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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

By way of summary, and to avoid wrong conclusions, I’ll emphasize these two points:

1.- If a cancellation shows up on a measurement due to a reflection, we need to be aware of the fact that the cancellation at that frequency only occurs at the specific point where the microphone is at, and therefore it should not be equalized because we’ll be unnecessarily coloring the response at the rest of positions.

2.- If, despite that, we applied equalization, even though the software shows recovery of the magnitude response, we are being fooled by the background noise or reverberation, which is what we are really measuring at the cancellation frequency.

If we EQed the cancellation produced by a reflection we would “fix” with electronics a “problem” that occurs only at that specific position and which is not coming from the system but from an incorrect interpretation of the graph and an incorrect evaluation of the environment where the measurement is taking place, creating a problem where there wasn’t one. As stated before, the software does not lie, but we have to learn how to interpret it.

Microphone And Sound Card

A range of affordable measurement microphones is available that can be used for this type or work.

It is advisable that, either the microphone has perfectly flat magnitude response, or else comes with a file for magnitude and phase response compensation. These compensation files, which are normally text files that are created specifically for each individual microphone, can be loaded onto the measurement software so as to counteract the small irregularities in the response, normally a high frequency roll-off.

Since we’ll be taking equalization decisions based on it, it will be good if you can make an effort and not buy a microphone that’s too inexpensive. I would also not recommend purchasing a very delicate and/or expensive microphone.

And I would recommend that, the moment you take it out of its box, when it is still new (and has not yet taken a fall to the ground), you take a reference measurement with a loudspeaker that you have around and do not use much, in repeatable measurement conditions, and compare the response from time to time so as to detect any deterioration in the response of the microphone.

If you were only to measure phase, i.e., make adjustments to phase, any microphone you may have will suffice. From your typical vocal electrodynamic microphone to a wireless microphone. Perhaps it would be good to start practicing with a conventional microphone that you may already have and, over time, when you gain fluency and feel more confident measuring, invest in a measurement microphone with its corresponding compensation file.

As to the sound card, in theory we can use any. For taking these kind of measurements we do not need a card of exceptional quality. We’re not recording audio, just comparing the signal that goes into a channel with the signal that goes into the other.

What we will definitely need is one with at least two channels, XLR microphone inputs, phantom power and level controls for input and output. It would also be advisable that it had 1/4-inch phone line level inputs for electrical measurements. Most current cards come with XLR-1/4-inch combo connectors.


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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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