Can Rockbox make the iriver ihp-140 to work as a SPL meter?

[Edil]

I was wondering if it could be possible to create a Sound pressure level meter plugin with Rockbox. I think that the internal microphone of the iHP-140 is not good enough to do that but possibly with an external microphone. That way I can use my DAP as an SPL with my home theater.

In other words use Rockbox to make the to behave like a

Probably I'm dreaming to much!  

[Datman]

It should be very simple to do that, but precision is the problem... It depends on the frequency response and the sensitivity of the intenal microphone. You think to use an external microphone, but if you want to buy a good measure microphone you can certainly buy a good spl meter.
Also consider that only a few DAPs have a microphone input. I have a H120, but my Sansa Fuze has only internal mic.

A cheap microphone like the Behringer ECM8000 is not good: years ago I bought two of them as cheap stereo pair (I have Shure and Sennheiser Microphones, but it was an experiment). Frequency response was very different at low and high frequencies; also sensitivity was different...

If you need a spl meter you can buy for only 60$ something like that:
http://www.tnt-audio.com/clinica/spl-meter_e.html

A computer based spl meter may have a calibration file, but you should know the response of that unique microphone or calibrating the instrument.

If we want a simple gadget and it is simple to program it could be interesting. It could have A and C weighting, optional calibration file (for he who can use it) and a frequency meter for single test tones.

Also a spectrum analyzer should be simple to do, using mp3 algorithms. A calibration file could be done by some people of us (like me) who has or can use for a few minutes a studio monitor/home stereo set/anechoic room :-) and play a pink noise mp3 file, then sharing the calibration file in the forum, telling the set he used.

[LADave]

Electret mic elements usually have wide and flat frequency responses, but level calibration remains an issue.  Still there are some very good sound level meter apps available for Apple's iPhones and other iOS devices.  Probably aided by Apple using mic elements with consistent performance. 

The calibration problem doesn't seem like sufficient reason not to enable this in RockBox.  Some folks will have access to calibrated equipment and sound booths so side-by-side calibration can be done.  For the rest of us, necessity-driven invention is in order.  The basic principles are simple; perhaps a practical DIY way can be found to implement.

[saratoga]

Isn't that exactly what the peak meter on the recording screen does?

[LADave]

Lots of detail differences.  SLMs are mono.  They usually have numeric readouts to 0.1 dB besides simulating an analog reading.  They have fast and slow integration and at a minimum A and C weightings. The  iOS app SPLnFFT https://itunes.apple.com/us/app/splnfft-noise-meter/id355396114?mt=8 ups the game by adding flat, B and ITU-R 468 weighting,  and 1/3 octave spectrum analysis.

[saratoga]

Have you looked at what is already included?  spectrogram, mono, integration are already available...

[LADave]

Searching "spectrogram" doesn't turn up anything.  "fft" is slightly productive.  There seems to be a plug-in but I couldn't find anything about it in the half-dozen manuals I looked at.  There doesn't seem to be a mechanism for (globally) searching manuals comparable to searching forums.

VU meters are different from SPL meters.  For one thing, VUs are tied to gain vs. clipping levels whereas SPLMs are calibrated to absolute sound pressure; 0 dB = 20 micropascals.

It's difficult to say how RockBox's VU integration settings correspond to "fast" and "slow" integration on SPLMs.  Perhaps someone who has worked on this would know, but given my previous quibble that may be moot.

[LADave]

iPhones 4 and later have MEMS microphones (I'm not sure whether the electret-to-MEMS switch actually came earlier).  Compared to electrets, MEMS microphones are fabricated as chiips to ultra-exact tolerances.  They tend to have flatter, smoother frequency response, plus better consistency in sensitivity (millivolts per pascal) from sample to sample and batch to batch, and change less with age and environmental variability.  This predictability lets iPhone SLM apps like SPLnFFT be pretty accurate without calibration to individual iPhones. 

If Rockboxes have electret mics they will need calibration for accurate results and calibration is a tough nut to crack.  Most (all?) Rockboxes lack the cylindrical microphone snouts that conventional meter calibrators attach to, and the average user is unlikely to have ready access to a calibrator anyhow.  The alternative method appears to be side-by-side calibration versus a known accurate SLM, preferably in a special room setup where sound levels don't vary over a few inches of separation.  Not many users have access to these either.