How Acoustic Ventilation Works


Because of the sound generated by traditional ventilation systems, many people don't properly air out their home.
Because of the sound generated by traditional ventilation systems, many people don't properly air out their home.
helenecanada/iStockphoto.com

Most of us are aware of the dangers of air pollution outside of the home. Every year, we hear of efforts to contain that pollution, protect the environment and preserve the ozone layer. Interestingly, much less focus is placed on indoor air quality, despite the fact that pollution levels are generally two to five times higher indoors than out and can be up to 100 times higher in big cities. Couple this with the fact that Americans spend more than 90 percent of their time indoors, and we can immediately see that our focus must shift to improving indoor air quality if we're going to protect our health and well-being [source: USGBC].

Poor air quality can be caused by mold, dust, off-gassing associated with building materials, radon, carbon dioxide, tobacco smoke, cooking fumes, bathroom exhaust, and simple metabolic processes of humans and animals, where CO2 and other by-products are released simply through breathing.

Most of us will notice when our homes are too stuffy, too humid, or if there's some strange smell we can't seem to figure out. But surprisingly, few of us know that the key to controlling these problems lies with our ventilation systems. In fact, a Seattle-based study done by Ecotope, Inc., revealed that more than 25 percent of people had no idea whether or not their houses were even equipped with a ventilation system. Of those who were aware of their systems, 70 percent had no idea how to use them or how they operated [source: Bower].

While proper ventilation systems are important to our health and comfort, many people are hesitant to use them because ventilation systems can be very noisy. To help combat the noise problems, we turn to acoustic ventilation. This process helps us bring fresh air into our homes, remove pollutants and control humidity levels, all while it is invisible to the ears, eyes, nose and touch. We shouldn't feel air from ventilation systems creating drafts. It shouldn't blow papers off the desk or billow the drapes. We shouldn't be able to smell exhaust fumes or outside odors. And we shouldn't be able to hear the fresh air being distributed through our homes while the stale air is being removed.

To understand why ventilation systems tend to create noise and how we can control it, we must first understand how sound travels and what sound control technologies are available.

How Sound Works

Before we can determine how to keep noise to a minimum, we must first look at how sound travels. It all starts with movement, whether it's our vocal cords vibrating as we speak, a bell ringing or the inner workings of a radio speaker system.

When one of these objects moves within a space, it creates a vibration in the air around it. This vibration forces air particles in the surrounding area to move away from the source of the vibration. These air particles, in turn, travel in the form of sound waves. When they reach your ear, they transfer the initial vibration to your eardrums, which the mind translates into sound. Everything from the size and shape to the speed of the waves determines how they will sound when they reach our ears.

We measure how loud sounds are in the form of decibels (dB). The decibel scale starts at 0, which is the very lowest level of sound a human being can hear. A normal conversation will measure around 50 decibels, while the average concert can vary between 90 and 100 dB [source: Truax].

In order to control sounds, we must slow down or block the sound waves as they travel through the air, before they can reach our ears. To do so, we use items such as batt insulation. These fiberglass sheets are placed in walls, under roofs and around ductwork to help trap the air particles that make up sound waves. Those particles that aren't trapped are at least dissipated by the dense fibrous surface, resulting in a lower noise level when the sound wave finally reaches our ears.

Of course, batt insulation is just one example of a sound control product. Insulation can vary from rigid sheets to spray foam to Earth-friendly recycled denim or cotton. Mechanically, we can lower noise levels by controlling vibration from the equipment we use, either by using better-built, higher quality pieces or by stabilizing the equipment on a vibration control base.

The biggest secret to noise control, however, is to reduce the need for it in the first place. By practicing source control and smart design techniques, we can create systems of all kinds that require only moderate noise control measures to keep them quiet. In the next section, we'll look at acoustic ventilation techniques that allow us to enjoy the benefits of a well-ventilated home without all the noise.

Acoustic Ventilation Materials and Methods

There are three major ways that ventilation systems create noise. First, the fans and other mechanical equipment can create vibration and noise as they operate. Second, air moving through ductwork creates noise and vibration. Third, for ventilation systems to bring fresh air into the home, there must be air intake grilles on the home's exterior. These grilles not only bring in air, but also noise from the outside, including traffic and pedestrian noises, wind and background noise.

