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Question:
STC means how well the product stops or blocks sound from getting into or out of the room. Is noise absorption the level of noise that is generated from within the area that bounces off the material/wall? I want to make sure I understand the difference between them. Also, which one is more important to people?

ANSWER: Thanks for sending us your question. Noise absorption is one of those words/phrases that take on many levels and meanings. Noise absorption does typically refer to the absorbent type of materials that one uses on ceilings, walls, and floors to reduce the noise reverberation (which is sound bouncing off surfaces). However, the technical term to compare apples to apples and STC to STC is actually referred to as NRC. NRC = Noise Reduction Coefficient.
NRC is a representation of the amount of sound energy which is absorbed while striking a particular surface or material. For example NRC of 0 means ideal reflection. Acoustic properties NRC of 1 indicates perfect absorption.
You’ll note the NRC variable on a frequency chart which typically is 250 Hz, 500 Hz, 1000 Hz, and 2000 Hz which generally encompass fundamental frequencies of typical human speech. NRC essentially simplifies the quantification of how a surface will absorb human speech. The best materials for improving speech intelligibility and absorbing these sounds are noise absorption products (bringing us to a full circle to how the term is often used as a less technical reference to NRC – Acoustic properties Coefficient.
As to which is more important to people…that is a question that requires information about the acoustic problem trying to be corrected. In general speech/human voice are important for applications such as churches, office settings, corporate, retail, and telemarketing scenarios. You’re trying to improve things like worker productivity and customer experience through improving acoustics – speech intelligibility, reduction of reverberation, echo, and other such problems. This is where NRC is most likely a priority.
STC is a priority in industrial settings where you’re attempting to block a noise sound source (equipment in plants, noisy facilities, manufacturing, warehousing, etc). Most people looking for this are preventing sound from transmitting from the source and/or preventing the sound from traveling into neighboring spaces, office/work areas, etc. STC is also synonymous with OSHA regulations and the Health and Safety of workers and constant exposure to particular decibel levels can create health problems.
All Noise Control – Noise Control & Soundproofing Specialists
www.AllNoiseControl.com