Free Field Acoustic Wave Propagation
Acoustic free field is a situation where there are no obstacle is close to the sound source, the receiver, and the sound path.
Under free field condition:
- Sound wave from omnidirectional point sound source will be spherical
- Sound pressure level, Lp, is a function of sound power level, Lw, of the source and the distance, d, from the source to receptor.
Where Q is the directivity factor of the source:
| Q | Type of source | Directivity Index, DI (dB) | # of Reflective surface |
|---|---|---|---|
| 1 | omnidirectional source | 0 | None |
| 2 | omnidirectional source over a reflective plane | 3 | 1 |
| 4 | omnidirectional source in a corner of 2 reflective plane | 6 | 2 |
| 8 | omnidirectional source in a corner of 3 reflective plane | 9 | 3 |
and D is the extra attenuation (e.g.: barriers, ground effects etc.):
Types of Wave Propagation Situations
| Situation | Remark | Attenuation Term | Standard |
|---|---|---|---|
| Terrain (ground effect) | Complicates calculation through screening or reflection |  |
(ISO 9613) |
| Room (very small) | All dimensions are small compared to wavelength of sound source | Stationary waves | - |
| Room (normal) | Free path is large compared to wavelength of sound source (practically constant reverberant field) |  |
- |
| Room (very large) | Spatial sound level decay is present | Empirical model exists, but usage of simulation models are advised | - |
Where:
- h_m = the average height above ground of the direct sound propagation path, in m
- α = mean absorption coefficient
- A = total area of absorption, in m^2
References
[1] M. Asselineau, Building Acoustics. Boca Raton, Florida: CRC Press, 2015.
[2] D. A. Bies, C. H. Hansen, and C. Q. Howard, Engineering Noise Control, 5th ed. Boca Raton, Florida: CRC Press, 2017.