Transfer Function Instructions
Hardware Setup
Measuring Time Delay
Setup Screen
Operating Transfer Function
Transfer Function is a powerful two-channel acoustic measurement tool.
To use it, you need an audio interface that supplies two discrete
channels of audio.
The Basics
Transfer Function is an acoustic measurement tool that mathematically
compares a reference signal, which can be pink noise or program
material, with the same signal after it has gone through a signal chain
and speakers. Since it is looking at the difference between the
signals, factors such as room reverberation and noise are minimized.
Normally, you will want to use a two-channel input device, such as
iAudioInterface2. However, you can use most of the features of Transfer
Function with the built-in iOS device microphone. In this case, you
will be using the internal pink noise generator as the reference test
signal. Route the headset output to the sound system input. The
generator signal will be looped back to the input internally.
When you are using a two-channel audio input device, you will route a mono line-level source signal to the right
channel input, and send the same signal to your speakers. Connect a microphone to the left input, as the test
signal input. With this configuation, the app can subtract the reference source
from the test signal, and show both the magnitude and phase of the
transfer function, so that you can correct system EQ and other
parameters for optimum system performance.
Since the source signal is always changing, the app needs to know very
accurately the time delay between the source and test signals. Transfer
Function includes a sample-accurate Impulse Response-based Delay Finder
for this purpose.
Hardware Setup - iOS Device Microphone
The first step is to setup the hardware. If you are using
the built-in iOS mic, make an 1/8" cable using a stereo 2-conductor
plug, like the kind used on headphones that have no microphone
included. On the other end of this cable, you can use either a balanced
or unbalanced connector. Make sure you match the generator setting to
the type of cable you are using.
Connect this cable to the input of your sound system, and route the
signal to the loudspeakers. The app will loopback this signal
internally to provide the reference test signal. The iOS mic provides
the test signal.
Skip ahead to the "Measuring Time Delay" section for the next step.
Hardware Setup -Generic Two-channel Input Device
In both cases, connect your microphone to the left input of the device, and if possible set it up on a mic stand so
that its position will not change during the test. Set the mic input
range to avoid clipping.
Pink Noise
Route the output of your device to the sound system input. Control the pink noise from the app.
Program Material
Send the
music, in mono, to the speaker system that you wish to test, and also
split the same signal out and route it to the right input of the device.
That's it. You are ready to setup the software for the Transfer Function measurements.
Hardware Setup - iAudioInterface
We are assuming that you will be using a microphone, so in both cases
you would have the mic plugged into the XLR input, as the Measure
source, and if possible set it up on a mic stand so
that its position will not change during the test. Set the mic input
gain range to avoid clipping.
Using the Internal Pink Noise Generator
For pink noise, on the Transfer Function Setup page, select Ref Source,
and on the top, select Mic/Gen. Then just run a cable from the Line
Output / Headphones jack to your system input. Turn on or off the pink
noise using the sine wave icon on the main screen. Note that pink noise
will only run when either the Delay Finder is on, or when the
measurement is running.
Using Program Material as the Reference
You may be in a situation, such as during a live sound event, when it
is not possible to run pink noise. In this case, you can use the actual
sound from the event as the reference signal.
To set up for this, select Mic / Line on the Reference source page, and
run an output from the sound system that is a mono mix of the audio
going out to the room into the Line Input jack. Probably Mid gain will
work, just check the little M R meters on the main screen for the
levels. Now set the delay, and you are all ready to measure the system
frequency response and phase while the music is playing.
Using the Stereo Line Input of iAudioInterface
If you are using iAudioInterface2, you can use the
line input in stereo mode, to route both the measurement signal and the
reference signal to the transfer function.
To do this, tap the "Ref Source" button on the Setup page to bring up
the Reference Source page, and then turn on "Use Stereo Line Input".
This will switch the hardware so that the line input will be feeding
the transfer funciton. The left input will be the measurement signal,
and the right input will be the reference signal in this mode.
Measuring Time Delay
Accurate time delay is the key to good Transfer Function results. Time
delay offsets will be a function of distance to the speakers from the
microphone, and also may be introduced by DSP processors in the signal
chain. It is important to set the time delay before performing any
Transfer Function measurements.
To set the time delay, tap the time delay value, which appears on the
top right of the iPhone display, and the bottom right of the iPad
display. In either case, the Time Delay setup screen appears:

First, select an FFT size that will accommodate the longest delay that
you expect. The maximum delay time is shown and changes as you change
the FFT size. Remember that delay is approximately equal to a foot per
ms, or a meter per 3 ms.
Next, select the number of averages you wish Delay Finder to take for a
reading. For pink noise, a couple of averages are plenty, although you
may want more for music, or if the noise level in the area is an issue.
Now tap the Find Delay button, and wait for the results to be computed.
The result will be shown in the text field on the top of the screen.
If the result looks reasonable to you, tap Apply to save it and to put that delay time into effect.
If you know the actual delay time from another method, you can enter it directly in the field.
Delay Finder resolves delays to 1 sample at 48kHz, or about 21us.
Setting System Latency
To get the best results, you will want to set the overall system
latency, which includes the delay from the internal codec, any DSP
processors in the chain, or any other sources of delay.
