Sensors on your device

Science Journal uses the sensors in your device to allow you to measure the world around you.

Sensor List

Accelerometer X, Y, and Z (m/s2)

The acceleration of the phone to the left and right (X-axis), up and down (Z-axis), and forward and back (Y-axis), in m/s2.

What's Going On?

Objects have a tendency to stay put, or to keep moving if they're moving—we call this tendency inertia. Newton's First Law expresses this idea formally: An object continues in its state of motion or rest unless acted on by an unbalanced force.

When an unbalanced force does cause an object to budge, we say the object accelerates: its velocity, or speed, changes—either by speeding up, slowing down, or changing direction. Acceleration is measured as a change of velocity (meters per second) in time, or meters per second squared (m/s2).

Your phone has a device to measure these changes in motion—an accelerometer. Inside an accelerometer, small suspended masses are free to move. Changes in motion causes these masses to shift, much as your own head tends to flop forward when you're in a car that stops suddenly. Measuring these subtle inertial shifts, an accelerometer in a phone can detect changes in motion and orientation, useful for switching the screen from landscape to portrait mode, for playing games on your phone, and more.

You probably noticed a persistent acceleration in the Z axis, even with the phone sitting still on a table. This is the acceleration we experience here at the Earth's surface due to the pull of gravity, approximately 9.8 m/s2.
 

Barometer (hPa)

The atmospheric pressure measured in hectopascals (hPa).

What's Going On?

Though we rarely notice it, all of us live at the bottom of a deep ocean of air: Earth's atmosphere, which extends upwards of 40 kilometers (130,000 feet) high. The weight of all that air pushing down on us from above us creates what's called atmospheric pressure.

At sea level, atmospheric pressure applies 14 pounds of force on every square inch—that's the equivalent of about 1,300 pounds on an area the size of a standard sheet of paper.

We don't notice this crushing pressure because we're used to it—that is, until it changes, even slightly, as it does when we visit high altitudes or ride in airplanes. Atmospheric pressure also varies with weather conditions, dropping in the presence of stormy, unstable air, and rising when air stabilizes and skies clear.

The digital barometer in your phone measures atmospheric pressure in hectopascals. (A hectopascal is 100 pascals, where a pascal is one newton of force applied over a square meter of area.) You've probably heard weather forecasters give atmospheric pressure in millibars—this friendlier-sounding unit is equivalent to hectopascals.
        
You might be wondering: Why would a phone measure pressure? Atmospheric pressure changes measurably with even slight changes in elevation, say, when you climb a flight of stairs. So a digital barometer in a phone can help detect location changes, for example, telling you what floor of a building you are on, or whether you are driving on the upper or lower deck of a bridge.

Brightness (Lux or EV)

The amount of light reaching the device.

On Android, brightness is measured in Lux, which measures the amount of light reaching the device's ambient light sensor.

On iOS, because the ambient light sensor is inaccessible, brightness is instead measured in Exposure Value (EV), which measures the amount of light reaching the front-facing camera.

What's Going On?

The ambient light sensor measures light in lux, a measure of illumination that depends on the amount of incoming light and the area over which it is spread.

A full moon provides about 1 lux of illumination, a typical lamp-lit living room about 50 lux, classroom lighting and sunrise and sunset about 400 lux, daylight (indirect sun) over 10,000 lux, and direct sun over 30,000 lux.

Lux is an unfamiliar unit for most of us. When we buy lightbulbs, we often shop by watts—but the wattage of a bulb doesn't tell you how bright a bulb is, only how much energy it uses. Lumens are a better measure for light shopping; they tell you how much light a bulb will actually produce. Lux, meanwhile, tells you how much of that light arrives at a particular area.
 
Since light spreads out as it travels, the number of lux dwindles as you back away from a light source, even though the source is still emitting the same amount of light in lumens. An area that is tilted away from a light sources also receives less illumination—which happens to be why the equatorial regions here on Earth are so much warmer than the chilly poles.

By the way, the purpose of the light sensor in your phone is to control the light level of the screen, adjusting accordingly to dim and bright environments. Inside the light sensor, a tiny semiconductor responds to incoming light by producing a small but measurable electrical current—a phenomenon known as the photoelectric effect. Similar sensors are used in some streetlights to turn them on when it gets dark outside.

Compass (degrees)

The orientation of the phone with respect to earth’s magnetic field.

What's Going On?

You probably already knew that the Earth has a magnetic field. What you may not have realized is that your phone contains a device designed to detect magnetic fields: a magnetometer. By combining data from your phone’s magnetometer with the phone’s orientation in space, we can create a digital compass.

An old-fashioned compass is a low-tech magnetometer, with a magnetized needle that turns to align itself with the surrounding magnetic field. There's no tiny compass inside your phone, but instead a tiny three-axis sensor that uses electric currents to detect the magnetic field strength in the three dimensions of space. But this alone is not enough to create a compass!

Why does a compass need to combine the data from the magnetometer with the phone's orientation? Knowing the phone's orientation allows us to compute the magnetic rotation even when the phone is not held flat in the same plane as the Earth's magnetic field. We know that gravity is perpendicular to Earth’s magnetic field, so we can use the direction of gravity from the accelerometer sensors to determine how much of the magnetic field is in the same plane as the Earth's magnetic field.

As you may have noticed, the compass responds not just to the Earth's magnetic field, but also to any and every magnetic field that happens to be around. Magnets, electric currents, and metal objects will easily steer your phone's magnetometer this way and that, causing the compass reading to change. Stand in an open area—preferably outdoors and far from power lines— if you want to be sure that your compass's red tip is aligning with the Earth's magnetic field. It helps to move your phone in a figure 8 pattern to calibrate the magnetometer in your phone.

