Index of content:
Volume 129, Issue 5, May 2011
- ACOUSTIC SIGNAL PROCESSING 
Estimation of signal coherence threshold and concealed spectral lines applied to detection of turbofan engine combustion noise129(2011); http://dx.doi.org/10.1121/1.3546097View Description Hide Description
Combustion noise from turbofan engines has become important, as the noise from sources like the fan and jet are reduced. An aligned and un-aligned coherence technique has been developed to determine a threshold level for the coherence and thereby help to separate the coherent combustion noise source from other noise sources measured with far-field microphones. This method is compared with a statistics based coherence threshold estimation method. In addition, the un-aligned coherence procedure at the same time also reveals periodicities, spectral lines, and undamped sinusoids hidden by broadband turbofan engine noise. In calculating the coherence threshold using a statistical method, one may use either the number of independent records or a larger number corresponding to the number of overlapped records used to create the average. Using data from a turbofan engine and a simulation this paper shows that applying the Fisher z-transform to the un-aligned coherence can aid in making the proper selection of samples and produce a reasonable statistics based coherence threshold. Examples are presented showing that the underlying tonal and coherent broad band structure which is buried under random broadband noise and jet noise can be determined. The method also shows the possible presence of indirect combustion noise.
129(2011); http://dx.doi.org/10.1121/1.3575553View Description Hide Description
This study examined the time course of cochlear suppression using a tone-burst suppressor to measure decrement of distortion-product otoacoustic emissions (DPOAEs). Seven normal-hearing subjects with ages ranging from 19 to 28 yr participated in the study. Each subject had audiometric thresholds ≤15 dB HL [re ANSI (2004) Specifications for Audiometers ] for standard octave and inter-octave frequencies from 0.25 to 8 kHz. DPOAEs were elicited by primary tones with f 2 = 4.0 kHz and f 1 = 3.333 kHz (f 2/f 1 = 1.2). For the f 2, L 2 combination, suppression was measured for three suppressor frequencies: One suppressor below f 2 (3.834 kHz) and two above f 2 (4.166 and 4.282 kHz) at three levels (55, 60, and 65 dB SPL). DPOAE decrement as a function of L 3 for the tone-burst suppressor was similar to decrements obtained with longer duration suppressors. Onset- and setoff- latencies were ≤4 ms, in agreement with previous physiological findings in auditory-nerve fiber studies that suggest suppression results from a nearly instantaneous compression of the waveform. Persistence of suppression was absent for the below-frequency suppressor (f 3 = 3.834 kHz) and was ≤3 ms for the two above-frequency suppressors (f 3 = 4.166 and 4.282 kHz).