Noise may be defined as any unwanted sound. Sound becomes noise when it is too loud, unexpected, uncontrolled, happens at the wrong time, contains unwanted pure tones or unpleasant. In addition to being annoying, loud noise can cause hearing loss, and, depending on other factors, can affect stress level, sleep patterns and heart rate. The primary object for determining subjective estimations of loudness is to present sounds to a sample of listeners under controlled conditions. In heating, ventilation and air conditioning (HVAC) systems only the ventilation fan industry (e.g., bathroom exhaust and sidewall propeller fans) uses loudness ratings. In order to find satisfaction, percent of exposure to noise is the valuable issue for the personnel who are working in these areas. The room criterion (RC) method has been defined by ANSI standard S12.2, which is based on measured levels of in HVAC systems noise in spaces and is used primarily as a diagnostic tool. The RC method consists of a family of criteria curves and a rating procedure. RC measures background noise in the building over the frequency range of 16-4000 Hz. This rating system requires determination of the mid-frequency average level and determining the perceived balance between high-frequency (HF) sound and low-frequency (LF) sound. The arithmetic average of the sound levels in the 500, 1000 and 2000 Hz octave bands is 44.6 dB; therefore, the RC 45 curve is selected as the reference for spectrum quality evaluation. The spectral deviation factors in the LF, medium-frequency sound and HF regions are 2.9, 7.5 and -2.3, respectively, giving a Quality Assessment Index (QAI) of 9.8. This concludes the QAI is useful in estimating an occupant's probable reaction when the system design does not produce optimum sound quality. Thus, a QAI between 5 and 10 dB represents a marginal situation in which acceptance by an occupant is questionable. However, when sound pressure levels in the 16 or 31.5 Hz octave bands exceed 65 dB, vibration in lightweight office construction is possible.
Keywords: Duct, quality assessment index, room criteria, ventilation system
|How to cite this article:|
Forouharmajd F, Nassiri P, Monazzam MR, Yazdchi M. Estimating occupant satisfaction of HVAC system noise using quality assessment index. Noise Health 2012;14:135-9
| Introduction|| |
The primary objective for acoustical design of in heating, ventilation and air conditioning (HVAC) systems and equipment is to ensure that the acoustical environment in a given space is not unacceptably affected by HVAC system-related noise or vibration. Sound and vibration are created by a source, are transmitted along one or more paths and reach a receiver. Treatments and modifications can be applied to any or all of these elements to reduce unwanted noise and vibration, although it is usually most effective and least expensive to reduce noise at the source.
Sound is a propagating disturbance in a fluid (gas or liquid) or in a solid. In fluid media, the disturbance travels as a longitudinal compression wave. Sound in air is called airborne sound or just sound. It is generated by a vibrating surface or turbulent fluid stream. In solids, sound can travel as bending, compressional, torsional, shear or other waves, which, in turn, are sources of airborne sound. Sound in solids is generally called structure-borne sound. In HVAC system design, both airborne and structure-borne sound propagation are important. 
Noise may be defined as any unwanted sound. Sound becomes noise when it is too loud, unexpected, uncontrolled, happens at the wrong time, contains unwanted pure tones or unpleasant. In addition to being annoying, loud noise can cause hearing loss, and, depending on other factors, can affect stress level, sleep patterns and heart rate. Predicting the response of people to any given sound is, at best, only a statistical concept, and, at worst, very inaccurate. This is because response to sound is not only physiological but psychological and depends on the varying attitude of the listener. Hence, the effect of sound is often unpredictable. However, people respond adversely if the sound is considered too loud for the situation or if it sounds "wrong." Therefore, criteria are based on descriptors that account for level and spectrum shape. The primary object for determining subjective estimations of loudness is to present sounds to a sample of listeners under controlled conditions. Listeners compare an unknown sound with a standard sound.
In HVAC systems, only the ventilation fan industry (e.g.,bathroom exhaust and sidewall propeller fans) uses loudness ratings. 
Further improvements on the room criteria (RC) methodology have been suggested by Blazier, resulting in Room Criteria Mark II (RC Mark II) (ASHRAE 1999). It has been developed and published by Blazier (1997).  The RC Mark II curves are identical to the RC curves, with the exception that the Mark II curves are slightly less lenient in the 16 Hz octave band. The RC Mark II also includes a Quality Assessment Index (QAI) that provides an estimate of occupant evaluation, ranging from "acceptable" to "objectionable." The QAI is found using spectral deviations between the measured levels and the RC contour levels. 
