Stereo 1/3-octave analysis now available in SignalScope Pro

With the release of version 2.2, the Octave tool in SignalScope Pro for iOS can now analyze two input channels simultaneously. This new capability is only available on devices running iOS 5 or later, and it requires a stereo or two-channel dock connector audio accessory.

     

The Octave tool offers whole and 1/3-octave real-time spectral analysis with A, C, or flat frequency weighting and fast, slow, or impulse time weighting. Time exponential averaged level (Lp) and equivalent level (Leq) measurements are both supported.

The Octave tool available via in-app purchase in SoundMeter 3.0 and SignalScope 3.2 also offers two-channel measurement capability on iOS 5 or later. SoundMeter 3.0, SignalScope 3.2 and SignalScope Pro 2.2 are all available for immediate download on the App Store.

Improvements in SignalScope 3.2 and SignalScope Pro 2.2 include:

  • For users running iOS 5, or later, the Octave analyzer tool can now be operated as a stereo RTA, analyzing two input channels simultaneously (with compatible input hardware). (The Octave tool is available within SignalScope via in-app purchase.)
  • Data can now be exported to CSV files from the FFT, Octave, and Oscope tools. CSV files can be opened directly in Numbers, or other spreadsheet apps that support it, from within SignalScope. Tab-delimited text files can still be created by giving the file name a .txt extension. (Data file export requires the data acquisition upgrade available within SignalScope via in-app purchase.)
  • Corrects a potential crash when accessing the image picker on iPad.
  • SignalScope uses less memory on iPhone and iPod touch.
  • Twitter support has been restored (in iOS 5) for tweeting level meter data. (The Level Meter tool is available within SignalScope via in-app purchase.)

SoundMeter 3.0 offers in-app upgrades

  SoundMeter 3.0 is now available for download on the App Store,

offering in-app purchases of the same tools found in SignalScope Pro. Available upgrades include dual-channel Octave RTA, Oscilloscope, and FFT Analyzer tools, as well as a data acquisition upgrade.

Dual-channel analysis in the Octave tool enables SoundMeter to be used as a stereo 1/3-octave RTA. This dual-channel capability requires iOS 5, and is also coming soon to the Octave tool in SignalScope and SignalScope Pro, but it is available now in SoundMeter. (In SignalScope, the Octave tool is also available via in-app purchase.)

SoundMeter’s new data acquisition upgrade enables the user to export Octave, Oscope, and FFT data to CSV, tab-delimited text, and MAT files, and to access those files from a web browser on another device. This upgrade also enables additional units, such as V, A, g, and ips to be assigned to input signals.

Download SoundMeter 2

SoundMeter 2.0 is on the App Store

As in recent updates to SignalScope and SignalScope Pro, SoundMeter now runs natively on iPad, as well as iPhone and iPod touch.

SoundMeter now supports drawing on an external screen with a compatible video output adapter. The original iPad supports external screen resolutions up to 720p and the iPad 2 supports resolutions as high as 1080p and even 1920×1200 with compatible hardware. Video output support is not supported on iPhone or iPod touch.

SoundMeter also supports saving the sound level display to PDF files, which can be accessed via iTunes File Sharing on a Mac or PC. The same black, blue, and white color schemes as found in SignalScope Pro 2 are also supported in SoundMeter 2.

Download SoundMeter 2

SoundMeter 2.0 Screenshot

How about an iPhone-ProTrack SLM

Alesis finally got around to making shields for securing an iPhone or iPod touch to the ProTrack stereo iPod recording device. This means that you can insert your iPhone (any model) or iPod touch (2G or later), plug in a phantom-powered measurement microphone, run SoundMeter, and turn the unit upside down for a handheld (albeit somewhat large) sound level meter. Of course, if you use SignalScope Pro, you’ll also have access to 1/3-octave and narrowband spectral analysis.

SignalScope Pro on iPhone with a ProTrack

One downside is that the headphone output is completely blocked for the iPod touch. The shield itself blocks the headphone output of the iPhone, although a drill could solve that problem fairly easily.

iPhone Microphone Frequency Response Comparison

With the advent of sound level meter apps for the iPhone OS (of which SoundMeter was the first) people began to ask, “How flat is the frequency response of the iPhone’s microphone?” Early testing indicated that the built-in microphone of the original iPhone was not a good candidate for sound level measurements, but that the iPhone’s headset microphone enjoyed a fairly flat response. Since then, additional iPhone models have arrived on the scene, each with its own set of weaknesses with respect to microphone frequency response. Additional Apple and third party headset microphones have also been introduced.

At long last, some relevant frequency response measurements are presented here for the benefit of those who would really like to “see” how flat a particular microphone is. These results have implications on the use of certain microphones for making sound level measurements, as well as on the use of these microphones for spectral analysis in which relative amplitudes need to be determined with some degree of accuracy.

The following measurements were made relative to a Type 1 precision microphone in a fairly quiet room. These measurements were not made in an anechoic chamber and although the coherence was very good across the audio band, the measurement error is non-negligible at high frequencies, because of diffraction effects.

Built-in Microphones

Built-in iPhone Microphone Frequency Response

Built-in iPhone Microphone Frequency Response Comparison

As I have often said, “The built-in microphone of the original iPhone is not recommended for sound level measurements.” Now, you can really see what I mean. Interestingly, the built-in microphone of the iPhone 3GS isn’t recommended, either, unless you don’t care about frequency content below 200 Hz. This behavior is consistent with the headset input frequency response of the iPhone 3GS (I suspect that the built-in microphone signal goes through the same high-pass filter that gets applied to the headset input). The iPhone 3G microphone’s response is clearly the best of the bunch, but its low end rolls off by 15 dB or more at 20 Hz. Not surprisingly, none of the iPhone models rivals a lab-grade sound level meter with its built-in microphone, but either of the 3G models can potentially give you a decent ball-park estimate of the current sound level, although the low frequencies will be de-emphasized.

Headset Microphones

The goal, here was not to measure every headset microphone on the market, but to take a look at some of the more common options. These measurements were made of each microphone’s electrical output, so they do not include the response of any iPhone input or output circuitry. The microphones included in these measurements are:

iPhone Headset Microphone Frequency Response Comparison

iPhone Headset Microphone Frequency Response Comparison

In the world of headset microphones (at least those that are presented, here), the official iPhone headset microphone and the SwitchEasy ThumbTacks microphone win the day. The USBFever microphone also exhibits a flat response between 20 Hz and 2 kHz, although its response appears to break up more severely by the time it gets up to 10 kHz. In light of recent headset input frequency response measurements, the best case scenario for inexpensive sound level measurement might be to use the ThumbTacks microphone with the original iPhone.

These results are also interesting, in that they strongly suggest that the newer Apple headsets, which are designed primarily for iPods, shouldn’t be used for sound level measurements, either. Their response certainly seems to follow an apparent trend with Apple’s microphone-related circuitry to de-emphasize low frequencies.

It may be important to keep in mind that the goal, here, is to see what makes sense in terms of using iPhone OS devices as inexpensive, portable sound level and spectrum analysis tools. Obviously, there was never an expectation that the iPhone’s inexpensive microphones would perform in a manner consistent with precision measurement mics that are (justafiably) much more expensive. It is possible to connect such high-end microphones to an iPhone, though (via the dock connector)–more on that, later…

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