Sound Power Vs Sound Pressure

sound-level-comparison
EXAIR Intelligent Compressed Air Product dBA ratings as compared to other sounds

When trying to explain or state a number associated with how loud a sound or noise is it can be somewhat confusing or at the very least, ambiguous.  This blog will help to make it clear and easy to understand the difference between Sound Power and Sound Pressure.

Sound Power is defined as the speed at which sound energy is radiated or transmitted for a given period of time.  The SI unit of sound power is the watt. It is the power of the sound force on a surface of the medium of propagation of the sound wave.

Sound Pressure is the sound we hear and is defined as the atmospheric pressure disturbance that can vary by the conditions that the sound waves encounter such as furnishings in a room or if outdoors trees, buildings, etc.  The unit of measurement for Sound Pressure is the decibel and its abbreviation is the dB.

I know, the difference is still clear as mud!  Lets consider a simple analogy using a light bulb.  A light bulb uses electricity to make light so the power required (stated in Watts) to light the bulb would be the “Sound Power” and the light generated or more specific the brightness is the “Sound Pressure”.  Sound just as with the light emitting from the bulb diminishes as the distance increases from the source.  Skipping the math to do this, it works out that the sound decreases by 6 dB as the distance from the sound source is doubled.  A decrease of 3dB is half as loud (Sound Pressure) as the original source.  As an example sound measured at 90 dB @ 36″ from the source would be 87dB at 54″ from the sound source or 84dB at 72″.

We at EXAIR specialize in making quiet and efficient point of use compressed air products, in fact most of our products either meet or exceed OSHA noise standards seen below.

OSHA Noise Level

EXAIR also offers the model 9104 Digital Sound Level Meter.  It is an easy to use instrument for measuring and monitoring the sound level pressures in and around equipment and other manufacturing processes.

If you have questions about the Digital Sound Level Meter, or would like to talk about any of the quiet EXAIR Intelligent Compressed Air® Products, feel free to contact EXAIR or any Application Engineer.

Steve Harrison
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.