In the simplest of metric terms, a decibel is one-tenth of a bel. But, historically, bel was a unit created to honor Alexander Graham Bell who invented the telephone. In the early days with telephone wires, they noticed that the signal strength would decay over a long distance. In order to determine power requirements to connect people for communications, they determined that they could use the ratio of power levels. As a start, it had to be based on a minimum amount of power required for a person to hear on the telephone. They found that the signal power level to generate an angular frequency of 5000 radians per second would be that minimum value as determined by an average number of people. They used this mark as a reference point in the ratio of power levels. Because of the large variations in values, they simplified the equation on a base-10 log scale and dividing the bel unit by 10. Thus, creating the measurement of decibel.
Today, this same method is used to measure sound. Like frequency waves that travel through the telephone wires, pressure waves travel through the air as sound. This sound pressure is what our ears can detect as loudness, and it has a pressure unit of Pascals (Pa). As an example, a small sound pressure would be like a whisper while a large sound pressure would be like a jet engine. This is very important to know as high sound pressures, or loudness, can permanently damage our ears.
With sound pressures, we can determine the Sound Pressure Level (SPL) which is measured in decibels (dB). Similar to the equation for the telephone power signals above, the SPL also uses a ratio of sound pressures in a base-10 logarithmic scale. For a minimum reference point, an average human can just start to hear a sound pressure at 0.00002 Pa. So, the equation for measuring sound levels will use this minimum reference point as shown in Equation 1.
L = 20 * Log10 (p/pref)
L – Sound Pressure Level, dB
p – Sound pressure, Pa
pref – reference sound pressure, 0.00002 Pa
Why is this important to know the decibels? OSHA created a chart in 29CFR-1910.95(a) that shows the different noise levels with exposure times. This chart was created to protect the operators from hearing loss in work environments. If the noise level exceeds the limit, then the operators will have to wear Personal Protection Equipment (PPE), or suffer hearing damage. EXAIR offers a Sound Level Meter, model 9104, to measure sound levels in decibels. It comes calibrated to accurately measure the sound to determine if you have a safe work environment.
There is a term that is used when it comes to loud noises, NIHL. This stands for Noise Induced Hearing Loss. Once hearing is damaged, it will not come back. To keep your operators safe and reduce NIHL, EXAIR offers many different types of blow-off products that are designed to decrease noise to a safe level. So, here’s to Alexander Graham Bell for creating the telephone which can be used to contact EXAIR if you have any questions.
Energy…all day (and night) long, we humans are surrounded by – and bombarded by – all kinds of energy. Sometimes, the effects are pleasant; even beneficial: the warmth of the sun’s rays (solar energy) on a nice spring day is the sure-fire cure for Seasonal Affective Disorder, and is also the catalyst your body needs to produce vitamin D. Good things, both. And great reasons to get outside a little more often.
Sometimes, the effects aren’t so pleasant, and they can even be harmful. Lengthy, unprotected exposure to that same wonderful sun’s rays will give you a nasty sunburn. Which can lead to skin cancer. Not good things, either. And great reasons to regularly apply sunblock, and/or limit exposure if you can.
Sound is another constant source of energy that we’re exposed to, and one we can’t simply escape by going inside. Especially if “inside” is a factory, machine shop, or a concert arena. This brings me to the first point of today’s blog: sound power.
Strictly speaking, power is energy per unit time, and can be applied to energy generation (like how much HP an engine generates as it runs) or energy consumption (like how much HP a motor uses as it turns its shaft) For discussions of sound, though, sound power level is applied to the generation end. This is what we mean when we talk about how much sound is made by a punch press, a machine tool, or a rock band’s sound system.
Sound pressure, in contrast, is a measure of the sound power’s intensity at the target’s (e.g., your ear’s) distance from the source. The farther away you get from the sound’s generation, the lower the sound pressure will be. But the sound power didn’t change.
