Sound level apps for both iOS and Android abound, turning any smartphone into a sound level meter. Given the low cost of these apps, why would you purchase a commercial sound level meter like the Go Direct® Sound Sensor, order code GDX-SND, $89, Sound Level Meter, order code SLM-BTA, or Sound Level Sensor, order code SLS-BTA, $69?
The primary advantage to a commercial sound level meter is accuracy you can trust. Depending on the expertise and approach of the app developer, a smartphone app may or may not accurately measure sound levels. Additionally, given the variability in smartphone microphone elements and filters, even the accuracy of a "good" sound level app may vary from phone to phone.
Researchers at the CDC evaluated the performance of a wide variety of smartphone apps and their potential use to monitor noise levels in work places (see article). Their study looked at 130 iOS and 62 Android apps across four iOS and six Android devices, although it focused only on ten iOS and four Android apps that met their criteria for occupational evaluations. Of those apps, some performed very well with average accuracies within 2 dB of a commercial sound level meter, others were much worse (+/- 5 dB or greater). In general, they found iOS apps to perform better than Android apps; developers could more easily bypass the speech and noise cancelling filters applied to microphone signals.
A commercial sound level meter, on the other hand, is designed with known hardware characteristics and uses software tailored to them. So, the accuracy is reliable. While a smartphone app may be helpful for impromptu sound level monitoring, scientific sound level investigations require a trustworthy meter.
Update: A follow-up evaluation found that adding an external, calibrated microphone to an iOS device could improve sound level readings to within 1 dB of a commercial sound level meter, showing again that accuracy improves when known hardware is used.