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New NIOSH Sound Level Meter App

Posted on by CAPT Chucri (Chuck) A. Kardous, MS, PE, and Metod Celestina, B.Sc. EE
SLM
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 So How Accurate Are These Smartphone Sound Measurement Apps?

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.

The NIOSH Sound Level Meter (NIOSH SLM) app for iOS devices is now available on iTunes 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.[1]  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. 2016).

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 video for a demonstration of the app’s features. YouTube Preview Image 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 NIOSH SLM app page.

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.

 

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. B. (2014). Evaluation of smartphone sound measurement applicationsThe Journal of the Acoustical Society of America135, EL186 (2014)

Roberts, B., Kardous, C., & Neitzel, R. (2016). Improving the Accuracy of Smart Devices to Measure Noise ExposureJournal of occupational and environmental hygiene. DOI 10.1080/15459624.2016.1183014

Kardous, C. A., & Shaw, P. B. (2016). Evaluation of smartphone sound measurement applications (apps) using external microphones – A follow-up study. The Journal of the Acoustical Society of America140 (4), EL327 (2016)

 

 

[1] 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 by CAPT Chucri (Chuck) A. Kardous, MS, PE, and Metod Celestina, B.Sc. EE

74 comments on “New NIOSH Sound Level Meter App”

Comments listed below are posted by individuals not associated with CDC, unless otherwise stated. These comments do not represent the official views of CDC, and CDC does not guarantee that any information posted by individuals on this site is correct, and disclaims any liability for any loss or damage resulting from reliance on any such information. Read more about our comment policy ».

    Thank you for the question, Asmaa. We discuss this issue on the frequently asked question section of our page https://www.cdc.gov/niosh/topics/noise/app.html. Basically, due to the fact that the Android marketplace is fragmented among many manufacturers with each having different hardware and software tools, it made it nearly impossible to verify the performance and accuracy of such an app on every model on the market today. If you’d like more information, see one of our first publications listed in the references section.

    Wonderful information. I already download this app in my phone. thanks you for the sharing more information. CAPT Chucri

    This article states “Of the 192 apps the team examined, only 4 iOS apps met the outlined criteria for functionality, features, and calibration capability”. Unless I am misreading your cited source, the number of iOS apps that met the criteria should be 10, with 4 being the number of the Android apps partially meeting the criteria. Please correct me if I’m wrong though.

    Thank you for the comment, Andrew. You are correct, we looked at 192 iOS and Android apps, we selected 10 iOS apps, and of those 10, only 4 iOS apps met our testing criteria. We modified the second paragraph above to improve clarity.

    Please see our study on external microphones http://asa.scitation.org/doi/full/10.1121/1.4964639, we examined the MicW i436 and the Dayton Audio iMM-6. Both microphones performed well in our study, though each has its pros and cons which we detail in the study. We have also evaluated the Etudige EIM-003 for our study, though that microphone does not fit the typical ¼” adapter for acoustical calibrators. Please note that references to products and services do not constitute an endorsement by NIOSH or the U.S. government.

    Love the app and I am using it on an IPAD. However, I noticed when I reset the calibration or changed it to OSHA, it said “measure must be stopped before changes…” After I make the adjustment, the app stops taking measurements. How do you restart it? I close the app and go back and still not measurements are recorded.

    The only way to get it to work is when I uninstall and re-install the app.

    app loaded fine and worked well. then stopped working. deleted and reloaded the app several times. on the main reading screen (Instantaneous level (dBA)) I have red text that says “overload.” Now what?

    A new update of the app will be released tomorrow and will address this issue. Sometimes there is a conflict when another app is trying to use the microphone. In the meantime, we suggest closing the app completely (press home key twice then swipe the app screen upward to close it) and then try again.

    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. government.

    With regards to android, couldn’t you just release it and ask folks to either use a calibrated external microphone or to calibrate their device’s mic?

    Also, have you seen audiotool? It was around when you guys did the 1st study, on both Android and iOS, and looks like one of the more serious, full featured apps available. Puzzled why it wasn’t included.

    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.

    I downloaded the app and let it run for a few minutes in the office. Worked fine. The next time I tried the app It did not seem to work and shows in red letters “overload”. What is going on and how can I restore operability?

    New update to address this issue should be available today, in the meantime, try closing it completely by pressing the home button twice and swiping the app screen up.

    Great app! I have downloaded it to my iPad too. No problems whatsoever. I love the way it goes immediately to the SLM mode . Is there a way to attach a picture to the email?

    Thank you for the input, David. You can share the noise report, in pdf format, via email. If you like to attach a picture of the screen, then I would take a screenshot (using the home and ON button) and then in the email screen that has the pdf report, I would add the picture as an attachment. It’s a great suggestion, we may consider making this process simpler in upcoming updates!

    Excellent app, I tested it in the worked areas with different nivels of noise, using an IPhone 7 plus, and comparing the results with a professional equipment; the results was very close (+/- 1dB compared with the professional equipment). I found a little bit more difference in closed areas than in open areas.
    Nice app!!

    Great idea and achievement. However, the fact that it is only for iPhones limits its use. Also, it sounds like recommending the iPhone. I know that this is not your intention at all, but still. Please review your decision to exclude the Androids!

    Thank you kindly Alberto, and great to hear from you! We provide a brief explanation in the FAQ section here: https://www.cdc.gov/niosh/topics/noise/app.html 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.

    How can I get permission to have the article posted in Canadian Audiologist, where I have a column (“Noisy News”)?

    Thanks

    All of our products are in the public domain and available for use free of charge. Please feel free to reprint the blog post. We just ask that you provide the correct attribution.

    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 am getting an error message that says “Overload” when I open th application. How do I remove or clean up the “Overload?”

