Describe audiometric calibration for air conduction. What equipment is used?
- Type 1 Sound level meter, with condenser microphone
- Pistonphone or calibrator
- specified frequency and intensity
- 6-cm 3coupler (supra-aural earphones) - artificial ear, or 2-cm3coupler (insert earphones)
- Weight or pressure gauge (for supra-aural ep’s)
- Oscilloscope, voltmeter, artificial mastoid
- needed for full calibration
- needed for full calibration
Describe the set up for audiometric calibration. What is calibrated as a preliminary measure and how is this done? How is the equipment for calibration set up?
- Calibrate sound level meter (SLM)
- Check pistonphone(hear signal; if B&K pistonphone–listen for pitch change)
- Place pistonphoneover micon SLM
- Follow instructions for specific SLM
- e.g., B&K: set to “linear” scale, read specified level
- Equipment set-up for calibration:
- Place earphone over coupler with microphone in place
- Place 500 gm element over earphone
How is level calibrated on the audiometer? What is presented? Where is the output of the level checked? What is the tolerance for calibration?
• Present known signals from audiometer (in dB HL) to earphone –measure output on SLM in dB SPL
• Check output level at each frequency
- Input: 70 dB HL
- Measure output in dB SPL (linear scale)
- Compare to standard
- Tolerances: +/-3 dB, 125 –5000 Hz; +/-5dB >6000 Hz
- Same tolerances used for earphones, bc, and speakers
- Out of calibration if even 1 measure is out of tolerance
How is linearity calibrated? At what frequency and why? Where do you start? What is the tolerance?
• Check attenuator linearity at 4 kHz (4kHz because it is remote from background noise)
- Start at high level input
- record output in dB SPL
- Decrease in 5 dB steps
- note change in output
- tolerance = 1 dB/5 dB step; maximum deviation shall not exceed 3/10 of the indicated interval, or by > 1.5 dB from the indicated HL value (-10 dB -> 0 dB HL), 1.4 dB (5-40 dB HL) and 1.2 dB (45 dB HL and up), whichever is smaller.
What is harmonic distortion? How do you check for harmonic disortion? What signal is presented and why? What is then looked at? What is the standard? How is harmonic distortion standard calculated?
Harmonic distortion - something present at output that was not present at input - when output signals are integer multiples of input
• Check for Harmonic Distortion
- Present high intensity signal (110 dB HL or highest output of audiometer) and measure output of f0 (in dB SPL)
- High intensity because harmonic distortion occurs at high levels and want worst case scenario
- Now look at output level of f2 and f3, using filter
- Compare output of f2 to f0 (fundamental)
- Standard: f2 should be 2% less than f0 (2% = -34 dB)
- Repeat for f3
• Check frequency of output: must be +/-1% of nominal frequency (Types 1 and 2), +/-2% ( types 3 and 4).
• Input high level signal (see Table 3)
• Read output in dB SPL
• level of 2nd harmonic is at least 2% less than output in dB SPL dB level = 20 log .02 log .02 = -1.7 20 (-1.7) = -34 dB
What equipment for bone conduction calibration? How is the equipment set up? What should SLM be read in?
• Equipment: artificial mastoid, simulates thickness of skull, texture and Z of skin
• Set-up: place b.c. oscillator on artificial mastoid
- B.C. vibes ->
- artificial mastoid ->
- has discs that vibrate in proportion to b.c.->
- converts to electrical signalcan be read by SLM
• Connect artificial mastoid to SLM
- Adjust reading on SLM to read in force level/1 dyne
- Place 550 gm mass on top of bc vibrator (on artificial mastoid)
How is the expected output calculated for bone conduction? What is the tolerance?
• Send specified input to b.c. vibrator
- Usually 40 dB HL input, read output in dB re 1uN
- Check correct HL output (Table 8)
- 1kHz, 40 dB HL input,
- RETFL at 0 dB HL for 1000 Hz, re: 1 uN= 42.5
- Output, re: 1uN = 82.5 force levels
- Check correction factor on frequency response curve (at 1k Hz, correction = -.5 dB
- Expected output: 42.5 + 40 -.5 = 82 dB force level (*note: our SLM reads force level/1 dyne; 1 uN= .1 dynes] In dB: dB = 20log(.1); log .1 = -1; dB = 20(-1); dB = -20
- Thus, expected output = 82 dB force level –20 dB = 62 dB (force level, re: 1 dyne)
- Tolerance: +/3dB, 125 –5000 Hz
- Tolerance: +/3dB, 125 –5000 Hz
How is speech calibrated on the audiometer? What is the RETSPL for speech? What should the VU meter be set to? What level should input be? What is the tolerance? For distortion?
