Saturday, January 21, 2017

Silencers, Sound Levels, Decibels, and How to Know

A few days ago, I posted a little on the Hearing Protection Act.  If you've paid attention to the SHOT show coverage, you've seen people talking about not assuming it's going to pass this year but we're likely to get it in the not-too-distant future.

Coincidentally, a friend who doesn't follow my blog sent me a link to the National Institute of Occupational Safety and Health, NIOSH, telling about a free iPhone app they provide that will use your phone's microphone to measure noise levels in the standard dBA format.  Naturally I went to the iTunes store and grabbed a copy to play with.
(NIOSH photo)  The app is called NIOSH SLM (sound level meter) and is intended for checking sound levels in an occupational setting - this is NIOSH's domain, after all - so they intend for folks to survey their work area and then get more accurate readings if needed.

SLM runs only on the iPhone and they say there's no android version in development.  Android discrimination?  There's a very logical reason for them to go to iPhones, even if they are going to add android later.  The iPhone is one product with known standards for how the microphone works, the levels it responds to and all the nitty gritty details that are necessary to do this.  Android is actually an open source platform and there are several phones or tablets out at any time.  Different manufacturers use different hardware that may or may not respond the same way.  If Apple changes the hardware in the phone, it's going to comply with the same specifications as earlier phones, as long as they want to keep backwards compatibility.

NIOSH claims that with an external microphone and calibration it will meet the accuracy of higher end instruments, but as it comes with the iPhone it's accurate to "within +/- 2 dB "of a type 1 sound level meter", defined as "Precision" for use in the field or laboratory.  That's pretty amazing.  Like many (not all) audio measurements, the results are expressed as dBA, where A is frequency response curve, sometimes called "A weighting".  The A weighting is supposed to match the response of the human ear to audio frequencies and there are several weighting curves.  A site with a lot of handy information is Noise Help, who provides this handy chart
As a guide for when hearing protection is needed you can combine that table with one that the NIOSH website provides.  This is the permissible exposure to sound powers that are considered damaging to hearing.  As a rule of thumb, 85 dBA is around where most people feel the need to raise their voice in a conversation.  If you spend your day having to raise your voice to be heard over something that's running, you probably need to be wearing hearing protection. 
Time-Weighted Average (TWA)Time to reach 100% noise dose
85 dB(A) 8 hours
88 dB(A) 4 hours
91 dB(A) 2 hours
94 dB(A) 60 minutes
97 dB(A) 30 minutes
100 dB(A) 15 minutes
Notice that for a 3 dB increase or decrease in the dBA level the time halves or doubles respectively.  If you look at the right column, you'll see a table saw comes in at 105 dBA.  The chart stops at 100, so what's safe?  103 dBA would cut that to 7-1/2 minutes and 106 dBa would cut it to 3-3/4 minutes (3:45).  You shouldn't run a table saw for more than that in a day without hearing protection.

Impulse noise, like that from a gun, is different and seems to affect hearing differently, causing a loss of high frequency hearing faster than continuous noise does.   Since noise levels for everything past a .22 rifle (130 dBA) is in the 150 to 155 range, hearing protection is absolutely required - but you knew that.  If putting a suppressor on a gun drops 155 to below 130, by everything I can read, you should still wear hearing pro. I'm unable to find anything that says that suppressors do anything more for hearing than simply changing the dBA level of the shot; that is, I see nothing that shows that modifying the shape of the noise impulse helps.  You will read that some agency said that if the crack of the gunshot impulse noise is less than 140 dBA that hearing protection is not needed. 

How does the App work?  You can simply open it up, and there are help screens accessed by tapping on "?" on the page.  Sitting here typing, with the phone's microphone pointed at my keyboard, typing puts the sound levels into the mid 50s to over 60dBA, which is pretty quiet.  When I stop typing, the noise level drops to the around 40.  When the air conditioner starts, that goes up by 3 dB.  Proximity to the microphone is important, so they say the SLM is accurate when it's a meter away (call it one yard) from the source.  The display responds to things like hitting one key on the keyboard, but whether it will give accurate readings on gunshots is a question that has to wait.  


  1. Useful tool, and widespread use could be beneficial to accelerating passage of the Hearing Protection Act.

    IIRC, the suppressor industry has agreed on testing standards regarding microphones, specifically, type, distance and positioning, to uniformly measure dbA reductions from suppressors; some sort of recommendation for similar standards for all noises, not just gunshots, would be useful as part of the NIOSH SLM package. Without it, we'll descend into a useless swamp of "my keyboard/dishwasher/gizmo is quieter than yours" garbage data.

