Dsl & compression
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The document discusses the dynamic range of hearing and how it is reduced with hearing loss. It describes how compression hearing aids aim to place soft, average, and loud sounds within the residual dynamic range by providing different amounts of gain based on input levels. It also discusses how the Desired Sensation Level (DSL) fitting method uses long-term average speech spectrum targets measured at the ear to fit compression hearing aids.
Dsl & compression
- 1. 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.
- 2. DSL & COMPRESSION Audibility vs. Loudness 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.
- 3. DSL & COMPRESSION Audibility vs. Loudness 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.
- 4. DSL & COMPRESSION Audibility vs. Loudness The challenge, as a hearing instrument specialist, is to place the perception of soft, average, and loud sounds into this residual dynamic range area.
- 5. DSL & COMPRESSION Audibility vs. Loudness 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.
- 6. 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.
- 7. DSL & COMPRESSION Audibility & HI Gain 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.
- 8. DSL & COMPRESSION Audibility & HI Gain 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
- 9. 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.
- 10. DSL & COMPRESSION Compression & Loudness Growth 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.
- 11. DSL & COMPRESSION Compression & Loudness Growth 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.
- 12. DSL & COMPRESSION Compression & Loudness Growth 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.
- 13. DSL & COMPRESSION Compression & Loudness Growth 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).
- 14. DSL & COMPRESSION Compression & Loudness Growth Digital hearing instruments will often employ two types of compression within their algorithms. They are: 1. Compression output limiting 2. Wide Dynamic Range Compression
- 15. 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.
- 16. 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.
- 17. 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.
- 18. 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!
- 19. 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.
- 20. 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.
- 21. 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 headset 2. Real ear to coupler difference (RECD)--ANSI 2cc coupler 3. Microphone location effect (MLE)--Style of HI microphone location/placement
- 22. 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
- 23. 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 decibels 2. Moderate inputs presented at seventy decibels 3. Loud inputs presented at eighty-five decibels
- 24. 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).
- 25. 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