• RETSPL for speech: 12.5 dB> RETSPL for 1k Hz
- HL dial = 0 dB, VU meter at 0 dB, for 1k Hz tone
- Should read out 19.5 dB (TDH 39)
- Take measures with high level input (60 dB HL), expected output = 79.5 dB SPL
• Calibration of VU meter: dynamic characteristics
• Fidelity: output across frequency (200 –4000 Hz) should be +/-3dB re: level measured at 1000 Hz
• Distortion: distortion products should be 2.5% below fundamental (-32 dB)
What are some reasons to obtain Sound Field measures? What do loudspeakers transduce? What varies in speakers? What does this affect? What frequency response should speakers have?
• Reasons for SF measures:
- Assessing thresholds in young children
- Assessing performance with hearing aid
- Assessing performance in simulated “real world” conditions (noise, reverberation, head shadow)
• Loudspeakers: transduce electrical signal to acoustic signal
- Frequency response of spkr may vary in range and levels within the frequency range
- Affects relative frequency emphasis of broadband signals (speech)
- Speakers should have flat FR, 200 –6000 Hz
What is the head shadow effect? Where is the loss greatest?
When signal comes from one side, the signal crossing to other side of the head gets a 6 dB loss (on average, in speech frequencies)
Transmisson loss is greater in high frequencies
Describe some near field-far field issues. Where is the signal that is transduced through the loudspeaker? What is the ideal room location dependent on? What do small changes in the near field lead to? What is the far field governed by? Based on this, where should the subject be seated? If not?
• Near field-far field issues
- Signal transduced through loudspeakers -> a near field and a far field
- Location: determined by size of spkr & diaphragm
- Near field: small changes in distance -> large changes in SPL
- Far field: governed by inverse square law
- Want subject seated in far field
- If not, head position must be controlled
What does presentation of pure tones in the sound field lead to? What sounds interact? What do reflected sounds create? What is RT? What is the RT for sound-attenuating rooms? What are the direct and reverb sound fields each dominated by? Based on this, where should the listener be located?
• Environmental Issues
- Presentation of pure tones in SF -> reflections
- Reflected sounds interact with incident sound -> standing waves
- Reflected sounds also create reverberation (prolongation of sound, expressed in RT in sec)
- RT -> time it takes for signal to reduce 60 dB after you shut sound off
- RT in most sound-attenuating rooms is short (.1-.2sec)
- Direct vs. reverb SF
- Direct SF: dominated by incident sound (SPL predicted by ISL)
- Reverb SF: dominated by reflected sound waves, not predictable
- Want listener located in direct SF
What stimuli are used for sound field testing? What are FM tones defined by? Define each. What is bandwidth? What are FM tones preferred to and why?
• Stimuli: want frequency-specific stimuli or speech
• FM tones: defined by carrier freqy, modulation rate, and frequency deviation
- CF: center, or nominal frequency; usually modulated sinusoids
- Mod. rate (in Hz): # times/sec the frequency is swept (should be 4-20 Hz)
- Freqy deviation: frequency range over which the instantaneous frequency varies during ea. modulation (should be 5-25% of carrier frequency)
- Bandwidth: of FM signal depends on frequency deviation
• FM tones are preferred to NBN –have a narrower bandwidth and produce a consistent level throughout room
To minimize standing waves
What equipment is used to calibrate the sound field? What are the requirements for the sound-attenuating booth?
• Calibration equipment: SLM + 1/3 octave band filter, and free field mic(1/2”)
• Requirements for sound-attenuating booth
- Ambient noise level inside must not exceed maximum permissible ambient noise levels (ANSI S3.1 –1999, R 2008)
- Higher levels interfere with thresholds
- Frequency response in SF: SPL between 250 – 4000 Hz, should be +/-3dB level developed at 1kHz, at 100 dB SPL
How are measurements of RETSPLs made in SF testing? What is used for RETSPLs for speech? How does this vary depending on number of speakers? What is used for RETSPLs for frequency specific stimuli?
• RETSPLS for SF testing (Table 9; ANSI S3.6 –2010)
- Measurements made with substitution method (position of subject’s head; at least 1 m from spkr)
• RETSPLS for speech: use masking noise, SLM in “A weight” position
- 2 spkrs, located at either 45 or 90 degrees azimuth
- 1 spkr, located at 0 degrees azimuth
- Note RETSPLs for speech for binaural testing (Table 9)
• RETSPLs for frequency-specific stimuli: use NBN or WTs
- Thresholds obtained in SF adjusted to = thresholds obtained under earphones, for young normal listeners
What are MAP and MAF? Which is lower and by how much? Why? What is the reference SPL for spondees in SF?
• MAP: least intensity pressure measured under earphones
• MAF: least intensity pressure measured in SF
• MAF is 6 dB lower than MAP (missing 6 dB)
- Head baffle effect (4 dB) - build up in pressure because head is in path
- Ear canal resonances, above 1500 Hz
• Thus, reference SPL for spondees in SF (14.5 dB) is approx. 6 dB below RETSPL for speech under earphones
• Standard takes this difference into account