Indoor noise ratings have a long history. This evolution began with designing instruments that could measure sound repeatedly. Frequency discrimination against octave band sound measurements paved the way for advances in understanding the impact of noise on communication and hearing.

The methods currently used in the literature for room noise rating include: Noise Criteria (NC), Preferred Noise Criteria (PNC), Room Criteria (RC), Balanced Noise Criteria (NCB), and RC Mark II. These methods are intended to accommodate the complexity of the tone and temporal character of the voice into quantitative and qualitative descriptors. In the following, several methods have been developed in determining the noise rating.

1924 – Equal Loudness Contour

The earliest reference to Equal Loudness Contours comes from Fletcher and Steinberg in 1924 (Kryter, 1985). However, the most famous reference source for Equal Loudness Contours comes from Fletcher and Munson in 1933.

1949 – A, B, C, D Weighting

The weighting standards were first listed in the ASA Standard Z24.3-1944 (Beranek, 1949 and 1988). In its use the A and C weights are quite familiar. This is different from the B and D weights which are still rarely used.

A-weighting is a weighting designed to reflect the response of the human ear to noise. Meanwhile, C weighting places more emphasis on low-frequency sounds than A weighting. In addition, peak sound pressure measurements usually use C weights.

1957 – Noise Criteria (NC)

Figure 3 shows the first NC curve published in 1957 (Beranek, 1957). This curve uses the equal loudness contour curve as a reference. The NC curve is usually used by the tangential method in evaluating the sound pressure level spectrum. The widespread use of NC is often associated with manuals issued by the American Society of Heating, Ventilating, and Airconditioning Engineers (ASHRAE). This book is used by most mechanical-electrical engineers as their guide in designing.

1971 – Preferred Noise Criteria (PNC)

The Preferred Noise Criteria (PNC) curve developed in 1971 is shown in Figure 4. This curve was published after an evaluation that the NC curve could not be used in the observation of hiss (sounds that sounded hiss) and rumbles (sounds that sounded rumbling). In contrast to the NC curve, this curve is less steep at low frequencies and steeper at high frequencies (Beranek, 1971). Although this curve has a better balance between low, medium, and high frequency sounds, in fact the PNC curve has a tighter range of values ​​at low frequencies. As a result, to overcome noise at low frequencies will require more extensive noise control. This is usually in line with the generally higher cost of constructing a mechanical system when compared to using NC curves. In addition, experienced consultants stated that the denser low-frequency limits on the PNC curve were unnecessary and impractical to apply to most buildings. On the basis of this and the fact that they have never been incorporated into standards or practical guidelines, PNC curves have never been widely used.

1981 – Room Criteria (RC)

In an effort to better understand the implications of the shape of the spectrum on the suitability of background noise in buildings generated by building mechanical systems, ASHRAE conducted a background noise survey in building spaces in the mid-1970s. Blazier used this survey to develop a method for evaluating the suitability of background noise in building spaces based on their use (Blazier, 1981). The development results in a set of Room Criteria (RC) curves that are straight, and have parallel lines with a constant slope of –5 dB/octave. This form is described as perceptual neutral, that is, it does not have a dominant tone in one frequency range.

The RC method involves determining the RC rating and spectrum quality descriptor to indicate an imbalance or dominance of sound in a certain frequency range and cause the sound spectrum to be perceived as hiss or rumble. RC curves and methods for assessing a room’s sound spectrum are listed in the American National Standard S12.2-1995, “Criteria for Evaluating Room Noise.”

The RC assessment method was first proposed by Blazier (Blazier, 1981) and has now been standardized in ANSI S12.2-1995, “Criteria for Evaluating Room Noise.” Figure 5 presents a set of RC curves along with a typical sound pressure level spectrum. The RC curve starts from RC-25 to RC-50, and is designed to cover typical background noise in buildings in the frequency range of 16 Hz to 4000 Hz. The RC curve has constant parallel lines with a slope of -5 dB per octave.

This shape is based on Blazier’s observations which are believed to be the average form of the spectrum commonly found in buildings that are the object of a survey conducted by ASHRAE.

This sound pressure level spectrum rating follows the general form RC XX(YY), where XX is the RC rating and YY is one or more descriptors indicating spectral balance as discussed below. The sound pressure level spectrum assessment using the RC method consists of two steps, namely:

1. Determine the mid-frequency average level (LMF) which is the RC rating value itself which is further defined as:

LMF = (L500 + L1000 + L2000)/3.

