Sound – It Adds Up! How to Calculate Decibel Levels

Keeping noise levels in check and at safe levels is very important to ensure employee safety and well being.  OSHA (the Occupational Safety and Health Administration) through standard 29 CFR-1910.95(a) has studied the situation and set Maximum Allowable Noise Exposure limits in Hours per Day based on the Sound Level, in dBA, of exposure.

For existing processes, a Digital Sound Meter is a valuable tool to measure the sound level to ensure that the source of loud noises can be quickly identified and isolated for immediate corrective action.

For new processes, or changes to an existing process, it is important to estimate the sound level prior to installation and start-up, so that precautions can be taken as needed.

For example, let’s say we are going to add a blow off station to clean off a part on a conveyor to improve the process and increase the throughput.  A typical set-up might be a 12″ Super Air Knife (model 110012) blowing off the top and a pair of Super Air Nozzles (model 1100) to blow off the sides.

SAK and ASAN
12″ Super Air Knife and Super Air Nozzle

If we look at the performance data for the (2) different blow off devices, we find that the Super Air Knife is rated at 69 dBA and the nozzles at 74 dBA, when operated at 80 PSIG of compressed air supply.

SAK and ASAN

When asked, “what is the sound level for (1) of the knives, and (2) of the nozzles” a little Acoustic Engineering is in order. The decibel scale is logarithmic, and determining the total sound level when all (3) devices are in operation is not as easy as adding up the three sound level values (which would equal 218 dBA, way off the charts!).  Thankfully, both the actual sound level and the numerical value are determined another way.  I’ll spare you a lot of the math but the equation is as below.

Capture

… where SL1, SL2, SL3, … are the sound levels in dBA of the each sound makers, for as many that are being combined (in our example SL1 = 69, SL2 = 74 and SL3 = 74)

Plugging in the numbers into the equation, the combined sound level works out to be a quiet 77.65 dBA — well within the OSHA limit for exposure for a full 8 hour period.

To discuss your application and how an EXAIR Intelligent Compressed Air Product can make your process better and quieter, feel free to contact EXAIR and myself or one of our other Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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The Decibel

The decibel is a unit of measurement that relates the ratio of a physical value to another value and is expressed on a logarithmic scale.  The common symbol for decibel is dB.  The decibel is used as a measure for many parameters in science and engineering such as acoustics (sound), electronics (power levels) and control theory.

The decibel originates from methods used to express performance and loss in telegraph and telephone circuits.  The term ‘bel’ was coined in honor of Alexander Graham Bell, and the decibel, being 1/10th of a bel was established.

For most of us, the decibel is the familiar term relating to how loud a sound is.

With sound, the sound pressure is typically what is measured and is the local pressure deviation from the base or equilibrium atmospheric pressure, caused by a sound wave. In air, the sound pressure can be measured by a standard microphone, and is measured in pascals (Pa.)

To get to the common decibel reading we are familiar with, a little mathematics comes into play.

Capture

  • where Lp is the Sound Level in dB, prms is the measured sound pressure, and pref is the standard sound reference pressure of 20 micropascals.
  • The prms is what is measured by a microphone

Below are some representative sounds and the decibel rating – Note that sounds that are above 85 dB can cause hearing issues, and proper protection should be taken.Decibel Scale Still Photo

Some other interesting blogs about sound for you take a look at-

Measuring and Adding Sounds

Sound Power Level and Sound Pressure

Super Air Knife Math – When 72  + 72 = 75

If you would like to talk about sound or any of the EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR and myself or one of our Application Engineers can help you determine the best solution.

Brian Bergmann
Application Engineer

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Sound Power Level and Sound Pressure

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:

Sound Power Level Equation

Where:

Wis 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:

Sound Power Level to Sound Pressure Equation

Where:

Lis 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, 2πrwhere 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:

Working in areas that exceed these levels will require hearing protection.

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.

 

Dollar Savings: Open Pipes vs EXAIR Air Nozzle

Early one morning we received a call from a local metal stamping company that had a problem. They had outstripped the volumetric capacity of their (2) 50 HP air compressors.

