Image of the main screen of the NIOSH SLM app (shown with a MicW i436 external microphone) Imagine if workers around the world could collect and share workplace (or task-based) noise exposure data using their smartphones. Scientists and occupational safety and health professionals could rely on such shared data to build job exposure databases and promote better hearing health and prevention efforts. In addition, the ability to acquire and display real-time noise exposure data could raises workers’ awareness about their work environment and help them make informed decisions about potential hazards to their hearing.
The idea was so intriguing that in 2014, the NIOSH hearing loss team evaluated 192 sound measurement applications (apps) for the iOS and Android platforms to examine their suitability and accuracy in relation to professional sound measurement instruments (Kardous and Shaw, 2014). Of the 192 apps the team examined, 10 iOS apps met the outlined criteria for functionality, features, and calibration capability, and of those, 4 iOS apps met our testing criteria. Read more about that study in the blog Realizing that most of the apps on the market are oriented at the casual user and lack the accuracy and functionality necessary to conduct occupational noise measurements, NIOSH hearing loss researchers collaborated with an app developer, EA LAB, to create an iOS based sound level meter app that measures and characterizes occupational noise exposure similar to professional instruments.
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The NIOSH Sound Level Meter (NIOSH SLM) app for iOS devices is now available on freely to the occupational safety and health community as well as the general public. The app was subjected to the same testing requirements that were established in the NIOSH laboratory study. It met the testing criteria (± 2 dB mean difference from the reference type 1 sound level meter). In our most recent study on the accuracy of apps when used with external calibrated microphones, the 4 apps from our original study achieved closer agreement (within ± 1 dB) of the reference type 1 sound level meter (Kardous and Shaw 2016).
The NIOSH SLM app, when used with an external calibrated microphone, measured sound levels within ± 1 dB of the reference SLM over the testing range of 65 -95 dB SPL in our laboratory. While the app is not meant to replace a professional sound level meter or a noise dosimeter or be used for compliance purposes, we recommend that those interested in making proper noise measurements use an external microphone that can be calibrated with an acoustical calibrator for improved accuracy (Roberts et al. The NIOSH SLM app has many important features, it provides a readout of the sound level using the built-in microphone (or external microphone if used) and reports the instantaneous sound level in A, C, or Z-weighted decibels. View the for a demonstration of the app’s features. The weighting is user-selectable and can be accessed in the “Settings” screen. The app also reports the main metrics that are of importance for proper occupational noise measurements – mainly the run time (total time), the A-weighted Equivalent Sound Level (LAeq), the Maximum Level measured during the current run time, the C-weighted Peak Sound Pressure Level (LCpeak), the Time-Weighted Average (TWA) and Dose.
The app also contains some basic information on noise and hearing loss prevention. In addition, the app allows the user to save and share measurement data using the smartphone other communication and media features. If location services are enabled, the app can utilize the GPS feature to provide an exact geospatial location of the location of the noise measurement. A full list of the features and functionality can be accessed on the.
Your input on the new app is appreciated as we try to improve it and make it widely accessible. Help us spread the word about this new tool for protecting workers’ hearing.
June 2018 Update: EA LAB and NIOSH researchers have evaluated the app’s performance as part of a system (iPhone + external microphone) for compliance with type 2 requirements of IEC 61672/ANSI S1.4 standard: Sound Level Meters – Part 3: Periodic Tests. The results were published in the Applied Acoustics Journal Celestina et al.
Celestina, M., Hrovat, J., & Kardous, C. Smartphone-based sound level measurement apps: Evaluation of compliance with international sound level meter standards. Applied Acoustics, 139, 119-128. CAPT Chucri (Chuck) A.
Kardous, MS, PE, is a senior research engineer in the NIOSH Division of Applied Research and Technology. Metod Celestina, B.Sc. EE, CEO at EA LAB References: Kardous, C. A., & Shaw, P. The Journal of the Acoustical Society of America, 135, EL186 (2014) Roberts, B., Kardous, C., & Neitzel, R.
