Sound Criteria, Attenuation Techniques and Preventive Measures to Limit Sound Problems (FA/123-03)
January 22nd, 2004
Sound is a very important consideration in the selection and application of fans. If not properly evaluated, fan sound can turn an otherwise completely acceptable application into a disaster. In spite of this, fan sound continues to be one of the most misunderstood topics in the air handling industry.
This is the fourth article in a series of four articles on sound. The topics included may help you better understand how fan sound is developed, rated, applied and controlled.
Part 1 Understanding the Development of Fan Sound Data and the Product Line Rating Process (FA/120-02)
Part 2 The Basics of Sound (FA/121-03)
Part 3 Radiated Sound (FA/122-03)
Part 4 Sound Criteria, Attenuation Techniques and Preventive Measures to Limit Sound Problems (FA/123-03)
This article contains descriptions of the most common sound criteria and the approaches used to attenuate sound if the criteria is not satisfied. Also included are some common sense approaches to making sure the likelihood of sound problems are minimized.
Fan sound ratings are usually provided on a sound power level basis in each of eight octave bands. It has been emphasized that these ratings are independent of distance and environment not only of the source, but also the listener. Sound power also cannot be measure directly by instrumentation.
On the surface this would seem to be a contradiction since instrumentation as well as our ears react to changes in sound pressure levels. Because instrumentation only measures sound pressure levels, all sound criteria is provided on a sound pressure level basis. Otherwise, how would you be able to determine whether sound levels are satisfactory or whether attenuation is necessary to either silence the source or acoustically treat the environment?
The success of calculating sound pressure levels from sound power levels varies considerably depending upon the sensitivity of the application. Sound pressure level predictions for a library or music concert hall should be performed by an acoustical consultant. Non-critical applications such as a gymnasium, kitchen, etc. may use simplified approaches including “default assumptions” allowing use of some sound pressure levels contained in catalogs. (Part 2 of this series illustrates simplified calculations for converting sound power to sound pressure.)
Simplified Sound Pressure Level Criteria
The following sections describe several different criteria used to evaluate the acceptability of sound pressure levels.
1. OSHA permissible noise exposure
Sound power levels can be converted to sound pressure levels in each of eight octave bands. These sound pressure levels can then be “A weighted” and combined into a single dBA sound pressure level number. This process is covered in Part 2 of this series. OSHA Standards are a means of limiting exposure to various dBA sound levels so that loss of hearing does not occur. The following table illustrates the number of hours per day allowed for a specific dBA level.
Many specifications assume the strictest interpretation by specifying a maximum
of 90 dBA. In reality, it is very unlikely that a person will spend a full eight
hour day at any one level. The combined effect should be considered rather than
the individual effect of each exposure. Also keep in mind that this is the exposure
the listener experiences at the dBA level indicated, not the dBA level of the
fan that may be located someplace else. Some specifications specify that the
fan must reach a dBA level from the table knowing that the listener will be
at another location removed from the source.
|Typical NC values|
|Conference rooms||25 - 35|
|Hospitals / Libraries||30 - 40|
|General offices||35 - 45|
|Factories||50 - 70|
2. Recommended noise criteria (NC levels)
Sound pressure levels in each of eight octave bands at a specified distance
from the source may be plotted on a Noise Criteria chart. This chart automatically
compensates for any “AWeighting” due to the shape of the NC curves
being higher in the lower octave bands. The maximum penetration of any one NC
curve is the NC level for that sound spectrum. Typical NC values are tabulated
for reference. An NC chart is also illustrated for reference.
keep in mind that the sound pressure level spectrum must correspond to that
in the actual application. In other words, it is not appropriate to apply a
NC criteria for an office to a fan located in an equipment room without considering
the characteristics of the ductwork and other acoustical considerations.
3. Room criteria (RC)
Another sound pressure level criteria is used to evaluate HVAC systems as a whole, not components such as a fan. The main difference between NC and RC curves is an emphasis on the lower frequencies (16 Hz, 31.5 Hz) and not the higher frequencies (8 KHz). These curves represent a well-balanced neutral type of system. There are descriptors that identify the perception of the sound as low frequency “rumble”, mid frequency “roar” and high frequency “hiss”. The sound pressure level spectrum is plotted on the RC curves in a manner similar to the NC curves.
