## The Trick To Adding Sound Levels Of Multiple Sources

If I put a 10 pound weight on a scale, the scale will read 10 pounds. If I put another 10 pound weight next to it, the scale with now read 20 pounds.

If I have \$10 bill in my pocket…well, that’s a LOT more cash than I usually carry. But if I somehow come into possession of another \$10 bill and put it in my pocket, now I have \$20. And it probably won’t be for long.

If there’s an EXAIR Model 1100 Super Air Nozzle supplied with compressed air at 80psig, and my Sound Level Meter is reading a level of 74 dBA, and I hook another one up right next to it, my Sound Level Meter now reads about 78 dBA.

Wait, what? Did we just break math there? You, and your ears, will be happy to know that there’s perfectly valid math behind the third (as well as the first two) statements above. The third one’s just a little different, that’s all.

See, sound power (that’s “how loud” sound is, as measured at the point of generation) and sound pressure (“how loud” it is, as measured at the point where it’s heard) are both quantified in units called decibels. And, unlike mass or wealth (the first two examples above), which are linear & additive, measurement of sound power & pressure is done on a logarithmic scale. That means simple arithmetic won’t work…we have to use a logarithmic equation to ‘add’ those sound levels together. It looks like this:

Combined Sound Level (dBA) = 10 x log10[10SL1/10 + 10SL2/10 + 10SL3/10 …]

Where “SL1”, “SL2”, “SL3”, etc., are the sound levels, in decibels, for the “noisemakers” in question. So, for the two Model 1100 Super Air Nozzles, generating 74 dBA each:

10 x log10[1074/10 + 1074/10] = 77.65 dBA

And just to prove the math works, we made a video of a real live test:

All EXAIR Intelligent Compressed Air Products are engineered & manufactured to be safe, efficient, and as quiet as possible. If you’d like to find out more, give me a call.

Russ Bowman, CCASS

Application Engineer
EXAIR Corporation
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## Measuring and Adding Sound Levels

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:

Where:

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:

Where:

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.

Tyler Daniel
Application Engineer
E-mail: TylerDaniel@EXAIR.com