The Impulse Response module for AudioTools provides an easy way to capture an IR audio file on the iPhone / Touch, and also calculates the most-needed metrics from the data. Impulse Response is an iPhone port of a powerful mathematical model developed on desktop computers by Daniel Valente, Ph.D Architectural Acoustics, Rensselaer Polytechnic Institute. In seconds, a complete set of measurements is made that describe the acoustic character of a room.

Impulse Response on the iPhone will give you very good results, even with the built-in mic. Of course, you are limited to moderate SPL levels, and the lowest frequency bands will not be as accurate as when used with iAudioInterface, but Impulse Response will give you accurate reverb decay times and other measurements.

Be sure to check out our Impulse Response demo videos.

The Impulse Response can be computed either from recording an actual impulse, like a balloon pop or handclap, or from recording a swept sine wave which is the converted to an impulse through deconvolution, all from within the iPhone.

Important: make sure you download our IR sweep files below, and play them from another iPod or a CD when recording an impulse response sweep.

Recording an Impulse Response

You can record either an actual impulse, or a swept sine wave that the app will deconvolve into an impulse. If you plan on recording a swept sine wave, download our three excitation signal files and burn them to a CD or put them on another iPod.

See our demo video on recording an impulse response.

To record your impulse response, go to the Impulse Response module in AudioTools, and tap the sine wave icon, which will bring up the Recording screen. The first time that you access this screen, the three reference swept sine signal files will be downloaded from our server and saved on your device. You can always re-download them by tapping the "reload" button. This could become necessary if we change the sweep files in the future.

Next, select the type of impulse response that you will be recording: Impulse, 3 second sweep, 7 second sweep, or 14 second sweep. 

Recording an Impulse

To record an impulse, just start recording, and create the impulse. Make sure that the Clip indicator does not appear. You can also listen to the impulse to check it. When you are happy with the recorded impulse, tap Done to return to the calculations screen.

Recording a Swept Sine Wave

You can select a 3 second, 7 second, or 14 second sweep. The tradeoff is that the shorter files process much more quickly, and require less device memory (although we have not yet found a situation where any device has actually run out of memory), and the longer files generally will give you better a better signal to noise ratio (S/N). You may want to start with a 3-second sweep as you get your setup working properly, and then switch to a longer file. 

Next, setup your playback system with the CD or iPod playlist that you created earlier. Connect to a system with only one speaker active, and try playing the sweep. Listen for obvious distortion or other problems.

When you are ready to start recording, tap the Record button on the Recording screen, and then start the sweep playing. You will hear a short sync tone, followed 5 seconds later by the sweep. Wait until the minimum time displayed on the screen is reached before tapping Stop to end the recording.  Watch the level meter as the sweep runs.

Check that "Clip" does not appear on the screen, and that the sweep got more than half-way up the level meter. The yellow area is perfectly fine. If you like, you can play back the sweep and listen to it.

At the end of the recording, a message box will appear asking whether you want to Deconvolve the sweep, or Cancel. If you are happy with the recording, choose Deconvolve. A series of calculations will begin, that make take up to several minutes, depending on the speed of your device. The new models are much faster. 

If all goes well, you will see the word "Done" in green, and you can tap Done and go on to the calculations screen. If there is a problem, it will be shown on the screen. The most likely problem is that the sync pulse may not be found. The sync pulse must occur at least one second after starting the recording, and must be within 20dB of the loudest part of the sweep. Also, the recording must be long enough to include at least 5 seconds of decay after the sweep. Normally, just waiting for the final sync pulse will allow enough time.

When you go back to the calculations screen, the ETC will be computed and shown on the screen. From here you can look at other parameters and metrics, or trim (edit) the IR.

Editing the Impulse Response

Often you can get better results by trimming the start and end of an impulse response. If you have recorded an actual impulse, it is highly recommended that you trim the result. If you have recorded a swept sine, sometimes the resulting IR will need some trimming before the impulse, and the noise after the decay curve can be trimmed to shorten the screen window for a better look at the data. 

When running a swept sine, if the sample rate of the playback device is slightly different than the sample rate of the device that the app is running on, even by just a few Hz, you may see a shorter false peak before the actual impulse. It is important to trim this for the best results.

To edit an IR, go to the ETC screen, and use the pinch/zoom gesture to expand the display. Your goal is to remove extraneous sound before the actual impulse begins, without cutting into the impulse itself, and to remove noise after the decay ends, without cutting into the decay curve itself. The result needs to be at least 0.5 seconds long. 1.0 second is fine.

When you have trimmed the ETC, tap the Edit button and select Trim. This will non-destructively trim the ETC and cause calculations to be re-done. 

At any time, you can undo the trim and return to the original file.  

After trimming, you can then select one of these calculations to display in a bar chart, either by octave band or 1/3 octave band. Calculations are done as needed when you change graph selections.

Octave and 1/3 Octave Bar Charts

T30 --  T30 is the decay rate of the impulse response or reverberation time, the time that is takes for a sound to decay by 60 dB is a space. By using a noise compensation technique to avoid inaccurate representations of reverberation time, the decay rate is calculated by determining the slope of the decay function from -5 dB to -35 dB, and extrapolating the time to a decay of 60 dB.

C50 – The Clarity Factor (50ms), expressed in decibels, is the ratio of the early energy (0-50 ms) to the late reverberant energy (50-end of the decay of the IR). C50 is perceptually aligned with speech perception.

C80 – The Clarity Factor (80ms), expressed in decibels, is the ratio of the early energy (0-80 ms) to the late reverberant energy (80-end of the decay of the IR). C80 is perceptually aligned with music perception.

EDT – Early decay time represents the decay function of the IR in the slope of the early part of the energy decay curve. It is the slope of the curve limited from 0 dB to -10 dB extracted to a decay of 60 dB below to stopping of the direct sound energy. EDT is typically associated with the perceived reverberation time in a room.

CT - Center time or Ts is the time after the onset of the direct sound to the time where half of the energy has decayed in the IR. It is the time corresponding to the "balancing point" or center of gravity of the squared IR. Center time is correlated with reverberation time, so center time increases as a function of reverberation time. 

Definition – Similar to the C50 metric, definition represents the ratio of sound arriving in the first 50 msec of the IR compared to the rest of the IR. It is expressed as a percentage and correlates to speech perception in a room

.

S / N – Signal-to-Noise ratio represents the difference between the measured signal captured by Impulse Response versus the noise floor present in the room under test. It is a good tool to verify that the measurement signal that is being used will yield quality results. Aim for a S/N ratio above 50 dB whenever possible or the reliability of the derived metrics of the IR will be reduced.

Wide-Band Plots

Also, these plots are available, for the wide-band signal:

Schroeder Plot – The backwards integration of the decay function of the impulse response.

ETC -- This is the time-domain plot of the decay response  of the IR

FFT -- The FFT of the ETC results in the frequency response graph for the signal.

Summary Page

We have a summary page that shows a set of basic measurements:

T30/EDT/C50/C80 Mid Values – This is the average value for the Reverberation time, and Clarity in the measured room taken at the 500 Hz and 1 kHz Octave Bands.

S/N Broadband – The overall signal-to-noise ratio, expressed in dB of the measured IR.

Bass Ratio – The metric that correlates to the “warmth” of a room. It is calculated by adding the T30 values for the 125 Hz and 250 Hz octave-bands and dividing them by the sum of the T30 values for the 500 Hz and 1 kHz octave bands.