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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


The image below shows the connections required to measure a loudspeaker with a 2-channel FFT-based system.

For clarity, on the diagram we placed the outputs on the right-hand side of the sound card and the inputs on the left. The connections are as follows:

Left Output: Connected to the amplifier that will, in turn, be connected to the loudspeaker.

Right Output: Looped back into the Right Input. It will be the Reference Signal.

Warning! The signal of the two outputs must be the same. We could not use, say, the two channels of a stereo wav file. Later in this article we’ll explain how to configure Smaart v8 to have the same signal on both channels.

Figure 3: Connections for a two channel electro-acoustical measurement.

Left Input: Connected to the measurement microphone. That is the Measurement Signal.

The connection diagram shows that, as explained earlier, the signal generated by our measurement system goes to the loudspeaker on one side and on the other one goes back to the right channel of the sound card. This is the signal we called Reference Signal, and denoted in the diagram by a red arrow.

The signal that gets picked up by the microphone, which we called Measurement Signal, is denoted by a green arrowhead and goes into the left channel of the sound card.

Warning! Many sound cards have a control knob, normally labelled “Mixer,” with two positions, minimum and maximum, often labelled “Input” and “Playback” respectively. To be able to perform the measurements described in this article this control should be set at the “Playback” position, which will typically be the maximum clockwise position. Otherwise feedback is introduced from the inputs to the outputs, resulting in an unreal measurement of the system.

Propagation Delay

As stated before, 2-channel measurement FFT-based systems compare the Reference Signal to the Measurement Signal from the microphone. This is what is referred to as finding the Transfer Function.

The Reference Signal, that comes out of the sound card and is looped back into it by means of a cable, gets to the input immediately, practically at the speed of light. However, sound that comes out of the loudspeaker travels very slowly, at the speed of sound, some nine hundred thousand times slower, before getting picked up by the microphone.

To be able to obtain a readable phase curve from the Transfer Function we’ll need to introduce a delay on the input channel for the Reference Signal that is equal to the time that the wave takes to travel between the output and the input of the left channel. i.e., basically the acoustical wave propagation time.

This delay time will be calculated by the measurement system itself, only when we tell it to do so, and is a result of the comparison between the measurement signal and the reference signal.

In Smaart, this propagation delay will be calculated when we press “Find.”

Figure 4: Smaart will calculate the Propagation Delay when we click on “Find.”

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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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