The information provided in this answer is derived from the British Society of Audiology (BSA) Recommended Procedure for Tympanometry (page 15).
1000 Hz: Ear canal volume and middle ear pressure
The value of ear canal volume should be disregarded when high-frequency probe tones are used because it will not be precise.
Both ear canal volume and middle ear pressure should be disregarded when using a 1-kHz probe tone. At 226 Hz, mmho and cm3 are interchangeable; however, this is not the case at higher frequencies where it is not possible to accurately calculate cavity volumes. Without an accurate measure of volume it is not possible to calculate pressure, since pressure = density/volume.
The exception is for use as an indicator of a possible blockage (i.e. very small volume given), although this should be verified (e.g. otoscopy or checking the probe). It is recommended that the traces recorded are classified as normal or abnormal using a classification system reported by Baldwin (2006; adapted from Marchant et al 1986).
1000 Hz: Tympanogram interpretation
- Draw a baseline on the trace at pressure extremes (–400/–600 to +200 daPa)
- If the trace disappears below the x axis, the baseline should be drawn to the x axis, as shown in Figure 1
- Identify the main peak, which can occur at any middle-ear pressure
- Draw a vertical line from the baseline to the peak of the trace
- If the peak is above the baseline it is a positive peak and normal
- If the peak is below the baseline it is a negative peak and abnormal
- If there is a positive and negative peak the trace should be classified as positive (i.e. normal)
- A positive peak at a positive or negative middle-ear pressure is classified as normal, whereas a flat or “trough-shaped” i.e. negative peak is abnormal
- If the conditions are good and the outcome is clear, repetition is not always necessary to draw a conclusion. However traces should usually be repeated if possible to check for reliability. Repeated traces should be classified in the same category of positive or negative. If the outcome is not clear the trace should always be repeated.