In a previous video, I talked about frequency response and what it tells us about an audio
system. Click here to watch that video and learn what frequency response is and why it's important.
This video answers a slightly different question: "How is frequency response measured?".
I'll show you a basic technique and free software that you can use to measure the frequency response
of your room, your speakers, your microphone, or any other audio system. But if this is our
first time meeting, my name is Kyle. You can learn audio production online by checking out the weekly
videos i post to the Audio University YouTube channel. Learn more at AudioUniversityOnline.com.
The method we'll use in this video to determine the frequency response of a system is called a
transfer function measurement. A transfer function of a system is a mathematical
function which models the system's output for each possible input. In this case, we'll be
measuring how a system responds to various input frequencies. By sending each frequency
equally to the system's input and measuring the signal again at the output of the system,
we should in theory be able to determine the effect that the system has on the signal.
A basic setup for making acoustical measurements consists of a computer, a measurement software,
an audio interface, a speaker, and a microphone. There's no need for a special computer. You can
use whatever you currently have access to. In fact, there's even a free measurement
software called Room EQ Wizard, or REW. I've put a link to the REW download page in the
description below this video. You'll need an audio interface with at least one microphone input,
one line input, and two line outputs. The speaker that you choose will depend on what
you're measuring. If you're measuring the transfer function of a room or a microphone,
you'll want to choose a speaker that has as close to a flat frequency response as
possible so that it can create each frequency evenly, or at least have a speaker that you
know the frequency response and can adjust your measurement accordingly. The same goes for the
microphone. If you're measuring something other than the microphone itself, you should choose
a mic that has a known frequency response or a microphone with a very flat frequency response.
Otherwise, the sound of the microphone will color the end measurement. Check out the list of
tools that I use for acoustical measurements and tuning systems in the description of this video.
To set up a basic measurement, I'll start by opening Room EQ Wizard.
In the top left corner, click "Measure". This window will pop up. You can see that
the software will test the audio system by sending a frequency sweep signal. This will test how the
system responds to each frequency along that sweep. Let's set up the inputs and outputs. I
want the test signal to play through Output 1 or the left output of my audio interface.
That's the output that I'll connect to the input of the system I'm testing.
I'll connect the output of the system I'm testing to Input 1 on my audio interface.
Select Input 1 as the input in the REW software. We'll also need to set up a path for a reference
signal. In a basic test, this is simply a cable that connects Output 2 of the audio
interface directly into Input 2 of the audio interface. This allows the software to ignore
any effect that the output and input of the audio interface have on the signal, because both the
test signal path and the reference signal path will include these elements. More on this later.
Before testing, it's important to check levels. The test signal will play out of the two outputs
we've selected. The goal is to adjust the gain until the test input signal and the reference
input signal are the same level. Once you've got the levels dialed in, click "Start Measuring".
After the test signal plays, you'll see the results of the test.
This graph should look familiar if you've watched my videos on frequency response graphs.
To learn how to interpret this graph, click the link in the top right corner of your screen.
Unfortunately, it's sometimes difficult to get accurate measurements - especially acoustical
measurements. That's because there are so many variables at play that need to be accounted for.
To understand this better, let's take a look at a very simple electrical measurement.
In a basic measurement of an electronic device, it's much simpler. The test signal path includes
the interface output, the electronic device (such as an equalizer), and the interface input.
The reference path only includes the interface output and the interface input.
This makes it possible for the software to measure only the changes caused by the electronic device.
However, consider all of the complexities that exist in an acoustical measurement that don't
exist in an electrical measurement. Let's say I want to measure the transfer function of my
speaker. The path of the test signal includes not only the speaker, but the room and the microphone
too. The acoustic properties of the room could result in cancellations, resonances, and other
anomalies that we have no way of accounting for in the reference signal. The microphone also has
a frequency response of its own. If we know the frequency response of the mic we're using, we can
adjust for it. But if we don't know the frequency response, it will affect the results of our test.
Luckily some of the most brilliant minds in the audio industry
have come up with ingenious ways of overcoming these obstacles.
Many tests of speakers and microphones are done in anechoic chambers - spaces
specially designed to eliminate reverberation, resonance, and echo. Acousticians, researchers,
and system techs also use specially designed speakers and microphones that either have a very
flat frequency response or a known frequency response that can be accounted for later.
Let's face it: most of us don't have access to laboratories or environments
that are specifically made for acoustical measurements, let alone the expensive tools
required to get the most accurate measurements possible. But that certainly doesn't mean that
we wouldn't benefit from experimenting and trying to get good measurements on our own.
If you're a student of audio, you'll definitely benefit from going through this process.
Just the act of considering all the different components of your system and brainstorming
ways to isolate the measurement to only measure a specific component is a great exercise. Do your
best using the steps in this video to set up a measurement that helps you understand the effect
a single component of your system has on the signals passing through it. If you don't have
the tools required to measure only one component of your system, measure the system as a whole
but consider the effects that each individual component will have on the results of that test.
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