Page 16 - Summer2017

Acoustical Measurements with Smartphones
It is important to note that no smartphone- based sound level measurement solution has been shown to meet all the electrical and acoustical requirements for professional SLMs, as specified in the American National Standards Institute (ANSI; 1983) and Inter- national Electrotechnical Commission (IEC; 2013) standards, although some individual components of such a system, such as apps or microphones, may have been designed to meet those specifications. This is an important consideration because the entire system would need to be properly qualified by passing nu- merous tests as required by the standards. If a smartphone-based app were to be upgraded or a microphone replaced, the measurement sys- tem would potentially need to be requalified (not just recalibrated) for standards compli- ance. In light of the previous consideration of the acoustic signal path through a smartphone, even a routine update to the OS of the device could necessitate a requalification of the mea- surement system according to the standards.
Built-In Microphones
Smartphones typically employ MicroElectroMechanical Sys- tems (MEMS) microphones. Although MEMS microphones exhibit a flat frequency response comparable to what can be expected of microphones used in type 2 instruments and can capture signals between approximately 30- and 130-dB sound pressure level (SPL), their signal-to-noise ratio (SNR) is currently limited to about 60 dB, which can have implica- tions on the quality of acoustical measurements. The speci- fication for sound level meters, published by ANSI, desig- nates various types of sound level meters according to their level of accuracy. In the S1.4-1983 standard, a type 1 meter is designated a “Precision” instrument and a type 2 meter is designated a “General Purpose” instrument (ANSI, 1983).
In light of the aforementioned limitations, NIOSH research- ers chose to investigate sound level measurement smart- phone apps in terms of their ability to measure sound levels with an overall allowable error of ±2 dB(A) (Kardous and Shaw, 2014). After several criteria were established for the se- lection of smartphone apps, based on the interest of NIOSH in occupational noise exposure, over 130 sound level apps for iOS and 62 apps for Android were identified, from which 10 iOS apps and 4 Android apps were selected for closer
Figure 5. Images of different test configurations within the National Institute for Occupational Safety and Health (NIOSH) acoustic test chamber. The chamber is designed to establish a diffuse sound field to minimize the effects of microphone size, orientation, and location on the results of the study. a: SPLnFFT app running on two iPhones with iMM-6 microphones set up for comparison to a Larson Davis 2559 (type 1) reference microphone. b: An expanded look at the reverberant test chamber. c: SoundMeter and i436 microphones connected to 4 different iPhones, with a Larson Davis 831 type 1 SLM for comparison. Image labels have been edited from the original. Republished from Kardous and Shaw (2016), with permission.
evaluation. There were no Windows-based apps that met the selection criteria. Tests were conducted on a representative selection of popular smartphone devices.
From a host of measurements with different apps and de- vices, over a test range of 65- to 95-dB SPL, three iOS apps were found to have unweighted mean differences within ±2 dB of a type 1 reference SLM: NoiSee, SoundMeter2, and SPLnFFT. There were also three apps found to exhibit A- weighted mean differences within ±2 dB(A) of the reference: Noise Hunter, NoiSee, and SoundMeter. Similar compari- sons were not made with the four selected Android apps, in part because they only partially met the desired criteria. The testing performed with the Android apps also showed high variability in measurements across different devices. Even on iOS, with a much larger assortment of sound meter apps, an app may indicate a sound level, but it may not be mea- suring or calculating that sound level with any reasonable degree of accuracy.
It is important to note that this work used nominal micro- phone sensitivities provided by app developers; no calibra- tions were made. This allows for potentially more accurate
2 SoundMeter is a product of Faber Acoustical, LLC, which is owned by the author.
14 | Acoustics Today | Summer 2017