Abstract
The paper presents an investigation simulating transformations of an acoustic field at low frequencies indoors, when the sound source is outdoors. The investigation was performed using a simplified and 5-times-smaller physical model. The paper presents measured spatial sound pressure level (SPL) distribution at 1/3 octave bands as well as at discrete frequencies (at various room modes). Measurements inside the physical model (at a total of 2,565 points) confirm that when exposed to outdoor broadband noise, low-frequency sound pressure levels at 1/3 octave bands inside the room can differ by more than 30 dB, while at discrete frequencies measured SPL can vary by 50 dB or more. Below the calculated lowest room mode, due to resonant vibrations of physical model walls, large differences in sound pressure levels inside the model (up to 20.7 dB at 100 Hz 1/3 octave band and up to 32.4 dB at discrete Hz frequencies) were found. The investigation also includes analysis of levels in the corners of physical models compared to average sound pressure levels in the whole model space or some cross-sections, which shows that sound pressure levels in corners can be up to 10 dB lower. Calculation of the indoor average sound pressure level at low frequencies according to empirical formulas specified in standard ISO 12354-3 showed conformity between measurement and calculation results only in a part of the investigated range of frequency bands. Calculations using FEM at discrete frequencies gave more adequate results of sound pressure levels and their spatial distribution. FEM calculations proved that calculation of the average sound pressure level from measurements at points every 25 cm (every 5 cm in the physical model) can produce results close to the average of the sound pressure level of the room if it were measured at every possible position.
Highlights
With the development of newer economic activities and the resulting increase in environmental noise pollution, it becomes increasingly important and relevant to predict the indoor noise level in newly-constructed buildings
To assess indoor noise levels coming from the outdoors, formulas found in architect-orientated manuals or textbooks, e.g.: [1,2,3] or the ISO 12354-3 [4] standard, are used, which are based on evaluation of the apparent sound reduction index of the building façade and include corrections according to the noise source, the incident sound wave angle, the volume of the enclosed space or the sound absorption properties of the enclosed space’s walls
The formulas rely on average sound pressure levels (SPL) and equivalent SPL and are adapted for predicting sound levels, presuming that a diffuse acoustic field is formed indoors
Summary
With the development of newer economic activities and the resulting increase in environmental noise pollution, it becomes increasingly important and relevant to predict the indoor noise level in newly-constructed buildings. The formulas rely on average sound pressure levels (SPL) (indoors and outdoors) and equivalent SPL (if the sound levels change over time) and are adapted for predicting sound levels, presuming that a diffuse acoustic field is formed indoors. It means that the formulas do not take into account the possible forming of a standing wave effect, which appears at low frequencies and directly depends on the dimensions of the enclosed space. STUDY OF MEASURED INDOOR LOW-FREQUENCY NOISE LEVELS RESULTING FROM OUTDOOR NOISE SOURCES, USING A SIMPLIFIED PHYSICAL MODEL AS WELL AS EMPIRICAL AND FEM CALCULATIONS. For the investigation of room modes and resonant frequencies of walls, for FEM calculations analysis of discrete frequencies was used
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