Essentials
SELELE MASHILO
Selele Mashilo has a mechanical engineering diploma from Tshwane University of Technology and
a refrigeration and air-conditioning diploma from Unisa. His experience includes over a decade in
government as deputy-director building services before rejoining the private sector in 1998 as HVAC&R
project engineer. He is the former chairperson of the Refrigeration and Air Conditioning Empowerment
Forum of SA (RAEFSA), the Air Conditioning and Refrigeration Industrial Council of SA (ACRICSA), and
Black Energy Services Companies (BESCO).
SOUND CHARACTERISTICS
(CONTINUED)
By Selele Mashilo
We managed to go through all the requirements contributing towards
attenuation of noise in a particular occupancy, so now we move on to
noise levels.
T
he noise level is different for various occupancies and
as indicated, if attenuation is not handled properly, it
can cause an uncomfortable situation even possibly a totally
uninhabitable environment. Think about recording studios where
air conditioning is a requirement for comfort but still noise must
be controlled to about 25 decibels.
REQUIRED SYSTEM ATTENUATION
Table 1 indicates the calculations leading to various indicators
of noise and its reflections on its path. From the initial
design, a fan of 630mm in diameter satisfied all air delivery
at a calculated pressure drop of the duct system. Noise is
considered at the nearest air discharge from the fan. Table 1
consists of a fan with its various decibels at various noise
octave bands. The fan, attenuators, ducting, and the first
grille outlet will give sound pressure levels radiated into the
occupied space.
The size of the room and the type of the material used for the
room will give effect to room sound pressure level reverberant.
The room direct effect sound pressure level depends upon on the
listener’s distance from the air outlet; in this case, it was 1m.
Depending on the noise criteria (NC) level required (in this
case, NC45 was used), we will finally arrive at the attenuation
required. This information will be given to the attenuator
manufacturer.
ROOM PRESSURE LEVELS
Figure 1 is a graphical indication of sound pressure levels (SPLs)
against various frequency octave bands. The calculated NC is
reflected by the blue line on the graph. Only at lower octave
bands will it rise above NC 45 noise criteria, but it will satisfactorily
do the work at higher bands. RACA
References: ASHRAE
Figure 1: Room pressure levels.
66
RACA Journal I January 2019
www.hvacronline.co.za