Sound field audiometry: recommended stimuli and procedures

Comprehensive recommendations are presented for conducting sound field audiometry with frequency specific stimuli. These recommendations are primarily based on a series of investigations by the authors. The rationale for each recommendation is presented, together with a brief overview of the supporting research. The preferred stimuli are frequency modulation tones, triangularly or sinusoidally modulated at a rate of about 20 Hz, or suitably generated narrow bands of noise. The optimal bandwidths of the stimuli, expressed in percentages of the center frequency, vary with frequency. Stimuli suitable for most purposes have bandwidths ranging from about 30% at 0.25 kHz to about 10% at 4 kHz. Stimuli having narrower or broader bandwidths are desirable for some special purposes. The test room should be as nonreverberant as possible and the subject should be seated on an adjustable height chair with headrest. The control microphone method of calibration is preferred but a method is also presented for carrying out the traditional precalibration procedure. The SPL of the complex stimulus should be taken as the peak deflections on a sound level meter set to "RMS-FAST." A conversion table is presented which allows thresholds obtained in the sound field to be expressed as dB HTL. With the materials and methods described here it is possible to achieve the same reliability for sound field testing as for audiometry under earphones.