Text of card field “Info” of the Masking Model Page 1 of 3

Masking Model

The purpose of this model is to demonstrate phenomena related to masking: cross hearing, the occlusion effect, and effective masking.

The following abbreviations are used:

Tac: Threshold by air conduction

Tbc: Threshold by bone conduction

ABG: Air-bone gap

S: The signal level

N: The noise level

MMN: Minimum masking for normals

HC: Headphone coupling factor

MR: Meatal resonance

IA: Interaural attenuation

OE: The occlusion effect

Audiometer Controls

To select the right or left ear, bone conduction, etc. just click the button. Increase or decrease the tone presentation level by clicking the up arrow or down arrow (or press the "a" key or "z" key). To increase or decrease the noise presentation level press the "s" or "x" key. Note: to use these keys, clear the check from the "Normal Keyboard" button.

The tone or noise are presented only when their buttons are hilited. Click the noise button to turn it on permanently (clicking it again will turn it off).

Change Thresholds

To change Tac or Tbc just click on the box that contains these names. The default values for MMN, HC and MR can be changed by clicking in the boxes that contain these names. Setting MR to 20 or 25 dB simulates testing at a low frequency (where the occlusion effect is present). Setting MR to 0 dB is like testing at a high frequency where there is no occlusion effect.

The Interaural Attenuation (IA) & Occlusion Effect (OE)

The abbreviation HC stands for headphone coupling factor, and MR stands for meatal resonance or radiation. HC represents the difference between the level of the air conduction (AC) signal delivered by the headphone, and the bone conducted (BC) signal also delivered (to the skull) by the headphone. So,

HC = AC - BC

HC is a measure of the efficiency of the earphone cushion as a bone conduction vibrator (i.e., a larger value for HC implies a less efficient coupling).

When a bone conducted signal is delivered to the skull, sound is radiated into the ear canal from vibrations of the osseous walls of the ear canal. If SPL_occluded is the dB SPL with the ear canal occluded, and SPL_unoccluded is the dB SPL with the ear canal open, then the MR is the difference:

MR = SPL_occluded - SPL_unoccluded

Together with the audiometric air-bone gap (ABG), HC and MR define the IA and OE according to the following equations:

OE = MR - ABG, OE >= 0

and

IA = HC - OE, IA <= HC

The OE is the meatal resonance reduced by the ABG. However, the OE is never less than 0 dB. The IA is the headphone coupling factor reduced by the OE, but the IA is never greater than the HC since OE cannot be less than 0 dB. It is assumed (for purposes of the simulation) that HC and MR are dependent upon properties of the skull, and are different for each patient. But, HC and MR are the same for each ear of the same patient. However, since ABG varies for each ear, the IA and OE are ear dependent (when the ABG is not the same for both ears).

For example: if HC=60 dB and MR=20 dB, and there is no ABG, the effective interaural attenuation is 40 dB, the conservative value conventionally given to the IA. If there is a 5 dB ABG, then the OE will be 15 dB, and IA will be 45 dB.

The Flow Chart

When the tone button is clicked the model shows the "flow" of tones and noise through the auditory system, mastoid, and skull. The dB level of the tone and noise is shown in the yellow fields. The fields are formatted so that:

• If a single number appears, it's the tone dB ("10" means there is a 10 dB tone;

• If tone and noise is present (like the tone is 10 dB and the noise is 15 dB), it's "10/15," as in S/N, the signal to noise ratio;

• If only noise is present (say 15 dB of noise, but there is no tone), it's displayed as "--/15";

An air conduction signal passes through the middle ear (where it will be attenuated by an ABG) to the cochlea. If the signal (dB level of the tone) at the cochlea is >= to the Tbc then one condition for a response is satisfied:

S >= Tbc

The second condition is that the tone must be above the "floor" set by the noise. The noise floor is the dB level of the noise reaching the cochlea minus MMN:

Noise_Floor = N - MMN

If the signal level exceeds the noise floor (S >= Noise_Floor), then the tone is audible in the noise. The tone is not heard if S < Noise_Floor.

If both conditions are true then the tone is heard (i.e., "Heard RE" is hilited if audible in the right ear or "Heard LE" is hilited if audible in the left). For example, in the right ear:

If (S >= Tbc) and (S >= Noise_Floor) then "Heard RE"

If an air conduction tone is delivered (say to the right ear) there are five possible ways the tone could be heard. There are three paths to the right cochlea and two paths to the left cochlea. So there is often more that one signal level presented to each cochlea. The most intense signal (or noise) is selected and put into the Signal, Noise field, "S = the most intense signal, N = the most intense noise."

When a bone conduction signal is delivered to the mastoid (skull) there are four possible ways the tone could be hear: two paths for the right ear and two paths for the left ear.

For an illustration of these paths, and more information about the model, click "Masking Paths To Cochlea."

The End