ETC measures room decay time graphically, by showing the decay of a pulse over time. Typically, you will make an impulse sound by clapping your hands, popping a balloon, or even snapping your fingers. The graph will show the decay of this signal over time, and from this, the RT60 time can be computed.
See our new demo video of this module.
How to use ETC
Here is the ETC main screen:
** Flipside Screen **
Here is the flipside screen:
First, flip the screen over and setup the app. Here are the fields that you can change:
Internal / External mic
Select Internal if you are using the iPhone's built-in mic, or even if you are using the headphone mic. This mode turns on the correction curve for the microphone, to flatten out the frequency response, and selects the calibration factor for this mic.
Select External if you are using the Studio Six Digital iAudioInterface microphone. This mode turns off the iPhone mic compensation curve, and selects the calibration factor for this microphone.
External Trigger
If you will be generating the impulse sound to trigger the graph, select External Trigger. Turn External Trigger off if you want to use the internal impulse. In this mode, you have to route the audio output of the iPhone to your sound system input. ETC will start the graph from the exact time that the pulse is sent from the iPhone. You can then see the time that it takes for the pulse to reach the iPhone from the speaker. Use this to set speaker delay times.
Threshold Level
The Threshold Level is used to set the audio level at which the graph will trigger in External Mode. It has no effect in Internal mode. Set this value high enough above the ambient noise level (which can be seen in the dB window of the calibration section) to prevent inadvertent triggering, but low enough to allow the graph to be triggered when your impulse occurs.
Units
You select ms (milliseconds), or a unit of distance: feet or meters. If a unit of distance is selected, then the time in ms is converted to distance using the speed of sound. In most cases, you will want to use ms, but if you are interested in distance, for example in measuring speaker delay in Internal mode, you may want to use distance.
Calibration
You can use the Trim field to adjust the dB level that is shown when a particular sound level is received. There are actually two factors, one for the internal (built-in) mic, and another for the external mic (this refers to the iAudioInterface mic, not the headphone mic).
The typical iPhone 3G mic Trim value is +7.0 dB. If you have access to a calibrated Sound Level Meter (note: we do NOT recommend using a Radio Shack meter for this purpose!), you can compare the sound level read on the meter to the sound level shown on the dB field, and adjust the Trim field until you get a match.
NOTE: Make sure that you select the same weighting on both the sound level meter and the ETC app, ie A-Weighted.
** Main Screen **
On the main screen, you have one button to select the weighting, another to select the time window, and one more that lets you save a snapshot of the screen to your photo roll. There are also cursors that can be used for reading out exact dB, time, and RT60 times. And, the screen resolution and scale can be adjusted using gestures.
Weighting
Touch this button to cause a picker to appear with the weighting choices. You can select ANSI A-weighting (think:voice), C-weighting (think:music), any octave band weighting, which isolates that octave, or no weighting. For general use, we suggest using A-weighting to filter out rumble that would otherwise raise the ambient noise level and reduce the available decay range.
Time Window
Touch this button to select the time window for the graph. Use shorter windows for smaller rooms, and larger windows for larger, more reverberant rooms. After you have seen the decay curve, if it doesn't decay all the way around, select a longer time. If it decays in a small part of the screen, select a shorter time window.
Note that if you have selected a distance unit on the flipside screen, this field will still be shown in ms, although all areas on the screen will be converted to the equivalent distance, using the speed of sound.
Cursors
If you slide one finger horizontally across the screen, a cursor will appear that shows the exact dB level and time or distance for the point that intersects the cursor.
If you use two fingers, you get a second cursor. By placing the two cursors at the start and end of the decay curve, the equivalent linear RT60 time is computed and displayed on the second (right-hand) cursor. To get the most accurate reading, make the green decay line as parallel as possible to the actual decay.
Graph Scaling and Resolution
You can use your finger to slide the screen up and down, to properly fit the graph on the screen.
You can also use a vertical pinch gesture to compress or expand the graph, to see more detail, or to fit a graph on the screen.
Capturing Images
To capture an image of the graph on the screen, including any cursors that are showing, touch the "Save Image" button. The current graph will be stored in a file in your photo roll.
iPhone Hardware
See our our page on iPhone hardware that explains the differences between hardware models, and the effects on performance of our apps.
Audio Test Equipment
If you are looking for more advanced audio and acoustics test equipment, check the the SoundPro line at www.Sencore.com