Dsl & compression

DSL & COMPRESSION

Audibility vs. Loudness

The patient/client hearing thresholds up to the loudest sounds the patient/client can tolerate is defined as the patient/client’s dynamic range of hearing ability—for each ear.

DSL & COMPRESSION


The typical dynamic range for the mid-frequencies is about one hundred decibels. Soft—barely apparent sounds to loud but tolerated sound falls within this typical dynamic range area.

DSL & COMPRESSION


With hearing loss, the soft sounds become more difficult to hear; while loud sounds continue to be tolerated at the same decibel level.Thus, hearing loss results in a reduced dynamic range for the hearing impaired patient/client.

DSL & COMPRESSION


The challenge, as a hearing instrument specialist, is to place the perception of soft, average, and loud sounds into this residual dynamic range area.

DSL & COMPRESSION


Some types of hearing loss even reduce the tolerance for loud sounds which even further reduces the residual dynamic range area.When this abnormal growth of loudness (recruitment) is encountered, this creates a greater and more critical necessity for compression.

DSL & COMPRESSION

Audibility & HI Gain

As previously discussed, various fitting formulae described a single gain target for linear hearing instrument fittings.With compression hearing instruments, three gain targets are normally described/defined. They are: Soft, Average, and Loud.

DSL & COMPRESSION


A linear fitting formula would provide enough gain for soft speech however, when a loud sound was received it was too much gain.Remember: Gain + Input = Output (this formula is true for both compression and linear HIs)

Output is what the patient/client perceives as comfortable sound.

DSL & COMPRESSION


Compression based fitting formulae provide for different amounts of gain based upon the instrument’s received input intensity levels. These gain targets are based upon Soft, Average, and Loud inputs.

Remember: Gain + Input = Output

DSL & COMPRESSION

Compression & Loudness Growth

HI’s can not replace normal cochlear function.However, the goal for fitting most HI’s is to restore normal loudness growth for mild to moderate hearing loss. As we learned, the cochlea is a WDRC amplifier. Thus, WDRC HI’s are appropriate for mild to moderate hearing loss encountered by the hearing instrument specialist.

DSL & COMPRESSION


Let’s review Venema, figure 4-1, on page #67 and figure 4-2, on page #68.

Notice the impaired loudness growth curve for the fifty-five decibel hearing loss and the perceived loudness is the same as the normal loudness growth curve.

DSL & COMPRESSION


From figure 4-2 in Venema, please notice how the loudness growth frequency contours for normal hearing begin to flatten with increased intensity levels. At one hundred decibels the frequency contour is almost flat.

DSL & COMPRESSION


These psychoacoustic loudness growth curves reveal that the cochlea is a non-linear organ. They also reveal that the physical acoustic properties of the outer and middle ear perceptually influence the minimal audibility frequency curves in loudness growth measurement.

DSL & COMPRESSION


Most digital compression instruments have algorithms designed to increase gain for soft level inputs and reduce and/or produce little to no gain for high level inputs (similar to the amplification function of outer hair cells in a cochlea).

DSL & COMPRESSION


Digital hearing instruments will often employ two types of compression within their algorithms. They are:1. Compression output limiting2. Wide Dynamic Range Compression

DSL & COMPRESSION

DSL Fitting Method

The Desired Sensation Level fitting formula was based upon creating audibility for speech information in order to enhance language development of hearing impaired children.

DSL & COMPRESSION

DSL Fitting Method

The goal was to place as much of the conversational speech input signal into the residual dynamic range of the children’s hearing ability using compression hearing instruments.

DSL & COMPRESSION

DSL Fitting Method

They chose to measure long term average speech (LTASS) as received at the ear (hearing) rather than had been previously measured at the mouth (spoken word).This resulted in more low and high frequency outputs required for this fitting formula as contrasted by others.

DSL & COMPRESSION

DSL Fitting Method

This long term average speech spectrum (LTASS) information is, of course, to be presented to the auditory system without distortion and comfortably—not too loud!

DSL & COMPRESSION

DSL Fitting Method

What sets this fitting method apart from others, is that its final interest is not necessarily with the gain component of the formula but, with the hearing instrument’s output. This is why you will often see its targets measured in dBSPL not dBgain.

DSL & COMPRESSION

DSL Fitting Method

In other words, this formula looks at the output delivered to the eardrum (both acoustically and electroacoustically)--not just the gain for audibility, or the in-situ gain.

DSL & COMPRESSION

DSL Fitting Method

When attempting to transform dBSPL output accurately into a fitting formula, there are three transforms to consider. They are:1. Real ear to dial difference (REDD)—

audiometer headset2. Real ear to coupler difference (RECD)--ANSI

2cc coupler3. Microphone location effect (MLE)--Style of

HI microphone location/placement

DSL & COMPRESSION

DSL Fitting Method

Let’ review Venema, pages #76 thru #79.This will provide information regarding the output transforms as well as the measurement decibel levels for soft, average, and loud speech signals

DSL & COMPRESSION

DSL Fitting Method

To measure outputs of wide dynamic range hearing instruments, Canada and the United States have chosen three common levels of standardized measurement input. They are:1. Soft inputs presented at fifty decibels2. Moderate inputs presented at seventy

decibels3. Loud inputs presented at eighty-five decibels

DSL & COMPRESSION

DSL Fitting Method

The three input levels used to measure HI output are measured by frequency using the long term average speech spectrum (LTASS).

DSL & COMPRESSION

DSL Fitting Method

These decibel levels by frequency result in the DSL fitting formula. Output is recorded from consistent input levels (50dB, 70dB, 85dB) using the LTASS frequencies.

GAIN + INPUT = OUTPUT

Education

Dsl & compression