A hybrid method for impulse response measurements with synthesized musical tones and Masked-MLS Stimuli
Impulse responses or transfer functions are descriptions of acoustic and audio transmission channels, which completely characterise point-to-point propagation of sound in room acoustics or input-output relationships of linear systems in electroacoustics. Measurements of impulse responses or transfer functions are routinely carried out first to determine critical room acoustics parameters of enclosures such as concert halls, theatres and auditoria, or technical specification of electroacoustic transducers, i.e. loudspeakers and microphones. In room acoustics measurements, tradition techniques employ noisy testing signals as probe stimuli, which are unpleasant and intolerable to audiences. This hinders occupied measurements to be taken in many cases. Predicted in-use parameters from unoccupied measurements are known to be unreliable and problematic. It is also well appreciated in room acoustics research community that the use of musical or music-masked probe stimuli can mitigate problems of occupied measurements. It is therefore hypothesised as a starting point of this thesis that the use of musical tone like stimuli or musically masked testing signals can be used to determine impulse responses or transfer functions. Based on the above hypothesis, this thesis develops a new hybrid technique, in which narrow band linear chirps, called “presto-chirps” centred on musical notes are used to measure impulse responses in low to mid frequency bands, and music-masked maximum-length-sequences are deployed to obtain those in higher frequency bands. Broadband impulse responses are then obtained by combining the measured lower and higher frequency impulse responses. To test the hypothesis and identify the potential and limitations of the developed technique, mathematical formulation and analysis, computer simulations and real room measurements have been carried out and documented in this thesis. Investigation results show that purposely tailored and windowed narrow chirps that emulate musical tones can be used as probe stimuli to measure impulse responses or transfer functions with an uncompromised accuracy. It is found that Hanning windows are almost optimal for this application. This method covers frequency ranges commonly quoted in room acoustics investigations. Music-masked maximum-length sequences are found to be able to obtain in impulse responses or transfer functions in higher frequency. However, if completely masked stimuli are sought, the resulted signal to noise ratios in the measurements is limited, or the required averaging is going to be overly prolonged. Nevertheless, the masking music can still potentially be used as a distracter to make the audience more forgiving to the hissing noise from maximum length sequences, facilitating the occupied measurements.
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