10 - Acoustic Immittance Cont'd Flashcards
In tympanometry, why do we use a 226 Hz Probe Signal?
It simplifies the calibrations:
Under standard atmospheric conditions, the acoustic admittance of a 1 cc cavity = 1 acoustic mmho at 226 Hz
At 226 Hz, the acoustic admittance (Ya) of a 1 mL (=1cc) hard-walled volume + ___ mmho
1.0 mmho
How does acoustic admittance calibration work?
Calibration cavities of known volume and therefore known admittance (mmhos) are used:
- the speaker presents the same 226 Hz probe signal voltage in each cavity (0.5 cc, 2 cc, 5 cc)
- microphone records the dB SPL and the “reference” voltage for each calibration cavity
After, for a hard-walled cavity of unknown size (OE canal), the microphone and ref meter can measure the voltage of the signal and compare this voltage and dB SPL to reference values to infer cavity volume and equivalent Ya
1 mmhos = 1 cubic cm of _____
Air
What does EECV stand for?
How do we find it?
Equivalent Ear Canal Volume
- not measured directly
- inferred from the measurement of admittance
How do we determine clinical admittance?
Use calibration data to convert from dB SPL/voltage level of the 226 Hz probe signal to acoustic admittance
Some systems will use _____ daPa instead of +200 daPa to create a hard-walled cavity resulting in a slight difference in EECV
-300
O.E. Ya is often smaller, and M.E. Ya is larger when using -300 or -400 daPa compared to +200 daPa
What can an abnormal O.E. Canal Equivalent Volume tell us (what is it’s clinical value)?
Can indicate:
- perforation
- narrow canal (stenosis) or growths
What does the “Peak Compensated Static Admittance” refer to?
Calculating and removing the outer ear component from the total admittance value by removing the effect of the ear canal
- probe tip measures the total immittance of the ear (OE and ME effects)
- to find Peak Compensated Static Ya, subtract Baseline Ya at 200 daPa from the total Ya at the tympanogram peak TPP
OE canals vary in size and complicate measures of ME admittance, so how do we normalize all OE canal cavities to “0” mmhos?
The acoustic immittance instrument uses a 226 Hz probe signal that is calibrated to a specific probe tone level (e.g. 85 dB SPL) in cavities of known size. It then uses special circuitry to set specific probe tone level (e.g. 85 dB SPL) in the hard-walled ear canal cavity before the test begins
If an ear canal volume is less than the calibration volume (e.g. 2cc), admittance at +200 daPa will show that ____ (more/less) voltage is needed to maintain the 85 dB SPL probe level. What will be used to calculate decrease in admittance relative to calibration cavities?
Less
Voltage difference is used to calculate decrease in admittance relative to calibration cavities
What are compensated tympanograms good for?
They allow easy comparison of tympanogram data between patients, and for the same patient on repeat visits
What is the tympanometry baseline?
The baseline measure before the pressure sweem tympanometry begins
-Absolute Ya and EECV is provided by patient’s own OE hard wall cavity at +200 daPa
What is a compensated tympanogram?
When the baseline is normalized to 0 for admittance measures during the pressure sweep
- the 226 Hz probe level is adjusted to the same probe level (e.g. 85 dB SPL)
- baseline compensation removes the “absolute” admittance from the tympanogram graph by normalizing to “0” mmhos (cc)
- the effect of the OE canal Ya is removed
Describe how the Immittance Instrument works during a compensated tympanogram (specify parts of the equipment)
Speaker - generates 226 Hz probe signal into hard-wall cavity of unknown size
Mic and Ref Meter - measure voltage of 226 Hz probe signal
-Use calibrated ref voltages/dB SPL values
Ref and Balance Meters - infer cavity volume and equivalent Ya for OE cavity
- calculate appropriate speaker voltage
- adjust and generate 85 dB SPL probe signal in the OE cavity (baseline normalization)