Fortunately, all of these sources easily can be controlled using relatively simple acoustical techniques. To minimize noise from fans, choose Quiet Fans, which are available from almost any fan manufacturer. These models operate between 10 and 20 dBs on average [source: Bower], which is no louder than a whisper between two friends [source: Truax] Designed with smaller, more efficient motors, these fans operate at surprisingly low energy levels while moving tremendous amounts of air. They're tightly constructed to minimize rattling and vibration and often can be installed with the motor in a separate compartment from the fan itself, allowing the fan to move air within a living space while the motor operates behind the scenes.

To keep vibration and air noises quiet, choose ventilation ducts that are lined with insulation. According to ASHRAE (the American Society of Heating, Refrigeration and Air Conditioning Engineers), a bare ventilation duct has a sound absorption coefficient of 0.05, meaning that 5 percent of sound that comes into contact with the material is absorbed. Duct lined with 1inch (25.4 mm) of insulation, however, has a sound absorption coefficient of 0.68 [source: Dolgin].

Use the largest duct that can fit into an available space to slow down airflow, further reducing noise. Also, minimize the number of duct bends and elbows and seal all joints using duct mastic to keep air flowing properly and prevent whistling or whooshing noises from ducts.

To keep noise from entering the house through fresh air intake units, choose grilles specifically designed for acoustic ventilation solutions. These grilles are lined with insulation and modified to include sound dampers that keep noise from entering the home. Be sure to locate these grilles away from the street and from other noise sources for maximum impact.

Applying Acoustic Ventilation to Your Home

The first step in designing a ventilation system is to determine how much air is required. Interestingly, there are very few regulations relating to ventilation requirements, but the most widely accepted standard is that developed by ASHRAE. It recommends ventilation be geared towards providing 15cfm (0.425 cubic meters per minute) or 0.35 ACH per person living in the home. To put these numbers into more easy to understand values, 15 cfm means that 15 cubic feet of air, or 0.425 cubic meters, are being circulated through the house each minute. A ventilation rate of 0.35 ACH means that 35 percent of the air is being completely changed out for fresh air every single hour [source: Bower].

The most effective type of ventilation system for the home involves a balanced central system with local exhaust fans at the kitchen and restroom. Local fans should be carefully selected based on noise levels produced. To create a successful acoustic ventilation system, choose place existing fans away from the spaces they serve. A bathroom or kitchen exhaust fan can be placed in the attic and vented through the roof.

The central system should be comprised of two fans: one for supply and one for exhaust. Intakes grilles should be lined and supplied with sound dampers or attenuators designed for acoustic ventilation systems [source: Building Services and Environmental Engineering]. Again, fans should be placed away from the main living areas, such as bedrooms and sitting rooms. Lined ducts will carry air throughout the home and it will enter the room through ceiling-mounted supply grilles[source: MBS]. Exhaust vents are most useful in areas subject to pollutants and moisture, including bathrooms and kitchens.

Ventilation systems require very little actual maintenance, but you can prolong the life of your equipment and protect your investment by cleaning fan blades, cabinets and intake grilles semi-annually.

For all the benefits they provide, acoustic ventilation systems are surprisingly affordable. The most expensive balanced acoustic ventilation systems run around $1,000, as of early 2009 prices. Add in costs to install lined ducts, intake grilles, and supply vents, and you're still left with a relatively inexpensive project that costs well under $3,000 [source: New York State Energy Research and Development Authority].

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Sources

  • Bower, John. Understanding Ventilation. Indiana: The Healthy House Institute, 1995.
  • Building Services and Environmental Engineering. "Acoustic Ventilation from Greenwood." March 8, 2005. April 8, 2009http://www.bsee.co.uk/news/fullstory.php/aid/3181/Acoustic_ventilation_from_Greenwood.html
  • Dolgin, Craig. "Cutting the Clamor in Classrooms." Building Operations Management. 2005. March 10, 2009.http://www.facilitiesnet.com/bom/articlePrint.asp?id=3225
  • Modern Building Services (MBS). "Acoustic Ventilation Delivers Fresh Air Without the Noise." August 2006. April 8, 2009http://www.modbs.co.uk/news/fullstory.php/aid/2312/Acoustic_ventilation__delivers_fresh_air__without_the_noise.html
  • New York State Energy Research and Development Authority. "Homeowner's Guide to Ventilation." 2008. April 9, 2009.
  • Truax, Barry. Handbook for Acoustic Ecology. 1999. April 8, 2009http://www2.sfu.ca/sonic-studio/handbook/Decibel.html
  • U.S. Green Building Council. LEED for New Construction and Major Renovations. 2008. March 11, 2009.http://www.usgbc.org/ShowFile.aspx?DocumentID=1095