Nal & compression
Published on: Mar 3, 2016
Transcripts - Nal & compression
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 Formost 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 Infact, 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 thresholdsincrease, the hearing impaired tend to havemore effective audibility with less audibility.For example: for those with a severe or greaterhearing loss, a small sensation level foraudibility might give speech understanding;while a greater sensation level will not providemuch more effective audibility.
NAL & COMPRESSION Audibility vs. Effective AudibilityWhen attempting to calculate frequencyspecific NAL gain targets for speech, asevere hearing loss will not have a gaintarget available—the target gaincalculation just disappears!
NAL & COMPRESSION Gain + Input = OutputLet’s compare the two compression fittingformula (NAL and DSL).The gain for audibility calculations will beestablished between these two formulaeusing a sixty decibel flat configurationhearing loss.
NAL & COMPRESSION Gain + Input = OutputWe will convert the gain for audibility tooutput for comfort curves to bettercompare/contrast NAL and DSL fittingformulae.Let’s review Venema, pages #84 thru #86and compare/contrast these two formulaeusing four different audiogramconfigurations.
NAL & COMPRESSION NAL vs. DSLYou will notice that the greatest differencebetween these two compression basedfitting methods concerns the lowfrequency 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 Thefitting of hearing instruments continues to be both an art and science.