Journal of occupational and environmental hygiene. DOI 10.104.20 Kardous, C. A., & Shaw, P. The Journal of the Acoustical Society of America, 140 (4), EL327 (2016) Please note that professional sound level meters must comply with a host of acoustical and electrical tests to meet national and international standards.
As of today, no smartphone or smartphone-based app has met the requirement of such standards. Although we tested and verified the accuracy and functionality of this app at the NIOSH Acoustics Laboratory (over a specific testing range), this app does not comply with any national standard.
We are currently conducting a study to evaluate the app’s performance in various workplace settings. In addition, the app was not designed to calculate noise exposure metrics based on environmental or non-occupational noise limits. Posted on January 17, 2017 by CAPT Chucri (Chuck) A. Kardous, MS, PE, and Metod Celestina, B.Sc. You might expect a manufacturer of noise measuring instrumentation to launch into a negative, stereotypical tirade but at Casella we understand the ubiquitous nature of the phone app and we fully support anything that generates more awareness of noise in the workplace.
Two comments however. Firstly I would recommend the use of an acoustic calibrator such as our CEL-120/2 with a 1/4″ microphone adaptor. Secondly had you considered resurrecting the Type 3 Indicator grade of instrument classification that existed in ASNSI S1.4-1971 (and BS EN 60651) back in the day? Since current US & EU noise legislation would not recognise the results of a type 3 instrument, it would both legitimise and caveat their use. Thank you for your comment. We agree regarding the use of acoustical calibrators, at the expense of repeating ourselves, we wrote above “we recommend that those interested in making proper noise measurements use an external microphone that can be calibrated with an acoustical calibrator for improved accuracy.” The issue regarding resurrecting ANSI S1.4-1971 would be up to ANSI. We remind readers that references to products and services do not constitute an endorsement by NIOSH or the U.S.
Thank you for the comment, it’s not as simple as releasing an app on Android and then instructing people to use it with an external mics – some people may not go through the NIOSH links or see our instructions. To release this app, we had to test it on all available iOS devices and ensure that they all perform according to our criteria, with the built-in microphone and with external microphones. As for your other question, If you are referring to the AudioTools app, we did include that app in our studies. If you are referring to AudioTool, that app did not meet our selection criteria for occupational type of noise measurements.
Thank you kindly Alberto, and great to hear from you! We provide a brief explanation in the FAQ section here: on why the app is only available on iOS devices and not for Androids. Basically, there are many Android manufacturers and each manufacturer has their own set of specs and different parts and chips that they use, not to mention the many different Android OS’s out there.
In addition, some Android makers use their own audio processing, some use third party, which can introduce latency in some devices. On the other hand, Apple devices have a common software architecture called Core Audio that makes dealing with audio signals uniform across all its devices. In our studies (see the blog references above), we would find the same app provide different readings on different devices and this could also explain why the sound measurement apps market on Androids is much less developed as far as features and performance than you can find on iOS. To release the app on Android devices, we would basically have to test and verify the performance of the app on every Android device out there, all with different combination of Android OS’s as well, an impossible task at this time. We are exploring several options at the moment since we recognize the importance of the Android market. If you, or any of our blog readers, have any suggestions or ideas for dealing with this issue, we definitely welcome them.
First I want to thank Captain Kardous and his colleagues for the studies and hard work that lead to the development of this app. @Alberto Behar As explained by the Captain, recording apps for Apple devices are designed around a very well build low-level API (Core Audio). Apple gives the app developers access to this low-level API through a framework (Audio Toolbox or Audio Unit). This architecture makes it easier to deal with raw PCM data (audio signals) recordings uniformly across all the devices as Core Audio isn’t affected by the hardware. Microphones and other electronic components used in the iDevice is what can change.
However, Apple is doing a good job at choosing good quality parts and variations between iDevices (for the kind of measurement we are looking for (peak values of PCM data)) can be corrected with proper calibration. Apple has much less SKUs than the entire Android market, making it easier to find the variations in the lab and correct them. Also, Android devices are subject to more latency in the audio signal processing. The main reason for that is that the low-level API (OpenSL ES) for audio recording is not very well integrated across all the operating system versions. Also, OpenSL ES is missing some features that requires some hard work in C and C language to code the desired calculations.
