14 
In order to ascertain when this correction term can be 
approximated by unity, consider the sound pressure ata 
distance o(x, y) from a point source emitting spherical 
waves; then the sound pressure is given by 
- 
= -00 kCA 0 Hi 40 3K 
Py = Ge, peers Lue: kote, yt 9 
where the symbols have the meaning ascribed to them in 
the nomenclature. 
The sound pressure amplitude is 
me |= Oo eA 
fe) Oo)” Saw 
and at a distance D- 
Se OGiCA 
ey Weg = 
The sound pressure amplitude drop between D and P(x, y) 
by the wave is given in db by 
Fy H D 
2010g, 5 em = 2010g, 5 o (x, y) 
D ’ 
D : : 
ao : etal 
If 0 oS Ga) <1.1, then, the amplitude error is +ldb or 
less, which is considered small enough in most cases so 
D NRK eok 
that 1 4 
a Ga) can be replaced by unity 
*Reference 4, page 30, e.g. (1.41) 
**Reference 5, page 457 
