Acoustic Measurements

This topic concerns the development of new or improved measurement procedures and measurement uncertainty for which ARU research has impact on the development of International and European Standards.

An alternative method of measuring the sound insulation of acoustic louvres has been developed using impulse response analysis to circumvent the limitations of other methods [1]. Sound transmission coefficients are measured at different angles of incidence to determine angular-dependent transmission loss. In the low-frequency range, transmission is governed by a mass layer effect. For the mid- and high-frequencies, diffraction, interference and absorption determine the angle-dependent transmission. Transmission at the angle of incidence which corresponds to a line-of-sight through the louvre blades is the dominant contribution to the angular-average value and a single measurement at the pitch of the louvre can be used to estimate the overall transmission loss.

Research has been carried out to improve procedures for the field measurement of low-frequency airborne sound insulation between small rooms with non-diffuse fields [2]. New procedures have been proposed for sound pressure level and reverberation time measurements for the 50, 63 and 80Hz one-third octave bands. The sound pressure level measurement combines corner microphone positions with positions in the central region of each room to provide a good estimate of the room average sound pressure level with significantly improved repeatability. The 63Hz octave band can be used to measure reverberation times to avoid problematic measurements of short decays that often occur in timber-frame buildings. These procedures have been incorporated into the latest revision of the International Standard on field sound insulation testing (ISO 16283-1:2014).

In architectural acoustics, noise control and environmental noise, there are often steady-state signals for which it is desirable to measure the spatial average, sound pressure level inside rooms using manual scanning [3]. In comparison with mechanical scanning devices, the human body allows manual scanning to trace out complex geometrical paths in three-dimensional space. To determine the efficacy of manual scanning paths in terms of an equivalent number of uncorrelated samples, an analytical approach is solved numerically. The benchmark used to assess these paths is a minimum of five uncorrelated fixed microphone positions at frequencies above 200 Hz. For paths involving an operator walking across the room, potential problems exist with walking noise and non-uniform scanning speeds. Hence, paths are considered based on a fixed standing position or rotation of the body about a fixed point. In empty rooms, it is shown that a circle, helix, or cylindrical-type path satisfy the benchmark requirement with the latter two paths being highly efficient at generating large number of uncorrelated samples. In furnished rooms where there is limited space for the operator to move, an efficient path comprises three semicircles. These procedures have been incorporated into the latest revision of the International Standard on field sound insulation testing (ISO 16283-1:2014).

Knowledge of the sound field in front of, and close to a building façade is important for the measurement and prediction of environmental noise and sound insulation. For simplicity it is often assumed that the facade can be treated as a semi-infinite reflector, however in the low-frequency range (50Hz to 200 Hz) this is no longer appropriate as the wavelengths are similar or larger than the façade dimensions. Scale model measurements and predictions using integral equation methods have been used to investigate the effect of diffraction on the sound field in front of finite size reflectors [4]. For the situation that is commonly encountered in front of building façades, the results indicate that diffraction effects are only likely to be significant in the low-frequency range (50Hz to 200Hz) when the façade dimensions are less than 5m. This assumes that there is a point source close to the ground and microphones at a height of 1.2m or 1.5m, at a distance between 1m and 2m in front of the façade.

Inside occupied spaces there are numerous transient sounds which need to be assessed using maximum sound pressure levels in octave or one-third octave bands; hence quantifying the error in the measurement is important for standards and regulations. Maximum levels are also required for the measurement of vehicle noise emissions in Directive 97/24/EC. Research has been carried out on signal processing errors due to Fast or Slow time-weighting detectors when combined with octave band filters, one-third octave band filters or an A-weighting filter [5]. For 6th order Butterworth CPB filters the inherent time delay caused by the phase response of filters is quantified using three different approaches to establish the following rules-of-thumb: (1) time-to-gradient/amplitude matching occurs when Bt»1, (2) time-to-peak matching occurs when Bt»2 and (3) time-to-settle matching occurs when Bt»4 for octave band filters, and when Bt»3 for one-third octave band filters. Four different commercially-available sound level meters have been used to quantify the variation in measured maximum levels using tone bursts, half-sine pulses, ramped noise and recorded transients. Tone bursts indicate that Slow time-weighting is inappropriate for maximum level measurements due to the large bias error. The results also show that there is more variation between sound level meters when considering Fast time-weighted maximum levels in octave bands or one-third octave bands than with A-weighted levels. To reduce the variation between measurements with different sound level meters, it is proposed that limits could be prescribed on the phase response for CPB filters and A-weighting filters.

Selected publications

[1] Viveiros EB, Gibbs BM, Gerges SNY (2002) Measurement of sound insulation of acoustic louvres by an impulse method. Applied Acoustics vol 63 pp 1301-1313.

[2] Hopkins C and Turner P (2005) Field measurement of airborne sound insulation between rooms with non-diffuse sound fields at low frequencies. Applied Acoustics vol 66 pp 1339-1382.

[3] Hopkins C and Lam Y (2009) Sound fields near building facades - comparison of finite and semi-infinite reflectors on a rigid ground plane. Applied Acoustics vol 70 issue 2 pp 300-308.

[4] Hopkins C (2011) On the efficacy of spatial sampling using manual scanning paths to determine the spatial average sound pressure level in rooms. Journal of the Acoustical Society of America vol 129 issue 5 pp 3027-3034.

[5] Robinson M and Hopkins C (2014) Effects of signal processing on the measurement of maximum sound pressure levels. Applied Acoustics vol 77 issue C pp 11-19.