The best uses of this guide are to measure the OINR and the AOITL(θ) or OITL(θ) at specific angles of incidence. By measuring the AOITL(θ) or OITL(θ) at several loudspeaker sound incidence angles, by energy-averaging the receiving room sound levels before computing results, an approximation of the diffuse field results measured with Test Methods E90 and E336 may be obtained.

The traffic noise method is to be used only for OINR measurements and is most suitable for situations where the OINR of a specimen at a specific location is exposed to an existing traffic noise source.

5.3 The OINR, AOITL(θ), and OITL(θ) produced by the methods described will not correspond to the transmission loss and noise reduction measured by Test Methods E90 and E336 because of the different incident sound fields that exist in the outdoors (1) . All of these results are a function of the angle of incidence of the sound for two reasons.

5.3.1 The transmission loss is strongly influenced by the coincidence effect where the frequency and projected wavelength of sound incident at angle, θ, coincides with the wavelength of a bending wave of the same frequency in the panel (2, 3, 4, 5). This frequency and the angle of least transmission loss (greatest transparency) both depend on specimen panel stiffness, damping and area mass. In diffuse-field testing as in the laboratory, the effect is a weakness at the diffuse field average coincidence frequency that is dependent on material and thickness, often seen around the frequency of 2 500 Hz for drywall and glass specimens. For free field sound coming from one direction only, the coincidence frequency varies with incidence angle and will differ from the diffuse-field value (5). Near or at grazing (θ <=90º) it will be much lower in frequency than the diffuse field (E90 and E336) value, and will increase with reducing θ to be considerably above the diffuse-field frequency when θ is 30º or less. Wood panels (that is, doors) and masonry walls exhibit lower coincidence frequencies while sheet steel exhibits higher coincidence frequencies.

5.3.2 The OINR is influenced by the angle of incidence of free field sound coming from a specific angle as compared to a diffuse field. This is because the intensity of free field sound incident across the specimen surface S is reduced by cos(θ) when the sound is not incident normal to the surface. Additionally, when the sound of level L arrives as a free-field from one direction only, and that is normal to the surface, the resulting sound intensity in this direction is 4 times that due to diffuse-field sound of the same level, L. These factors are reflected by the cos(θ) and 6 dB terms in Eq 6.

5.3.3 The methods in this guide should not be used as a substitute for laboratory testing in accordance with Test Method E90.

5.4 Of the three methods cited for measuring the outdoor sound field from a loudspeaker, the calibrated loudspeaker and flush methods are most repeatable. The near method is used only when neither the calibrated speaker nor the flush method are feasible.

5.5 Flanking transmission or unusual field conditions could render the determination of OITL(θ) difficult or meaningless. Where the auxiliary tests described in Annex A1 cannot be satisfied, only the OINR and the AOITL(θ) are valid.

5.6 When a room has multiple surfaces exposed to outdoor sound, testing with just one surface exposed to test sound will result in a greater OINR than when all surfaces are exposed to test sound. The difference is negligible when the OITC of the unexposed surface is at least 10 greater than the OITC of the exposed surface.