Not all noise is of the same nature. In the cacophony of urban living, unwanted sound can vary from the low thud of heavy machinery to high pitch sirens of a passing police car. How much transmission of sound is allowed by a wall? Can a single number measure them all?

In this post, we will explore some myths behind the terms STC, NC and NCB, which are commonly used terms in soundproofing and acoustic treatment.

What is Sound Transmission Class (STC)?

STC stands for Sound Transmission Class. Before we can understand how it works, we need to look at some other terms.

First, let us look at the term dB, or decibels. A dB value simply measures how loud or soft a sound is. A loud rock band at a club is typically around 110dB, while the noise level in a quiet office is probably around 30dB.

When installing “soundproofing” solutions – say a wall, door, or window, this dB value can be affected. The reduction in this value is known as Transmission Loss. For example, imagine we are at that same club where noise levels are measured to peak at 110dB. After shutting the door of the club and standing outside, the meter reads 90dB. We could then say that the shut door, walls, and other related components of the club has resulted in a transmission loss of 20dB in general.

But wait! I can still hear that loud kick drum and bass wobble, even though the guitars are hard to make out. If we sent a pure sine wave through that same door and changed the frequency each time, we will find that the transmission loss value is different in each case.

This means that transmission loss is geluidsoverlast buren dependent on the frequency measured. Frequency is measured in Hz. The loud boom of the kick drum was probably from 20Hz to 60Hz, while the now inaudible guitars were probably 500Hz. The human ear is able to detect sounds ranging from 20Hz to 20,000Hz.

By taking 16 standard frequencies from 125Hz to 4000Hz and measuring their transmission loss, a curve can be plotted. There are several standard reference curves. If the plotted curve approximately matches an STC 45 curve, then that wall or structure being tested is said to have a Sound Transmission Class of 45.

The Myths…

1) If Wall A has a Sound Transmission Class of 60 (STC 60) and Wall B also has an STC 60 of 60, they are exactly the same.

As we can tell from the process used to rate the Sound Transmission Class of a wall, it is possible to have 2 walls with slightly different curves but both having the same STC rating. Therefore, not all walls with the same STC rating perform exactly the same.

2) A wall with a high Sound Transmission Class rating will block everything, including bass.

Another issue is that Sound Transmission Class values do not include frequencies below 125Hz. This means the

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