Science With Your Smartphone : Decibel Meter

Introduction

Did you know that you can use your phone as a scientific instrument to explore the world around you? Your phone contains tons of built-in electronic sensors that can measure things like sound, light, motion, and more! In this project you’ll use your phone’s microphone to examine the loudness of different sounds in your environment. How quiet is a library? How loud is that truck roaring by? Try this activity to find out!

Background

You’re probably familiar with the units we use to measure everyday quantities like length or temperature. You wouldn’t bat an eye at someone saying they are 6 feet tall or that it’s 70 degrees outside. But how do we measure sound? You might describe a sound as “quiet as a whisper” or “louder than a jet engine,” but you probably don’t use a number. Sound is measured using a unit you’re probably a little less familiar with. We measure the loudness of sound in decibels, abbreviated dB*. The decibel scale is a little unusual because it is logarithmic, not linear. What does that mean? For every increase of 10 dB, the loudness of the sound doubles. For example, a 30 dB sound is twice as loud as a 20 dB sound. A 40 dB sound is twice as loud as a 30 dB sound, and four times as loud as a 20 dB sound, etc. 0 dB doesn’t mean there is no sound at all – rather, 0 dB is chosen as a reference level at the threshold of human hearing. Sound confusing? Don’t worry – here’s a list of reference sounds and their approximate decibel levels:

• 0 dB: threshold of hearing
• 20 dB: rustling leaves
• 40 dB: quiet library
• 60 dB: normal conversation
• 80 dB: screaming baby
• 100 dB: chainsaw
• 120 dB: live rock concert
• 140 dB: jet engine

Sound levels above 80 dB can cause hearing damage over long periods of time, and sound levels above 120 dB can cause immediate damage. That’s why hearing protection is recommended for people using equipment like lawnmowers. Note that the loudness of a sound also depends on your distance from the source of the sound (it will get quieter as you get farther away) – so to do a direct comparison of different sounds, you have to keep this distance constant.

What does all this have to do with your phone? Previously, if you wanted to measure sound levels, you would need to buy a standalone decibel meter – a device with a microphone and a screen that would display the sound level in dB. However, modern smartphones (which already contain built-in microphones) have apps available that will display the sound reading in dB directly on your phone’s screen. So if you want to explore the sounds of the world around you – all you need is your phone!

*The complete explanation is a little more complicated than this. There are different physical quantities related to sound that can be measured, like sound pressure level (measured in pascals), sound intensity (measured in watts per square meter), or the electronic signal used to generate sound in a speaker (measured in volts). All of these quantities can be expressed in decibels, and you might see a subscript after the “dB” indicating which one is being measured (like dB_SPL). Human perception of “loudness” roughly corresponds to the sound pressure level, although it also depends on the sound’s frequency. For a more detailed explanation, see the links in the More to Explore section.

Materials

Preparation

Procedure

Observations and Results

Using everyday items around your house, you could probably measure sounds in the range of roughly 20-80 dB. Even in a perfectly “quiet” room, background noises like the hum of a computer, or even your own breathing, could make it hard to get below about 10 dB. If you’re in a busier location with lots of people, or your house is close to a street with lots of traffic, the background noise level was probably much higher. Loud appliances like a vacuum cleaner or power tools could exceed 80 dB. Human screams can be quite loud, possibly exceeding 100 dB (as of March 2019, the world record is 129 dB!) – but you probably wanted to avoid that, because screams that loud hurt your ears! You should also have found that sound levels drop off quickly as you get farther away from the source. This is why people who will be very close to a consistently loud sound all day (like someone who mows lawns for a living) should wear hearing protection.

More to Explore

• dB: What is a decibel?, School of Physics, UNSW Sydney
• Tune Up Your Rubber Band Guitar!, Scientific America
• Sound Science: Make Your Own Harmonica!, Scientific American
• Make Sprinkles Dance, Scientific American
• Crash, Clunk, Thump - Let's Make Some Noise!, Scientific American
• Science Activities for All Ages!, from Science Buddies

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Credits