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Equipment for Measuring Hearing and Calibration Lecture 4 Outline Equipment: Audiometers Types Components Reference Level Calibration Purpose Types Audiometers: Screening units Components Power source Frequency selector dial Hearing Level dial Ear Selector Stimulus selector Audiometers: Diagnostic Unit 2 channel audiometer Stimuli CD recordings Microphone - Speech Tones Masking Components - Diagnostic Additional components Menus for setting up Response indicator VU meter Talk over Special test features Transducers with Audiometers Air Conduction Supraural earphones Insert earphones Speakers in soundfield Bone conduction Vibrator Air Conduction ( AC ) Performed via TDH-39 headphones – placed over pinna Can also use with insert earphones in the ear canal Through loudspeakers Checks validity of entire ear system Tests one ear at a time (ipsilateral) if using headphones or inserts Parameters for testing with AC Parameters vary depending on equipment Upper Limits for AC: maximum output ranges between 90-110 dB HL Lower Limits for AC: -10 dB HL Standard frequencies tested: 250, 500, 1000, 2000, 4000 and 8000 Hz test interoctaves if >20 dB difference between octaves Bone Conduction (B/C) Bone vibrator placed on mastoid process (behind the pinna) OR forehead Checks validity of inner ear (cochlea) Reflects the condition of the better Parameters for testing with BC Placement is on the skull forehead or mastoid Upper limits for BC : maximum output ranges between 50-80 dB HL Limit depends on frequency Lowest limit for BC: -10 dB HL Standard frequencies tested: 250, 500, 1000, 2000, 4000 Hz “Think-pair-share” Calibration What is calibration? Why is calibration important? Calibration – defined Checking the accuracy of the output of a measuring instrument Electroacoustic of psychoacoustic determination that an audiometer is performing properly in terms of its acoustic output, attenuator linearity, frequency accuracy and distortion See Table 4.1 Calibration If signal too strong, then hearing assessments will be ______________ If signal is too weak, then hearing assessments will be ___________________ Types of calibration Electroacoustic Calibration – completed by a licensed specialist at least annually Formal paperwork provided to document completion Biological (Human) Calibration – can be completed by clinician whenever equipment used Biological Calibration ID power button Visually inspect headphones Look/feel for frays/breaks in cords Perform Listening check Ear accuracy with an audible signal Attenuator function Ear phone cord – with a continuous stimulus Electroacoustic Calibration Process Earphone (from audiometer) placed over “an artificial ear” that simulates an ear canal volume. A microphone is placed on top of the artificial ear 1. 2. A. B. 3. 4. 5. 6. The artificial ear is connected to a sound level meter The earphone is connected to the audiometer A signal is sent through the earphone from the audiometer The output of the signal is measured from the SLM that connects to the artificial ear Audiometer is set at 70 dB HL Output values obtained at SLM are compared to standards for each frequency tested (Table 4.1) Calibration Process Coupler that connects with earphone Earphone placed over coupler Earphone remains connected to Audiometer SLM used to Measure actual SPL values Weight to hold earphone in place Procedure to calibrate and example • Signals sent from audiometer to earphone and read by SLM • Each frequency from 250-8000 Hz is measured with a 70 dB HL input signal • Output at each frequency measured During calibration what should the recorded SPL be when the sound level meter is measuring 1000 Hz? ANSI Standard +/- 3 dB SPL is defined as “within calibration” Output of signal measured Frequency of signal measured Calibration sheets All information is recorded on calibration sheets Units MUST be calibrated annually. NO EXCEPTIONS ! Critical documents Self Assessment questions calibration What are the typical limits of audiometers for AC and BC? Why are there limits? Can I explain the need for calibration ? Why is there a difference between SPL and HL? At 70 dB HL, what should the SPL value be on the sound level meter if the audiometer is at 1000 ? 2000 ? 4000 ? Etc. 1 minute evaluation What can you apply What was the most important thing you remember learning today? What one question would you like reviewed related to what was discussed ? when you leave this room?