The effect of changes in ambient temperature on the reliability of acoustic rhinometry data

The effect of changing ambient temperature on the reliability of acoustic rhinometer data was examined. The acoustic rhinometer was set up in a climate chamber, and connected to a simple cylindrical model containing a constriction. This constriction was at 22.5 cm from the microphone. This would be the position of the tip of a 7.5-cm nose piece, relative to the microphone, when attached to the rhinometer. The ambient temperature was increased from 10 degrees C to 40 degrees C. The position of the constrictions as recorded by the acoustic rhinometer was compared in the same stable model at intervals during the temperature increase. The point of identification of the constriction varied with ambient temperature and the change almost perfectly followed the expected changes in the readings given the relationship of the speed of sound in air to ambient temperature. A shift of approximately 1 mm along the X-axis per 2.5 degrees C change in temperature is seen for this constriction. In a human subject the whole acoustic rhinometry trace would shift along the X-axis to the same degree when using a 7.5-cm nose piece. Volume estimates are calculated between two fixed points on the X-axis and may be profoundly affected by even a small shift of the acoustic reading along this axis. Acoustic rhinometry data should always be collected under the same stable environmental conditions.