    Please see the January 24th response to Marc. An update is now available for the app which addresses this issue.

    I found a bug at the report.
    After configuring the threshold limiter to 80dB.
    At TRESHOLD information is written 40dB instead 80dB

    If you download the latest update (3/2/17) you’ll see that the issue has been corrected. Thank you.

    I have noticed that if I use my Iphone 6 without using an external microphone, I get noise levels which are much lower than the actual sound levels.

    If I use my ipad ai without using an external microphone, I get much better results and higher noise levels.

    Anyone else, notice this?

    I have noticed this as well. I also get higher readings with my iPad to a degree of about 10 decibels. I don’t use external mice but have ordered one to see if there is a difference as well. Which one should I use?
    Thanks.

    Hi all,
    I’ve reinstalled the app as I also had problems with the message ‘overload’. However, the message is still there! What should I do to make this work?

    Some users have reported this ‘overload’ problem and it’s due to the interface not refreshing. We’re aware of this bug and we’re working on a fix, expect a new update soon. In the meantime, please try to turn off your iPhone and let it reboot and it should have resolved it.

    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.

    I just tried to download the App. However it is not available from the iTunes nor from the App Store. Any reason?

    It is available, you will need to search under NIOSH or NIOSH SLM and it should be the first result. Or you can click on the direct link above.

    Thank you Capt Chuck and Metod for this great app! Is there a way to display the frequency spectrum of measured noise in this app? If not, can it be added in future versions?

    Thank you for the kind comment, Dr. Deshpande. There is no current plan to display the frequency spectrum though we may consider it if there’s enough interest. There are some apps out there that offer frequency spectrum, check our earlier study on smartphone apps http://asa.scitation.org/doi/full/10.1121/1.4865269, several of the apps we tested offer various displays. For instance, NoiSee from EA LAB, our partner, offers octave band display. SPLnFFT (Paid app) offers Octave and FFT displays.

    Some users have reported this ‘overload’ problem and it’s due to the interface not refreshing. We’re aware of this bug and we’re working on a fix, expect a new update soon. In the meantime, please try to turn off your iPhone and let it reboot and it should have resolved it.

    Endless possibilities makes me even more curious. Wonderful insights here at you blog.
    Very informative post. Nice collaboration, keep it up.

    Good to see the NIOSH app. I compared it using the SoundLog app on an iPhone 5S and the standard internal microphone. The NIOSH app seemed to be consistently low by about 8dB compared to an inexpensive sound level meter, a UNI-T UT353.The SoundLog results were within +/- 1 dB of the UNI-T.
    Sound levels were provided by a 1000Hz tone produced by a loudspeaker.

    All uncalibrated – so the could explain a lot 🙂

    You might want to test the SoundLog app. It has some nice features/

    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.

    https://twitter.com/NIOSHNoise/status/822186936093868035

    https://twitter.com/NIOSHNoise/status/822174014063972357

    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.

    Thanks. I will have another go at my comparisons. I understand that you have done lots of tests for the NIOSH SLM showing it is a robust app 🙂

    GUI of this /sound level meter app is fabulous, I found its user interface something like M Indicator app even you can compare it

    When I use my iphone with no external mike I get a lower reading then when I use in the same way my iPad Pro (around in average 10 decibels)How can this be explained? My iPhone is a SE.

    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?

    This product was developed under a Memorandum of Understanding with an outside partner so the code is not in the public domain.

    Thanks for sharing this post. Android’s source code is released by Google under an open source license, although most Android devices ultimately ship with a combination of free and open source and proprietary software, including proprietary software required for accessing Google services. Android is popular with technology companies that require a ready-made, low-cost and customization operating system for high-tech devices.

    What type (make/model) of external microphone and calibrator should be used to achieve the + or- 1 dB error rate?

    Hello Emily and thanks for your interest. In our study, we evaluated two microphones, the MicW i436 and the Dayton Audio iMM6. Both microphones performed well over the 65-95 dB (pink noise) testing range in our lab, when calibrated. We used the Larson-Davis CAL200 acoustical calibrator in our study.

    No, app is only available on iOS devices. Please see one of our comments above regarding the reason the app is not yet available for Androids.

    Great app. That said, I tried using it to measure rifle gunfire with and without a suppressor and the results were too close. Is there suggested settings for short peak impulse sounds? Thanks!

    Thank you, Max. Measuring peak impulse sound levels, especially from gunfire, require some dedicated and specialized equipment. We’ve done some studies in the past where all of our noise dosimeters clipped above 143 dB SPL. Gunfire can measure anywhere from 160-170 dB, and the highest level we were able to capture with the app is 139.2 dB SPL. In other words, this app – or any smartphone app for that matter – cannot capture peak sound levels accurately.

    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.

    In your response to Martin you state “It doesn’t look like it offers a lightning port adapter so will not work with iPhone 7 and later.” Is that true? iPhone 7, 8 and X all come with an adapter for “lightening port-to-audio jack.” Wouldn’t that solve the problem and make it work for the later iPhone models?

    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.

    See photos 1 and 2

    Love the app, but I found it only after doing an online search for sound meter apps. Your app would likely find a broader audience if the AppStore listing made it more findable. For example, consider calling it “NIOSH SLM Sound Level Meter.” Cheers!

    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!

    Will the app recognize microphones plugged in directly to the Lighning port like the MicW i437L mentioned above? We had tested a consumer Sennheiser microphone with a lightning connector on an iPhone8 about 4 months back, and it wasn’t recognized at all.

    Thanks.

    Yes, the app will recognize an external microphone automatically, we have tested it with the MicW i437L and SSD iTestMic2. Please let us know if you still encounter any issues, and the model of the Sennheiser microphone you use.

    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

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