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 app to use when calculating signal levels to deter- mine an accurate sound level. Unfortunately, it can be difficult for many users to accurately measure the built-in microphone sensitivity, often because the user lacks a properly calibrated SLM to use as a reference and/or sufficient know-how to avoid significant measurement errors. Some smartphone apps include nominal sensitivities for select smart- phone models that allow the user to make sound level measurements with ballpark accuracy with- out any additional equipment or effort.
When working with external measurement mi- crophones, calibration can potentially be simpli-
fied or even obviated, depending on the required
level of measurement accuracy. Often, a calibrated measurement microphone includes a certificate indicating the sensitivity of the microphone at a frequency of 1 kHz in units of millivolts per pas-
cal (mV/Pa). If, as in a case such as this, the mi- crophone sensitivity is known and it is connected
to an analog audio input, then the input sensitiv-
ity must be determined in units of volts relative to
the full-scale digital value of the ADC or V/FS. Once this is done, then the sensitivity of the microphone may be com- bined with the sensitivity of the input device to arrive at an overall sensitivity in Pa/FS, which can then yield an accurate overall sound level. The relationships between these differ- ent sensitivities are shown in Figure 4, which also illustrates the path through which the signal must pass for basic sound level measurements.
As before, if better than ballpark accuracy is critical, then a direct calibration of the microphone sensitivity, as part of the complete measurement system, is recommended. This becomes a simpler task when working with a measurement microphone that is designed to fit an acoustic calibrator with a standard (nominal) 0.25-, 0.50-, or 1-inch-diameter open- ing. With smartphone apps that support it, this approach makes the calibration procedure much easier for the user. The user simply needs to follow a process such as the following:
1. Connect the microphone to the smartphone.
2. Insert the microphone into the calibrator.
3. Tell the app the reference level of the calibrator.
4. Start the calibrator.
5. Press a “Calibrate” button in the app to go ahead and
calibrate the sensitivity based on the actual acoustic pressure being applied to the microphone.
Figure 4. The stages through which the original acoustic signal must pass in order to be used for measurement within a smartphone app. Properly cali- brated transducer and electronics sensitivities allow the smartphone app to perform accurate acoustical measurements. SPL, sound pressure level; FS, full- scale value.
 Suitability for Calibrated Measurements
Perhaps the most obvious answer to the first question, “What can I measure with it?” is that of overall sound levels. If sound levels can be determined easily enough with suffi- cient accuracy, the widespread use of smartphones presents the potential for unprecedented access to sound levels (and other acoustic parameters) in various environments across the globe. Researchers at the National Institute for Occu- pational Safety and Health (NIOSH) saw this potential and took action to begin to answer the second question, “How good could the measurements be?” and to determine the feasibility of acquiring widespread samples of occupational noise exposure. They have published their initial findings in two separate articles in The Journal of the Acoustical Society of America Express Letters (JASA-EL). The first publication presented research aimed at identifying specific smartphone apps that met certain requirements and then testing the performance of those apps with the smartphones’ built-in microphones to determine their reliability for occupational noise measurements (Kardous and Shaw, 2014). Their sec- ond body of research focused on assessing a handful of in- expensive external microphones that could be connected to a standard 3.5-mm headset jack on a typical smartphone (Kardous and Shaw, 2016).
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