This app can be very accurate and useful if it's calibrated.

Microphone calibration:

The calibration is a decimal number that corrects the readings of your device's microphone multiplied by the amount entered on the "Mic calibration value" option in Preferences. This decimal number can range from 0.1 up to around 10 depending on the device and it's microphone.

Calibration procedure:

Use pink noise generator or a web site that offers a pink noise generator in a quiet room. Adjust the volume in the room using a real sound meter to be 70dB at the place you stand. Don't change your position, take your device in your hands and point the mic towards the sound source. Now, enter custom calibration values until you match 70dB on the Sound Meter's scale. This is a basic calibration procedure.

Measuring problems:

Every device is different and has different capabilities for sound level measurement. Some devices have microphone noise resulting in Sound Meter showing considerable amount of noise even if you sit in a very quiet room. This is not app's fault. Some devices have a microphone limiter that limits maximum sound level that can be measured. In some cases the limiter can limit the measuring range even as low as 90dB. This is also not app's fault. Also, devices that use AGC (Automatic Gain Control) and this app cannot automatically bypass will have measuring non-linearity meaning that can show higher readings in relatively quiet rooms and lower readings in very noisy environments. This is again not app's fault.
Some of those problems can be reduced by enabling the option "Bypass AGC" in Preferences (it is enabled by default) and by using the "Headroom extension" option in Preferences

Headroom extension:

Headroom extension is an algorithm that enables the Sound Meter app to measure more than your device's microphone is capable, analyzing the amount of distortion levels when your device's microphone is saturated and working above it's limits. In this way it is possible to measure as much as 12dB+ above the microphone's limit but the actual range increase is device dependent, meaning some devices will have more range increase and others will have less range increase.
Headroom extension must be enabled in Preferences and calibrated in order to be used on your device. Headroom extension calibration must be performed only after the basic calibration procedure described above.

Headroom extension calibration:

Headroom extension calibration is quite simple. First, disable the "Use headroom extension" option in Preferences (it's disabled by default). Next, you blow air into your microphone hole simulating loudest possible sound that your microphone can register and take note on the Max dB level achieved. Enter this number into the "Max device dB measure" option in Preferences.
Now enable the "Use headroom extension" option in Preferences and your device is now set to use headroom extension feature.
Have in mind that headroom extension is not as accurate as the main measuring range of the Sound Meter so use it only as an indication of the maximum loudness level that is detected.

Some values for previously calibrated devices are:

Alcatel mPop (1.15)
Asus Eee Pad (1.1)
HTC Desire (1.19)
HTC G1 (0.975)
HTC Hero (0.975)
HTC Legend (0.935)
HTC One (1.2)
HTC One V (1.29)
HTC Wildfire (0.975)
Lenovo S720 (0.79)
LG Nexus 5 (1.2)
LG Optimus One (0.981)
LG Е610 (1.27)
LG G3 (1.1)
Motorola DroidX (1.151)
Motorola Flipout (1.151)
Samsung Galaxy Beam (1.22)
Samsung Galaxy Beam2 (1.22)
Samsung Galaxy Note (1.14)
Samsung Galaxy Note2 (1.14)
Samsung Galaxy Note3 (1.09)
Samsung Galaxy Golden (1.1)
Samsung Galaxy S (1.23)
Samsung Galaxy S2 (1.2)
Samsung Galaxy S3 (1.24)
Samsung Galaxy S4 (1.16)
Samsung Galaxy Tab (1.14)
Samsung Galaxy Mini2 (1.2)
Samsung Galaxy Young (1.29)
Samsung Vibrant (0.915)
Sony Xperia T (1.21)
Sony Xperia X8i (0.961)



     
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