386
8 Waves
(ii) λ =
v − v s
f
=
340 − 15
514
= 0.63 m
(iii) λ =
v + v s
f
=
340 + 15
514
= 0.69 m
8.3.6 Shock Wave
8.77 (a) If an object flies with a supersonic speed (speed greater than that of
sound) a shock wave is emitted, a booming sound. In the two-dimensional
drawing, Fig. 8.9, the wave fronts CB and DB represent the V -shaped
wave. In three dimensions the bunching of the wave fronts actually forms
a cone called the Mach cone. The shock wave lies on the surface of
the cone.
(b) The half-angle θ of the cone called the Mach cone is given by
sin θ =
vt
v s t
=
v
v s
=
1
2
The Mach number =
v s
v p
= 2
(c) The Mach angle θ = 30 ◦
Fig. 8.9 Shock wave
8.3.7 Reverberation
8.78 If V is the volume, S the surface area and K the absorption coefficient then
the reverberation time t R is given by
t R =
0.16V
i k i S i
(Sabine Law)
V = 10 × 18 × 4 = 720 m
3
8 Waves
(ii) λ =
v − v s
f
=
340 − 15
514
= 0.63 m
(iii) λ =
v + v s
f
=
340 + 15
514
= 0.69 m
8.3.6 Shock Wave
8.77 (a) If an object flies with a supersonic speed (speed greater than that of
sound) a shock wave is emitted, a booming sound. In the two-dimensional
drawing, Fig. 8.9, the wave fronts CB and DB represent the V -shaped
wave. In three dimensions the bunching of the wave fronts actually forms
a cone called the Mach cone. The shock wave lies on the surface of
the cone.
(b) The half-angle θ of the cone called the Mach cone is given by
sin θ =
vt
v s t
=
v
v s
=
1
2
The Mach number =
v s
v p
= 2
(c) The Mach angle θ = 30 ◦
Fig. 8.9 Shock wave
8.3.7 Reverberation
8.78 If V is the volume, S the surface area and K the absorption coefficient then
the reverberation time t R is given by
t R =
0.16V
i k i S i
(Sabine Law)
V = 10 × 18 × 4 = 720 m
3