What is sound pressure level (SPL)?
Sound Pressure Level (SPL) is a term we hear a lot in acoustic engineering. But what is it exactly? In the most basic sense, SPL measures how loud something is.
Sounds easy enough. Unfortunately, this is where it gets technical. If you’re familiar with SPL you’ll know that it can be a difficult concept to get your head around. The best way to start is by identifying what we mean by ‘sound pressure’.
What is sound pressure?
Sound pressure results from pressure variations in the air caused by sound waves. At one end of the spectrum is the lowest sound which can be heard by humans, called the hearing threshold. At the other end of the spectrum is the pain threshold.
As you can tell from the name, this is the highest possible sound pressure the human ear can endure. Sound pressure at the pain threshold is one million times higher than pressure at the hearing threshold. It’s the point at which hearing damage can occur.
From sound pressure to SPL
Because hearing is subjective sound levels are perceived differently by different listeners, so there needs to be a standardised unit of measurement for getting an objective read.
Now that we’ve got the basics down, a more technical definition of sound pressure is the sound force (N) acting on the surface area (m²) perpendicular to the direction of sound. Sound pressure uses a unit of measurement called Pascals (Pa), the basic formula for which is:
Pa = N/m²
Because of the long range between the hearing threshold and the pain threshold, it’s more convenient to express SPL using a compressed logarithmic scale expressed in decibel (dB). On the decibel scale the hearing threshold has a reading of 0 dB, and the pain threshold takes it up to around 120 dB.
Knowing how loud something is, is useful
Sound is usually measured using microphones which respond proportionally to the sound pressure.
To get more of an idea of sound pressure level, it’s interesting to look at decibel readings from a variety of typical sources:
Mosquitos - 20 dB.
Whispering - between 30-40 dB.
Talking loudly and laughing - 65 dB.
Vacuum cleaner or background noise a busy café - 70 dB.
Loud machinery and industrial noise like blenders, jackhammers, drilling, bulldozers and factory noise - 90 dB.
Diesel trucks, lawn mowers and car horns - 110-117 dB.
Chainsaws and power saws as well as large aircraft verge on the pain threshold at around 120 dB. Anything higher can cause permanent or semi-permanent hearing loss or damage. If it’s louder than 120 dB, you’d want to be wearing ear plugs.
As you can see from this list, measuring sound pressure levels gives us an idea of what’s safe to listen to, which is useful in engineering. Think for example of civic venues like cinemas, shopping centres, kindergartens and sports stadiums. These are venues in which sound needs to be controlled so that it does not reach unbearable limits.
SPL is used to ascertain the maximum recommended level for rooms where noisy activities take place.
Noise and attenuation
Noise is usually defined as unwanted sound.
We use SPL in HVAC systems to control and silence noise. To do this we conduct detailed acoustic analysis.
For example, it’s basically impossible to directly measure the sound power level of a fan. We can, however, undertake acoustic analysis which considers how the fan will be used and the environment it will be used in - such as the size and recommended sound pressure levels of a room - to get a better idea of how to attenuate the noise it makes.
Our engineers rely on SPL to measure a range of factors which influence attenuation. When you purchase one of our HVAC Attenuation products you know it’s been exposed to a rigorous testing and design process. Our silencers are made to order right here in Australia, so you can be sure that each product is designed with detail in mind.
Hear more about how we can help with your next project.