ACOUSTIC IMPEDANCE METER/BRIDGE

Typical Number in Hospital: 2

Cost Bands: 4

References: 3

A device for measuring the integrity of the sound conduction mechanism between the ear drum and the oval window to the inner ear by measuring the acoustic impedance of the ear drum. In the normal ear, sound applied to the ear drum will readily pass through,and a low acoustic impedance is measured. If sound will not pass through easily (due to a defect in the middle ear) then a high acoustic impedance may be recorded. The acoustic impedance is recorded while varying the pressure in the outer ear (typically from +2 kPa to -6 kPa). A minimum impedance is recorded when the pressure in the outer ear is the same as that in the middle ear (i.e. where there is no pressure differential across the ear drum causing it to stiffen), thus allowing the middle ear air pressure to be measured indirectly. In the normal case the middle ear pressure is close to atmospheric whereas in many pathological conditions it is lower.

For clinical use in the ENT or audiology department, the measurements are recorded on an X-Y plotter (which may be part of the acoustic impedance meter) on which the Y-axis shows compliance (the inverse of impedance) expressed in equivalent air volume (e.g. 0-4 ml), and the X-axis shows air pressure applied to the outer ear.

The apparatus normally consists of a low-pressure air pump and the necessary controls, a sound source and ear piece, an oscillator (usually about 220 Hz), and an amplifier and metering circuit. The unit may contain or be connected to an X-Y plotter.