I have accumulated several SPL/Spectrum Analyzer apps over the years and today I did a quick comparison between them to show how accurate they really are in the bass range, since that is where I tend to do most of my tweaking. It is well known that the iPhone automatically rolls off frequencies below around 150Hz, in order to minimize wind noise being picked-up by the mic while making a phone call.
The apps are as follows:
- RTA (full/paid version)
- JL Audio Tools
- Decibel Ultra
The reference curve ("actual") is taken with a RadioShack digital SPL meter with the reading having been corrected through the use of correction tables available on the internet. Tones were played individually from a CD. I'm sure it is not perfect, but the readings should be within 2 or 3 dB of the actuals.
Results show that most apps are not reliable whatsoever below 160Hz (as expected). The absolute readings are also all over the place, but for this purpose they don't really matter. It is the accuracy of the relative readings that is important. Above 160Hz, Decibel Ultra and SPLnFFT are fairly accurate, but below 160Hz only SPLnFFT is accurate and only down to about 63Hz. Below that, it's a crapshoot!
For the record, the iPhone was held in head position with the mic pointing straight up.