Noise Control Performance Indices & Ratings
In building acoustics, there are numerous noise control strategies to reduce the background noise level to the targeted noise criteria (NC) or dBA–the unit for the overall A-weighted equivalent sound pressure level. Since noise control measures vary in multiple aspects, one lesson is clearly not enough to talk about this topic alone.
But don’t worry, in this lesson you will learn the fundamental principles about noise control. We are going to discuss the noise control performance indices and ratings. From here, we hope that you will get the idea of how to choose the right noise control strategy.
Sound Transmission Loss (TL)
The performance of airborne noise insulation of a vertical partition between two rooms in a building is defined as the difference between the sound pressure levels in the source and receiver room, and it’s called sound transmission loss (TL).
TL is measured in a laboratory according to ASTM E90. If you read any building material specification datasheet, you will see a graph or table that covers 16 different measured TL values spanned over 16 standard frequencies from 125 Hz to 4,000 in dB, according to ASTM E413.
Now, let’s take a simple example of sound measurement in a laboratory at a single frequency. Assume, in the source room, you generate 100 dB sound at 250 Hz. Then, measure this sound in the receiver room that is adjacent to the source room. Let say your sound pressure level meter shows 65 dB. Here, we could conclude that the sound transmission loss of the vertical partition between those two rooms is 100 dB – 65 dB = 25 dB at 250 Hz.
Sound Transmission Class (STC)
Commonly, when you see the specification of sound insulation material, you also find another number, and that is the STC value. Next question is, what is the STC value?
Sound transmission class (STC) is a single number rating to represent the performance of airborne sound transmission insulation of a vertical partition between two rooms. This value is derived from the laboratory-measured sound transmission losses in different frequencies, like the one we mentioned in the previous example.
Even though we are not going to talk about how we can derive the STC value. It’s good to know that a single STC value represents a unique curve that can be used as the reference for rating the performance of a room partition. Below is the example of an STC graph with a comparison of the one from the measurement results.
The problem with STC is that it only considers the frequency range of human speech sound, which is between 125 Hz to 4,000 Hz. But if you deal with low-frequency noise, your acoustic engineer should work harder to make a prediction.
After construction, the STC of the installed vertical partition will be measured, and the result is known as the field sound transmission class (FSTC). In most cases, FSTC values are usually lower compared to the predicted STC that is measured in the laboratory value due to sound leakage because of the imperfection of the construction.
Outdoor-Indoor Transmission Class (OITC)
The OITC rating was created to provide a single number rating for facades (exterior walls) and facade elements (windows and doors) that are subjected to outdoor airborne noises such as traffic noise, aircraft noise, and railway noise. Exterior airborne noise tends to be at lower frequencies than interior airborne noise (such as human voices). It means that the OITC rating is curated to include low-frequency sound in its calculations. As the OITC value increases, the better the sound insulation performance of a facade or building element.
The OITC value is calculated over the frequency range of 80 to 4,000 Hz at each one-third octave band frequency. This OITC standard can be seen in ASTM E1332-90 Standard Classification for Determination of Outdoor-Indoor Transmission Class.
In general, the STC rating system is still the preferred rating method when comparing the product’s soundproofing abilities. However, if low-frequency sound like neighbor’s bass music or noise from low-flying planes causes the most disruptions in your home or office, you should consider the OITC rating for any interior or exterior barrier.
The following table shows insulating glass constructions that meet specific sound level ratings. Data is derived from test reports and published information.
Now, what about the airborne noise you hear from footsteps? We need to know what causes the noise in the first place. Noise from footsteps is indeed airborne noise, but what causes it is structure-borne noise. That’s why to get a satisfying result, the noise control strategy should focus on the structure-borne noise. Below, you will learn indices and rating that are usually used in reducing the level of structure-borne noise.
Impact Insulation Loss & Impact Insulation Class (IIC)
Impact insulation loss is an index that quantifies the loss between two rooms separated by a floor-ceiling assembly due to an impact on the floor surface at the upstairs room. Some examples of impact noise here are high-heels footsteps and solid material falling on the floor.
How well an impact sound insulation in this assembly works is defined by what ASTM call as the impact insulation class (IIC) (ASTM E989).
The IIC number is derived from sound attenuation values tested at 16 standard frequencies from 100 to 3,150 Hz. The bigger the IIC number, the better the performance of the floor-ceiling assembly at preventing the transmission of impact noise.
Damping Loss Factor
Damping loss factor is another useful measurement index in tackling structure-borne noise a.k.a vibration in solid objects. Damping loss factor–often shorted as loss factor–is commonly used in material testing to measure the damping performance.
Loss factor is defined as the ratio between the energy dissipated per radian and the energy associated with the vibration. From this definition, you get the idea that the higher the energy dissipated by a material in the vibration of interest, the higher its loss factor. In conclusion, the higher the loss factor of a material, the higher its damping performance.
Loss factor is dependent on many parameters; two of them are the vibration frequency and temperature. The table below depicts the loss factor values for typical materials around room temperature.
Table 2. Loss factors for various material around room temperature (Das et al., Engineered elastomeric bio-nanocomposites from linseed oil/organoclay tailored for vibration damping)