RTA is a full-featured one-third octave Real Time Analyzer, available for the iPhone 3G or the second-generation iPod Touch. Note that you must be running iPhone software version 2.2, you will have to upgrade your iPhone if you have an earlier version installed.
NOTE: If you plan to purchase iAudioInterface2, you should instead download AudioTools, which includes this app in the base purchase price, and gives you a lot more.
Don't know if you should download FFT or RTA? Check out our comparison page for more information.
New in RTA in AudioTools 3.5 -- Transmission Loss test option.
RTA uses filters conforming to ANSI and IEC specifications. It can be calibrated, and uses either the built-in mic, the headset mic, or the Studio Six Digital iAudioInterface microphone, available Summer 2009.
*** NEW: Check out our new demo video of RTA.
See our "dB vs. dB" page to understand what the SPL numbers mean, and why they might be different on different modules.
How to Use RTA
Here is the RTA main screen:
Band Size
To use RTA, select the band size that you want, either Octave or 1/3 Octave.
Decay Modes and Speed
Select the decay mode and speed. You can select one of the exponential decay modes, which are labelled in seconds. The seconds indicate the amount of time for a bar to decay by 20dB.
Also, you can select Peak Hold, which holds the highest value that a bar has reached. To reset Peak Hold, re-select Peak Hold mode.
There is one more mode, Average. This mode performs a continuous equal-weighted average, from the time you start the RTA with the Play button in this mode, until you Pause the RTA. Even then, you can continue the average by pressing Play. To reset Average mode, re-select it.
Play / Pause
Touch the Play/Pause icon to begin. Anytime one of the parameters is changed, the RTA will revert to Pause so that the new parameters can be in effect for the most accurate readings.
Screen Scaling & Resolution
Once the graph is running, you can adjust the screen dB scale by sliding the graph and up and down. You can also change the dB resolution showing on the screen by pinching -- pinch together to get more dB on the screen, or pinch apart to get less vertical dB, and thus more detailed resolution.
Saving a Screen Image
You can take a snapshot of the screen by touching the Save Image button. The image will be saved in your photo roll, and can be emailed or transferred to your computer using various third party apps.
Noise Curve Overlays
To display a noise curve overlay, flip the screen over by touching the info icon in the navigation bar, and select one of the overlays: NCB -- Balanced Noise Curve, RC -- Room Curve, PNC, NR, or Noise Rating, used in the EU, and the cinema large-room X-curve.
The overlay will appear on the graph. To read the noise curve value, find the lowest curve that does not have any bars touching it. You can interpolate between curves to estimate an exact curve number.
Starting in RTA in AudioTools 4.0, now see the overall noise overlay curve value -- in octave mode only, since the noise curve values are only defined for octave band mode.
Max / Min Bars
You can turn on this option on the setup page. When active, the maximum and minimum levels of each bar is stored, and shown as blue and green bars on the screen.
To reset the bars, just double-tap the screen.
Overall SPL
The overall unweighted dB SPL level is shown just above the graph, on the right side. Choose unweighted SPL, or A or C weighting on the setup screen.
Calibration
In AudioTools, all calibration is adjusted in a single location, Setup->Microphone calibration.
In the standalone RTA app, you can calibrate the mic SPL level by flipping over to the setup page, and using the + and - buttons to adjust the number in the Trim field. This value is added to the computed dB levels, both for the overall dB level that appears just above the graph to right, and to the RTA level bars. For a typical iPhone, the Trim value should be 7.0. If you have access to a calibrated SPL meter, you may be able to get a more accurate reading.
To calibrate the SPL level, first generate a known dB level. You can do this by playing pink noise through your system, and measuring the pink noise with a calibrated SPL meter. To get the best match, you should be not be using weighting. If your SPL meter does not have that option, select C weighting. Now adjust the Trim field by using the + and - buttons until the level shown in the dB field matches the level shown on the SPL meter. The RTA level is now calibrated.
There is a provision for selecting the internal or external microphone. In this case, the External selection is for the Studio Six Digital iAudioInterface microphone. When using the iPhone mic or headset, select Internal.
Pink Noise
The next version will incorporate pink noise, which is the typical test signal used with an RTA. There will be a switch on the flip side window that lets you turn pink noise on or off. Pink noise has equal energy in each octave (or 1/3 octave), so in theory if your system is flat, you will see a flat line on the RTA display.
Room Tuning
In practice, the RTA is really showing the sum of the direct sound as well as the reflections, so this is not 100% correct. Also, most people roll off the high end somewhat, and many like to boost the low end. So, use the RTA and it's information, but also use common sense, and don't make extreme EQ changes. You're usually better off making conservative changes in EQ, but you should definitely try moving speakers and subs around, and watch the effect in the RTA.
Here are some links to learning the basics on room tuning with an RTA:
http://mixonline.com/live/applications/audio_getting_room_tune/
http://www.studiotips.com/rta.html
iPhone Hardware
See our our page on iPhone hardware that explains the differences between hardware models, and the effects on performance of our apps.