Audio Engineering

96 Chapter 3

Box 3.1 (Continued)

The Sound Pressure Level (in decibels of a sound)—20 times the logarithm to the

base 10 of the ratio of the pressure of this sound to the reference pressure. Unless

otherwise specifi ed, the reference pressure is understood to be 0.00002 N/m^2 (20

micropascals or 20 μ Pa).

The Velocity Level (in decibels of a sound) — 20 times the logarithm to the base 10

of the ratio of the particle velocity of the sound to the reference particle velocity.

Unless otherwise specifi ed, the reference particle velocity is understood to be

50 1 0 –9 meters per second (m/s).

The Intensity Level (in decibels of a sound) — 10 times the logarithm to the base

10 of the ratio of the intensity of this sound to the reference intensity.

Unless otherwise specifi ed, the reference intensity is 10 –12 watts per square meter

(W/m^2 ).

10 log 20

10 W

0.1 W

 dB

Therefore the LP drops by 20 dB also

100      10 dB/^2010 % reflected LP.

In other words, 10% of the LP returns as a refl ection. If the sound source had directed an
LP of a 100-dB signal at the wall of the chamber, a signal of 80 dB would be refl ected
back. Remembering how dB are combined, we can see that this refl ection will not change
the 100-dB reading of the direct sound by a discernible amount on any normal sound
level meter.

The desirability of a refl ective surface can be seen when it is realized that the direct sound
and the refl ected sound from a single surface can combine to be as much as 3 dB higher
than the direct sound alone. If the loudspeakers are directed to refl ect off the ground
during the cool early morning hours, then when the refraction effect of the sun on the
hard surfaces causes the sound to bend upward during the hot part of the day, the sound
bends up into the grandstand area. Most of the time, the refl ected sound is assisting the
direct sound, thereby saving audio power.