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Sound meter apps, which you can download on your smartphone, measure noise levels using the built-in microphone. However, the accuracy of these apps can vary due to several factors:

1. Microphone Quality:
   - Smartphones: The microphones in smartphones are primarily designed for voice recording, not for accurate sound level measurements. They often have frequency response limitations and may not pick up certain frequencies accurately.
   - External Microphones: Some sound meter apps allow the use of external calibrated microphones, which can significantly improve accuracy.

2. Calibration:
   - Calibration of the App: Some apps allow you to calibrate the sound level meter against a known sound level using a professional sound level meter. Proper calibration can make the app more accurate, though it still won't match the precision of professional equipment.
   - Pre-calibrated Apps: Some sound meter apps come pre-calibrated based on the average microphone sensitivity of a specific smartphone model. These might offer better accuracy for that particular device but can still vary across different units of the same model.

3. Environmental Factors:
   - Background Noise: Background noise, wind, and the position of the microphone can affect the readings. Professional sound level meters often have windshields and other features to minimize these effects, which smartphones lack.
   - App Software: The app’s algorithm and how it processes the sound data also play a role in accuracy. Different apps might yield different readings under the same conditions.

4. Comparison with Professional Sound Meters:
   - Studies and user reports often show that while some sound meter apps can be within 2-5 decibels (dB) of a professional sound meter in ideal conditions, others can be off by a wider margin.
   - For professional use, regulatory compliance, or precise measurements, a dedicated sound level meter is always recommended.

Summary:
Sound meter apps can provide a rough estimate of noise levels and might be suitable for casual or non-critical measurements. However, they are not as accurate or reliable as professional sound level meters, especially in environments where precise measurements are necessary. For general, non-critical use, sound meter apps can be useful, but for anything requiring high accuracy, it's best to use professional equipment.