This guide is a resource for learning about noise-induced hearing damage and hearing protection. Reviewing this information can help you understand how hearing protection devices are rated and how they can help prevent hearing loss.
The physics of sound, anatomy of the ear, and how the two interact to produce hearing loss are complex topics. This guide provides a brief introduction to some of these concepts using simplified terms. Ask your Audiologist if you have questions.
How Can Noise Affect Hearing?
Exposure to excessive noise can damage your hearing. Common types of hearing damage include noise-induced hearing loss (NIHL), tinnitus (commonly referred to as “ringing in the ears”), and hyperacusis (perception of normal sounds as unbearably loud).
Noise-Induced Hearing Loss
Noise-induced hearing loss (NIHL) is simply a reduced ability to hear sounds caused by noise exposure. While some NIHL is temporary, excessive noise exposure may lead to permanent and irreversible damage.
Noise enters the ear and acoustic energy hits the eardrum, is translated into mechanical motion, which then in turn enters the fluid-filled cochlea, where mechanical deflection of the stereocilia on the hair cells (Auditory Hair Cells) classify the frequency/intensity information along the auditory nerve to the brain, where it is intepreted as hearing.
Auditory hair cells can become damaged from noise that is too intense, or when exposed to noise for too long. Once damaged, the hairs can no longer send signals to the brain to be interpreted, which ultimately leads to NIHL.
Since auditory hair cells respond to different sound frequencies, hearing loss may occur for only certain frequencies. Typically, the loss of hearing at a certain frequency is associated with exposure to noise at that frequency. Most commonly, noise-induced hearing loss affects high frequency hearing (3-6 kHz) first.
Typically, NIHL is permanent and cannot be corrected, but may be treated with hearing aids.
Not all hearing loss is noise induced, and factors other than noise can increase the risk of hearing loss. Factors that can contribute to hearing loss, including:
- Diseases and disorders
Tinnitus is the perception of noise when no such sound is present. The perceived noise can sound like ringing, hissing, static, crickets, screeching, whooshing, roaring, pulsing, ocean waves, buzzing, etc.
There are many medical conditions that can lead to tinnitus, but it is perhaps most commonly associated with NIHL.
According the American Speech-Language-Hearing Association (ASHA): “Hyperacusis is a rare hearing disorder that causes sounds which would otherwise seem normal to most people to sound unbearably loud. People who suffer from hyperacusis may even find normal environmental sounds to be too loud. Hyperacusis is not discomfort around loud sounds. Individuals with hyperacusis may find a car engine, dishes clanking, rustling of paper, or even loud conversation uncomfortable enough to avoid such situations.”
A variety of medical issues can cause hyperacusis, including excessive noise exposure.
What is Noise?
In the context of hearing damage and hearing protection, noise usually refers to excessively loud sound, whether it is pleasant, like music, or unpleasant, like the sound of a jackhammer. Noise has three key characteristics: frequency, volume, and duration.
Frequency is measured in Hertz (Hz), which is the number of vibrations per second. At birth, humans can hear a wide range of frequencies -- from about 20 Hz to 20,000 Hz. Low frequencies correspond to low pitches (bass, in musical terms) and high frequencies to high pitches (treble, in musical terms). Almost all sound is formed by a combination of frequencies.
Intensity, also called sound pressure level, is measured in decibels (dB). This is measurement of the energy of a sound wave. This generally corresponds to loudness, which is our perception of how loud a sound is. However, our perception does not perfectly match the intensity of the sound - our perception of loudness is also influenced by factors such as the frequency of the sound, the relative intensity compared to other sounds, hearing loss, etc.
Here are some everyday noises and decibel values, according to the U.S. Centers for Disease Control and Prevention (CDC).
- 20 dBA - watch ticking
- 30 dB - Soft whisper
- 40 dB - Refrigerator hum
- 60 dB - Normal conversation
- 70 dB - Washing machine or dishwasher
- 80-85 dB - Gas-powered lawn mowers and leaf blowers
- 95 dB - Motorcycle
- 100 dB - A car horn from 16 ft away
- 110 dB - Maximum volumes for personal listening devices, shouting in the ear
- 120 dB - Standing beside or near sirens
- 140 dB - Firecrackers
Decibels measure intensity on a logarithmic scale, which means that every increase of 10 dB represents multiplying the sound pressure level by 10. This compresses an enormous range of numbers into a shorter scale. For example, compared to 60 db, 70 dB is ten times as intense , 80 dB is one hundred times as intense, and 90 dB is one thousand times as intense!
