Science with a Smartphone: Decibel Meter


Editor’s Note: We have strived to maintain accessibility in our Bring Science Home activities by limiting the required technology. We recognize technology is now more widely available than it was when we began this series in 2011 and that it can add value to scientific exploration. This is our first activity that requires the use of a smartphone or tablet. Please let us know what you think! E-mail [email protected] with feedback about the use of technology in this—and future—Bring Science Home activities.

Key concepts

Physics

Sound

Measurement

Logarithmic

Introduction

Did you know that you can use a smartphone as a scientific instrument to explore the world around you? Smartphones contain many built-in electronic sensors that can measure phenomena such as sound, light, motion and more! In this activity you’ll use a 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, such as length or temperature. You wouldn’t bat an eye at someone saying they are six feet tall or 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 wouldn’t use a number. Sound is measured using a unit called decibels, abbreviated dB. The decibel scale is a little unusual because it is logarithmic rather than 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, etcetera. Zero 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: human hearing threshold

20 dB: rustling leaves

40 dB: quiet library

60 dB: normal conversation

80 dB: screaming baby

100 dB: chain saw

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 such as lawn mowers. Note 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 a smartphone? If you wanted to measure sound levels previously, you would have had to buy a stand-alone decibel meter—a device with a microphone and a screen that would display the sound level in dB. Modern smartphones, however, (which already contain built-in microphones) can run apps that will display the sound reading in dB directly on the phone’s screen. So if you want to explore the sounds of the world around you, all you need is a phone!

Materials

  • Smartphone or tablet with internet access and permission to download and install an app
  • Adult (to help verify and download the app)
  • Other people whose voices you can measure (optional)
  • Multiple locations to take the phone (optional)


Preparation

  • Ask an adult to help you search for a “decibel meter” or “sound meter” app on a smartphone or tablet. There are plenty of free options available, but some apps may have ads or in-app purchases enabled.
  • Get to know your decibel meter app. Some apps will just display a number on the screen whereas others will display a meter or a graph. Some will also let you record data. Make sure the app is working: Talk at a normal volume, and you should see the numbers fluctuate.


Procedure

  • Determine the level of background noise. Put the phone down, sit perfectly still and hold your breath. What is the decibel level? Does it fluctuate with background noises, such as a car driving by or a bird chirping?
  • Now explore your own voice. Try whispering, talking and even yelling at the phone. You can also try other sounds such as whistling or humming. Does the whisper even register or is it drowned out by the background noise? How loud is your yell?
  • If there are other people around, try measuring their voices as well. Is everyone’s “normal” voice the same decibel level? Who can yell the loudest?
  • Now test different sounds. This can be as simple as clapping your hands or knocking on a door. There are plenty of other everyday sounds you can try as well—for instance, running a faucet or a clicking light switch. You can also try running some appliances such as a microwave or vacuum. How do all the different sounds compare? Which ones are the loudest?
  • Find out how distance from the sound source affects the sound level. Try to find a relatively constant sound such as a running faucet or a person humming. Start out with the phone right next to the source and then slowly walk away. How does the decibel level change as you get farther away?
  • Try measuring background noise levels in different locations. Take the phone into different rooms, a library or to a playground or park. Where is the quietest place you can find? The loudest? Are noise levels loud enough anywhere that they could pose a danger to your hearing?
  • Extra: You can also download apps to measure the frequency, or pitch, of sounds. Frequency is measured in hertz (Hz). The range of human hearing is from about 20 to 20,000 Hz. As we get older we tend to lose our ability to hear sounds at the higher end of that range. Some animals, such as dogs, can hear all the way up to 45,000 Hz. What’s the frequency range of your voice? What about all the other sounds you measured earlier?
  • Extra: Measure the sounds made by various musical instruments. If you don’t have any instruments handy, you can make your own (see the “More to explore” section)!

Observations and results

Using everyday items you could probably measure sounds in the range of roughly 20 to 80 dB. Even in a perfectly “quiet” room background noises, such as 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 you are close to a street with lots of traffic, the background noise level would probably be much higher. Loud appliances such as 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 want to avoid that because screams that loud can hurt your ears! You should also have found sound levels drop off quickly as you get farther from the source. People who will be very close to a consistently loud sound all day (such as someone who mows lawns or works near jet engines for a living) should wear hearing protection.

More to explore

dB: What Is a Decibel?, from the University of New South Wales, School of Physics

Tune Up Your Rubber Band Guitar!, from Scientific American

Sound Science: Make Your Own Harmonica!, from Scientific American

Make Sprinkles Dance, from Scientific American

STEM Activities for Kids, from Science Buddies

This activity brought to you in partnership with Science Buddies

Science Buddies

Cludo Reports

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