Methods of Measuring Impulse Responses in Architectural Acoustics

Projecte final de carrera realitzat en col.laboració amb la Technical University of DenmarkNew methods of measuring impulse responses based on carefully designed deterministic signals can further improve the performance o ered by classical methods. In fact, these methods are particularly interesting when measuring long impulse responses as the ones analyzed in architectural acoustics. However, the e ects of background and impulsive noise, distortion and time-variance are known rather qualitatively. For this reason, the ISO 18233 encourages to develop a deeper understanding of the theoretical bases of these techniques. In this sense, this project presents an in depth analysis of two di erent methods of measuring impulse responses: the linear convolution of sweep signals with the inverse lter and the circular crosscorrelation of maximum length sequences (MLS) and inverse repeated sequences (IRS). The results of this work reveal that the sweep technique can provide signi cant reduction of distortion compared to MLS/IRS technique but, unlike what is explained in the literature, sweep signals cannot reject all distortion artifacts from the causal part of the impulse response. Besides, it is proved that IRS sequences are immune to distortion of even order. On the other hand, it is con rmed that synchronous averaging procedure improves the SNR at the microphone position by 3 dB per doubling the number of averages. Alternatively, it is also proved that the noise contaminating the measured impulse response is reduced by 3 dB every time that the length of the excitation signal is doubled. In terms of impulsive noise, the sweep technique only contaminates speci c frequency bands of the system's impulse response, whereas the MLS/IRS technique uniformly distributes all impulsive noise artifacts over the entire measured impulse response. Finally, it is also shown that MLS/IRS measurements are more vulnerable to time-varying systems than sweep measurements

Spread-spectrum techniques for environmentally-friendly underwater acoustic communications

PhD ThesisAnthropogenic underwater noise has been shown to have a negative impact on marine life. Acoustic data transmissions have also been shown to cause behavioural responses in marine mammals. A promising approach to address these issues is through reducing the power of acoustic data transmissions. Firstly, limiting the maximum acoustic transmit power to a safe limit that causes no injury, and secondly, reducing the radius of the discomfort zone whilst maximising the receivable range. The discomfort zone is dependent on the signal design as well as the signal power. To achieve these aims requires a signal and receiver design capable of synchronisation and data reception at low-received-SNR, down to around −15 dB, with Doppler effects. These requirements lead to very high-ratio spread-spectrum signaling with efficient modulation to maximise data rate, which necessitates effective Doppler correction in the receiver structure. This thesis examines the state-of-the-art in this area and investigates the design, development and implementation of a suitable signal and receiver structure, with experimental validation in a variety of real-world channels. Data signals are designed around m-ary orthogonal signaling based on bandlimited carrierless PN sequences to create an M-ary Orthogonal Code Keying (M-OCK) modulation scheme. Synchronisation signal structures combining the energy of multiple unique PN symbols are shown to outperform single PN sequences of the same bandwidth and duration in channels with low SNR and significant Doppler effects. Signals and receiver structures are shown to be capable of reliable communications with band of 8 kHz to 16 kHz and transmit power limited to less than 170.8 dB re 1 μPa @ 1m, or 1W of acoustic power, over ranges of 10 km in sea trials, with low-received-SNR below −10 dB, at data rates of up to 140.69 bit/s. Channel recordings with AWGN demonstrated limits of signal and receiver performance of BER 10−3 at −14 dB for 35.63 bit/s, and −8.5 dB for 106.92 bit/s. Piloted study of multipath exploitation showed this performance could be improved to −10.5 dB for 106.92 bit/s by combining the energy of two arrival paths. Doppler compensation techniques are explored with experimental validation showing synchronisation and data demodulation at velocities over ranges of ±2.7m/s. Non-binary low density parity check (LDPC) error correction coding with M-OCK signals is investigated showing improved performance over Reed-Solomon (RS) coding of equivalent code rate in simulations and experiments in real underwater channels. The receiver structures are implemented on an Android mobile device with experiments showing live real-time synchronisation and data demodulation of signals transmitted through an underwater channel.UK Engineering and Physical Sciences Research Council (EPSRC): PhD Doctoral Training Account (DTA)