They were using open copper tubes to facilitate separating the part from the die on the upstroke and then blow the part backwards into the collection chute. The (5) 1/4” copper tubes were all connected to a single manifold with a valve to control each tube.  Compounding their compressed air shortage was that this setup was duplicated on approximately (8) presses.  Per the plant they run the presses for approximately (4) hours per day.  The volume of air required for one press was calculated as:

One 1/4” open copper pipe consumes 33 SCFM @ 80 PSIG, therefore:

EDV Blog.JPG

Due to the award winning design of EXAIR’s engineered air nozzles the plant achieved faster separation of the part from the die and greater efficiency moving the part to the collection chute, while averting the need to purchase a larger air compressor. They are saving air, reducing energy costs and lowering the noise level in their facility.

If you would like to discuss saving air and/or reducing noise, I would enjoy hearing from you…give me a call.

Steve Harrison
Application Engineer
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OSHA 29 CFR 1910.95 – Standard on Occupational Noise Exposure

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)

OSHA Noise Level

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.

SoundMeter_new_nist225

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.

Brian Bergmann
Application Engineer

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A Sound Reckoning Of The Super Air Knife

In May of 1976, The Who performed a concert in London that Guiness’ Book of World Records used to certify them as the World’s Loudest Band. A sound level of 126 decibels was recorded and documented at a distance of 32 feet from the stage. That’s right at the boundary of the threshold of pain.  Which I’m sure is what they were going for.

There are a variety of charts available that relate common noises to the decibel levels they could be expected to produce. For instance, a DC-9’s engines produce a sound level of about 120 decibels at takeoff or landing. Now, imagine if such a plane were to land at the aforementioned concert: would the sound level, at a given distance, be equal to those two decibel levels added together?

The answer, of course, is no, because we’re talking about sound pressure level. It’s not EXACTLY the same as fluid pressure, but a decent analogy is that, if you have an air compressor supplying your system with 100psig compressed air, turning on your other, identical air compressor won’t result in 200psig in your system.

I mention this for a couple of reasons. One; I’m a BIG fan of The Who, and I heard one of my favorite songs of theirs on the radio this morning: “You Better You Bet,” from their Face Dances album, which came out in 1981 and hence would not have been played at the 1976 Loudest Band concert, but I digress.

The other reason is because of a conversation I had with a caller about the sound levels produced by our Super Air Knives. The published sound pressure level is 69 dBA. “dB” is short for decibels; “A” means the unit is weighted to express the relative loudness of sounds as perceived by the human ear. Anyway, the caller was interested in knowing how much louder our longer Super Air Knives were than their shorter counterparts. The answer is, of course, they’re not louder…for the same reason that your second air compressor doesn’t double the air pressure in your system, which is the same reason that the fictional jet landing at the rock concert wouldn’t double the sound level.

Now, a couple of things to consider: the sound pressure levels that we publish were measured at a distance of 3 feet to the side of the Super Air Knife. Sound levels at a closer distance, and/or in front of or behind the Air Knife, will be different. Also, the Super Air Knife was blowing into free air. If the air flow is impinging on a surface, there will be a sound level associated with that as well. If it’s in excess of the 69 dBA that the Super Air Knife is producing, then that’s what your ears are going to be subject to.

All things considered, though, the Super Air Knife is INCREDIBLY quiet, considering the amount of air flow it’s producing. The science behind this has to do with what makes them so efficient with their use of compressed air: their entrainment ability. The Super Air Knife’s design allows it to use the primary compressed air flow to entrain enormous amount of air from the surrounding environment. This entrained air not only multiplies the resultant flow rate produced, but forms an attenuating boundary layer, which effectively reduces the sound level produced by the high velocity compressed air.

The Super Air Knife entrains air at a rate of 40:1, relative to its compressed air consumption.
The Super Air Knife entrains air at a rate of 40:1, relative to its compressed air consumption.

If you’d like to find out more about how EXAIR Intelligent Compressed Air Products such as the Super Air Knife can reduce your air consumption AND your sound levels, give me a call.

Russ Bowman
Application Engineer
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Video Blog: Features and Positioning of EXAIR’s Digital Sound Level Meter

This video explains a few of the features on the EXAIR Digital Sound Level Meter. It also illustrates proper positioning of the meter when taking sound level readings. This sound level meter is an important tool to quantify noise exposure of employees and identifying the sources of noise.

Lee Evans
Application Engineer
LeeEvans@EXAIR.com
@EXAIR_LE