Handbook of air conditioning and refrigeration / Shan K

Control at Design Stage
Structure-borne transmission is actually a combination of sound and vibration effects. This problem
is usually solved through the combined effort of the structural and HVAC&R designers.
The evaluation and control of all noise transmission paths should take place at the design stage, and
it should be a cooperative effort of the architect and the structural and mechanical engineers. If an
excessive noise level occurs due to improper design—the designer fails to analyze the potential
noise problem, there are mistakes in analysis, or data are used improperly—remedial measures to
reduce the noise level are usually expensive and are often less effective.
One effective measure to reduce noise in buildings is to locate the fan room or other sound
source away from critical areas, such as conference rooms and executive offices. For purposes of
noise control, the fan room may be located near rest rooms, stairwells, or copy rooms.
For noise control, theoretical estimation and prediction schemes are mainly guidelines. Previous
experience and field performance are important. Especially for projects with strict acoustic requirements,
field/laboratory tests and checkouts are often necessary.
Recommended Procedure for Noise Control
Before the analysis and evaluation of sound levels, the indoor design NC or RC criteria are usually
determined, and the necessary data to evaluate sound levels at various points of interest are collected
and investigated. The recommended procedure for noise control uses the following approach:
The basic procedure is as follows:
Source : Path (attenuation) : Receiver
SOUND CONTROL 19.3
1. Determine the sources of the noise from the fan, compressor, and pump, and the sound power
level generated. Use certified manufacturer’s data.
2. Carefully analyze all the possible sound paths that can transmit noise from the source to the
occupied zone. An overlooked sound path may affect the final results.
3. Calculate all the sound attenuation and transmission losses in each sound path during transmission.
4. Determine the airflow noise L af, in duct-borne path due to dampers, elbow, or junctions and
the attenuated fan noise near the damper L at, fan, both in dB. If L at, fan � L af � 8 dB, L af can be
ignored. Only when L at, fan � L af � 8 dB should L af be added to L at, fan for duct-borne transmission
calculations.
5. For duct-borne paths, determine the attenuated sound power level of fan noise at the supply
outlet (system noise or room source L wr), in dB re 10 �12 W, that affects the receiver. Convert L wr to a
sound pressure level received by the receiver at a chosen point in a room L pr or at a plane 5 ft (1.5
m) from the floor L pt, both in dB re 20 �Pa.
6. For radiated sound transmissions or airborne transmissions, the sound power level of the
sound radiated into the receiving room should be converted to a room sound pressure level.
7. The resultant sound pressure level L �p at the center frequency of each octave band, in dB, that
is perceived by the receiver at a chosen point or at a plane 5 ft (1.5 m) from the floor level is the
sum of sound pressure levels from all sound paths. Determine the NC or RC L �NC, in dB, from the
calculated L �p in each octave band.
8. Compare L �NC with the design criteria NC or RC L p,NC. If L �NC � L p,NC, then one should add
a silencer (or an active noise control device, which is discussed in later sections) to the duct-borne
sound path, change the configuration of the duct system, or build structures to bring L �NC down to
or slightly below L p,NC.