Mechanics of sound absorption
There are different types of sound absorption, but what most experts refer to when talking about sound absorption is frictional or porous absorption. This alone begins to describe what is needed from a material like a ceiling panel, island, or baffle for it to absorb sound. First, the sound energy must be able to penetrate the ceiling panel’s surface. This means the surface must be porous. Otherwise, the sound energy reflects off the surface and continues to propagate around the room.
Ceiling panel manufacturers take great care when painting their panels to not close the pores and decrease absorption. This typically requires special paint and mechanized sprayers, or curtain-coaters applying unbelievably thin coats of paint. The important concept is that painting ceiling panels in any other conditions—for example, by contractors in the field to match a custom color—will most likely decrease the sound absorption by as much as half.
Surface porosity alone is not enough. The sound energy must get trapped inside the core material until it is converted to heat energy via friction in the internal pores or cells. This requires a tortuous, maze-like fiber structure, similar to those found in stone wool. For example, a piece of wood with 12.7-mm (1⁄2-in.) diameter holes drilled through it is very porous, but not very sound absorptive. The sound energy simply passes through the holes; it does not get trapped inside and converted to heat. Essentially, tortuosity results in greater airflow resistance, which relates strongly to sound absorption. This can be tested by trying to blow air through a material. If it is easy to blow through it, then the internal structure is not tortuous enough. Conversely, if air can be blown through it, but only with effort or force, it is probably a good sound-absorber.
The thickness of the material is also important. Thinner materials are efficient absorbers of high- and mid-frequency sound, but their performance at low frequencies quickly diminishes. That is why ceiling panels, which are thicker than carpeting and wall coverings, are generally much better sound absorbers. Sound absorption varies with frequency or pitch, and most porous absorbers less than 50 mm (2 in.) thick are efficient absorbers of mid- and high-frequency sound.
There is an extra benefit with ceiling panels when they are suspended in a metal grid with a plenum above (i.e. an architectural E mount). That airspace helps boost the low frequency absorption of the entire ceiling system. It is important to assess the sound absorbing capability of materials in the actual application in the building. If the ceiling panels are glued directly to a solid substrate such as gypsum board (i.e. an architectural A mount), one should review and specify the correct test report data. The information from the manufacturer or laboratory should define the mounting type as E-mount, A-mount, or another.
Quantifying absorption performance
The most common method of characterizing and specifying the absorption performance of a material is by noise reduction coefficient (NRC), which is a single numerical metric that varies between 0 and 1. A higher number means more sound absorption. While values of 0.50 and 0.60 may seem like they represent decent absorption, the acoustics industry generally refers to these as ‘sound reflective’ because so much sound energy is being bounced off them. NRC performance can be broken down into:
- NRC-0.90: best (high performance);
- NRC-0.80: better;
- NRC-0.70: good; and
- NRC-0.60: poor (reflective).
NRC is a calculated average of the absorption coefficients in the 250-, 500-, 1000-, and 2000-Hz octave bands. That average is then rounded to the nearest 0.05. This means two materials having NRC values differing by only 0.05 should not be viewed as substantially different. The difference could just be the result of rounding the average. Two materials that have the same NRC rating could, and likely do, perform differently at the individual octave bands. In some rooms, such as those used for group education, the differences in absorption at various frequencies may be important to speech intelligibility.
The octave bands used in the calculation of NRC (250 to 2000 Hz) relate to speech intelligibility. That does not mean absorption performance below 250-Hz and above 2000-Hz octave bands is unimportant. In some applications, such as those rooms intended for music or multi-media presentations, low-frequency absorption performance may be important.
Specifiers might need to specify absorption coefficients by individual octave bands down to the 63-Hz octave band as opposed to relying on a single, multiband indicator such as NRC. Manufacturers cannot get NRC values without starting with the individual octave band data. Any manufacturer publishing NRC ratings should be able to provide specifiers with a detailed absorption test report per ASTM C423, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method. Those test reports should be from an independent, third-party, National Voluntary Laboratory Accreditation Program (NVLAP)-certified acoustics laboratory.