    1. That's an excellent point.

      There's nothing to prevent us from spinning up a user community of this little app and decide how we're going to standardize. For example, at the indoor and outdoor ranges I go to, I can't always put my SLM a meter from the gun. But there's nothing to say we can't all standardize on 18" or some other number. If we measure suppressor sound levels from less distance than the pros do, it takes a couple of measurements to verify the difference between measuring at their distance vs ours.

      The essence of repeatable measurements is repeatable setups.

      I'm also interested in seeing the noise levels I get in my shop from things that are quieter than a table saw, but that I stand closer to.

  2. I don't use an iPhone (antique Android instead) so I don't know what accessories are available; a remote microphone that's easily attachable to various surfaces would be a very useful addtion.

    Way back when I worked for (someone), periodically we'd get the dreaded "monitoring team" visit where we had to wear air sampling devices on the belt with collar-attached intakes one week, sound measurement boxes with similar but dual collar-mounted microphones the next, in addition to the usual devices.

    There was some variance tolerated in positioning, but clothing requirements being what they were it was less than a couple inches variance between nose/mouth and ear distances for everyone, well within "normal" human body dimension differences.

    Were electronic earpro manufacturers all reading from the same page it would be simple to monitor what the microphones on muffs pick up. Lacking that, an accessory dual microphone tool that easily attaches to the top outside face of all earpro muffs might be the ticket (I say "top" because too low and different shooting positions might muffle input with clothing). Not perfect, but close enough to minimal variance to be statistically useful, and very obviously directly related to the precise problem suppressors are meant to address.

  3. This is a great app that I will be sure to download and check out. I am constantly arguing with a local hall that rents out for parties and the sound crews have no concept of how loud the sound is that their equipment generates. When I go inside to complain, it is so loud, you cannot have a conversation. The attendees do not realize it but their hearing is being compromised. Thank you for this.

    1. When I was a teenager, I spent many hours in a garage band. Drummers are loud, amps are needed to be heard over the drummer, and everything echoes like crazy off the garage walls. I probably have some hearing damage, but pretty much all the musicians we listened to as kids have hearing damage now. As well as young musicians early in their careers. But it's not just rock and roll. Classical musicians get it, too.

      It's ironic that the people who enjoy listening to the most subtle dynamics in music get that ability taken away by taking part in playing it.

    2. Great idea for musicians, set up an iPad in the corner on a stand and keep an eye on the app, anything above 85 decibels is supposed to be harmful to hearing!

  4. I know the exact moment in time when I harmed my hearing. July 1966 I was qualifying with the M16 along with 20 or so other guys. 80 rounds later and my ears were ringing. They never stopped. My hearing is actually pretty good but not at that frequency of the ringing. My left ear is worse. Why? Because, suprisingly, it wasn't the muzzle noise that was loudest it was the action and other sounds related to it so my left ear was about four feet from my fellow shooters rifle and my left ear was in direct line with the opening ejection port and exiting spent gasses of his rifle. On a summer night if crickets are chirping I can hear them but if I lay my right ear against the pillow I hear nothing.

    I notice that a lot of TV drama shows play background music that is raised and lowered to match the drama. For me, and I assume a lot of others my age, this makes it very difficult to hear what is being said. I'm not sure the TV producers know this. I can hear the ads and the news perfectly just can't hear most of my favorite shows. When something significant happens my wife will laugh or respond in some way so I say "what did they say?". She explains it but I'm sure she thinks I'm going deaf.

    The other day some kind of high pitched alarm went of. My wife said aren't you going to fix that? Me: Huh! Her; That alarm can't you hear it? Me: what alarm? She got up and shut it off whatever it was I never heard it. Just the right frequency I guess.

  5. Thanks for posting SiGraybeard. This same NIOSH outfit did a lot of work on impulse noise from firearms and if you dig into one of their studies, even some of the best sound level meters out there are not capable of capturing a real impulse from a gunshot which can go up to 170-180 decibels, SPL. They ended up designing their own instrument.

    So I doubt this app can capture high level impulses, maybe up to 130-140 with an external microphone (if calibrated correctly) and that's a big if. It would be interesting to ask them on their blog what they think and give us some pointers on what it takes to establish uniform test standards.