Developers are using public JAVA APIs instead (AudioRecord or MediaRecorder) that has huge audio latency and might not be suitable for real-time measurements. As for the hardware part of the issue, there are more than ten popular Android phone brands on the market, all using various components. All things considered, it is indeed an impossible task to test and verify the performance of the app on every Android device on the market to find a proper calibration. I have asperger and workshop in an Office environment. I would like to use this tool to measure the total sound dosis received every day. For concentrated work the advised level is about 45db or less.
That is a much lower than the level for hearing damage. Is it possible to add these lower levels to the app so we can use it also for office sound level dose? Is there a way that I calculate the dose myself for this lower level based on 80 db? I’m also thinking about sharing the data between colleagues. Would be handy if the data could be exchanged in binary format with some kind of service for reporting on a larger scale. Think about the best office to work for contest.
Thanks for your comment, Tauvic. The app was designed to measure noise exposure in the workplace and is aimed at industrial hygienists or occupational safety and health specialists. The 45 dB is a non-occupational limit for office environments and indoor spaces and thus it is beyond the scope of our mandate at NIOSH. There are instruments out there that allow you to set the threshold at 40 dB (or no threshold at all), there is no way to calculate the dose otherwise, unless you find a way to record the actual sound waveform in your office and do the calculations based on that recording of the raw signal. Thank you for the suggestion for using binary data, this is something that we might consider in future updates. I haven’t downloaded and tried the app yet, but it seems intended mainly for handheld use with a user in constant attendance.
Some kinds of noise exposure happen infrequently or unpredictably. For example, neighborhoods near me have started experiencing increased noise from overflights connected with a large airport within five miles, as air traffic routings and protocols are modified to exploit the potential of GPS. Noisy overflights happen unpredictably, so it would be useful to have the measuring device in standby mode, tracking noise levels, and springing into more intensive action when a threshold is exceeded. Beyond measuring the levels for a single weighting, it would be more useful if enough information could be retained so the levels could be calculated and displayed under multiple weighting schemes and different metrics, e.g.
Instantaneous maximum, L10, L90 and so forth. It would also be good to save recordings of noise incidents, perhaps including some seconds before and after the actual threshold-exceeding period. The overflight example is only one scenario.
Others that have come up in my neighborhood have been late-night street racing, construction noise, loud custom mufflers and freeway noise. A smartphone-based application for extended monitoring would be extremely helpful in all these instances and surely many others. Hello Dave and thank you for the comment, you bring up some interesting points. It’s true, the app is intended for the user to be in constant attendance. Our aim is to help the occupational safety and health specialist make an easy and quick noise measurement in the workplace – the app measures and calculates the relevant occupational noise exposure metrics and reports noise levels and averages in A, C, or Z weighed decibels. The user can also save measurements, generate reports, or upload/share those measurements.
As for your suggestion to have the app running in the background constantly and then records events when certain thresholds are exceeded, while possible, it would most likely require a dedicated smartphone – no phone calls or running any apps that need access to the microphone, very careful handling (can’t be taken in and out of pockets and such), and most importantly, different features since our app is intended to measure occupational noise instead of environmental noise. I believe there are some studies that have used smartphones as dedicated sound measurement tools. Some researchers have explored the use of older, refurbished smartphones with a dedicated app, to build a cheap noise monitoring network, mostly in urban settings. I believe limitations regarding battery life, proper calibration, and reliability continue to hinder their wide adoption for more than conducting research studies though. We have tested the NIOSH SLM app in our lab to verify its operation over 65+ dB range, see videos in links below when we compared it to a type 1 professional grade sound level meter. The SoundLog is an excellent app from the National Acoustic Laboratories, an Australian government research institution that conducts research on noise and hearing loss, similar to the NIOSH hearing loss prevention program.