Linear accelerometer (m/s2)

The total acceleration of the phone, excluding the force of gravity, in m/s2.

What's Going On?

Objects have a tendency to stay put, or to keep moving if they're moving—we call this tendency inertia. Newton's First Law expresses this idea formally: An object continues in its state of motion or rest unless acted on by an unbalanced force.

When an unbalanced force does cause an object to budge, we say the object accelerates: its velocity, or speed, changes—either by speeding up, slowing down, or changing direction. Acceleration is measured as a change of velocity (meters per second) in time, or meters per second squared (m/s2).

Your phone has a device to measure these changes in motion—an accelerometer. Inside an accelerometer, small suspended masses are free to move. Changes in motion causes these masses to shift, much as your own head tends to flop forward when you're in a car that stops suddenly. Measuring these subtle inertial shifts, an accelerometer in a phone can detect changes in motion and orientation, useful for switching the screen from landscape to portrait mode, for playing games on your phone, and more.

The combined linear accelerometer sensor records the total acceleration in all three axes of the phone, excluding gravity.

Magnetometer (μT)

The strength of the ambient geomagnetic field in Microtesla (μT). 

What's Going On?

Your phone contains a device designed to detect magnetic fields: a magnetometer. This tiny, three-axis sensor that uses electric currents to detect the magnetic field strength in the three dimensions of space. By combining data from all three axes we can create a total magnetic field strength sensor.

Some magnetometers detect magnetic fields using the Hall effect—the tendency of magnetic fields to deflect moving charge as it flows. Newer magnetometers rely on magnetoresistance, metal alloys that change their resistance in response to magnetic fields.

Why does a phone need a magnetometer? Knowing your orientation in space is useful for built-in compass and navigation apps, especially those that seek to give turn-by-turn directions.

 Magnets, electric currents, and metal objects will easily steer your phone's magnetometer this way and that. But even in the middle of an empty field, the magnetometer in your phone always shows a low, non-zero reading: this is the Earth's magnetic field.

Pitch (Hz)

The pitch of the sound, measured in hertz (Hz), reaching the sound sensor or microphone.

What's Going On?

Sounds are made by vibrations. Your voice, for example, comes from vibrations in your throat's vocal cords. These vibrations create alternating zones of high and low air pressure that travel outward—much like the expanding circular ripple made by a pebble thrown into a pond.

All sounds can be described in terms of their frequency and intensity. The frequency (also, pitch or tone) of a sound wave is equivalent to its rate of vibration. The faster an object vibrates, the higher the pitch of the resulting sound. We measure these vibrations in Hertz (Hz) where 1 Hertz = 1 vibration/second.

Humans can hear sounds in the range of 20 to 20,000 Hz. The lowest key on a piano produces a sound with a frequency of 27.5 Hz and the highest key produces a sound with a frequency of 4186.01 Hz.

The red dot indicates how close the detected pitch is to the nearest musical note. If the pitch is lower or “flatter”, the dot will appear to the left. If the pitch is higher or "sharper" than the red dot will appear to the right.

Sound intensity (dB)

The intensity of sound, measured in decibels (dB), reaching the sound sensor or microphone.

What's Going On?

Sounds are made by vibrations. Your voice, for example, comes from vibrations in your throat's vocal cords. These vibrations create alternating zones of high and low air pressure that travel outward—much like the expanding circular ripple made by a pebble thrown into a pond.

All sounds can be described in terms of their frequency and intensity.

Intensity is what you measure here with the Science Journal sound intensity sensor, in units of decibels (dB). Intensity, or loudness, depends on the distance that a vibrating object moves each time it vibrates; we hear greater intensity as increased loudness.

The frequency (also, pitch or tone) of a sound wave is equivalent to its rate of vibration. The faster an object vibrates, the higher the pitch of the resulting sound. The Science Journal sound intensity sensor does not measure frequency—only loudness.

One important thing to know is that the microphone in your device is designed to be very sensitive to differences in sound waves over time, but not the absolute size of each wave.  We have tried to choose a code that is likely to produce numbers similar to the reference numbers shown below, but each device may give consistently higher or lower numbers.  Scientists will usually calibrate a sensor like this to a known measurement.  For now, Science Journal can tell you whether your next train ride is louder than your next rock concert, but only if you bring the same phone to both events.

Another thing to know is that there are important differences between “sound intensity”, “sound intensity level”, and “sound pressure”.  We are treating them as the same thing, but if you want to dig deeper, there’s many online resources about the differences.

The quietest sound that the average human ear can detect is defined as 0 dB. Ordinary conversation corresponds to about 60 dB, and sounds above about 140 dB are painful to the human ear. But sounds don't have to be painful to be harmful. Continued exposure to sounds of 90 dB—about the loudness of a vacuum cleaner—can eventually cause hearing loss.

The decibel scale is logarithmic, which makes for some trickiness: A sound source of 40 dB isn't twice as intense as one with 20 dB—it's 100 times more intense. Meanwhile, if one ringing alarm clock produces 70 dB, two ringing alarm clocks produce not 140 dB, but 73 dB. Like we said, it's a strange scale.
        

  • 10 dB rustling leaves.
  • 20 dB whispering at 5 feet.
  • 30 dB soft whisper.
  • 50 dB rainfall.
  • 60 dB normal conversation.
  • 90 dB blender.
  • 100 dB car without muffler.
  • 110 dB shouting in ear.
  • 120 dB thunder.
  • 130 dB jackhammer.
  • 140 dB airplane taking off.
Tip: To learn more about each sensor, on your device, tap Info  next to the sensor indicator.
Note that not every sensor is available on every device.

 

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