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Transfer Function Measurements With Smaart v8

Why measurements are a quicker and safer way to optimize a sound system than doing it by ear...

By Joan La Roda January 30, 2019

Image courtesy of Rational Acoustics

If we matched the levels of the Measurement Signal and the Reference Signal (M and R) well, we’ll see a magnitude curve practically flat around 0dB at the bottom and its corresponding phase response around 0º at the top. And, if the propagation delay was right (under normal conditions, it will be) the coherence trace, which appears on the upper half of the magnitude graph, will be 100% across the spectrum.

Now we can, in this case, move the sliders in the graphic equalizer to see its real effect on our signal. We can do the same with mixing consoles, signal processors and so on. I encourage you to measure the electronic devices you work with.

Now that we know how to measure an electronic device, it would be advisable to repeat the measurement, this time connecting the card’s output to its inputs, so as to check our measurement system. If both channels of the card have the same performance, we should see two practically flat lines on both the magnitude and phase graphs.

First Electro-Acoustical Measurement

The connection diagram would be like the one we saw earlier in “Connections”.

We’ll assume that Smaart is already configured as described above and that we are in “Transfer Function” mode. We can use any loudspeaker, even a hi-fi speaker that we may have at home.

Next we’ll do as follows:

1.- For this first measurement let’s place the microphone some 2 meters from the loudspeaker, at a point as distant from reflections as possible.

2.- Once in “Transfer Function” mode, we’ll click on the arrowhead to activate the audio, as in the previous case.

3- Adjust Averaging, Phase Smooth and Mag Smooth also as in the previous case.

4.- Select “Pink Noise” and click on the “On” button to start the measurement.

If we want to perform the measurement using music instead, we’ll do as follows:

Options–> Signal Generator

Figure 20: Changing the Signal Generator.

Under “Signal”, select “File” from the drop-down menu.

Figure 21: Selecting “File” from the drop-down menu.


Then click on “Browse” and select a WAV format file from the computer. It is not recommended to use music to measure subwoofers.

Figure 22: Selecting the track we want to play from our hard disk.

5.- Adjust the levels for the two input channels on the sound card so that they have the same unclipped level. The input VUmeters are the ones labelled M and R (which stand for “Measurement” and “Reference”) and can be seen in green and blue in figure 23. Yellow input VUmeters will still produce a good measurement. The Measurement Signal goes into the left channel (labelled “M”), and the Reference Signal goes into the right channel (labelled “R”).

The output signal levels need to be such that the sound from the loudspeakers is above the background noise in the venue where we are measuring.

6.- Click on “Find”, wait and accept if a reasonable delay time is presented for the distance of two meters between the loudspeaker and the microphone. Typically the Propagation Delay proposed will be correct when measuring a mid-high loudspeaker, specially with pink noise.

If we matched the levels of the Measurement Signal and the Reference Signal (M and R) well, we’ll see a magnitude curve centered around the 0dB line at the bottom and its corresponding phase response at the top. If we see the phase response ‘disappearing’ at the bottom and ‘appearing’ at the top (or the other way around), making reading of the phase trace more difficult, we can manually fine tune the Propagation Delay here:

Figure 23: Adjusting the Propagation Delay to avoid truncation on the phase trace.

This way we add or take away delay until an easily interpretable un-truncated phase trace is obtained.

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Fedele says

For now i have just read only introduction and... desagree.
Sofwares are just tools based on numbers! Calculations on numbers, made across many kayers of approximation and errors.
Reality, the Sound Phenomena, the Human way to ear, are more complex tan that.
In a concert, there are many forms of energies that analyzer don’ t measure ( remember...also just calculations on numbers and a 1/4 capsule here or there) but that caracterize our complex sensation.

Seth Morth says

Fedele is missing the point, and should have read the rest of the article before commenting.

This is a helpful guide in how to use Smaart8 to get the sound system to an (objectively) calibrated baseline. Occasionally this is referred to as "flat" but that is rather misleading; consider it "time aligned, checked for phase, and compensated for detrimental artifacts".

From this milestone you can then trust your ears to make (subjectively) changes as desired to have it sound the way you would like it to.

I would in fact like to hear more from Joan La Roda, as well as José Brusi, Xesc Canet, Pepe Ferrer, José Moldes, Germán Ramos, James Woods, and Jamie Anderson. They always have an open invitation to visit my workplace and command my attention when they publish an article.

Fedele says

And also.....
There are other conditions for cancellation!! It is not only problem of relative time As it is writed.

Fedele says

Good evening.
In addition to what I have already written, and which I will soon write,...
I also have to underline how, quoting this part of the text:
"When two acoustical waves reach the same point with a time difference, there will be a total cancellation ...etc"
There are other conditions for cancellation!! It is not only problem of relative time.
Fedele De Marco

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