SoundLog displays the LAeq (A-weighted, equivalent sound level), LCpk (C-weighted peak sound level) and LAeq.8h (A-weighted, equivalent sound level averaged over 8 hours). Our app displays the instantaneous sound pressure level as a default (in A, C, or Z weighting), but you can select to display the LAeq, Max Level, LCpeak, TWA, and Dose. So when you do comparisons, I suggest comparing the LAeq on both apps, I just ran a quick test (pink noise through speakers) and the LAeq on both apps are with 1 dBA of each other’s and with the Larson-Davis model 831 Type 1 sound level meter. We have tried to see if we can get different readings on our iPads and iPhones in our lab to measure various noises, but have not seen similar results. Only at very low ambient background levels, there’s some discrepancy between the iPad and the iPhone readout but that can be explained by the different sensitivity of the microphones. Are you running the same OS on both devices?
Also, the iPads have a back-facing noise-cancelling microphone, in addition to the front-facing one that may be causing some issues for some users. Try to go into Settings, General, Accessibility and see if the Noise Cancellation is enabled or disabled on both devices? Hi Martin, thanks for bringing that specific microphone to our attention. Not having tested that specific mic, I can’t offer any empirical judgment. However, I would tend to think it will work as well as the mics we tested, it has excellent specifications and the manufacturer claims it is IEC 61672 class 2 compliant.
The main point with any of these external microphones, is the ability to calibrate them using commercial acoustical calibrators and available adapters (a 1/4” adapter would work perfectly with this mic). It doesn’t look like it offers a lightning port adapter so will not work with iPhone 7 and later. Great question, Frank. Yes, it is true that newer Apple products do come with a lightning port to 3.5 mm headphone jack adapter. The issue with that arrangement is that it will leave the microphone ‘dangling’ from the adapter. To make a good noise measurement, the orientation of the microphone is important and a dangling cable (and microphone) may present some issues to how the user handles and orients the microphone. Some microphone manufacturer make microphones that can plug directly into the lightning port (e.g., MicW’s i437L and Studio Six Digital’s iTestMic2).
The microphone mentioned above by Martin, does not seem to offer a lightning port adapter that can be directly plugged into newer Apple devices. Thank you for the kind endorsement of the app, Steve. We have received similar comments in the last several months from users having a hard time finding the app, and now with the introduction of iTunes 12.7, Apple has removed access to the App Store from its desktop versions, so that makes it even harder for users to find our app without using specific search terms. We are still learning how App Store users search and find apps so this suggestion and feedback is very valuable to us and we will consider it in the new update we’re currently working on!
Dear Captain Kardous, Could you please speak to changes in the newest update, specifically regarding the “dose?” While in the immediately previous version, “dose” was a running tally, and “projected dose” an extrapolation of that measure over 8 hours, the basic “dose”measure now appears to be the latter. For example, in the older version, if I whistled directly in to the mic, dose would slowly rise 1, 2, 3, etc.%. Now doing so spikes the dose well over 100%. This value drops the longer the recording continues, as quieter readings are averaged in. The documentation does not seem to address this change (although in the help page of the new version there is only one landing page and no further info?).
All best, and thanks to your team for their hard work on this app! Dr Ian Howell New England Conservatory Boston. Hello Roberto, the MEMS microphones used in smartphones (or if you use an external microphone) can measure noise levels as low as 20 Hz according to their manufacturers. For low frequency measurements, you will need to choose Z-weighting from the settings instead of the default A-weighting. However, for such types of measurements, it is best to use a professional sound level meter, with an appropriate microphone, that can provide 1/3 octave band measurement features, so you can see/display the exact contribution from such sources.
Hi Roberto, there shouldn’t be an issue with the software of the app, all it’s doing is taking the electrical signal from the microphone, digitizing it, and applying some calculations to convert those signals into decibels. The microphone’s suitability for low-frequency measurements is the most critical component here. For a preliminary assessment, the app should be fine. You should plan to make two sets of measurements, one with the default A-weighting and one with the Z-weighting selected. You can then compare the two measurements to understand the low-frequency noise contribution from the transformers.
