COVER STORY
orientation or positioning of the device has been shown in previous studies to be a significant factor upon the frequency response of the measurements. Most smart phones use a microphone that is positioned at the lower edge of the device, others now use multiple microphone capsules spread over the device to allow for noise cancellation.
The location of the microphone on tablets is even more variable as these are not intended to be used for calls. And most modern devices will auto-rotate the display as the user moves the phone with the result that the microphone could possibly be in one of four different positions. A quick sample of users of smart phones and tablets showed that unless that were made aware of the position of the microphone they would hold the device with the microphone typically facing their body, sometimes with the body of the device touching their clothing, further affecting the frequency response of the device.
Taking all this into account there has to be huge caveats surrounding their use in the workplace. Most sound level meters currently being manufactured comply with IEC 61672-1, often with the subsequent Type Approval to IEC 61672-2 and then periodic verification to IEC 61672-3. The standards, regulations and guidelines to which most of us are working will specify that an instrument should meet Class 1 or Class 2 of this standard.
Even the user instructions on some of the sound level meter apps contain disclaimers that emphasise the sound apps are not designed to meet such rigorous standards or type 2/Class 2 standards.
An evaluation of 25 sound level meter apps on the Google Play Store showed that only six labelled the metric being displayed correctly. Where this wasn’t the case, the majority were using SPL, often with no indication of the frequency or time weighting being used.
Where some form of averaging was available, only eight of the apps used displayed Leq or TWA as a metric. The others used Average or Mean as a descriptor. Using a quick, visual check using a controlled noise source that stepped by +20dB, some even appeared to be using a simple linear average.
Another recent app test using a new iPad showed a discrepancy of up to 10dB between measurements. This appeared to be caused by the use of multiple microphones on a device.
This effect was more pronounced at low levels below 40dB(A) where this could significantly impact upon environmental noise measurements and calculations where the statistical distribution of the noise is an important metric.
If using a mobile phone or tablet there are so many more variables to take into consideration than a static personal dosimeter attached to an employee’s shoulder. The
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The clear risk here is that the user may not be aware of whether the app is providing the appropriate metric or whether the processing of raw data to provide those metrics is correct. Both of which would prove manna from heaven for a personal injury lawyer looking for an easy win.
For any health & safety managers who have recently used a mobile or tablet-based app to take measurements the advice is to check that their equipment is compliant, accurate and trustworthy and consult with a noise at work specialist.
Cirrus Research is a UK developer and manufacturer of noise measurement equipment for occupational and environmental noise monitoring.
www.cirrusresearch.com #weknownoise. @cirrusresearch
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