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Sonar Utility App

Sonar Utility uses sound to measure distance with your iPhone. But it is so much more than a ruler. Use it to "see" in the dark. It is the first iPhone app for generating 360° sonic images that show surfaces like walls, obstructions, openings (e.g., doorways), and other features. Sonar Utility is also a thermometer that estimates the ambient temperature using the physics of sound.


Specifications:
  • Waveform: 5000-20000 Hz noise modulated "chirp"
  • Sampling Frequency: 44100 Hz (0.3 inch accuracy at 72 °F)
  • Range: 9 inches to 30 feet (0.23 - 9.14 meters)


How to Use

Launch the app and turn the device so the "home" button faces away from you. The screen auto rotates so you can measure the height of objects above or below, as well as left, right, and in front of you. Make sure the sound is at maximum volume. Hold the device a few feet away from a wall. Tap "Range" at the upper left. The graph is the auto-correlation function of the received signal. As the echo processing converges a peak forms at the distance of the object. The distance is displayed above the graph. Tap the upper right buttons to change from English to Metric units.

To measure temperature set up the device a known distance from a wall or other large acoustically reflective object. Measure the distance and enter as the reference range (top right text field). Tap the upper left button "Temp". The approximate temperature is displayed once the signal processing algorithm converges.

Tap "Map" to create 360° images of your acoustical environment. As you spin in a circle a map of the reflected sound is rendered on the screen. (See examples below.)

Technical Details

Like other sonar apps Sonar Utility uses echo location to measure the range of an object. If d is the time delay measured from when the chirp is emitted from the bottom speaker to when it is detected by the microphone next to the speaker, the range is

r = c (d / 2)

where c is the speed of sound. The speed of sound depends on temperature, which in air is given by

c = 20.0457 m · sec-1 (t + 273.15)1/2

where temperature is in degrees centigrade. For best results adjust the temperature in the app settings to within about 5 degrees of the true ambient temperature.

If instead you know the range, Sound Utility can estimate the ambient temperature by inverting the above equation:

t = (c / 20.0457)2 - 273.15

The accuracy depends on the reference distance, which you must first measure and then set. The longer the distance the better the estimate. For example at 3 feet we measured the temperature 67.9 °F with the Sonar Utility app. The correct temperature was 70 °F; measured with an indoor digital thermometer. At 7 feet the Sonar Utility estimate was 70.5 °F and the correct temperature was 70.2 °F.

Perhaps the most unique feature of Sonar Utility is the 360° sound map it computes from the received signal. A gyro computes the rotation as you spin and plots the relative acoustical energy that is returned as a function of direction. Energy is summed from the maximum distance selected to your position. The reference direction is in front of you when plotting begins.

The figures below show sound maps of several types of features.

Taken from middle of a small room:

From a corner of the same room:

In a narrow hallway:

Middle of L-shaped room:

In front of a small tree:

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