Just like the power made by an engine and used by a motor are both defined in the same units – usually horsepower or watts – sound power level (e.g. generation) and sound pressure (e.g. “use” by your ears) use the same unit of measure: the decibel. The big difference, though, is that while power levels of machinery in motion are linear in scale, sound power level and pressure scales are logarithmic. And that’s where the math can get kind of challenging. But if you’re up for it, let’s look at how you calculate sound power level:
Wo is reference power (in Watts,) normally considered to be 10-12 W, which is the lowest sound perceptible to the human ear under ideal conditions, and
W is the published sound power of the device (in Watts.)
That’s going to give you the sound power level, in decibels, being generated by the sound source. To calculate the sound pressure level:
Lw is the sound power level…see above, and
A is the surface area at a given distance. If the sound is emitted equally in all directions, we can use the formula for hemispheric area, 2πr2 where r=distance from source to calculate the area.
These formulas ignore any effects from the acoustic qualities of the space in which the sound is occurring. Many factors will affect this, such as how much sound energy the walls and ceiling will absorb or reflect. This is determined by the material(s) of construction, the height of the ceiling, etc.
These formulas may help you get a “big picture” idea of the sound levels you might expect in applications where the input data is available. Aside from that, they certainly put into perspective the importance of hearing protection when an analysis reveals higher levels. OSHA puts the following limits on personnel exposure to certain noise levels:
EXAIR’s line of Intelligent Compressed Air Products are engineered, designed, and manufactured with efficiency, safety, and noise reduction in mind. If you’d like to talk about how we can help protect you and your folks’ hearing, call us.
Last week, the EXAIR Blog featured an article about the OSHA Standard 1910.242(b) – Reduction of Air Pressure below 30 psi for Cleaning Purposes. This week, we will review another OSHA standard that affects many of you in manufacturing and other industries.
OSHA 29 CFR 1910.95 – Standard on Occupational Noise Exposure discusses the effects of noise and sets limits for exposure. Occupational noise can cause hearing loss, and also interfere with concentration and communication, disrupting the job performance. Below is a summary from the standard of the Permissible Noise Exposure (OSHA Table G-16)
From the chart, the time an employee can be exposed to loud noise is greatly reduced as the sound level goes up. The use of hearing protection is helpful but relies on the operator to use consistently and correctly. Ear plugs or ear muffs can be uncomfortable and hot, leading to possible reduced usage. OSHA can come on site, and if violations to the sound level exposure limits are found, they can impose fines and mandate corrective action be taken place.
The recommended course of action when an operator is subjected to sound exceeding those in the chart above is to enable feasible administrative or engineering controls. Engineering controls is the arena in which EXAIR can be a great resource.
The first step in understanding and addressing any sound level issues is to measure the sound. The easy to use Digital Sound Meter, model 9104 shown below, allows for accurate testing of noise levels throughout the facility. Noisy areas can be quickly identified, leading to review, design and implementation of the engineering controls.
Some of the worst offenders for noise violations is compressed air usage. A prime example would be inefficient blowoffs, used for cooling, drying, or cleaning. Open pipe, copper tube or drilled pipe are a few of the common culprits. Not only do they consume excessive amounts of compressed air, they can produce noise levels above 100 dBA.
EXAIR manufactures a wide variety of engineered products that utilize compressed air and deliver it in a controlled manner. This allows for the most efficient use of compressed air and keeps the sound levels much lower than the inefficient methods. A Super Air Knife can replace a drilled pipe, reducing sound by as much as 20 dBA, while using 50-70% less compressed air. An engineered Super Air Nozzle can replace an open pipe or copper tube and reduce sound levels down to 74 dBA, and even down to 58 dBA for the smallest available nozzles.
EXAIR has been providing Intelligent Compressed Air Products since 1983.
If you have questions regarding noise limits and how to solve any issue with an EXAIR Intelligent Compressed Air® Product, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.