30 Nov The Realm of Effective Acoustics Part 3: Measuring Noise with a Smartphone App
Enter the realm of effective acoustics. Here we will dispel common misconceptions and take a look at the proper way to solve problems related to noise and sound, from making a room sound better to controlling and measuring noise.
Last time we entered the realm, we examined sound isolation and room acoustics, and before that we met the armchair acoustician. Here in Part 3, we look at what happens when an armchair acoustician measures noise with a smartphone app.
Measuring Noise with an App
You’re at a park enjoying a picnic dinner when you hear the throbbing bass of a rock concert from a nearby outdoor venue. Maybe it’s Sleazy Joe, who reached 143.2 decibels at a show in Sweden in 2008. Or maybe it’s the soft-rock stylings of Toto. Either way, the music is loud enough to disturb your egg salad al fresco.
Midway through “Rosanna” (so it is Toto after all) you notice a guy by a tree holding his phone in the air. You recognize him right away: It’s the armchair acoustician from that cocktail party a few weeks back, and it looks like he’s measuring concert noise with a free-of-charge smartphone app.
This might surprise you, but many available noise measurement apps are built correctly (the free-of-charge ones less so). They analyze the input and show a resulting noise level based on the information they have. That means they’re accurate, right? Not quite. While the app itself might be built right, the information it is analyzing is probably wrong. Here are a few reasons why:
The microphone in your phone is made for one thing: picking up your voice. It is calibrated to capture the sound levels and frequencies of typical human conversations. (We’ve all heard the distortion when someone screams into their phone.)
Microphone limitations aside, people like to record concert footage on their phones. After receiving complaints about the distortion, phone manufacturers started including an automatic gain-setting function that limits the noise levels being recorded on a phone to preserve the signal. The microphones, however, remained pretty much the same.
Most of the latest and greatest improvements in this field are basically programmed padding, where algorithms try to guess what you want to record on your phone. The post-processing trickery gives the impression of a better recording without capturing an accurate sample of the actual noise levels around you.
We’re not saying the noise apps can’t circumvent all this fancy coding, but basically, once the gain trickery is removed and the sound levels around you reach the tolerance of the microphone, your phone can’t distinguish between loud and louder. The microphone simply caps out when the highest possible level is reached. This happens a lot sooner than you’d think.
The microphone in your cell phone isn’t great at capturing low-frequency noise (e.g., the bass beat rumbling your egg salad). So if you try to measure concert noise from a nearby park or balcony, your phone will most likely record noise from nearby road traffic or rustling leaves and ignore the bass beat.
The Shape of the Phone
Your phone’s microphone is internal. When you try to record noise, your hand and even the phone itself can get in the way—even if you point the phone at the perceived direction of the noise. It’s a small factor, but it adds to the uncertainty.
If the app itself is built correctly, why does all this matter? For quiet noises, studies show that app-based cell phone noise measurements are mostly within 0-5 dB, but as soon as the noise is louder, some apps could be within a whopping +/- 20 dBA of “the correct” number. This variance results from the inaccuracies described above.
A Detour into Math
To properly understand the significance of +/- 20 dB, let’s take a quick look at the math. First up: linear values versus logarithmic values.
A linear value behaves the way most people think a number should behave: Double 4 and you get 8. Then add 1 to make it 9. Simple, right?
But our ears don’t work that way (i.e., they don’t sense changes linearly). They are (roughly) logarithmic. Therefore, decibels are too: they are a representation of the actual measure, which is the pressure that sound waves put on your ear drum. These values have a massive linear range: human ears can sense pressures from 20 (0 dB) to 20,000,000 (120 dB) micropascals (which is the scientific measure of pressure). The scale of 0 to 120 decibels is proportional to 20 micropascals, as it represents the threshold of hearing for most of us (unless you attended that Sleazy Joe show).
Here are a few examples that show how the ratio scale works:
- Raising the pressure twofold raises the decibel value by 6
- Raising the pressure three times raises the decibel value by 10
- Raising the pressure tenfold raises the decibel value by 20
Which actually corresponds nicely with our ears:
- an addition of 6 dB (i.e., going from 10,000 mp to 20,000 mp) represents a well-noticeable difference perceived loudness in most humans (“I can clearly tell that it got louder/quieter”);
- an addition of 10 dB (i.e., going from 10,000 mp to 30,000 represents a perceived doubling of sound level (“twice as loud”);
- an addition of 20 dB (i.e., going from 10,000 mp to 100,000 represents a perceived quadrupling of sound level (“four times as loud”).
Think of it this way: If you’re having a dinner party and want to extend your dining room table from 55 inches to 65 inches to fit two more guests, adding a 20-inch leaf should do the trick. But if you lengthened your table by 20 “decibels,” it probably wouldn’t fit in your living room! (However, the guest capacity would be incredible!). So, with this in mind, a measurement error of +/- 20 dB is pretty significant, more than enough to nullify a formal complaint about noise from a Toto concert.
So, to keep it simple and to save us acousticians from writing a lot of zeros, we use decibels. Interested in decibels? Read more about the acoustical concepts behind the decibel scale here.
Advice for a Budding Home Acoustician
Because noise measuring apps are often limited by your smartphone’s microphone, adding an external mic can improve the accuracy of your smartphone noise measurements. While these add-on mics can be convenient, affordable, and usually work with a variety of phones, they aren’t as accurate as a properly calibrated sound level meter.
At BKL, we have invested in the latest instruments and software for measuring, analyzing, predicting, and assessing sound and vibration. In the hands of our experienced professionals, these tools deliver accurate results. Get in touch with us if you want to learn more about how we can help you solve problems concerning noise and vibration.
Written by Joonas Niinivaara