Active control of discrete-frequency noise from small subsonic fans

Abstract

High-level spontaneous otoacoustic emissions (SOAEs) were measured from 16 ears using both spectral and time averaging. The purpose was to determine the source of an upward shift in frequencies of synchronized SOAEs (SSOAEs) observed while using a subroutine of the ILO88 system of Otodynamics Ltd. An HP3561A signal analyzer performed spectral averaging to extract SOAEs with no external stimulation applied to the ear canal. Synchronized SOAEs were derived using the ILO88 system performing time averaging following click stimulation. The frequencies of all SSOAEs were shifted upwards by 6 to 21 Hz when compared to corresponding SOAE frequencies determined with spectral averaging. Additional measurements of signals in a cavity and of click-evoked otoacoustic emissions in selected ears indicated that the frequency shift is the result of an error in the ILO88 software. Incorrect cursor readouts in the program cause an apparent upward shift in frequency of 12.2 Hz. This error was confirmed by the manufacturer.Spontaneous otoacoustic emissions ͑SOAEs͒ represent narrow-band signals that can be recorded in the outer ear canal when no external acoustic stimulation is presented ͑see Probst et al., 1991 for a review͒. In general, two methods have been used to record them. In the first, the sound-pressure level in the ear canal is measured by a low-noise microphone with no stimulation applied. The microphone signal is averaged in the frequency domain ͑e.g., Whitehead et al., 1993͒. The second method consists of recording SOAEs synchronized by acoustic stimuli, for example clicks, using averaging in the time domain. This enables the detection of long lasting oscillations following click-evoked otoacoustic emissions ͑CEOAEs͒. It has been shown that for an ear with strong SOAEs, a CEOAE spectrum exhibits peaks corresponding to SOAE frequencies ͑Probst et al., 1986; Gobsch and Tietze, 1993͒. Software of a widely used commercially available instrument for measuring OAEs, the ILO88 ͑Otodynamics Ltd., Hatfield, UK͒, includes a subroutine for measuring synchronized spontaneous otoacoustic emissions ͑SSOAEs͒. Several recent studies have reported SSOAE data collected with the ILO88 system ͑Wable and Collet, 1994; Kulawiec and Orlando, 1995; Prieve and Falter, 1995͒. As part of an ongoing study of otoacoustic emissions in normal-hearing humans in our laboratory, we have measured SOAEs using both spectral averaging and the synchronization technique of the ILO88. In comparing the two results from the same ear, we have observed a slight but consistent difference in the frequencies of SSOAEs and SOAEs. Therefore, we sought to characterize this discrepancy further and to determine its source. Because of the widespread use of the ILO system, we believe that it is important that our findings be reported.Both ears of eight subjects from our laboratory pool who had known SOAEs that were at least 10 dB above the noise floor of the instrumentation were tested with two methods. In the first method, the sound-pressure level in the ear canal was me...

Active control of axial-flow fan noise

“…The patterns are clearly more uniform than in the unbaffled case, which is favorable from the ANC viewpoint. The directivity patterns on the outlet side of the fan were measured also 13 and found to be nearly identical to those shown in Fig. 4.…”

“…13 Again, the sound-pressure level at first two harmonics of the BPF was found to be reduced by 6 or more dB at the remote ͑and error͒ microphone positions. Sound power was not measured on this side of the baffle because of the proximity of the apparatus to the hard reflecting floor of the hemianechoic chamber.…”

“…5 and 6 has been carried out. 13 The results indicate that the algorithm of Fig. 6 operates only on causal, periodic signals.…”

“…1 have been obtained for off-axis positions of the microphone, the fan, and shaker in operation at the same time, and again with the flow direction reversed. 12,13 In all of these situations the shaken fan was found to create sufficient acoustic energy to warrant its use as an antinoise source. However, it was observed that when the flow from the fan was reversed and directed toward the microphone ͑which was 1-m away and uninfluenced by the flow͒, the shaken fan acoustic pressure amplitude was about 2 dB less than the case when the flow was away from the microphone.…”