4. Sone criteria
Non-ducted fans such as propeller fans often have sound presented in sones. The most definitive criteria for sones is contained in AMCA Publication 302, Application of Sones Ratings. A table is contained in this document titled, Suggested Limits for Room Loudness. Again, these limits are not for fan sones, but sones present at the location of a listener in a room. Fan sones are given at a distance of five feet from the fan, but this may not match the actual application. The limits are given in a range of sones for a particular location such as a hotel lobby (4 to 12 sones). This emphasizes the subjective nature of sound in that not all people find a particular sone level objectionable. The sone range given in some applications is sometimes quite large.
When the sound is louder than the allowable criteria, then some form of attenuation technique must be used to reduce the sound to acceptable levels. It is critical to determine the source of the objectionable sound and the path it is taking to reach the listener. This establishes the attenuation technique because in all instances, the sound path to the listener must be interrupted to reduce the sound level being experienced.
In general, there are four major sound paths to a listener and each sound path has its own most practical approach to attenuating the sound.
Sound Path #1 Airborne sound from a fan inlet or outlet radiating directly into the atmosphere
Typical attenuation: Select the quietest fan available for the intended service. This typically requires larger, lower speed, higher efficiency fan designs. Install acoustical barriers, acoustical louvers or create an acoustical plenum through treatment of walls, ceiling, etc. Install the equipment in an equipment room isolated from sensitive areas. An attenuator can be mounted directly upon an inlet or outlet, but the pressure losses through the attenuator or resulting system effect can be substantial. Some reduction in attenuation from catalog values will also result.
Sound Path #2 Airborne sound from a fan inlet or outlet traveling through a duct system
Typical attenuation: Select the quietest fan available for the intended service. This typically requires larger, lower speed, higher efficiency fan designs. Acoustically line the ductwork with duct liner. Insert dissipative attenuators into the ductwork making sure pressure drops and self-generated noise have been considered and taken into account. Dissipative attenuators incorporate absorptive material into their construction. There are also reactive attenuators that do not use absorptive material but are tuned by wavelength, and thusly are effective over a narrower frequency range. Active attenuators utilize electronics to reduce sound by creating sound opposite in phase to the offending sound. This cancels the offending sound. This technique is good when a narrow frequency range is present such as a tone like the blade frequency or a rumble due to air rolling over itself. This technique is used for lower frequencies up to 250 Hz.
Sound Path #3 Casing radiated sound
Typical Attenuation: The fan casing itself forms the first layer of attenuation assuming there are no flanking paths through the ductwork or flex connections. This is called a transmission loss and is a function of the type of material and its thickness. Additional attenuation can be obtained using leaded vinyl coverings. This is typically expensive on a per square foot basis and attenuates only the higher frequencies. The most effective attenuation technique is to place an enclosure such as the Greenheck Sound Vault around the fan. This type of enclosure is designed to reduce sound in all octave bands and attenuates motor drive noise as well as the fan sound. The flex connections are inside the enclosure so that break out noise is not a consideration. Special attenuated air passages allow for motor cooling. NC levels down to 35 are possible using this approach.
Sound Path #4 Structureborne sound
Typical Attenuation: Structureborne sound paths can usually be interrupted quite efficiently by using flex connections on the fan inlet and outlet and isolators under the fan. Isolators may be elastomeric for lighter fan equipment or springs for larger equipment. Isolation bases in combination with inertia bases can obtain 95% efficiency levels.
Preventive Measures to Limit Sound Problems
There are several common sense approaches which can be used to minimize the likelihood of a sound problem. Some of these include:
- Select the quietest fan for the application. The lower the sound at the source, the lower the sound at the listener.
- Establish the location of all low sound requirements and what levels are required under what operating times and under what operating conditions. Establish the appropriate sound criteria that applies to the application.
- Obtain sound power or pressure values in each of the eight octave bands and compare them to generally accepted criteria. Determine whether there is a likely problem right in the beginning.
- Establish and follow all possible sound paths that exist for the sound to travel from the source to the listener.
- For each sound path look for locations that may result in system effects at the fan inlet or outlet, result in excessive turbulence within the ductwork, or short circuits that would cause unwanted sound to leak from one location to another. Look for excessive velocities and/or pressure losses. Make sure areas requiring low sound are not located adjacent to loud sound sources. Locate storerooms or lavatories between loud sound sources and the listener.
If there is a question about sound requirements and whether they are being met, contact a sound consultant or expert in addition to other authorities or personnel who need to be made aware. It is always better to address problems up front so that a plan of corrective action can be instituted. Once the equipment is installed, it is often too late and too expensive to do much about it.