Stimuli Presentation and Data Acquisition

All experiments were performed in a double-walled sound-attenuating chamber (IAC Inc., Bronx, NY). Generation of sound stimuli and recording of responses was performed as described previously.^24,29–31 Briefly, stimuli were generated digitally, and presented via a bone conduction oscillator (Cochlear Ltd., Centennial, CO, U.S.A.) or a closed-field magnetic speaker (MF1; Tucker-Davis Technologies Inc., Alachua, FL) coupled directly to the ear with an ear speculum altered to accommodate flexible speaker tubing and to reduce sound escape. Both transducers were driven by an external sound card (Hammerfall Multiface II, RME, Haimhausen, Germany), and output directed to the loudspeaker was amplified with one channel of a stereo amplifier (TDT SA1). The sound intensity in the ear canal was measured with a probe-tube microphone (type 4182; Bruel & Kjær, Nærum, Denmark), which was also placed through the modified ear speculum. Baseline acoustic transfer functions were generated from presentation of short (1 second duration) tone pips between 100 and 14,000 Hz ramped on and off with one half (5 ms) of a Hanning window. Tones were presented at 70 to 120 dB SPL. Input from the microphone, LDV, and pressure sensors were simultaneously captured via the sound card analog inputs.