Understand the basics and importance of Noise Reduction Rating to protect your hearing.
This post focuses on providing everything you need to know about Noise Reduction Rating (NRR).
We will include information on:
- What exactly is noise
- When you need hearing protection for excessive noise
- Different type of Hearing Protection Devices
- Decipher what is NRR and whether it is reliable
- How to derate NRR
- The difference between SNR and NRR
Let’s dive right in.
What is Noise?
In simplest terms, noise is any sound that you do not need or want to hear. Our ears are excellent at identifying those unwanted sounds that tend to cause mild to major discomfort or irritation.
Noise is an inevitable part of our everyday lives; whether it’s the constant ticking of the wall clock across from you in the office, the sound of blaring horns and car engines during your commute or heavy machinery operating nearby. Noise is measured in units known as decibels (dB).
Types of Noise
There are four different types of noise and it is important to understand the differences between them before we can accurately select the right equipment and parameter for measuring it.
When is noise considered excessive?
The U.S.Occupational Safety and Health Administration (OSHA) regulation CFR 1910.95(c) posits that when noise level in the workplace exceeds 85 dB measured on the A scale over an 8-hour time-weighted average sound level (TWA), the employer must institute an effective and continuing hearing conservation program.
A hearing conversation program is designed to protect employees from experiencing hearing loss due to noise exposure in their occupation. Under the OSHA guidelines, feasible administrative or engineering controls should be put in place and if such controls are not enough to reduce sound levels, then Personal Protective Equipment (PPE) must be provided.
Other aspects include monitoring an employee’s exposure as well as conducting an audiometric testing program for said employees. There should also be an annual training program for all employees and an accurate record keeping of all employee exposure measurements.
The table below outlines the permissible duration per day at sound volume from 90 – 115 dbA.
Permissible Noise Exposures
|Duration per day, hours||Sound level dBA slow response|
|¼ or less||115|
Hearing Protection Devices
Hearing Protection Devices (HPDs) are designed to help prevent noise-induced hearing loss when exposed to loud, hazardous noise. When worn properly, they can significantly reduce (not eliminate) the dB level entering the ear by acting as a barrier. A hearing protection device may also offer protection against other effects of noise exposure, including hyperacusis and tinnitus.
There are three main types of HPDs available for use:
Per the OSHA hearing protection requirements, only HPDs that pass the ANSI S3.19 safety standard can be distributed for use. The ANSI S3.19 tests help make sure that a particular HPD provides the required level of protection for the noise exposure in a particular environment. In order to comply with the ANSI standards, the devices must be tested by an authorized testing facility before they can be sold with the appropriate Noise Reduction Rating (NRR) labeling.
What is Noise Reduction Rating?
The Noise Reduction Rating indicates the HPD’s ability to attenuate external noise coming into the ear. The number represents the amount of attenuation and is in units of decibel. In the NRR label shown above, the HPD has an NRR of 31 which means it can reduce the noise level by 31dB.
The higher the rating on the label, the more effective the device will be in reducing noise exposure.
However, do not take the NRR at face value.
Is the NRR Accurate?
The short answer is no. NRR can often be far too optimistic. The actual noise levels blocked by a rated HPD are actually lower than what it says on the NRR label. There are a few reasons:
- Testing of these devices are done in highly-controlled facilities so there is a gap between the testing environment and real-world application.
- Issues such as not wearing the HPD properly, getting hair or glasses in the way of the HPD will affect the effectiveness of the noise attenuation
How to get a more accurate protection number?
As a result of these inconsistencies, there was a need to “correct” the calculated value of the NRR into something more realistic. This process is known as derating. There are different derating calculations depending on the body overseeing the process.
OSHA derating method
For instance, OSHA applies a 50% reduction factor to the manufacturer’s labeled NRR for all types of hearing protection devices. Let’s see an example.
TWA(dBC): 100 dBC
TWA(dBA): 105 dBA
NRR: 28 dB
Derated NRR Value: 28 * 0.5 = 14dB
Estimated Exposure (dBC): 100 – 14 = 86dBC
Derated NRR Value: (28 – 7) * 0.5 = 10.5dB
Estimated Exposure (dBA): 105 – 10.5 = 94.5dBA
If the TWA is measured in dBA instead of dBC, the NRR has to be subtracted by 7 decibel before applying a 50% reduction factor.
NIOSH derating method
On the other hand, the National Institute for Occupational Safety and Health (NIOSH) applies its own derating calculation which takes into account of the type of HPD.
- Earmuffs: Subtract the NRR by 0.25
- Formable earplugs: Subtract the NRR by 0.5
- All other earplugs: Subtract the NRR by 0.7
The same “7 decibel subtraction” logic applies depending on the type of weighted decibel scale. Let’s take a look at the example.
Assuming the device in question is an earmuff.
TWA(dBC): 100 dBC
TWA(dBA): 105 dBA
NRR: 30 dB
Derated NRR Value : 30 * 0.75 = 22.5dB
Estimated Exposure (dBC): 100 – 22.5 = 77.5dBC
Derated NRR Value : (30 – 7) * 0.75 = 17.25dB
Estimated Exposure (dBA): 105 – 17.25 = 87.75dBA
How to get derated NRR for dual hearing protection?
In some cases, earplugs may be used together with earmuffs to provide a stronger level of protection. In this instance, determining the derated NRR involves first identifying the NRR for the higher-rated protector (NRRh). We will then add 5 dB to account for the secondary hearing protection.
- Estimated Exposure (dBA) = TWA (dBC) – (NRRh + 5)
- Estimated Exposure (dBA) = TWA (dBA) – [(NRRh- 7) + 5]
TWA: 110 dBA
NRR (Ear plug): 29 dB
NRR (Ear muff): 25 dB
Derated NRR: 29 – 7 + 5 = 27 dB
Estimated Exposure (dBA): 110 – 27 = 83 dBA
Single Number Rating
While the NRR is the standard for hearing protection attenuation across North America, the Single Number Rating (SNR) is the EU’s standard. Like the NRR, the SNR gives you an estimate of the level of noise reduction that a hearing protection device can provide. This means that if you come across a product with an SNR of 30, then it’s essentially saying that the product can reduce up to 30dB of noise exposure.
SNR VS NRR
The main difference between SNR and NNR is that the former’s ratings are typically a few decibels higher. There is currently no exact formula to convert the rating values from NNR to SNR or vice versa, but SNR values tend to be about 3 dB higher. As such, an HPD with an of 25 will have an SNR of about 28.
This difference in values is a result of the different testing procedures. SNR is also known to be paired with the HML values. This system provides hearing protection ratings at three different frequencies — High, Medium, and Low, which in turn allows the user to be able to select a device based on the risk levels of the frequency spectrum that they are exposed to.
For example, Etymotic earplugs come with an NRR of 12dB, but an SNR of 18dB and HML rating of
- High: 18dB,
- Medium: 16dB
- Low: 14dB
How is SNR derated?
Like the NRR, the SNR value also needs a slight correction. In this instance, the derating is done by deducting 4 dB from the SNR value on the label. This means if you purchased a pair of earplugs or earmuffs with an SNR value of 28, then the product will have a derated value of 24.
TWA: 110 dBA
SNR: 28 dB
Derated SRR: 28 – 4 = 24 dB
Estimated Exposure (dBA): 110 – 24 = 86 dBA
The dangers of exposure to high noise levels are real and you should, therefore, take the necessary steps to protect your hearing if you find yourself in one of these environments. With the NRR and SNR labels, you can at least have a rough idea of how much protection a particular hearing protection device offers.