A good rule of thumb is that sound pressure level doubles with every 3 dB increase. So 70 dB would be twice as intense as 67 dB. Decibels can also be difficult to add and subtract. For example, two speakers that produce 67 dB don’t add together to produce 134 dB. Instead they produce 70 dB -- or twice as much as one speaker.
Noise generally contains a combination of different frequencies. Each frequency may have its own intensity level. When sound level meters measure intensity, they typically use one of three formulas that average the intensity across frequencies to produce a single overall measurement:
- A-weighted decibels (dBA) — generally default for measuring exposure to occupational sounds, used for measuring lower level sounds.
- C-weighted decibels (dBC) — used for measuring higher level sounds.
- Z-weighted decibels (dBZ) — used for measuring very high level sound impulses.
Noise can also be characterized by its duration.
- Continuous noise is relatively consistent over a period of time or changes in volume slowly — like a car engine, a cheering crowd, or music. Sound level meters are well-suited to measure this kind of noise.
- Impulse noise is a short, transient burst of noise — like a clap, gunshot, or explosion. It is defined as the instantaneous change in sound pressure over a short period of time. According to OSHA, these can be harder for sound level meters to measure accurately. According to the CDC, “Considerable research has shown that impulsive noise is more likely to cause noise-induced hearing loss (NIHL) than continuous noise of equal energy” and “Exposure to high-intensity impulses can cause acoustic trauma and instant mechanical damage to the inner ear.”
According to the CDC, hearing loss can result from a single very loud sound or over time from repeated exposures to loud sounds. The louder the sound, the less time it takes for hearing loss to occur.
Daily Exposure Limits
Various organizations provide guidance about what volumes and exposure times can damage hearing. According to OSHA, the World Health Organization (WHO), and ASHA, hearing can be damaged when exposed to 85 dB for more than 8 hours in a day.
According to the CDC, “noise over 70 dB over a prolonged period of time may start to damage your hearing” and “damage is possible after 2 hours of exposure to 80-85 dB.”
In general, safe exposure time is reduced by half for every increase of 3 dB.
Immediate Hearing Damage
According to the CDC, noise above 120 dB (approximately equivalent to standing beside or near a siren) can cause immediate and permanent harm to your ears.
According to ASHA, impulse noises from gunfire or fireworks greater than 140 dB can cause immediate hearing damage. ASHA also indicates that almost all firearms create noise over 140 dB and as high as 175 dB.
What is Hearing Protection?
These kinds of hearing protection devices physically help block sound waves from entering the ear. This results in fewer decibels of sound that enter the ear.
Federal law requires that all hearing protection devices indicate the extent to which they reduce the amount of noise entering the ear. This rating is referred to as the Noise reduction rating (NRR).
NRR is expressed in decibels and represents a simple approximation of the protection the wearer of a hearing protection device might be provided.
In a best-case scenario, the noise entering the ear while wearing a hearing protection device is reduced by the number of decibels indicated by the NRR.
However, in many circumstances, one may not experience the full NRR.
Reasons for this include:
- Poor fit of the hearing protection device
- How noise was measured
- Characteristics of noise exposure
Fit of the Hearing Protection Device
Hearing protection devices are tested using a standardized test to measure how much sound gets reduced. This test is conducted in a laboratory setting, where experimenters fit the hearing protection. This ensures a proper fit, which can differ from real-world use. According to OSHA, “in most cases, this number overstated the protection afforded to workers.”
According to NIOSH, improper fit of hearing protection devices was a primary reason why users may not experience the full NRR, stating:
“[T]hese ratings may differ markedly from the noise reduction that a worker would actually experience in the real world. Specifically, studies have repeatedly demonstrated that real-world protection is substantially less than noise attenuation values derived from experimenter-fit, laboratory-based methods.”
As a result, NIOSH recommends reducing NRR by 25-75%, depending on the type of HPD, when proper fit is uncertain.
Characteristics of Noise Exposure
Noise can have a variety of characteristics, including frequency, intensity, and duration. Depending on the characteristics of a particular noise, one may not experience the full NRR.
