone-conduction testing, so we are going to have to create a far-fetched scenario. I’ll have to make Sarah an official Hamill for a moment, though I have no such niece. Hamills don’t give the extended family real Christmas gifts, we exchange gag gifts. The person who gives the best gag gift wins the coveted fish slippers, which are … slippers shaped like fish. The winner displays the fish slippers but never wears them, as they must be surrendered to the next year’s winner without toe jam contamination. Sarah’s Aunt Mary gave Sarah a pair of earphones that have giant dog ears attached, as shown in Figure 1-2. Now Sarah can hear sounds better than average so long as she’s looking in that direction of the sound source, and the cookie-cutting Sarah is indeed looking at the glass beaded door, wondering who her mother is talking to on the phone. (And Aunt Mary once again has custody of the fish slippers. Hey, I warned you the analogy was weak.)

Figure 1-2. Sarah’s foam earphones are mounted inside fur-covered giant plastic dog ears that enhance sounds from the direction in which she is looking.

Masking is a difficult topic to fully comprehend. This brief text, and its accompanying games, attempt to make masking easier to understand. We need to mask because sometimes when testing one ear, the signal could “cross over” to the non-test ear. We will always put masking in by air-conduction, and preferably using an insert earphone to produce that masking noise. If we aren’t using loud enough noise to mask the crossover, then we are “undermasking.” There are times when the masking noise is so loud it can “cross back” to the test ear and cause erroneous results – this is called overmasking.

Bone-conduction testing is especially challenging. All of the signal crosses to the non-test ear, and if you insert an earphone to start the masking process, for the low-frequency stimuli, you create a sound enhancement, called the occlusion effect. This means you will need even more masking noise in the non-test ear to nullify the occlusion effect. And… unfortunately that added noise makes cross back more likely.

There are two fundamental approaches to masking: plateau masking and formula masking. Plateau masking is akin to Sarah’s father putting on some music, checking that he can’t hear the phone conversation, then turning up the music a bit more to be sure. In plateau masking, you make incremental noise adjustments. In formula masking, you know how your masking is calibrated, and you can calculate how much noise is enough. You’ll double check your math – was that really enough (not undermasking), not too much (not overmasking)? This would be like the experienced father knowing that the soundtrack to Nemo (does Nemo have a soundtrack?) at volume 20 will mask the conversation if Sarah’s Mom is speaking at 50 dB HL behind a door.

You need to understand plateau masking well before learning about formula masking.

I’ll try to make this learning as fun as possible for you – or if that’s too high a bar then hopefully this e-book, the games, and the audiometer simulator will make the learning less painful.