The most acceptable frequency spectrum for HVAC sound is a balanced or neutral spectrum in which octave band levels decreases at a rate of 4-5 dB per octave with increasing frequency. This means that it is not too hissy (excessive high-frequency content) or too rumbly (excessive low-frequency content). Unfortunately, achieving a balanced sound spectrum is not always easy; there may be numerous sound sources to consider. 
| Methods|| |
Room criterion method
Room criterion (RC) measures background noise in a building over the frequency range 16-4000 Hz. The RC method is defined by ANSI standard S12.2, which is based on measured levels of HVAC noise in spaces and is used primarily as a diagnostic tool. The RC method consists of a family of criteria curves and a rating procedure.  The shape of these curves differs from the noise criteria (NC) curves to approximate a well-balanced, neutral-sounding spectrum; two additional octave bands (16 and 31.5 Hz) are added to deal with low-frequency sound and the 8000 Hz octave band is dropped. This rating procedure assesses background sound in spaces based on its effect on speech communication, and on estimates of subjective sound quality. The rating is expressed as RC followed by a number to show the level of the sound and a letter to indicate the quality [e.g., RC 45 (N), where N denotes neutral]. The RC rating value is the average of the levels in the 500, 1000 and 2000 Hz octave bands. Rumble imbalance exists if any levels in octave bands with center frequencies from 31.5 to 250 Hz are more than 5 dB above the RC rating curve, and a hissy imbalance exists if any of the levels in the octave bands with center frequencies from 1000 to 4000 Hz are above the RC rating curve by more than 3 dB. 
Room criteria Mark II method
The RC method was revised to the RC Mark II method to add additional parameters that further describe a measured sound.  Like its predecessor, the RC Mark II method is intended for rating sound performance of an HVAC system as a whole.  The method is primarily used as a diagnostic tool for analyzing sound problems in the field. Because the RC Mark II method is somewhat complicated to use, it is discussed in some detail below.
The RC Mark II method of rating HVAC system sound comprises three parts:
The number is the arithmetic average rounded to the nearest integer of the sound pressure levels in the 500, 1000 and 2000 Hz octave bands (the principal speech frequency region). The letter is a qualitative descriptor that identifies the perceived character of the sound: (N) for neutral, (LF) for low-frequency rumble, (MF) for mid-frequency roar and (HF) for high-frequency hiss. There are also two subcategories of the low-frequency descriptor: LFB, denoting a moderate but perceptible degree of sound-induced ceiling/wall vibration and LFA, denoting a noticeable degree of sound-induced vibration.  Regions A and B denote levels at which sound can induce vibration in lightweight wall and ceiling constructions that can potentially cause rattles in light fixtures, furniture, etc.
- Family of criterion curves
- Procedure for determining the RC numerical rating and the sound spectral balance (quality)
- Procedure for estimating occupant satisfaction when the spectrum does not have the shape of an RC curve (QAI).
Procedure for determining the RC Mark II rating
Step 1: Determine the appropriate RC reference curve. This is done by obtaining the arithmetic average of the sound levels in the principal speech frequency range represented by the levels in the 500, 1000 and 2000 Hz octave bands. [This is the preferred speech interference level (PSIL), which should not be confused with the ANSI-defined speech interference level (SIL), a four-band average obtained by including the 4000 Hz octave band level. The RC reference curve is chosen to have the same value at 1000 Hz as the calculated average value (rounded to the nearest integer)]. 
Step 2: Assign a subjective quality by calculating the QAI. This is a measure of the degree the shape of the spectrum under evaluation deviates from the shape of the RC reference curve. The procedure requires calculation of the energy-average spectral deviations from the RC reference curve in each of three frequency groups: low (LF; 16-63 Hz), medium (MF; 125-500 Hz) and high (HF; 1000-4000 Hz). However, when evaluating typical HVAC-related sounds, a simple arithmetic average of these deviations is often adequate if the range of values does not exceed 3 dB. The procedure for the LF region is given by equation (1) and is repeated in the MF and HF regions by substituting the corresponding values at each frequency. 
Quality assessment index
The energy-average spectral deviations in each of the mentioned frequency regions are calculated by the above equation. The ∆L terms are the differences between the spectrum being evaluated and the RC reference curve in each frequency band. In this way, three specific spectral deviations factors (∆LF, ∆MF and ∆HF), expressed in dB with either positive or negative values, and are associated with the spectrum being rated. QAI is the range in decibels between the highest and the lowest values of the spectral deviation factors. The spectrum is assigned a neutral (N) rating if QAI ≤5 dB. If QAI exceeds 5 dB, the sound quality descriptor of the RC rating is the letter designation of the frequency region of the deviation factor having the highest positive value. The arithmetic average of the sound levels in the 500, 1000 and 2000 Hz octave bands is 45 dB; therefore, the RC 45 curve is selected as the reference for spectrum quality evaluation. The spectral deviation factors in the LF, MF and HF regions are 2.6, 6.8 and -2.6, respectively, giving a QAI of 9.4. The maximum positive deviation factor occurs in the MF region, and the QAI exceeds 5, resulting in a rating of RC 45 (MF). An average room occupant would perceive this spectrum as marginally roar [Table 1].