Effect of Frequency on NRR
Hearing protection devices may reduce noise at some frequency more than others. This can be seen in standard NRR test results tables, typically published with hearing protection devices. For example, the SoundGear [product name] was found to have different NRR values for different sound frequencies in laboratory conditions.
|Total Frequency (Hz)||Mean Attenuation (dB)||Standard Deviation (dB)|
Effect of Intensity on NRR
Extremely loud sounds can vibrate the skull itself, allowing sound to bypass hearing protection devices and reach the inner ear. This phenomenon is referred to as bone conduction.
Effect of Impulse Noise
Impulse noise, like a gunshot, may not be protected in the same way as continuous noise. How to interpret NRR for exposure to loud impulse noises is not well understood.
According to OSHA:
“Although earplugs can offer protection against the harmful effects of impulse noise, and some earplugs are designed specifically to reduce this type of noise, the NRR is based on the attenuation of continuous noise and may not be an accurate indicator of the protection attainable against impulse noise.”
Other rating schemes besides NRR have attempted to provide information about noise reduction specifically to impulse noise exposure. For example, the EPA proposed a new rating method called IMPULSE, which was similar to concepts from an ANSI standard that uses a method called the impulse peak insertion loss (IPIL) method.
|IPIL (Impulse Peak Insertion Loss)|
|High Freq. Boost||17.5||36.2||34.6|
While IPIL and other ratings can help provide additional context for how hearing protection devices perform under impulse noise exposure, NRR remains the standard rating that is required by federal law.
How Noise is Measured
Noise typically has a combination of frequencies with varying intensities that sound level meters average using one of three common settings. If settings are used that are different from the NRR test, the NRR may need to be adjusted to estimate protection for that environment.
Since the standard NRR test uses C-weighted decibels, some organizations, like OSHA, advise reducing NRR by 7dB when sound levels of an environment are measured using A-weighted decibels.
Seeking out hearing protection devices is a great first step to protecting your hearing. If you are considering using SoundGear products to be a hearing protection solution for you, here are some practical steps you can take to further help protect your hearing.
Consider Wearing Secondary Hearing Protection
Some levels of noise exposure are too much for one hearing protection device to provide adequate protection. Wearing two hearing protection devices together can provide additional protection.
This typically involves wearing earmuffs over earplugs. According to OSHA, 5 dB can be added to the NRR of the most protective device when using both earplugs and earmuffs. For example, earplugs with an NRR of 22dB worn under earmuffs with an NRR of 30dB would have a combined NRR of 35dB.
ASHA recommends using both earplugs and earmuffs “when noise levels are above 105 dB for 8 hours or more” or “if you might hear impulse sounds that are more than 140 dBP”. ASHA also indicates that almost all firearms create noise over 140 dB and as high as 175 dB.
SoundGear devices may be sufficient for some situations where exposure to loud impulse sounds are infrequent, such as when hunting. If you choose to engage in activities that involve repeated exposure to loud sounds, like using firearms at a shooting range, additional protection may be needed.
However, even with secondary hearing protection, some very loud sounds could still damage hearing, such as using high-caliber firearms.
Consider limiting your exposure to the kinds of noises that can cause hearing damage (see above). If you don’t have a sound meter with you, ASHA provides some rules of thumb for whether sounds are too loud:
- You must raise your voice to be heard.
- You can't hear or understand someone 3 feet away from you.
- Speech around you sounds muffled or dull after you leave the noisy area.
- You have pain or ringing in your ears after you hear the noise, called tinnitus. It can last for a few minutes or a few days.
For shooting enthusiasts, consider further limiting exposure to impulse noise by:
- Standing farther away from other shooters.
- Shoot at outdoor ranges.
- Shoot at ranges with fewer people.
- Limiting the use of high-caliber firearms.
If you use a SoundGear device to amplify ambient sound or stream audio, limit your exposure in the same way you would when listening to music with headphones. Higher volumes and longer listening times can increase the risk of hearing damage.
Depending on the model, SoundGear devices may be capable of producing 90 dB of audio. Various organizations propose daily exposure time limits to noise levels around 90 dB.
- ASHA recommends limiting 88 dB of exposure to 4 hours and 91 dB to 2 hours.
- CDC recommends limiting 80-85 dB of exposure to 2 hours and 95 dB to 50 minutes.
- NIOSH recommends limiting 90 dB of exposure to 2 hours and 31 minutes.
Ensure Proper Fit of Hearing Protection Devices
Follow the instructions provided by the hearing protection device. Consider testing the fit of your hearing protection using the CDC’s online hearing protection test.
If you are wearing a device that was custom fit by an audiologist, have them verify proper fit.