To do this, you will want to bring the microphone (iOS or external mic)
directly to the front of the loudspeaker. Then, run the delay finder as
usual, and tap "Set as System Latency". This will store this value, and
all calculations done will now take this latency into account.
It's a good idea to set this before each set of measurements, to make
sure that nothing has changed since the last time this was set.
Relative Delay Calculations
The Delay Finder screen provides a method to calculate the delay
offset, or difference in delay times, between two different
loudspeakers. This is useful when setting up delay times for fill
speakers that are physically offset from the main loudspeakers.
To do this, from a single position, turn on only the main loudspeakers,
and route the reference signal to only those loudspeakers. Run the
Delay Finder, and tap the "Capture as Baseline" button. This delay
value will be stored.
Now, turn off the mains, and turn on the next set of fill loudspeakers.
Again run the Delay Finder, and observe the relative delay, which is
shown as the Delta.
Repeat this for as many fill loudspeakers as desired. If you move to a
new position, go back to the first step and capture the main
loudspeakers delay time before testing the next fill loudspeaker.
The Setup Screen

The setup screen has these fields:
- Meas Source - Select the microphone and gain range for the measurment input.
- Ref Source - Select the reference signal source. In most cases
this will be the right channel input, but you can also set Stereo Input
mode from this screen.
- FFT Type
Select
the size of the FFT to use. Larger FFTs resolve frequencies better, but
take longer to update. Number of Hz = Sample Rate / FFT Size.
- Display Type
Select Magnitude only, Phase only, or both.
- Peak Tracking
Show the frequency of the highest point on the magnitude graph, either on the plot itself, or in large numbers.
- FFT Windowing
Turn on to enable windowing. Windowing an FFT reduces edge effects, and results in a more accurate display.
- Coherence Trace
Turn
on to show the coherence trace in red on the graph.
- Coherence Blanking % - Magnitude and Phase plot values that are below this level of coherence will not be shown.
- Audio Pass Through
Turn on to listen to the microphone input.
- Difference Mode
Used to show the difference between the current plot(s) and a recalled plot.
- Lock Graph Scale
When turned on, the screen will not respond to touch GUI commands.
Operating Transfer Function
Transfer Function is a more powerful measurment than an RTA, since
it has the ability to analyze a system even in a reverberant space.
However, it is more complex to set up and use correctly.
The most critical factors involved in using transfer Function are
delay, averaging, and coherence. If these are setup correctly, you will
likely get good results.
Other factors that are important are input levels, FFT settings, and
graph smoothing. We will look at all of these, and explain how to use
them to get the best results.
Delay
Delay is very important, since the algorithm is comparing two signals
that are usually shifted in time. Use the Delay Finder, by tapping on
the delay time number on the main screen, to set the delay. If the mic
moves in relation to the speaker even small amount, it will effect the
results. Initially, you will see the highest frequencies bending up or
down.
See the section on Measuring Time Delay for more information.
Averaging
Averaging, which is set be tapping the button in the lower right
corner, is useful to get the plots to settle down, and show more stable
results.
Transfer Function supports averaging up to 16 FFTs, or you can also
select Fast or Slow, which are exponential decay averages, or Avg mode,
which is a continuous running linear average of all FFTs. To reset Avg mode, just re-select averaging.
You will want more averaging with music than pink noise. With pink
noise, 4-8 averages or Fast mode are reccommended, while 8-16 averages or Slow work better
for music.
Coherence
Coherence is a measure of how much one set of FFTs is like another.
Coherence is what you can use to determine if you have the other
settings, such as delay and gain, set correctly. A coherence of 100%
would be perfect, but if you can get an average above 70% that's fine.
While you are getting setup, and learning how to use Transfer Function,
it's a good idea to leave the coherence trace turned on (Settings
screen).
In fact, it is so important that we have implemented Coherence Masking, to essentially hide results that don't meet a minimum
coherence level. Try setting Coherence Masking on the Setup Screen to
at least 50%, or just touch the right edge of the display, and slide
the Masking marker up or down.
Time and Data Windows
Transfer function employs both time and data windows.
The time window is a side effect of the FFT length and sample rate.
Since we run at 48kHz, the time window in effect is simply the FFT
length / 48000. Ideally, the first reflections from the speaker
would lie outside that time window, to remove the effects of the room
on the measurement.
The data window is set on the Setup screen, as the FFT Window.
Input Levels
As in any other test measurement, if your input gain is not set
correctly, you will not get usable results. We have two small level
meters visible at all times, to help you see the input levels.
Try to get the level at the half to three-quarters area, and make sure you avoid clipping.
You can use the microphone gain settings, and the line level input gain
setting (available on the Levels screen) to get solid input levels.
Smoothing
Smoothing can help you visualize overall eq or phase patterns, by removing some of the detail from the graphs.
On iPad, you can set the magnitude and phase smoothing independantly, and on iPhone, one setting controls both plots.
FFT Settings
There are two settings that control the FFT itself. FFT size, and FFT Windowing. Both are set from the Setup screen.
FFT size is a tradeoff between speed and frequency resolution. For most
cases, we would suggest 8192 or 16384 points. Windowing helps remove
artifacts and increases dynamic range, and we suggest using it in most
cases. We have 3 different windowing algorithms, Hann is our personal
favorite.