Methods of Acoustical Treatment

Acoustic remedies are required if the sound pressure levels within the critical space exceed the sound pressure levels per frequency of the corresponding NC or RC curve. The most common acoustic attenuation method for HVAC duct systems is to install a silencer (i.e., sound attenuator or sound trap).

Silencers

A silencer attenuates sound when it is directly inserted in the ducted air stream (path). The silencer is basically a series of perforated sheet metal baffles (rectangular silencers) or bullets (circular silencers) placed inside a silencer single or double wall outer solid shell. The baffles/bullets are usually filled with sound absorbing material.

Acoustic performance of duct silencers is generally described in terms of “insertion loss”—the measure of noise level reduction determined by comparing the noise level without a silencer to the noise levels with silencer. Since the silencer itself can generate noise (because it disturbs the airflow), its self-generated noise has to be added to the attenuated sound level. Baffle and bullet-type silencers block a portion of the air stream and will cause additional pressure drop (PD). Manufacturers should always list values for insertion loss, regenerated noise and pressure drop.

An absorptive silencer is the most common type of silencer. It uses absorptive fibrous material within sound baffles or sound bullet cavity with perforated sheet metal facings that allow sound energy to pass through and be absorbed by the fibrous fill.

Rectangular and circular silencers have solid rounded nose cones on the air inlet end and tapered tails on the air discharge end of each baffle/bullet to minimize silencer pressure drop. The tapered end of the silencer baffle/bullet allows for static regain to occur, thereby offering the lowest silencer pressure drop for a particular level of attenuation. This is important because silencer pressure drop is in direct relation to a duct system’s lifetime energy costs.

Kinetic cut view of rectangular silencer showing internal acoustic media, nose cone, perforated baffle face and outer solid silencer shell Image courtesy of Kinetics Noise Control, Inc.

Manufacturers offer a selection of standard or custom engineered silencers that will satisfy the requirements of project specific applications.

Circular silencer with sound absorbing bullet and outer shell supply and return air systems Photo courtesy of Kinetics Noise Control, Inc.

Rectangular Silencer with circular end caps and absorptive baffles. Image courtesy of Kinetics Noise Control, Inc.

Circular silencer with absorptive outer casing and no sound absorbing bullet. Image courtesy of Kinetics Noise Control, Inc.

T-rectangular elbow silencer with absorptive side baffles and curved air passages. Image courtesy of Kinetics Noise Control, Inc.

Circular silencer with sound absorbing bullet and no absorptive outer casing Image courtesy of Kinetics Noise Control, Inc.

Circular silencer with internal sound absorbing bullet and absorptive outer shell. Image courtesy of Kinetics Noise Control, Inc.

Rectangular elbow silencer with extended body (casing) Image courtesy of Kinetics Noise Control, Inc.