What sound level do you get when you feed an EXAIR Super Air Nozzle at 80psig? What if there are two of them? Or three? Grab your scientific calculators, folks…we’re gonna ‘math’ today!
But first, a little explanation of sound power & sound pressure:
Strictly speaking, power is defined as energy per unit time, and is used to measure energy generation or consumption. In acoustics, though, sound power is applicable to the generation of the sound…how much sound is being MADE by a noisy operation.
Sound pressure is the way acoustics professionals quantify the intensity of the sound power at the target. For the purposes of most noise reduction discussions, the target is “your ears.”
The sound levels that we publish are measured at a distance of 3 feet from the product, to the side. The units we use are decibels, corrected for “A” weighting (which accounts for how the human ear perceives the intensity of the sound, which varies for different frequencies,) or dBA. Also, decibels follow a logarithmic scale, which means two important things:
- A few decibels’ worth of change result in a “twice as loud” perception to your ears.
- Adding sources of sound doesn’t double the decibel level.
If you want to know how the sound level from a single source is calculated, those calculations are found here. For the purposes of this blog, though, we’re going to assume a user wants to know what the resultant sound level is going to be if they add a sound generating device to their current (known) situation.
Combined Sound Level (dBA) = 10 x log10[10SL1/10 + 10SL2/10 + 10SL3/10 …]
Let’s use an EXAIR Model 1100 Super Air Nozzle (rated at 74dBA) as an example, and let’s say we have one in operation, and want to add another. What will be the increase in dBA?
10 x log10[1074/10 + 1074/10] = 77.65 dBA
Now, there are two reasons I picked the Model 1100 as an example:
- It’s one of our most versatile products, with a wide range of applications, and a proven track record of efficiency, safety, and sound level reduction.
- We proved out the math in a real live experiment:
Why do I care about all of this? My Dad experienced dramatic hearing loss from industrial exposure at a relatively young age…he got his first hearing aids in his early 40’s…so I saw, literally up close and very personal, what a quality of life issue that can be. The fact that I get to use my technical aptitude to help others lower industrial noise exposure is more than just making a living. It’s something I’m passionate about. If you want to talk about sound level reduction in regard to your use of compressed air, talk to me. Please.
We’ve blogged about sound and what exactly it is before, see the link. Understanding that sound is vibration traveling through the air which it is utilizing as an elastic medium. Well, rather than me continue to write this out, I found a great video to share that is written in song to better recap how sound is created.
Now that we have that recap and understand better what sound is let’s dig a little deeper to better understand why some sounds may appear louder to a person when they may not appear different on a sound scale that is shown by something like a Digital Sound Level Meter.
Loudness is how a person perceives sound and this is correlated to the sound pressure of the frequency of the sound in question. The loudness is broken into three different weighing scales that are internationally standardized. Each of these scales, A, C, and Z apply a weight to different frequency levels.
- The most commonly observed scale here in the USA is the A scale. A is the OSHA selected scale for industrial environments and discriminates against low frequencies greatly.
- Z is the zero weighting scale to keep all frequencies equal, this scale was introduced in 2003 as the international standard.
- C scale does not attenuate these lower frequencies as they are carrying the ability to cause vibrations within structures or buildings and carry their own set of risks.
To further the explanation on the A-weighted scale, the range of frequencies correlates to the common human hearing spectrum which is 20 Hz to 20kHz. This is the range of frequencies that are most harmful to a person’s hearing and thus were adopted by OSHA. The OSHA standard, 29 CFR 191.95(a), that corresponds to noise level exposure permissible can be read about here on our blog as well.
When using a handy tool such as the Digital Sound Level Meter to measure sound levels you will select whether to use the dBA or dBC scale. This is the decibel reading according to the scale selected. Again, for here in the USA you would want to focus your measurements on the dBA scale. It is suggested to use this tool at a 3′ distance or at the known distance an operator’s ears would be from the noise generation point.
Many of EXAIR’s engineered compressed air products have the ability to decrease sound levels in your plant. If you would like to discuss how to best reduce sound levels being produced within your facility, please contact us.
1 – Fun Science: Sound – @charlieissocoollike – https://youtu.be/xH8mT2IQz7Y
Noise-induced hearing loss, or NIHL, is one of the most common occupational diseases. This doesn’t occur overnight, but the effects are noticed gradually over many years of unprotected exposure to high sound levels. This is 100% preventable! Through proper engineering controls and personal protective equipment (PPE), NIHL can be prevented. It is irreversible, so once the damage is done there’s no going back. OSHA standard 19 CFR 1910.95(a) states that protection against the effects of noise exposure shall be provided when the sound levels and exposure time exceed those shown in the table below.
Intensity of the sound pressure level is expressed in decibels (dB). The scale is logarithmic, a 3 dB reduction cuts the sound level in half. A 10 dB reduction decreases it by a factor of 10, and a 20 dB reduction decreases the sound level by a factor of 100. To calculate the dB level, we use the following formula:
L – Sound Pressure Level, dB
P – Sound Pressure, Pa
Pref – reference sound pressure, 0.00002 Pa
For example, normal conversation has a Sound Pressure of .01Pa. To calculate the dB level:
dB = 20 log10 (.01Pa/.00002Pa)
= 54 dB
When designing a new blowoff process, it’s important to consider the sound levels produced before implementation. EXAIR publishes the sound level for all of our products for this very reason. If you’re implementing multiple nozzles, you’ll need to add the sound levels together. To do so, we use the following formula:
L1, L2… represent the sound pressure level in dB for each source
A customer was using ¼” open ended copper tubes for a blowoff application removing trim after a stamping operation. They had a total of (4) tubes operating at 80 PSIG. Not only were they VERY inefficient, but the sound level produced at this pressure was 94 dBA. To calculate the sound level of all (4) together we use the above formula:
L = 10 x log10(109.4+ 109.4 + 109.4 + 109.4)
L = 100 dB
At this sound level, permanent hearing loss begins to occur in just two hours of unprotected exposure. We recommended replacing the loud and inefficient copper pipe with our 1” Flat Super Air Nozzle, Model 1126. At 80 PSIG, the 1126 produces a sound level of just 75 dBA.
L = 10 x log10 (107.5 + 107.5 + 107.5 + 107.5)
L = 81 dB
At almost a 20 dB reduction, that’s nearly 100x quieter! Don’t rely on just PPE to keep your operators safe from NIHL. Replacing loud inefficient blowoff methods with EXAIR’s Intelligent Compressed Air Products will take it one step further in ensuring your creating a safe working environment for your employees.