Archive for the ‘Acoustics’ Category

HVAC-System Acoustics

Friday, June 12th, 2009

Methods of reducing unwanted sound associated with mechanical systems in commercial buildings

Though intended to provide comfort, HVAC systems often are a major source of noise in buildings. While the primary focus of designers understandably is temperature and humidity control, the more experienced among them know the importance of keeping sound and vibration in check.

This article provides an overview of acoustics as it applies to HVAC-system design.

Fundamentals

Sound levels are expressed in decibels (dB). Decibels cannot be added traditionally. For instance, 50 dB plus 50 dB plus 50 dB does not equal 150 dB. It equals 54.8 dB. To add decibels, one can go through the calculations of logarithmic addition or use Table 1, which provides reasonably accurate results.

TABLE 1. Simplified logarithmic addition.

Decibel values must be considered two at a time. Using the example of 50 dB plus 50 dB plus 50 dB:

1) The difference between 50 dB and 50 dB is 0 dB. Following Table 1, add 3 dB to the higher value:

50 dB + 3 dB = 53 dB

2) The difference between this result and the remaining 50 dB is 3 dB. Following Table 1, add 2 dB to the higher value:

53 dB + 2 dB = 55 dB

A difference of 3 dB barely is detectable by humans, while a difference of 10 dB is perceived as twice or half as loud. Therefore, a 60-dB source typically is perceived as being twice as loud as a 50-dB source of the same frequency.

When dealing with undesirable sound, a designer must consider (Figure 1):

FIGURE 1. Source, path, and receiver.

 

• The source of the sound. In HVAC systems, air-handling units are common culprits. Each of any other components contributing to noise should be considered a separate source.

• The path of the sound. A single sound can have many paths (Figure 2). Different paths present different problems and, thus, require different solutions. This article focuses mainly on the path associated with ductwork.

FIGURE 2. Paths of sound and vibration.

• The receiver of the sound. For this article, that is the person or persons occupying a building.

Weighting

In general, the human ear is less sensitive to low- and high-frequency sounds than it is mid-frequency sounds. Weighting is a method of reducing the contribution of low- and high-frequency sounds to attain a value corresponding approximately to what the ear perceives. The most common method of weighting is performed using the “A” scale, which produces values in units of dBA.

Table 2 shows “A” weighting for the center frequencies of the eight octave bands typically associated with HVAC-system acoustics. The more negative the “A” adjustment, the less sensitive the human ear is to the corresponding frequency.

TABLE 2. A-weighting scale.

Where HVAC is concerned, “A” weighting more often is used for outdoor sound calculations than it is indoor sound calculations. For indoor sound calculations, Noise Criteria (NC) is the more common method.

The NC method is fairly straightforward. Sound-pressure levels at eight octave-band frequencies are plotted on a family of curves defining the maximum allowable sound-pressure level corresponding to a chosen design goal. An NC rating is determined by the lowest curve exceeding the value at each octave band.

The downside of the NC approach is that the shape of a sound curve is not evaluated as a whole, and lower frequencies (below 63 Hz) are not evaluated at all. Still, the NC method is the most widely used basis for sound criteria for indoor HVAC applications. For office buildings, NC 35 usually is acceptable (Table 3).

TABLE 3. Sound-pressure levels for NC 35.

Room Criteria (RC) Mark II is the method recommended by the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). This is a relatively new method that evaluates an overall sound curve. Using this method can be difficult because information for the lowest octave band is not readily available from manufacturers. (To learn more, see Chapter 7 of 2005 ASHRAE Handbook–Fundamentals.1