Architectural Acoustics - Acceptable Room Sound Levels

It is important for architects to understand what sound levels are acceptable in different types of spaces. In addition, they need to understand which sound level measurements are relevant to what is trying to be achieved. We will cover the different sound measurements and end with a chart addressing the various levels of acceptability.

This article discusses acceptable sound levels within a room. Part of that is to control sound sources. Be sure to check our article Architectural Acoustics - Controlling Sound, which talks about design methods for controlling sound.

Measuring Sound Levels

Before you can determine the appropriate sound level for your space, we need to discuss the various ways of measuring sound. Unfortunately, it isn’t as simple as saying the sound in a bedroom should be 30 dB or lower. There are different measurements for different conditions, so let’s take a look at them.

Decibels [dB]: The decibel is used in acoustics as the standard unit of sound pressure level, or the loudness of a sound. Keep in mind that sound pressure increases on a logarithmic scale. As a general rule of thumb, an increase of 10 dB means the sound is perceived to be twice as loud – however this can vary based on the type of sound and the listening conditions. Humans can just barely detect a 3 dB sound level difference. They can easily detect a 5 dB change in sound level under most conditions.

Decibel A [dB(A)]: dB(A) is simply a filter that adjusts decibels for the frequency range that the human ear is capable of hearing, which is in the range of 1 kHz to 4 kHz. Outside of that general range, we aren’t concerned about whether the sound levels are very high or low (for architecture, at least).

Before you can begin to understand the various rating criteria, it is important to understand that humans detect varying frequencies differently because the human ear is less sensitive to very high and very low frequencies. For instance, a 40 dB sound at 1000 Hz frequency would seem louder than a 50 dB sound at 80 Hz frequency – even though the sound pressure is higher in the 80Hz sound, our ears are not as sensitive to it as they are to the 1000 Hz frequency. Since humans detect frequencies differently, sound criteria are measured with curves across a range of frequencies.

Sound Rating Criteria for Buildings

The main goal for acoustic design in a space is to keep the background noise levels low enough that normal speech (or other special sound, like music) is easily understood. A classroom will need to be quieter than a residential living room since the speech needs to reach people further away. Concert halls tend to need the quietest levels to allow people to hear the intricacies of the different instruments.

Keep in mind that we need to provide acceptable dB levels across a range of frequencies to account for all the different kinds of sounds in our environment. In addition, acceptable dB levels vary with the frequency of the sound due to the varying sensitivity of the human ear across frequencies.

The criteria and ratings below take help identify acceptable background noise levels caused by HVAC equipment, refrigerators, computer fans, etc. Background noise can be thought of as the general hum of the room or building.

Noise Rating [NR]

Noise Rating curves have been the international standard for indicating acceptable sound levels within a space. NR curves were developed by the International Organization for Standardization (ISO.) Each curve depicts the acceptable dB levels across a range of frequencies between 31.5 Hz and 8,000 Hz (8 kHz).

NR Curves are depicted in the graphic below.

Noise Criterion [NC]

Noise Criterion was developed in the 1950s and was most often used in the United States to depict the acceptable range of background noise in a space. It is measured in the range of 63 Hz to 8000 Hz (8 kHz).

NC Curves are depicted in the graphic below.

Unfortunately, NR and NC aren’t perfect. Recent studies indicate that the NR and NC curves allow for dB levels that are uncomfortable at very low or very high frequencies, such as the rumble or hiss of HVAC equipment. A couple of new standards have been developed and are gaining popularity around the world.

Room Criteria [RC]

Room Criteria is an alternative range of allowable background noise in a building or room that was developed in the 1980s. It is measured in the range of 16 Hz to 4000 Hz (4 kHz). Like NC, RC takes into account the general “hum” of the building. However, RC looks at sounds at much lower frequency levels to account for rumbling HVAC equipment. RC are depicted with straight lines of constant slope, which were observed to be the average spectrum seen in office buildings in the 1980s.

RC Curves are depicted in the graphic below.

Balanced Noise Criterion [NCB]: The new Balanced Noise Criterion curves, updated in ANSI S12.2-2008, accounts for sound frequencies down to 16 Hz, which will address issues from the low-frequency hum of energy efficient HVAC equipment. NCB also reduces the higher frequency levels to eliminate hiss.

The NCB curves are depicted below.

RC Mark II: This criteria is an improvement to the original Room Criteria. It is almost the same as RC, except that it takes into account the subjective response of room occupants to vibrations of very low frequencies, which are often caused by HVAC equipment. It was developed in the late 1990s.

The RC Mark II curves are depicted below.

Of course, there are other noise criteria that have been developed, however they haven’t caught on like those mentioned above. Some of the other criteria include eNC and PNC.

So Which Noise Criteria Should be Used?

In general, architects will not be selecting whether to use one criteria or another. However, they may be part of a discussion about whether a certain piece of HVAC equipment can be placed in the ceiling of a space or in an adjacent room. Our experience is that most HVAC equipment manufacturers are using the standard Noise Criteria (NC).

We expect that many acoustic experts will be pushing manufacturers toward the Blanaced Noise Criterion (NCB) and RC Mark II since these two criteria take into account the high frequencies that create a hiss and the low frequencies that can create uncomfortable vibrations.

Table of Acceptable Room / Space Sound Levels

The table below provides acceptable sound levels for various room types. This table covers a majority of the space types that an architect may be interested in.

ROOM / SPACE	DBA	NR	NC/NCB	RC/RCM2
Theaters, Concert Halls, Recording Studios	25-30	20	10-20	20
Bedrooms, Libraries, Religious Prayer Rooms	25-30	25	20-25	25
Living Rooms, Classrooms, Lecture Halls, Conference Rooms	30-35	30	30-40	30
Offices, Courtrooms, Private Work Rooms	40-45	35	30-40	35
Corridors, Open Offices, Bathrooms, Toilet Rooms, Reception, Lobbies, Shopping	45-55	40	40-40	40
Kitchens, Shopping, Common Spaces, Dining Halls, Computer Rooms, Workshops	45-55	45	40-50	45

The values above come from a variety of sources around the internet and in published books.  ANSI S12.2: American National Standard Criteria for Evaluating Room Noise serves as the basis, with slight adjustments based on other publications. We've also created groups to help simplify selection of criteria — check ANSI S12.2 or an acoustical engineer if you need value for a specific room.

References

In case you are interested in going more in-depth with room acoustics, we recommend the following articles, which were used to develop this article.

Fundamentals of Acoustics by Colin H. Hansen
Room Noise Criteria — The State of the Art in the Year 2000 by Gregory C. Tocci

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