Linear sweeps and the characterisation of linearly time-varying acoustical systems.

Sine sweeps are widely used as test signals in the experimental characterisation of linear and non-linear acoustical systems' responses. One of the reasons for their popularity is that their immunity against time variance is assumed to be superior to that of other types of test signals. This paper explores and quantifies the consequences of linear time variance on the characterisation of acoustical systems with linear sweeps. The first result is a novel compact closed-form expression for the spectrum of a linear sweep. Second, a similar expression for the frequency response of a linearly time-varying delay to a linear sweep is derived. Additionally, this work demonstrates the application of these expressions through a numerical investigation into the frequency response of a time-invariant bandpass filter when the measurement chain exhibits linear time variance and the attenuation between a ground-borne point source and a receiver in front of a façade under the assumption of a homogeneous atmosphere subject to realistic temperature changes.

Sound emergence as a predictor of short-term annoyance from wind turbine noise.

While sound emergence is used in several countries to regulate wind energy development, there is no published evidence that it is a relevant noise descriptor for this purpose. In the present work, we carried out two listening tests to evaluate the merits of sound emergence. Three definitions of sound emergence were considered: the one in ISO 1996-1, sound emergence under audibility condition eUAC, and spectral emergence eSP. We also considered the specific to residual ratio and loudness metrics. In each listening test, the sound stimuli consisted of 48 sound stimuli at three A-weighted sound pressure levels {30, 40, 50} dB and four specific-to-residual ratios {-10,-5,0,+5} dB. The results lead to the conclusion that short term annoyance is better predicted by the total sound pressure level than by sound emergence, whatever the definition considered for the latter, or than by the specific to residual ratio. Short-term annoyance is slightly better predicted by eUAC than by e, while e is a better predictor than eSP. The total sound pressure level and the loudness metrics performed similarly. Furthermore, the results provide evidence that sound emergence is a poor predictor of the audibility of wind turbine sounds.

Developing experimental skills: A hands-on course in acoustical measurement techniques at the Norwegian University of Science and Technology.

The course "Acoustical Measurement Techniques TTT4250," offered by the Acoustics Group at the Department of Electronic Systems, Norwegian University of Science and Technology, is a fourth-year course in the specialization of acoustics in the five-year master program "Electronics Systems Design and Innovation" or MTELSYS, and the two-year international master program "Electronic Systems Design" or MSELSYS. It is one of the four required courses for MTELSYS and one of the two required courses for MSELSYS. It offers a hands-on approach to acoustics. This paper outlines the topics covered in this course and the involvement of several academic staff members, as well as invited industry and research institute guest speakers, as teachers. The assessment of laboratory reports is described, and general lecture topics, including measurement uncertainty and statistics, the introduction of standards, and programming, are also described. All aspects of the course aim to maximize students' experience with a broad range of acoustic measurements and their interest in acoustics.

Challenges of the Use of Sound Emergence for Setting Legal Noise Limits.

In the vast majority of legislation on environmental noise, the metric used for expressing limit values is based on sound pressure levels. But some countries have introduced sound emergence limit values where the compliance of a noise-generating activity is defined as a maximum allowable difference between the sound pressure level with and without the regulated activity operating. This paper investigates the foundations and the merits of this kind of differential noise limit values. Our review of literature indicates that there is very little evidence supporting the use of differential noise limits over absolute ones. Moreover, while sound emergence limits seem to originate from consideration about audibility of the regulated noise source, they appear to give little insight into what is audible and what is not. Furthermore, both the definition and the practical measurement of sound emergence raise several challenges that compromise reproducibility. In addition, first, the reference to background noise makes it very difficult first to ascertain the conformity of noisy installations in the long run, second to effectively protect the community from excessive noise and third to evaluate conformity on the basis of simulations. When switching to another metric is not an option the paper makes recommendations toward a more reliable use of sound emergence.