When we refer to the admittance of the outer and middle ear system (i.e. its ability to convey sound energy) then we can consider the spring elements that compress and expand (like a spring) including some soft tissues and the air in the ear canal and middle ear. We can also consider the components of the outer and middle ear that move without compressing much, primarily the ossicular bones.
For different probe tone frequencies (or frequency components of the click stimuli used in WBT) the middle ear will be either spring dominated (sometimes called stiffness dominated) or mass dominated.
Mass resists high frequency oscillations and stiffness more easily accepts high frequency oscillations (and vice versa), meaning admittance of the middle ear to acoustic energy is mostly opposed by spring (stiffness) elements with low frequency probe tones (stiffness dominated), while admittance of the middle ear is mostly opposed by the mass elements with high frequency probe tones (mass dominated).
The (lowest) frequency at which the spring and mass elements of the outer and middle ear structures contribute equally to the admittance is the resonant frequency.
The resonant frequency will move upwards or downwards according to outer and middle ear pathologies that affect the mass and stiffness differently. An increase in stiffness increases the resonant frequency whereas an increase in mass decreases the resonant frequency.
Therefore, one major advantage of WBT is easy identification and analysis of the resonant frequency, for example via the absorbance display.
There are several ways in which resonant frequency can be calculated. In admittance terminology (Y,B and G components of the tympanogram), the method used in the Titan relates to analysis of susceptance (B) at peak pressure (i.e. pressure where absorbance is highest). The B component tympanogram should be zero mmhos when the stiffness elements (which produce positive mmhos) and mass (negative mmhos) are equal. Hence the lowest frequency at which B is zero mmohs (at peak pressure) is the resonant frequency.
For a review of these concepts, please see Shanks (1984)
Another major advantage of WBT is that the admittance tympanogram at resonant frequency is easily obtained. With single-frequency tympanometry it would be a time consuming process to obtain this.
n.b. the resonant frequency tympanogram is clinically relevant because the resonant frequency tympanogram should be sensitive to middle ear disorders affecting both mass and stiffness.
References and caveats
Shanks, J.E. (1984) Tympanometry. Ear and Hearing, 5 (5), pages 268-280
A range of researchers have looked to address this question, with resonant frequencies of the human middle ear often reported informally as typically falling between 800 and 1200 Hz (Shanks, 1984). More specific estimations are available, such as Valvik et al. (1994). These authors reported a mean resonant frequency of 1049 Hz (SD = 261) based on 100 non pathological ears. More recently, Polat et al (2015) described normative data from 110 normally hearing individuals from the Turkish population using Wideband Tympanometry (WBT). The mean resonant frequency reported in this study was 965 Hz (SD = 234).
References and caveats
Polat, Z. et al (2015) Wideband Tympanometry Normative Data for Turkish Young Adult Population. The Journal of International Advanced Otology, 11 (2), pages 157 - 162.
Shanks, J.E. (1984) Tympanometry. Ear and Hearing, 5, pages 268-280
Valvik, B-R. et al (1994) Multifrequency tympanometry. Audiology, 33, pages 245-253
The ‘wideband’ part of the name Wideband Absorbance (WBA) refers to the stimulus, which is a click i.e. a broadband sound. The ‘absorbance’ part of the name refers to how effectively the energy from a click sound is transmitted through the middle ear system. WBA is measured on a scale from 0 to 1, where 0 is no incident sound energy absorbed, and 1 is all incident sound energy absorbed. So, WBA is a measure of the effectiveness of the middle ear (to absorb sound) as a function of frequency. It has been shown that measuring middle ear function across frequencies (instead of just at a single frequency, or a handful of frequencies as in traditional tympanometry) can give clinicians a greater understanding of how certain disease processes affect the middle ear.
An overview of historical and recent developments in the field of wideband absorbance measurements can be found in the proceedings of the 2012 Eriksholm Workshop on Wideband Absorbance Measures of the Middle Ear, published in Ear and Hearing 2013, as a supplement to volume 34.
References and caveats
Lilly, D.J. and Margolis, R.H. (2013) Wideband acoustic immitance measurements of the middle ear: Introduction and some historical antecedents. Ear and Hearing, 34, Supplement 1, 4S – 8S.
For a brief description of absorbance please watch this video tutorial
For a more in-depth discussion please watch this webinar by Dr Lisa Hunter