Constant Position for Subjects:  measurement variability due to subject movement

The sound pressure level in a semi-reverberant test booth can vary with position due to interference between direct and reflected sound waves.  

Most clinics use a substitution method for calibrating the sound field (see section on sound field calibration), and as a result, if the subject moves from the calibrated test point, the sound pressure level (SPL) at the ears will vary as the sound field is very rarely uniform.  This variation can result in inaccurate threshold estimation and increased test-retest variability.  

During sound field testing, the examiner should be in another room (if possible) to avoid further reflection or absorption.  Obviously when testing a young child an examiner or parent may be present.  This will contaminate the measurements somewhat, but currently there is no standard solution to that problem. 

During sound field testing it is imperative that the subject's head remain still throughout testing.  According to Beynon et al (1995) as much as +/- 4.6 dB of variability can exist within the sound field at the test point.  This degree of variability is acceptable given that in clinical practice 5 dB steps are used during.  However, this variability is for a situation where the subject's head (or microphone, depending on the method of calibration) is fixed.  If movement occurs, then it is likely that this degree of variability will increase, causing  significant proportions of test-retest variation. 

Below is a picture of a chair which is used to prevent a subjects head from moving during sound field testing.  The extension on the back of the chair is adjustable to the height of the clients head.  The client's head is fixated by two foam cups.  Not every clinic setting will have this equipment, and not every client will be willing to sit in such a chair (especially children).

This picture was taken in an acoustics lab at the University of Western Ontario.  

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