NAL & COMPRESSION

The evolution of NAL fitting formulae have always been based upon the theory of creating enough gain for audibility.

It is a formula which evolved from it’s early days as a linear fitting formula with one gain target to it’s current compression formula with three gain targets.

NAL & COMPRESSION

The research used to evolve the linear formula into a compression formula revealed that, for gently sloping hearing loss audiograms ranging from mild to severe in degree, similar gain targets were required for sixty-five decibel inputs.

NAL & COMPRESSION

For most types of hearing loss, the mid frequencies of speech are aided so that they will sound similar in loudness to the lower and higher adjacent speech frequencies bands. This suggests that maximal speech intelligibility should be received.

NAL & COMPRESSION

Although normal hearing people hear vowels louder than high frequency consonants, NAL proposes that speech intelligibility is maximized if all frequencies of speech are equalized rather than normalized.

NAL & COMPRESSION In other words, NAL proposes to

preserve the loudness relationships (soft, average, and loud inputs) of the speech frequency spectrum; compared to DSL which proposes to preserve unaided loudness relationships of all soft, average, and loud outputs.

In effect “equalize” instead of normalize.

NAL & COMPRESSION

With this “equalization” approach to speech audibility, it is theorized that maximum speech intelligibility should occur. Simply preserving loudness relationships of all unaided speech frequency outputs has not been shown to improve speech intelligibility.

NAL & COMPRESSION

In fact, non-speech frequency information is not even used to calculate an NAL formula. This is why you will not find any NAL frequency gain targets above 4K.

NAL & COMPRESSION

Speech intelligibility for NAL is based upon the importance of phonemic information as calculated from the Articulation Index. However, formula exceptions are created based upon other factors such as:

1. Severe to profound hearing loss2. Ear canal volumes of children vs.

adults

NAL & COMPRESSION There are two primary features which

characterize the NAL compression fitting formula. They are:

1. Equalizing rather than normalizing the loudness of adjacent speech frequencies.

2. Providing less gain for frequencies where the hearing loss is worst and more gain where hearing is best.

NAL & COMPRESSION

Audibility vs. Effective Audibility If actual hearing thresholds are known,

audibility can be measured in terms of sensation level.

Effective audibility refers to how much speech information can be extracted from speech sounds, once they become audible.

NAL & COMPRESSION

Audibility vs. Effective audibilityIt is theorized, that as hearing loss thresholds increase, the hearing impaired tend to have more effective audibility with less audibility. For example: for those with a severe or greater hearing loss, a small sensation level for audibility might give speech understanding; while a greater sensation level will not provide much more effective audibility.

NAL & COMPRESSION

Audibility vs. Effective AudibilityWhen attempting to calculate frequency specific NAL gain targets for speech, a severe hearing loss will not have a gain target available—the target gain calculation just disappears!

NAL & COMPRESSION

Gain + Input = OutputLet’s compare the two compression fitting formula (NAL and DSL). The gain for audibility calculations will be established between these two formulae using a sixty decibel flat configuration hearing loss.

NAL & COMPRESSION

Gain + Input = OutputWe will convert the gain for audibility to output for comfort curves to better compare/contrast NAL and DSL fitting formulae.Let’s review Venema, pages #84 thru #86 and compare/contrast these two formulae using four different audiogram configurations.

NAL & COMPRESSION

NAL vs. DSLYou will notice that the greatest difference between these two compression based fitting methods concerns the low frequency gain/output.

NAL & COMPRESSION

From a physiological perspective, it is not presently possible to model what is really going on in the aided cochlea with any one particular fitting method.

NAL & COMPRESSION

WDRC hearing instruments are meant to imitate the role of the outer hair cells after being driven through the middle ear system.

NAL & COMPRESSION

The application of electroacoustic information to various hearing pathologies will always remain a challenge to standardized fitting formulae.

NAL & COMPRESSION

The fitting of hearing instruments continues to be both an art and science.