Calculating Sufficient Masking Levels
In this section, I’m starting to talk about formula masking, but there is one big exception. In this section I am going to talk about the formula for masking a given level of signal and “pretend” that is the one and only stimulus that I need to mask. So, we’ll use case examples such as presenting a 70 dB tone to one ear and ask how would we formula mask to keep that 70 dB tone from being heard.
That’s a good and necessary question to answer at this point. In many of these examples that 70 could be the test ear’s threshold level. I just want to “go on record” now as saying that in formula masking we will think about calculating the non-test ear noise level before we find the test ear threshold… we’ll have to guess the threshold. But that is too much information for now.
There are many articles on the average
occlusion effect, and differing opinions on whether one, when plateau masking,
should add a "pad" or extra safety factor of 10 or 15 dB. In
this tutorial, I have elected to use the occlusion effect values of:
250 Hz = 15 dB
500 Hz = 15 dB
1k Hz = 10 dB
With a "pad" of 10 dB. If you use other values, that's fine. I chose these because the majority of the students I am teaching at this moment happen to have been taught these values.
I will use an air-conduction interaural
attenuation value of 50 dB, and am assuming insert earphones are used.
1) Let's look
at several masking cases and determine if the masking level used was
sufficient. In this first case, the tympanograms are normal and acoustic
reflexes are present, so sensorineural pathology is likely. The tone is
presented by air conduction. Rather than using a plateau approach, the
audiologist desides to put in 40 dB of masking in the NTE
Is the 40 dB EM adequate?