Volume 103, Issue 5, May 1998
Index of content:
- ACOUSTIC SIGNAL PROCESSING 
103(1998); http://dx.doi.org/10.1121/1.422767View Description Hide Description
The problem addressed in this paper is whether higher order correlation detectors can perform better in white noise than the cross correlation detector for the detection of a known transient source signal, if additional receiver information is included in the higher order correlations. While the cross correlation is the optimal linear detector for white noise, additional receiver information in the higher order correlations makes them nonlinear. In this paper, formulas that predict the performance of higher order correlation detectors of energy signals are derived for a known source signal. Given the first through fourth order signal moments and the noise variance, the formulas predict the SNR for which the detectors achieve a probability of detection of 0.5 for any level of false alarm, when noise at each receiver is independent and identically distributed. Results show that the performance of the cross correlation, bicorrelation, and tricorrelation detectors are proportional to the second, fourth, and sixth roots of the sampling interval, respectively, but do not depend on the observation time. Also, the SNR gains of the higher order correlation detectors relative to the cross correlation detector improve with decreasing probability of false alarm. The source signal may be repeated in higher order correlations, and gain formulas are derived for these cases as well. Computer simulations with several test signals are compared to the performance predictions of the formulas. The breakdown of the assumptions for signals with too few sample points is discussed, as are limitations on the design of signals for improved higher order gain. Results indicate that in white noise it is difficult for the higher order correlation detectors in a straightforward application to achieve better performance than the cross correlation.
103(1998); http://dx.doi.org/10.1121/1.422768View Description Hide Description
Reference-beam detection is inherently superior to the knife-edge detection currently in use in scanning laser acoustic microscopy. This new detector makes use of a reference beam, retarded 90 degrees, which is mixed coherently in a photodiode with the acoustically obtained image-modulated beam. The new detector has an isotropic transfer function which is circularly symmetrical around its highest value, namely the zero-frequency point in the spatial spectrum. This property makes it possible to detectspatial frequencies in all directions simultaneously and with equal sensitivity and simplifies the associated electronics. It also makes possible the employment of acoustic evanescent-wave detection so that ultrasound of low temporal frequency can be used and at the same time high spatial frequencies can be detected for obtaining high resolution. Oblique insonification, required for best operation in the knife-edge detector, is thus not preferred in the reference-beam detector and the resultant Doppler shift in the detected frequency of the transmitted zero-order acoustic waves is avoided.
103(1998); http://dx.doi.org/10.1121/1.422769View Description Hide Description
Auscultation of lungsounds in patient transport vehicles such as an ambulance or aircraft is unachievable because of high ambient noise levels. Aircraft noise levels of 90–100 dB SPL are common, while lungsounds have been measured in the 22–30 dB SPL range in free space and 65–70 dB SPL within a stethoscope coupler. Also, the bandwidth of lungsounds and vehicle noise typically has significant overlap, limiting the utility of traditional band-pass filtering. In this study, a passively shielded stethoscope coupler that contains one microphone to measure the (noise-corrupted) lungsounds and another to measure the ambient noise was constructed. Lungsoundmeasurements were made on a healthy subject in a simulated USAF C-130 aircraft environment within an acoustic chamber at noise levels ranging from 80 to 100 dB SPL. Adaptive filtering schemes using a least-mean-squares (LMS) and a normalized least-mean-squares (NLMS) approach were employed to extract the lungsounds from the noise-corrupted signal. Approximately 15 dB of noise reduction over the 100–600 Hz frequency range was achieved with the LMS algorithm, with the more complex NLMS algorithm providing faster convergence and up to 5 dB of additional noise reduction. These findings indicate that a combination of active and passive noise reduction can be used to measurelungsounds in high noise environments.
103(1998); http://dx.doi.org/10.1121/1.422770View Description Hide Description
In scanning tomographic acoustic microscopy (STAM), projection error correction is necessary for high-resolution tomographic reconstruction. In this paper, both phase and alignment errors are examined. These errors arise from several sources, including the quadrature receiver channels, the unknown initial phase term, and any misalignment in the rotational scan. The data acquisition process and the image formation algorithm for the STAM are reviewed, and a description of error estimation and correction is also presented. Experimental results from the STAM are included to demonstrate the capability and effectiveness of the error-removal techniques.