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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

For the Signal Generator to come out on both the output channels, we’ll need to go to:

Options–> Signal Generator

On “Device”, select the sound card if it is not selected already.

On “Main” select the channel we named “Out 1” and, select the channel we named “Out 2” for “Aux.” This way, the generator signal, which will be the Reference Signal, will come out of the two channels. Click on OK and continue.

Figure 11: Selection of the card and the two channels through which the generator signal will be output.

First Electrical Measurement

The connection scheme would be like the one we saw earlier, but replacing the amplifier, speaker and microphone with the electronic device we want to measure. In this example it is a graphic equalizer. We can use the line input if the XLR input gets overloaded.

Figure 12: Connection diagram for an electrical measurement.

To perform a Transfer Function measurement, go to:

View–> Real Time Mode. On the bottom right click on “Transfer.” If we wanted to see the impulse response on the top part, we can also click on “Live IR.”

Figures 13 and 14: Selecting the “Transfer Function” mode.

Next we’ll do as follows:

1.- Click on “New TF” on the lower bar, give it a name, click on OK and activate audio by clicking on the arrowhead.

Figure 15: Active audio by clicking on the arrowhead.

2.- Adjust Averaging, Phase Smooth and Mag Smooth as seen in the image:

Figure 16: Initial setup of Averaging, Phase Smooth and Mag Smooth.

3.- If it isn’t already, select “Pink Noise” and click on “On”. By default the initial level in the generator is -48dB. Typically we’ll need to increase that so as to have enough signal.

Figure 17: Activating the pink noise generator.

4.- Adjust the levels for the two inputs on the sound card so that they have the same unclipped level. If the microphone input is overloaded, we can use the line level input. 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 18. 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 signal level should not overload the input of the device being measured. It can be adjusted via the card’s output potentiometer and the output level in the Signal Generator.

5.- Click on “Find,” wait and click on “Insert.”

Figures 18 and 19: Finding the Propagation Delay of the signal.

Caution! Make sure that the grey circle to the right of the delay field is not active (yellow). Otherwise Smaart will be continuously adjusting the “Propagation Delay” and we will lose the phase relationship between two measurements. The grey circle turns yellow when delay tracking is active.

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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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