The Z-weighted noise levels will higher, depending on how much of the noise is low-frequency. There are apps that offer octave and 1/3-octave band analysis, see the apps from our study on smartphone apps SoundMeter, SPLnFFT, NoiSee, and SPL Pro all offer some sort of frequency or octave-band analysis, at some nominal cost. A measurement with a professional SLM is suggested if you are trying to conduct a full assessment and isolate the noise emitted from the transformers, especially if you plan to implement some engineering controls to mitigate any harmful exposures to the workers. Hi John, it’s not likely that exposure to 72 dB is going to cause hearing damage.
Our recommended exposure limit is 85 dB (A-weighted), and that’s over 8-hours a day, 40 hours week, and years of exposure. We do recommend hearing protection for sound levels above 85 dBA. Questions I would ask, how far was your iPhone from the amplifier speaker, was it pointed (bottom side where the microphone is) at the speaker, I would also try to take an average, you can do that by selecting LAeq and let it run for 10-15 seconds. It could be that the measurement is too low. Having said all that, muffling in the ear is a symptom that often needs medical attention, especially if it happens repeatedly after you play your guitar.
We do recommend that you follow up with a health professional, your physician or an audiologist. Hi, we can’t comment or endorse any specific product, we use Type 1 and Type 2 acoustical calibrators from the main sound instrument manufacturers such as Larson-Davis, 3M Quest, and B&K among others. Those types of professional instrument calibrators are priced in the several hundred to over a thousand price range. There are cheaper options on the market but we cannot vouch for their accuracy or reliability, though they should be adequate for general purpose, Type 2 level of accuracy, noise measurements. I think it will be worth checking the manufacturer specifications for accuracy, some provide in the form of ± x dB, it is also worth checking whether the calibrator meets IEC 60942 standard or equivalent before making a purchasing decision.
One important factor in the calibration process is to ensure that you obtain a calibrator with the right adaptor. Most external microphones, especially the ones we tested, have a ¼” diameter so it is critical that the external microphone has good fit to ensure proper calibration. See image at 4:03 – 4:21 on this video. Hello Chris, regulatory agencies specify that a sound measurement instrument must conform to specific national (or international) standards. The sound level meter standard has 3 parts.
NIOSH is making concrete steps towards establishing conformance of the app with part 3 of the standard, periodic testing, and developing a framework for others to follow. See our recent publication where NIOSH established compliance with IEC 61672 (ANSI S1.4): Part 3 – Periodic Testing, “Smartphone-based sound level measurement apps: Evaluation of compliance with international sound level meter standards.” Applied Acoustics, 139, 119-128. NIOSH has also conducted several field-based studies that show excellent agreement with professional sound measurement instruments, when used with external, calibrated microphones. However, a sound measurement system must comply with a host of acoustical, electrical, and environmental requirements to fully meet standards. This is typically accomplished by sound instrument manufacturers.
Although we did develop the app software, NIOSH does not manufacturer the hardware that it takes to make the system, as a whole, compliant with the standard. Thank you for your interest in the app and your support. Hi John, great question. We have tested some apps that support downloaded calibration files and found that such files did not improve the accuracy of the calibration when compared with using acoustical calibrators.
It appears that such calibration files are based on some “nominal” correction values and are not specific to the external microphone’s sensitivity and frequency response – which would be required to establish proper calibration. The NIOSH SLM app will automatically select a “nominal” correction value when a user plugs in an external microphone, and that value is based on our testing and evaluation of external microphones’ sensitivities. We recognize that this is not an optimal solution for establishing proper calibration and that is why we continue to recommend calibrating external microphone with acoustical calibrators similar to how an industrial hygienist or an occupational safety and health specialist (our main target audience) would normally calibrate their instruments prior to use. We believe this is the best practice until a calibration standard for smartphone apps is established.
Hi John, yes we use a nominal correction for internal microphones. And yes, we provide a correction for external microphones based on our testing of 10+ external microphones. The use of an external microphone does not add to the accuracy per se, we recommend it because you can calibrate it with an acoustical calibrator. Out of the box, it is not necessarily better than the internal microphone on a new and well-maintained iPhone that is not exposed to the elements. However, on an older iPhone where the internal microphone has been exposed to various environmental elements (humidity, temperature variations, dirt accumulation at opening, etc.), the external microphone will be a better option.
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