Acoustic Duct Silencer – Introduction

A Duct silencer is a piece of ductwork that is designed and manufactured to attenuate sound transmitted through the ductwork. The common duct silencer is a dissipative silencer. It uses changes in duct cross section and sound absorbing materials to attenuate sound.

Dissipative duct silencers are available in a number of configurations including straight, elbow, tee, and transition. They are also available as a rectangular or round duct section. The illustration below shows a typical straight rectangular configuration.

Duct Silencer Section

In the illustration above, you can see the three stages of sound attenuation in the duct silencer.

1. A change in cross sectional area at the inlet causes a reflection of sound back towards the sound source.

2. Acoustic media in the baffles absorbs sound as it flows down the passage.

3. A change in cross sectional area at the outlet causes another reflection of sound back towards the source. This attenuation component is small compared to the other two.

Dissipative duct silencers have three measurable performance characteristics:

1. Acoustic Insertion Loss: This is a measure of the attenuation provided by the duct silencer. Insertion loss varies slightly (±2 dB) with air flow direction and velocity. It is expressed as a spectrum of dB values (insertion loss) for the octave bands from 63 Hz to 8 kHz.

2. Airflow Generated Noise: This is a measure of the noise generated by the silencer. The noise is generated by air turbulence caused by the silencer. This noise is a function of air velocity and silencer geometry. It is also expressed as a spectrum of dB values (noise levels) for the octave bands from 63 Hz to 8 kHz.

3. Air Pressure Drop: This is a measure of the total air pressure drop caused by air flow through the silencer. Just like all duct fittings, pressure drop is also affected by upstream airflow conditions (turbulence). Pressure drop is expressed as a single number in inches water column or kPa. ASHRAE recommends that this does not exceed 0.35 inch of water (90 kPa).

Increasing the insertion loss typically also increases the air pressure drop. Dissipative silencers must be selected to provide sufficient insertion loss while minimizing air pressure drop.

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Duct Branch Sound Power Division

When a sound wave traveling in a main duct encounters a branch, the sound power contained in the sound wave is divided between the main and branch ducts.  This acoustic phenomenon is referred to as branch sound power division.

The portion of sound energy that is transmitted into each branch is associated with the cross sectional area of the main and branch ducts.

If the total areas of the branch and main ducts after the junction are different from the main duct before the junction then there will be a reflected sound wave.  The reflected wave travels back up the duct.  That sound power is deducted from the overall sound power transmitted into both the branch and main.

The remaining sound energy is divided proportionate the cross sectional area of the branch and main ducts.  This sound wave continues down the main and branch ducts.

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How to Mitigate Noise Transfer between Hotel Guestrooms

Sound GrilleQ  If you have an existing hotel with excessive room to room noise transfer through the ventilation shaft, what can you do to fix it?

A  You must add some form of sound attenuation in the path between the two rooms.  In this case, the path is the ventilation system. 

This is how I solved that problem in an existing five star hotel. 

I reviewed the existing conditions and there were a number of constraints that had to be considered.

1. The visible portion of the grille had to compliment the interior design of the room. (The grille was mounted to a wall with a mirror finish.)

2. The solution could not interfere with the ventilation air flow or smoke management airflow (for which this shaft was also used).

3. The solution could not be applied to the inside of the shaft due to a lack of access.

4. The solution could not be applied to the very short duct from the shaft to the room because of the fire-smoke damper there.

I decided to replace the existing standard grille with a device to add a sound attenuating while not impeding airflow.  I searched for a product to provide this solution, but could not find anything suitable. 

Necessity Is the Mother of Invention

I designed a custom sound attenuator to replace the existing grille and provide the aesthetic the client desired.  I sent CAD drawings of the custom grille to the owner who had few prototypes made for mock-up.  The grilles were installed in three stacked rooms to test and obtain aesthetic approval from hotel management.

Innovative Solutions Provide Great Guest Experiences

The sound attenuating grille actually looked better than the original off-the-shelf grille. The new grille didn’t adversely affect the ventilation or smoke management systems.  Most importantly, sound transmission from guestroom to guestroom was reduced to a level that was not perceptible. When it comes to the guest experience in a five-star hotel, everything must be the best, including the acoustics.

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Outdoor Noise Barrier Basics

If you have a problem with noise propagation outdoors then an outdoor sound barrier may be the solution. Outdoor sound barriers attenuate sound between an outdoor sound source and receiver.

Outdoor sound barriers provide sound attenuation between a sound source and a receiver through multiple mechanisms. The illustration below shows what happens to sound when a barrier is placed between a source and receiver. The noise barrier interrupts the direct path from the source to the receiver.

Depending on the noise barrier material and surface treatment, a portion of the source sound energy is reflected or scattered back towards the source. The remaining sound energy is absorbed by the barrier material, transmitted through the barrier and diffracted at the top edge of the barrier.

Sound Barrier Geometry

The level of the attenuation provided by an outdoor sound barrier is based on the geometry of the barrier relative to the source and receiver. In general, the taller and wider the barrier, the greater the attenuation. The maximum theoretical sound transmission loss of any outdoor sound barrier is 20 dB.

The material that the noise barrier is composed of should have a transmission loss of 30 dB in the frequencies of concern. The barrier should not have any holes that would allow sound to pass directly through.

The sound reflected from the noise barrier (described above) can create new noise issues. If this is a problem, the sound barrier should be designed with a sound absorbing surface. Because this is a common issue, there are sound barrier products that also incorporate sound absorption. If a standard wall construction is used then sound absorbing panels should be added to provide absorption.

Non-acoustical design issues that should be considered when designing sound barriers include aesthetics and structural integrity. Wind loads should also be considered.

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Sound Attenuating Transfer Duct

A sound attenuating transfer duct is a duct used to allow air to flow through a sound wall, but not sound.  It is a necessity in buildings that have a return air plenum and full-height sound walls.

The duct should be constructed according to SMACNA HVAC Duct Construction Standards with no turning vanes in the elbows.

Line duct with 1” thick fiberglass with a coated surface equal to Johns Manville Linacoustic RC.

I typically layout transfer air ducts to provide multiple parallel paths of airflow from each space back to the air handling unit return air duct.  This minimizes pressure drop and provides for a good air balance.

transfer duct installation
Dimensions H and W are the clear dimensions from liner surface to liner surface.  H and W are calculated as required to achieve an airflow velocity of approximately 500 fpm (HxW=CFM/500).  H may be limited by space constraints in the plenum.

Dimension A should be equal to or greater than W.

Dimension B should be equal to or greater than 1.5 x W.

transfer duct

If you have a question or comment about sound attenuating transfer air ducts or their application, please leave it below.

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