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Author Topic: Most accurate db app?  (Read 20555 times)

Arthur Skudra

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Re: Most accurate db app?
« Reply #10 on: May 11, 2013, 03:06:03 pm »

Thanks Doug and Brian.  Does anyone think that an iPad app could be repeatable, within a given db range, calibrated to another know source?  Anyone done a series of side-by-side tests?
Love the irony here with some posts questioning the validity of using an iPhone for measuring dB, and yet in other posts a $50 Rat Shack meter is a suitable device (which doesn't even meet Class 2 requirements)??  :) Really, it boils down to what exactly you're measuring, and what error tolerance is acceptable.  Unless you're having to prove a case officially or legally, using inexpensive equipment such as a Rat Shack meter or an iPhone is perfectly fine, as long as it's calibrated correctly.  If you're going into a legal battle over SPL levels, then spend the coin on a full B&K setup complete with traceable calibration certificates that are up to date, that way a public official or lawyer will not have an argument against the validity of the measuring equipment.

I use AudioTools and Faber Acoustical dB meters with both my iPhone and iPad, calibrated using a custom adapter on my B&K 4231 calibrator.  It's very consistent and accurate when measured against a professional dB meter (in this case a B&K 2231), and calibrated correctly.  Every iPhone/iPad is different in the calibration level required to measure SPL correctly because of the manufacturing tolerances of the microphones used in these devices.  Also, make sure you have the latest iOS version, previous OS versions prevented app makers from bypassing built in microphone's high pass filter that limited the built-in microphone from measuring below 200Hz (or higher than 105 dB SPL).  This is no longer a limitation with the latest iOS.

However, there are two challenges in getting a correct calibration with an iPhone/iPad device:
1.  With the mic built into the case of the phone/iPad, it's challenging to get an airtight seal around the microphone that is necessary to get a proper calibration.  Depending on how tight this seal and position of the calibrator, you may see a variance of 1-2 dB based on my experience.
2.  Most calibrators are fixed in output based on the volume of space left in the measurement chamber with the mic/calibrator adapter in place.  Change the volume of this empty space, and your calibrated level will change by about 0.5 dB.  To get even more nit pickier, you also need to compensate dB calibrated level for barometric pressure as well, but we won't go there.  The exception here is using a calibrator like the B&K 4231, which has a tiny compensation microphone built into the chamber to ensure that the calibrator is outputting exactly 94 or 114 dB no matter what happens to the empty volume inside the chamber with the mic in place, nor the barometric pressure of the environment being measured.  This is why the B&K 4231 is so expensive!

So the take-home here is to first ask yourself whether a dB or two is an acceptable tolerance, and whether you have a reasonable means of calibrating your measuring device.  Some people use external calibrators, some simply get a known calibrated measuring device, feed a 1K tone through a speaker in a very quiet room, measure the level with the known calibrated device, then substitute it with the iPhone/iPad, and set the compensation level to get both devices reading the same dB.  This should get you close enough in the ballpark for most SPL measurement applications in live sound reinforcement.  If you need a higher level of accuracy or confidence with an iPhone/iPad, then consider getting the iAudioInterface2 or the iTestMic accessories from Studio Six Digital, and use an external calibrator.
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Jay Barracato

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Re: Most accurate db app?
« Reply #11 on: May 11, 2013, 03:35:37 pm »

Arthur,

My gut instinct is that the straight level calibration is probably solid because it requires sealing the mic. This is a reflection of accuracy.

However, while I use my iPad on occasion just as you mentioned, I have noticed some problems with repeatability (precision) which I attribute to differences in axial response. You can see a change that is greater than measurement noise with fairly slight rotation of the way the iPad is oriented.

But like you said, for quick and dirty relative comparisons, it seems adequate for its price.
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Jay Barracato

Arthur Skudra

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Re: Most accurate db app?
« Reply #12 on: May 11, 2013, 03:57:25 pm »

Arthur,

My gut instinct is that the straight level calibration is probably solid because it requires sealing the mic. This is a reflection of accuracy.

However, while I use my iPad on occasion just as you mentioned, I have noticed some problems with repeatability (precision) which I attribute to differences in axial response. You can see a change that is greater than measurement noise with fairly slight rotation of the way the iPad is oriented.

But like you said, for quick and dirty relative comparisons, it seems adequate for its price.
It really depends on where the mic is actually located (could be the bottom/top/front face depending on the iDevice), and whether you have a case that is in the way!  I do have to take the case off with mine!  I know that calibrating the mic on the iPhone 4S is a bit challenging to do, often requiring "3" hands to keep things steady and oriented correctly, while adjusting the calibration setting.  Even changing the orientation of the calibrator 15-20 degrees causes some change in level, it could be the seal around the calibrator adapter, it could be some "directionality" in the way the mic is mounted in the case.
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Alfredo Prada

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Re: Most accurate db app?
« Reply #13 on: May 11, 2013, 05:02:20 pm »

I have a decade old Radio Shack analog meter.  Anyone have an idea as to how accurate they are?

http://i1361.photobucket.com/albums/r663/AlfredoPrada/IMG_0084_zps3f83a4a4.jpg
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Dave Mason

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Re: Most accurate db app?
« Reply #14 on: April 19, 2015, 04:39:44 pm »

One advantage to the iPhone SLM apps is that Apple started using MEMS microphones in the iPhone 4.  These are chips with the actual mic parts etched in silicon.  Their characteristics are very stable from sample to sample, batch to batch, and over time, temperature and other factors that cause conventional electret mics to drift out of calibration.  Supposedly the better iPhone apps (SPLnFFT for example) are pretty accurate as installed, without further calibration.
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Lyle Williams

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Re: Most accurate db app?
« Reply #15 on: April 20, 2015, 01:38:13 am »

One advantage to the iPhone SLM apps is that Apple started using MEMS microphones in the iPhone 4.  These are chips with the actual mic parts etched in silicon.  Their characteristics are very stable from sample to sample, batch to batch, and over time, temperature and other factors that cause conventional electret mics to drift out of calibration.  Supposedly the better iPhone apps (SPLnFFT for example) are pretty accurate as installed, without further calibration.

The problem is that the app developers normally lack a wide enough sample of different hardware to determine correct levels.  What is right for an original iPad will be wrong for an iPhone 5, etc.
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Chris Johnson [UK]

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Re: Most accurate db app?
« Reply #16 on: April 23, 2015, 11:58:18 am »

I've been using the Faber Acoustical app with a micW iphone measurement mic with great success. The mic ships with a mV/Pa calibration value, and it can be easily calibrated with a pistonphone.

The result is a solid Class II (i think?) metering solution
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Lyle Williams

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Re: Most accurate db app?
« Reply #17 on: May 02, 2015, 04:49:43 am »

dBMeter Pro on my ipad thinks my ETX rig can only do 30dBa @ 1m.  I was wearing earplugs to get the measurement.  It seemed louder than that.
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Timo Beckman

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ProSoundWeb Community

Re: Most accurate db app?
« Reply #18 on: May 25, 2015, 05:51:46 am »


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