Top 3 Acoustic Misconceptions

Misconception 1: Managing sound and noise is the same, STC vs. NRC

Sound absorption is measured by the Noise Reduction Coefficient (NRC). The NRC is a measurement of a material's ability to absorb sound. NRC ratings vary from 0 to 1, with 0 indicating complete reflection and 1 indicating near perfect absorption. Some materials even have NRC ratings over 1, which has to do with the methods used for testing the materials. An NRC rating of 0.85 indicates that the material absorbs 85% of sound waves. NRC material affects the quality, reverb and clarity of sound in a space. For example, NRC materials would reduce the noise in a hotel lobby or a large meeting room in an office building. These materials would make it easier to hear someone or something and not have their voice be drowned out by other sounds in the room.

Sound reflection is measured with Sound Transmission Class (STC). STC is the barrier efficiency of a material as described by this rating. Sound transmission is reduced more effectively with higher STC values. The STC ratings are based on how much noise a partition can reduce in decibels. Loud speech, for example, can be heard through an STC 30 wall but should not be audible through an STC 60 wall. The rating evaluates the performance of airborne sound transmission across a frequency range of 125 to 4000 Hertz. This range corresponds to the speech frequency range. Spaces where noise transmission is an issue other than voice, such as music, require special attention. STC materials would manage sound from one room to another. For example these materials would keep an apartment quiet even if there was a busy street right outside or it would keep one hospital room quiet even if the room next to it was noisy.

Misconception 2: Acoustic products are always flammable/dangerous

Good architectural products are thoroughly tested and shown to be flame resistant. ASTM E84 Surface Burning Characteristics is the fire rating test that applies to building materials such as wall or ceiling-mounted acoustic panels. This test determines how rapidly the flame spreads across the material's surface, as well as how much smoke and heat are produced. This data is used to assess how a material may impact a building's safety in the case of a fire. You should never use packing foam or other non-architectural materials to control sound in a space.

Misconception 3: You do not need to factor diffusion into acoustic design

Sound Diffusion can be just as important as absorption or reflection depending on the use of the space. The process of evenly distributing sound waves over a room is known as sound diffusion. When a sound wave hits a flat, smooth surface, it bounces back at the same angle it was struck. It's similar to banking shots in a game of pool. When a sound wave hits an uneven surface, however, it reflects in many directions, equally dispersing the sound across the room. This is important because if sound is diffused more evenly across a space, you eliminate “dead” spots in a room and keep them from being swamped by echoes.

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