Criteria selection guidelines
Undesirable rumble can result if NC curves are determined mainly by low-frequency noise. Similarly, a hissing effect can result from NC level being controlled by higher frequency sounds.  To achieve a better balance between the low-frequency and the high-frequency components, RC curves have been established for which the objective is to design spectra that meet an RC curve within +2 dB at all frequencies. In general, these basic guidelines are important: sound levels below NC, NCB or RC 35 are not detrimental to good speech intelligibility. Sound levels at or above these levels may interfere with or mask speech. Even if the occupancy sound is significantly higher than the anticipated background sound level generated by mechanical equipment, the sound design goal should not necessarily be raised to levels approaching the occupancy sound. This avoids occupants having to raise their voices uncomfortably to be heard over the noise. 
| Results|| |
RC measures background noise in a building over the frequency range of 16-4000 Hz. This rating system requires determination of the mid-frequency average level and determining the perceived balance between high- and low-frequency sounds.
[Figure 1] show HVAC sound levels as determined RC mark II rating based on [Table 2] data. RC reference is plotted based on the ASHRAE applications handbook and the ANSI standard S12.2. A summary of results is demonstrated in [Table 1].
|Figure 1: Room criteria curves. ANSI S12.2 criteria for evaluating room noise|
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The spectrum plotted in [Figure 1] is processed in [Table 2]. The arithmetic average of the sound levels in the 500, 1000 and 2000 Hz octave bands is 44.6 dB; therefore, the RC 45 curve is selected as the reference for spectrum quality evaluation. The spectral deviation factors in the LF, MF and HF regions are 2.9, 7.5 and -2.3, respectively, giving a QAI of 9.8. The maximum positive deviation factor occurs in the MF region, and the QAI exceeds 5, resulting in a rating of RC 45 (MF). An average room occupant would perceive this spectrum as marginally roar according to [Table 3].
|Table 3: Definition of sound quality descriptor and quality assessment index to interpret RC Mark II ratings of HVAC-related sound – AHRAE 2009|
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| Discussion|| |
The study is a reason of achieving the job satisfaction of workers, especially for buildings with ventilation systems. Current methods include the A-weighted sound pressure level dB (A), noise criteria (NC), RC suggested by Blazier, balanced noise criterion (NCB) and RC Mark II based on ASHRAE 1999. It concludes that the QAI is useful in estimating an occupant's probable reaction when the system design does not produce optimum sound quality. A QAI between 5 and 10 dB represents a marginal situation in which acceptance by an occupant is questionable. However, when sound pressure levels in the 16 or 31.5 Hz octave bands exceed 65 dB, vibration in lightweight office construction is possible (and likely if levels exceed 75 dB). Even at moderate levels, if the dominant portion of the background sound occurs in the very low-frequency region, some people experience a sense of oppressiveness or depression in the environment. In such situations, the basis for complaint may result from exposure to that environment for several hours and, thus, may not be noticeable during short exposures.
Each sound rating method was developed from data for specific applications; not all methods are equally suitable for rating HVAC-related sound in the variety of applications encountered. It is also important to determine the purpose for which the rating system will be used, because each system has strengths and weaknesses. Tangency methods are typically best for design criteria, whereas some of the more complicated sound rating systems are useful for diagnosing the nature and magnitude of particular problems. The simplest sound rating systems may be appropriate for commissioning work, depending on project sensitivity to noise levels or annoyance. The degree of occupant satisfaction achieved with a given level of background sound is determined by many factors, including sound perception and acoustical privacy. Sound perception of desired sounds (e.g., speech and music) makes low background noise levels desirable. For privacy, higher background noise levels are desired to mask intruding sound, such as in open-plan offices, where a certain amount of speech and activity masking is essential. Large conference rooms, auditoriums and recording studios, in comparison, can tolerate only a low level of background sound. Therefore, the system sound control goal varies depending on the required use of the space. To be unobtrusive, HVAC-related background sound should have the following properties:
Unfortunately, there is no acceptable process to easily characterize the effects of audible tones and level fluctuations; therefore, currently available rating methods do not adequately address these issues. Some sound rating methods comprise two distinct parts: a single number related to the overall magnitude of the noise and a procedure for determining the quality of the noise (for instance the degree of frequency balance, which is based on a family of criterion curves specifying sound levels by octave bands).
- Balanced distribution of sound energy over a broad frequency range
- No audible tonal or other characteristics such as whine, whistle, hum or rumble
- No noticeable time-varying levels from beats or other system-induced aerodynamic instability
- No fluctuations in level such as a throbbing or pulsing.
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Department of Occupational Health Engineering, School of Public Health, Tehran University of Medical Sciences, Tehran
Source of Support: None, Conflict of Interest: None
[Table 1], [Table 2], [Table 3]