And when these problems are persistent, the calls pour in to builders and contractors. Better to deal with the issue early, during the design phase. Read on for a basic guide to the sources of sound in the typical duct system, and how to solve the problems.

 

The fundamental premise of noise control is the disruption of the source-path-receiver chain. The generator of the sound is the source; the sound travels via the path (the duct) before reaching the ears of the receiver - the disturbed office or home dweller.

 

In any building, mechanical equipment is a primary noise source. However, the characteristics of these ‘generators’ are rightfully focused more on their intended use (i.e. heating, cooling) than sound control. For example, opting for a quieter fan that doesn’t cool the building properly may win the sound battle – but lose the war for overall comfort. So, two factors of the source-path-receiver chain are generally off-limits to the HVAC designer – the source and the receiver.

 

Therefore, the HVAC professional must focus primarily on the noise path – on the ductwork itself. And here, the issues multiply.

 

The simple transmission of mechanical equipment noise through the duct into the ‘living space’ of the building is commonly referred to as ‘breakout:’ the noise flows through the duct, before breaking through the duct wall into the surrounding area.

 

However, mechanical equipment isn’t the only noise source that impacts the ductwork. There is also the problem of duct ‘break-in:’ sound that gets into the duct from the surrounding area and transmitted on before breaking out – commonly referred to as crosstalk, because the best example is office noise (i.e. phone conversations) carried into the next office by the duct.

 

The duct itself can also be a source of noise. When airflow fluctuates, due to a change in fan speed or other variables, it can make the duct surface vibrate. This rumble by itself can produce sound levels from 65 to 95 decibels at frequencies that range from 10 to 100 Hz. At the mid range, this noise is as noticeable as normal human speech at a distance of three feet – not easily ignored.

 

So, duct noise has several sources – but there are also several ways to minimize the problem.

First, select fans and other mechanical equipment that operate as near as possible to their rated peak efficiency when handling normal airflow and static pressure. This may seem obvious, but is often overlooked. Using an oversized or undersized fan can lead to higher equipment noise levels. The resulting variations in fan RPMs can also lead to airflow fluctuations and duct rumble, as described above.

 

Secondly, the ducts should be designed to minimize airflow resistance. Flow resistance causes the fan to work harder to meet demand, increasing equipment noise and duct breakout. Increased airflow around joints and elbows can also cause the vibration of duct fittings, creating additional duct noise.

Thirdly, take steps to make the duct itself resistant to sound transmission. Though it’s impossible to prevent the first part of duct breakout – equipment or other sounds getting into the duct – simple steps can be taken to stop sound from escaping.

 

The walls of the duct themselves provide some sound attenuation, at fairly low levels for unlined metal. The shape of the duct also makes an impact – generally, round ducts are less effective acoustically, as they tend to be more rigid and absorb less sound than rectangular ducts.

No unlined metal duct can be truly effective for sound control, however. Some lining or other duct wall material must be used for better sound attenuation.

 

Most duct insulation products are fabricated of mineral fiber, such as fiber glass. They can be applied to the external walls of metal ducts, as insulation board or wrap. Duct wrap products are composed of a flexible fiber glass blanket that is particularly well-suited to wrapping sheet metal ducts. These products can provide excellent thermal protection, but only slightly affect acoustical performance – external duct insulation doesn’t address duct rumble, for example, because turbulent airflow still has direct contact with the metal duct wall.

 

More effective is an internal liner. These products, also generally mineral fiber, must be moisture resistant to avoid microbial growth and should offer a facing or coating that resists fiber erosion (such as the tightly-bonded mat surface on Knauf Fiber Glass’ Duct Liner E-M). This surface absorbs duct sound directly, cutting duct breakout and reducing vibration of the metal duct wall.

 

However, the best acoustical solution is to eliminate the sheet metal altogether (and thus the threat of duct rumble) by fabricating the duct entirely of rigid fiber glass duct board. Generally available with a fire resistant facing on the external surface and a smooth internal airstream surface, duct board can be fabricated into a rectangular duct that offers the best of both worlds – outstanding acoustical and thermal performance. Figure 1 demonstrates the desirability of this construction.

 

Description	125	250	500	1000	2000	4000
Bare Sheet Metal* No perceived noise reduction	.1	.1	.1	.1	.1	.1
Wrapped Sheet Metal* No perceived noise reduction	.2	.2	.2	.2	.2	.2
Lined Sheet Metal* (1" thick) Significant perceived noise reduction	.2	.5	1.4	3.0	2.4	1.9
Fiber Glass Air Duct (1" thick) Very significant reduction. Perceived noice reduced to  less that 1/3 original level.	.4	1.4	3.3	3.9	5.0	3.7

Source: NAIMA

An efficient, effective HVAC system is key to providing occupant comfort in any residential or commercial construction. With some planning, an HVAC system can be designed and installed that achieves its primary purpose while also avoiding a major source of occupant complaints – noise. Using common sense when selecting mechanical equipment and plotting duct paths and taking advantage of available duct board and liner products are key elements to solving the acoustical challenge.