External Microphones Improve Sound Measurement App Accuracy
Calibrated external microphones greatly improve the overall accuracy and precision of sound measuring iPhone and iPad apps, a NIOSH study published in The Journal of Acoustical Society found. According to the study, external microphones remove most of the variability and limitations associated with built-in microphones on smartphones.
The research was a follow-up to a 2014 study by NIOSH, which evaluated sound level measurement (SLM) smartphone apps at the agency's acoustic testing lab to “examine their suitability and accuracy in relation to professional sound measurement instruments." According to NIOSH, almost all of the SLM apps lacked accuracy. Of the 130 iOS apps examined, only four were found to be “adequate” for certain occupational noise assessments.*
NIOSH conducts SLM testing on smartphone devices.
NIOSH’s follow-up study used the four iOS apps the researchers had deemed adequate in their initial study and examined their performance when used with external calibrated microphones. All four apps performed well using both sets of external microphones, NIOSH found.
The researchers said there appeared to be no substantial difference in the type of microphone selected—so long as it was “appropriately calibrated, preferably by using an acoustical calibrator instead of relying on the predefined profiles available from some developers.”
The study, however, has a limited scope as it only tested a limited range of sound levels. Level linearity, directionality, time and frequency-weighting responses, tone bursts, radio frequency interference, and the atmospheric and environmental conditions specified by the IEC-61672 standard (also ANSI/ASA S1.4)
Following the two studies, NIOSH released an iPhone sound measurement app for measuring sound levels in the workplace.
Although smartphone apps are “still unlikely to replace professional instruments or comply with applicable ANSI or IEC standards in the near future,” the NIOSH researchers said, “the results of this study indicate that, due to the advancements made in app design and external microphones availability, the gap between professional instruments and smartphone-based apps is rapidly narrowing.”
*The researchers initially set out to include Android apps in their research and determined that the variables created by various manufacturers’ divergent hardware made “ a comprehensive experimental design and analysis similar to the iOS devices and apps study was not possible.”