As spring moves into summer, the world starts to wake up. People spend more time outside talking and playing, and that pesky bird keeps waking you up at 4 am! With all this activity, you might find yourself thinking that everything seems so loud now. As you are probably aware, all this sound can be measured in units of decibels (dB), but did you know that the unit dB means multiple things? Read on to learn about how the decibel may not be as simple as you think.
Sound pressure or sound power? Often, when someone says that a sound is a certain decibel level, they are talking about sound pressure. But units of dB are also used to describe sound power, which means something different. Sound power is a measure of the total sound energy that a noise source generates, and it never changes. Saying that a source produces 60 dB of sound power is similar to saying that a lightbulb is 60 watts – that’s just the amount of energy that it puts out. Sound pressure, on the other hand, is a measure of what our ears perceive, or “loudness,” and it changes based on your distance from the sound source. Just like a lightbulb looks brighter when you stand right next to it versus standing 20 ft away, a loudspeaker will have a greater sound pressure when you are standing right next to it and less sound pressure when you are farther away.
Octave bands and dB weighting: As with all the different metrics we use as acoustic consultants, decibels vary by frequency. An overall sound pressure or sound power level can be presented with finer resolution in octave or one-third octave bands. For example, a noisy generator will often be louder at low frequencies, while a braking car will squeal at high frequencies. But the human ear does not hear every frequency equally, and a weighting factor can be applied to the dB level in each frequency band to yield an overall dBA or dBC value. dBA is used to represent typical listening conditions, whereas dBC is used for louder sounds, such as concert venues.
dB math is not like regular math: The decibel is a logarithmic unit, meaning that decibels cannot be added using standard arithmetic. When you are listening to a sound and then it suddenly seems to be twice as loud, this does not mean that it went from 60 dB to 120 dB, it means that it went up 10 dB, from 60 dB to 70 dB. And if you have two sound sources next to each other, their decibel levels do not add arithmetically where 50 + 50 = 100 or 50 + 60 = 110. They add logarithmically where 50 dB + 50 dB = 53 dB and 50 dB + 60 dB = 60 dB.
Clearly, the decibel is a more complicated unit than someone might initially expect – and this newsletter just scratches the surface. Luckily, we as acoustical consultants are well versed in the many quirks of the decibel. From noise ordinance requirements to mechanical system noise, we’ve got you covered!