I have my doubts that anyone reading this right now remembers the ubiquitous
"Can you hear me now" ads from one cell phone company in the US years ago.
That said, for many years I worked in a small office with no form of sound absorption whatsoever, and I hated any time I needed to have a phone conversation that everyone in the office would be able to hear me making #ConstructionAdmin demands or the like. Painted gypsum walls, concrete floor, and gypsum ceiling. A sum sound absorption of next to nothing...
So what, in fact, would have made my co-workers not hear every word I spoke, reflected off the painted 5/8" GWB wall and ceiling that I faced as I tried to ensure my projects went as they were supposed to?
#1 - Decibel (dB)
The unit used to measure the intensity of a sound. Decibel is a logarithmic ratio, which means that the higher the number the exponentially higher the sound level. That is IMPORTANT TO KNOW. Below are some examples of common decibel levels.
10 - Breathing
30 - Whispers
60 - Normal conversation (this is the general limit of comfortable sound)
80 - Busy street traffic
100 - Motorcycle or Lawnmower (prolonged exposure damages hearing)
[100 decibels is 100,000 times LOUDER than 50 decibels]
120 - Rock concert
150 - Jet Engine (from 100 feet away)
160 - Human eardrum explodes
#2 - Sabin
This is the unit of Sound Absorption. This unit ranges from 0 to 1. 1 Sabin is a completely sound absorbent material. Sabin is named after a scientist (named Sabine) who studied and defined sound absorption.
This unit is used to calculate the amount of desired sound absorption for a room. To get the total number of Sabins (unit of sound absorption) in space - you multiply the NRC of each surface (a) of the space, by the area of that surface (S), and add them together.
A (Sound absorption) = S1a1 + S2a2 + S3a3 ...
#3 - Sound Absorption Coefficient
This is the measure of how absorptive of sound a product is across several frequencies: typically at 500 Hz, 1000 Hz, and 2000 Hz.
Frequency is the number of times the sound curve passes a point in a given amount of time.
The Sound Absorption Coefficient can be high for a certain frequency and low at a different frequency. Higher frequencies are harder to absorb than lower frequencies. This differentiation is useful for different room uses - a wood shop will need sound absorbed at different frequencies than a radio station sound booth or an airport.
To make a general statement (that should seem semi-obvious), soft, porous materials will be more absorptive than hard, reflective surfaces. (I.e. Cotton curtains will absorb more sound than marble tile.)
The general sound absorption coefficient of painted drywall is: 0.1 and that is not very good.
That means painted gypsum walls reflect 90% of the sound against it, and absorbs only 10%. Unpainted CMU (concrete block) is a surprising 0.35. Cellulose insulation is 0.75.
#4 - Noise reduction coefficient (NRC)
This is the single number value ranging from 0.0-1.0 of the sound absorption levels of material across the various frequencies.
NRC 0 = complete reflection and NRC 1 = complete absorption,
This single value described the average sound absorption performance of a material
The Sound Absorption Average (SAA) and Noise Reduction Coefficient (NRC) values are both single number ratings that indicate the level of sound absorption of the material in question.
#5 - Sound Transmission Class (STC)
This is the measure of the effectiveness of a material or assembly of materials at blocking the transmission of sound through that material.
The higher the STC rating the more effective that material or assembly is at blocking the sound transmission through itself. STC is a typical measurement used for doors or window ratings, as well as wall assemblies.
There are many ways to improve the sound transmission class of partitions. The two easiest ways are by adding mass or increasing the overall thickness of the partition. Adding an extra layer of drywall to a wall assembly can increase STC by around 5 points.
1 layer 5/8" drywall on both sides of a 3 5/8 steel stud assembly has an STC of 40.
Adding 3 inches of fiberglass insulation between gypsum wall boards makes that wall STC 48.
Using 3 inches of mineral wool insulation makes that wall assembly STC 53.
The USG Fire rated assemblies catalog is a great reference that includes STC ratings of the assemblies
#6 - Impact Insulation Class
This is a single number measure of the ability of a ceiling/floor assembly to prevent noise from passing through to the space above or below.
This applies specifically to me because the IIC of the ceiling/floor to the apartment above mine is basically zero to the point where I hear the footsteps of my upstairs neighbor.
It is called Impact insulation because when one entity comes in contact with another entity it is called an impact, and that impact creates vibration, that will then travel through the affected assembly.
Think of a high heal hitting a ceramic tile floor, and the clicking noise you can hear from that.
BONUS (COMMON) SOUND TERMS:
Reflection – the repetition of a sound resulting from reflection of the sound waves.
Refraction – of sound waves involves a change in the direction of waves as they pass from one medium to another. Refraction, or bending of the path of the waves, is accompanied by a change in the speed and wavelength of the waves.
Echo – The reflection of sound arriving at the listener with a delay after the direct sound. The delay is directly proportional to the distance of the reflecting surface from the source and the listener.
Loudness – is the subjective perception of sound pressure.
Sound Masking – Is the acoustic treatment by which noise is masked by another layer of sound. This can be done using a water feature in a public space, with pre-recorded music played on a speaker system, or soundscape / white noise devices.