Applied Acoustics Laboratory activities

  • Outdoor sound propagation

The laboratory activities in this field consist in sound level metering and subsequent analysis aimed at verifying the compliance with by-law limits indoors and outdoors. Beside measurements, the construction of a complex software model can be required that, starting from the sound power level of the sources and from the geometric and orographic features of the location, allows to predict the distribution of the sound pressure level in the surrounding areas.

The Applied Acoustics Laboratory course was born to provide future engineers with the essential elements to understand and control sound and vibration problems.

Instrumentation: sound level meter equipment, class 1; simulation software SoundPLAN.

  • Indoor sound propagation / Room acoustics

This area includes different activities having in common the determination of how the sound propagates in a closed space.

To do so, the sound absorption behaviour of the given environment is evaluated by means of sound level measurements or software simulations that use large databases of material characteristics.

If the sound power levels of the sources are known, it is also possible to estimate the sound pressure level in any point of the closed space (acoustic mapping), as a prediction or verification tool.

The results obtained for a closed-space can also be used to estimate the noise impact of the structure on the surrounding area, provided that the sound insulation characteristics of the building envelope are known.

Another typical room acoustics activity is the evaluation of the acoustic quality of a given environment. All the parameters required to qualify a location, according to the scope it is meant to fulfill from an acoustic viewpoint, can be determined by measuring the impulse response in defined positions. Whenever a more sophisticated evaluation is required, for instance to remove system distortions, the time-reversal mirror technique is used.

Instrumentation: sound level meter equipment, class 1; sound card Motu Traveler; omnidirectional and eight-shaped measurement microphones; omnidirectional source; binaural acquisition system Head Acoustics; simulation software Ramsete; acquisition software Audacity; plug-in suite Aurora; simulation software SoundPLAN.

  • Sound reduction index and radiation efficiency of layered structures

This highly specialised branch of sound insulation field deals with insulation properties of complex materials.

To save long and expensive test campaigns in sound transmission rooms, correlations are available in the literature providing good estimation of sound reduction index for simple single-leaf structures, while complex multi-layered structures are still in the research phase. Recently, "hybrid" techniques have been developed that combine the results of accelerometer-based measurements on even large-sized beams or panels to mathematical models, providing encouraging results.

Once the critical frequency is known, the same measurements can be used to evaluate how efficiently the structure can irradiate sound energy in the surrounding space.

For further information on this topic, please check the Research section.

Instrumentation: sound intensity probe; single-axis accelerometers; impact hammer PCB; shaker; amplifier; impedance head; multichannel analyser Oros OR 36; sound intensity measurement acquisition and post-processing software Oros Sound Intensity; post-processing MATLAB language script.

  • Building acoustics

This group of activities consists in sound level metering to the aim of verifying the sound insulation requirements of buildings, with particular reference to façade, airborne and tapping noise insulation.

Instrumentation: sound level meter equipment, class 1; omnidirectional source; sound source for façade measurements.

  • Sound power level

The laboratory is equipped to perform sound power level measurements on sound sources with pressure methods according to ISO 3744 and ISO 3746 standards, and with sound intensity methods according to ISO 9614 series standards.

Instrumentation: sound level meter equipment, class 1; sound intensity probe; multichannel analyser Oros OR 36; sound intensity measurement acquisition and post-processing software Oros Sound Intensity.

  • Sound absorption

The diffuse-field absorption coefficient is an important acoustic parameter that is measured in sound transmission rooms on a 10 m2 material sample, according to the international standards. However, at an early stage when several materials must be examined and compared to make a choice, a preliminary selection can be made on the basis of the normal-incidence absorption coefficient, that can be determined in a quite simpler way by means of a special device called "impedance tube".

This approach is particularly interesting as it allows to save several test sessions in sound transmission rooms. Besides, it requires a circular sample only 60 mm in diameter. The sample is placed in the tube at one end, opposite to a sound source generating white noise. Two microphones within the tube measure incident and reflected sound intensity, thus allowing to determine the absorbed fraction.

Instrumentation: impedance tube BSWA; microphones BSWA; sound card M-Audio M-Track; acquisition software Audacity; plug-in suite Aurora; post-processing MATLAB language script.