A Practical Guide to Sound Transmission Class and Noise Criteria

A Practical Guide to Sound Transmission Class and Noise Criteria
There are different types of noises. They are not of the same nature. The harsh mixture of unwanted sounds that prevail in the cities contain several. It may be the low sound of a heavy machinery or the blaring high pitch sirens of a police patrol car. How much sound can a wall allow through? Is there a single index to measure all types of sound? The terms STC, NC, and NCB are commonly used when we talk about soundproofing and acoustic treatment.
What is Sound Transmission Class (STC)?
STC stands for Sound Transmission Class. To understand this term we may have to look into some more terms. First, the term dB, or decibels is used to measure the loudness or softness of a sound. To understand it better, a rock band in a disco is around 110 dB. A quiet office may have 30 dB sound level. When you install soundproofing measures, say, on windows, doors, walls etc., the value of dB will come down. This is called transmission loss. In the case of the above disco, if you shut the doors and windows and stand outside the dB reading which was 110 dB may now read 90 dB. It can be said that closing of the doors and windows and measures taken are the reasons for the transmission loss of 20 dB. It is also seen that with the change of frequency of the instruments like guitars played inside the club, the transmission loss is also varying. What is clear now is that transmission loss is dependent on frequency, which is measured Hz (Hertz). A loud drum boom has a frequency of 20-60 Hz, and a guitar around 500 Hz. Human ear can detect sounds at 20 Hz to 20,000 Hz. Consider 16 standard frequencies ranging from 125 Hz to 4000 Hz, note the corresponding transmission loss, and plot a curve. Compare this curve with several standard curves, and suppose this curve matches approximately an STC 45 curve, we can conclude that the space has a sound transmission class 45.
What is NC and NCB?
Noise Criterion – NC – level is a standard that indicates the loudness (dB) of a space with reference to a range of frequencies (Hz).
The NC rating is obtained from NC curves, by plotting the octave band levels against a given noise spectrum.
NCB Curves
Balanced Noise Criteria (NCB) curves are intended to replace NC curves and are used to specify permissible levels of background noise inside homes and other living spaces like air-conditioning and similar ambient noise.
Conclusion:
It can be said that STC reference curves help us decide effectiveness of a wall in preventing sound transmission between rooms. Noise Criterion, in both NC and NCB variants help us to identify the noise present in an environment like a room. Knowing these values is important in formulating solutions for noise control and soundproofing.