Is this what you refering… this is taken from an article written by pamela souza
example is shown in Figure 3. Suppose this patient is at a restaurant. (For now, ignore other hearing aid features like directional microphones or digital noise reduction). Let’s imagine that his dining companion’s speech reaches his hearing aid microphone at a level of 70 dB SPL. Now suppose someone seated at a nearby table is also talking, and the speech of that person (who is further away) reaches the hearing aid microphone at 50 dB SPL. For the distant talker, the output level is 68 dB SPL (gain 18 dB). For the close talker, the output level is 70 dB SPL (gain 0 dB). Although the desired effect of compression—that soft inputs receive more gain than louder inputs—is maintained, the output difference between the distant talker and the close talker is only 3 dB so the patient may perceive those sounds as of similar loudness.
Figure 3. Input output function for a hearing aid set for a compression threshold of 45 dB SPL, and compression ratio of 6:1.
If this patient comes back for a follow-up and complains that the distant speaker was as loud (or nearly as loud) as the close speaker, we might lower the compression ratio (Figure 4). Now the speech of the distant talker receives 18 dB gain, for an output of 68 dB SPL. The speech of the close talker receives 8 dB gain, for an output of 78 dB SPL. Now the output difference between the speech of the distant talker and the close talker is 10 dB, so the patient will perceive the close talker as louder. These compression ratios are higher than would likely be used in the clinic, but the concept is the same: the higher the compression ratio, the more similar the outputs for sounds of different intensity. The lower the compression ratio, the more dissimilar the outputs for sounds of different intensity will be.
Figure 4. Input output function for a hearing aid set for a compression threshold of 45 dB SPL, and compression ratio of 2:1.