2. The second step is to determine the perceived balance between low and high frequency sounds. Sounds with a rich spectrum of low frequencies (16 Hz to 500 Hz) are defined as “rumbly” sounds, while sounds with a rich spectrum of high frequencies (1000 Hz to 8000 Hz) are defined as “hissy”.

The “rumble” criterion is shown from the RC curve which is 5 dB higher than the neutral curve determined from the LMF and straight at a frequency of 16 Hz to 500 Hz. If the sound level at low frequencies exceeds the criteria curve, then the spectrum is considered to represent a rumbling sound, for example the sound coming from a heavy vehicle or an outdated cooling engine.

The “hiss” criterion where the RC curve has a value of 3 dB higher than the neutral criterion and extends from 1000 Hz to 4000 Hz. Sound spectrum whose value exceeds this criterion will be considered as hissy sound, for example the sound of water pipes or air leaking.

In addition, the two criteria curves provide the possibility that low frequency sounds will produce audible rattling sounds in light building elements such as suspended ceilings, lighting fixtures, doors, windows, air ducts, etc. In Figure 5, it is shown that there is a “moderately noticeable vibration” region or a vibration that is felt and is quite audible. There is also a region for “clearly noticeable vibration” or a vibration that is felt and heard clearly. There are several YY descriptors that can be used to express spectrum balance, including: : Neutral (N), Rumble(R), Vibration (RV), or Hiss(H).

Spectrums that do not exceed the criteria for marked rumble, hiss, or vibrations can be considered “neutral”, meaning that the spectrals have a relatively good balance between low, medium, and high frequency sound energy. This spectrum is followed by a quality descriptor (N). For example, it can be seen in Figure 5 where there is a spectrum that exceeds the “clearly noticeable vibration” criterion curve, then the spectrum will be designated as RC 46(RV) spectrum.

1989 – Balanced Noise Criteria (NCB)

The NCB curve has a profile that extends from 16 Hz-8000 Hz. The ANSI S12.2 standard defines values ​​for each individual curve from NCB-10 to NCB-65. The NCB-0 curve is defined as the audibility threshold for continuous sound in a diffuse plane. The NCB curve is derived by a different procedure from the procedure for defining the NC curve (Beranek, 1989).

The NCB curve uses the SIL (Speech Interference Level) value derived from the average of the four mid-high frequencies of the octave band. The NCB curve also satisfies the same interference test as the NC curve, that is, the loudness of the sound at low frequencies does not exceed the SIL value of more than 24 units. Like the RC rating, the NCB rating takes the form of NCB XX(YY), where XX is the NCB rating and YY is the spectral balance descriptor.

As with the RC rating, there are two steps that need to be taken to determine the value of the NCB rating

1. Calculates the Speech Interference Level (SIL) for the spectrum being evaluated. SIL is defined as followst:

SIL = (L500 + L1000 + L2000 + L4000)/4

The assessment of the spectrum to determine the NCB rating is the same as the SIL rounded to the nearest decibel. For example, the spectrum shown in Figure 6 has a SIL of 44 dB, and therefore the spectrum is defined as NCB-44.

2. Determines the perceived balance between low and high frequency sounds. The spectrum rich in low frequencies (16 Hz to 500 Hz) is defined as “rumble”. The spectrum rich in high frequencies (1000 Hz to 8000 Hz) is defined as “hissy”.

The previously described criteria for sufficient and clearly visible vibration are also used in the NCB rating. As with the RC method, any spectrum found not exceeding the visible rumble, hiss or vibration criteria will be considered a “neutral” spectrum. In this category, the spectral has a relatively good balance between low, medium, and high frequency sound energy.

The “rumble” criterion was defined as the NCB curve 3 dB higher than the defined neutral curve of the SIL and extending between 16 Hz and 500 Hz. Figure 6 presents a “rumble” criterion curve corresponding to the NCB-44 spectrum. Note that the spectrum exceeds the NCB-47 rumble criteria, therefore the spectrum displayed will be characterized as “rumble”. This also falls under the “clearly noticeable vibration” criteria.

The hiss criterion is somewhat more complicated to define as illustrated in Figure 7. The hiss criterion curve is the arithmetic mean of the three values ​​of the NCB curve that intersects the spectrum at 125, 250, and 500 Hz—in this case NCB-49. Note that the spectrum does not fall above the NCB-49 hiss criterion curve, therefore the spectrum is not “hissy”.

