Q: What is the difference between Tympanometry and Reflectance?
A: Tympanometry and Reflectance (a quantity derived from a Wideband Acoustic Immittance test) are both audiological tests that evaluate middle-ear status. Tympanometry and Reflectance both measure the ear canal acoustic admittance, albeit by somewhat different methods. Reflectance magntiude is easily interpreted as the square root of the power reflected from the tympanic membrane (TM) over the incident power. Reflectance is obtained over a wide frequency range (0.2 to 6 kHz), with the measurement taking only seconds to run.
Q: Why is Reflectance magnitude useful?
A: Reflectance magnitude directly provides what one wants to know about the TM admittance across a wide frequency range (0.2 to 6 kHz). Unlike tympanometry, it automatically and accurately removes the effect of the residual ear canal without pressurization. The residual ear canal is the volume between the probe tip and the tympanic membrane, which varies with probe insertion depth, and is generally considered to be a measurement confound.
Q: Why is reflectance magnitude not affected by the residual ear canal?
A: When measuring reflectance, the residual ear canal canal shows up as a delay (phase), thus taking the magnitude removes this delay in a natural manner, without the need for pressurization.
Q: How does Reflectance relate to Admittance?
A: Reflectance, R, is mathematically the same information as admittance, Y, based on the transformation:
where r is the constant: r=ρc/Ac, ρc=407 the characteristic impedance of air, and Ac is the ear canal area = 4πD2, D=.75 cm.
Q: What is the ear canal admittance (i.e., compliance)?
A: The compliance, C, of a cavity such as the ear canal is the volume, V, divided by 1.4 x 105, which is the acoustic stiffness of air at sea-level under normal conditions.
Q: How does Tympanometry estimate the ear canal admittance (i.e., compliance)?
A: Tympanometry estimates the compliance of the TM by measuring the total compliance, Cm, at the measurement location and subtracting the compliance of the residual ear canal, Cc, to obtain Ctm:
Ctm = Cm - Cc.
To estimate the residual canal compliance Cc, the ear canal is pressurized and it assumed that at 400 daPa Ctm is zero. If this assumption was true (it isn't*) then the TM compliance is given by
Ctm = Cm - Cm(400daPa)
namely the baseline at the extreme static pressure is subtracted out to remove the effect of the canal compliance.
Q: Why do the Mimosa systems not have Tympanometry?
A: Clinically we want to know the admittance of the TM. Reflectance directly provides this information, at all measured frequencies, without the complication of pressurization required to remove the residual canal compliance. Thus, the same information is more accurately provided over a much larger frequency range (0.2 to 6 kHz),** without the need to make the unnecessary and incorrect assumption that Ctm=0 at 400 daPa.
Rabinowitz, W. M. (1981). "Measurement of the acoustic input immittance of the human ear," J. Acoust. Soc. Am. 70, 1025-1035.
Shanks, J. E., and Lilly, D. J. (1981). "An evaluation of tympanometric estimates of ear canal volume," J. Speech Hear. Res. 24, 557-566.
ASHA (1988) "Tympanometry. ASHA Working Group on Aural Acoustic-Immittance Measurements Committee on Audiologic Evaluation," J. Speech Hear. Disord. 53, 354-377.
Robinson, S., Nguyen, C., Allen, J.B. (2013). "Characterizing the ear canal acoustic impedance and reflectance by pole-zero fitting" Hearing Research, 301, 168-182. (pdf).
For more information about how you can use Reflectance in your clinic, see our Guide for Clinicians.
Version 1.0, 2 May 2014.
Prepared by Prof. Jont Allen, CTO Mimosa Acoustics and Associate Professor at the University of Illinois.