Page 61 - The Master Handbook Of Acoustics

36 CHAPTER TWO

reflection from the nearest sidewall arrives at seat x 105 milliseconds
after the arrival of the direct sound. (A) How far does the reflection
travel to reach seat x? (B) What is the SPL of the reflection at seat x,
assuming perfect reflection at the wall? (C) How long will the reflec-
tion be delayed after arrival of the direct sound at seat x?
(A) Distance = (1,130 ft/sec) (0.105 sec)
= 118.7 ft
(B) First, the level, L, 1 foot from the tympani drum must be estimated:
55 = L – 20 log (84/1)
L = 55 + 38.5
L = 93.5 dB
The SPL of the reflection at seat x is:
dB = 93.5 – 20 log (118.7/1)
= 93.5 – 41.5
= 52 dB
(C) The reflection will arrive after the direct sound at seat x after:
Delay = (118.7 – 84)/1,130 ft/sec
= 30.7 milliseconds
A free field is also assumed here. In chapter 3, the 30.7 ms reflection
might be called an incipient echo.

Example: Combining Decibels
Let’s say it is warm in our studio and a fan is brought in to augment the
air conditioning (A/C) system. If both fan and the A/C are turned off, a
very low noise level prevails, low enough to be neglected in the calcu-
lation. If the A/C alone is running, the sound-pressure level at a given
position is 55 dB. If the fan alone is running, the sound-pressure level
is 60 dB. What will be the sound-pressure level if both are running at
the same time?
60
55
( )
10
10
Combined dB = 10 log 10 + 10
= 61.19 dB
If the combined level of two noise sources is 80 dB and the level with
one of the sources turned off is 75 dB, what is the level of the remain-
ing source?