1997 – Room Criteria (RC) Mark II

This rating method is similar to the RC rating method where the LMF value is used to calculate the rating value. However, there are things that distinguish this method from the RC rating method, especially in two respects. First, the RC curve used in the RC Mark II method is slightly different, at a frequency of 16 Hz-31 Hz the curve is flat and not sloping, as shown in Figure 8.

Second, the RC Mark II rating method differs in the way it calculates the qualitative characteristics of the sound. This new method uses two new quantities to calculate the qualitative characteristics of sound namely “energy mean spectral deviation factor” and “quality rating index”. As shown in Figure 8, the RC Mark II rating method divides the audible frequency range into three regions—low (16-63 Hz), medium (125-500 Hz), and high (1000-4000 Hz). Excess sounds in this range are perceived as “rumble”, “roar”, and “hiss”, respectively.

The RC Mark II qualitative rating method can be divided into three steps as follows:

1. Determining the RC rating using LMF as previously discussed.
2. Calculates the average spectral deviation of the energy in each of the three previously mentioned frequency regions. These are as follows:

Where the value of Lf is the difference between the value of the spectrum and the value of the RC curve at that frequency.

1. Determine the Quality Assessment Index (QAI). QAI is the difference between the highest and lowest energy mean spectral deviation.

• If the QAI is less than or equal to 5 dB, the spectrum is considered neutral, i.e. showing a good balance between the low, medium, and high frequency ranges. Thus, the qualitative descriptor following the RC rank is (N).
• If the QAI is greater than 5 dB, then the qualitative descriptor will be determined by the maximum energy mean spectral deviation and denotes (LF), (MF), or (HF).
• If the spectrum exceeds the moderate criteria or is clearly visible, a qualitative descriptor (LVA) or (LVB) will also be used. It is possible that two descriptors will be required, namely, one of (N), (LF), (MF), or (HF) and one of (LVA) or (LVB).

In addition to rating the RC Mark II, Blazier also provides a means of determining how the occupants of a room may respond to a particular spectrum. The subjective responses of the occupants were shown as: “acceptable”, “marginal”, and “unpleasant”. This response considers that the RC rating (which is LMF) is consistent with the recommendations for such a rating based on space use. Subjective responses are provided in Table 1 (Blazier, 1997). Table 1. Interpretation of the RC Mark II rating assuming a suitable spectrum for space use

Of the many noise rating methods developed in the room, currently there are 4 methods that are most often used by practitioners in the design process, namely Noise Criteria (NC), Balanced Noise Criteria (NCB), and Room Criteria (RC) and RC Mark II. NC curves which were previously developed, are more widely used and standardized through the technical literature. While the RC and NCB curves are defined in the American standard, in this case, ANSI S12.2 (ANSI, 1995).

Both methods provide a means of assessing the spectrum based on the arithmetic mean band octave of sound level, as discussed earlier, defined by the LMF and SIL parameters. In addition, both methods have qualitative descriptors that indicate the perceived spectrum balance between low and high frequencies. The RC Mark II method is a further development of the RC method and uses a curve that is nearly identical to the RC curve defined in ANSI S12.2. The RC Mark II rating differs greatly from the RC method in that it describes spectrum quality with a more comprehensive descriptor.

References

[1]   ANSI Criteria for Evaluating Room Noise, ANSI Standard, S12.2, 1995 .

[2]  ASHRAE Fundamental Handbook, ASHRAE Handbook, Chapter 48, 2001.

[3]  Leo L. Beranek, Ed. 1, Acoustics. New York: McGraw-Hill Book Company, 1954.

[4] Gregory C. Tocci, “Room Noise Criteria—The State of the Art in the Year 2000.”  Current Issues in Noise News International, vol. 8, no. 3, September, 2000.

[5] Blazier Jr., Warren E., “Revised Noise Criteria for Application in the Acoustical Design and Rating of HVAC Systems,” Noise Control Engineering Journal, pp 64-73, 1981.

[6] Blazier Jr., Warren E., “RC Mark II: A refined procedure for rating the noise of heating, ventilating, and air-conditioning (HVAC) systems in buildings” Noise Control Engineering Journal, vol. 445, pp 243-250, 1997.