Trust, But Verify – Response and Using Room EQ Wizard

Mike Rivers © 2019

Prologue

“Trust, but verify” is from a Russian proverb, a phrase that former US President Ronald Reagan frequently used when speaking about the Soviet Union’s compliance with a nuclear disarmament treaty. If you’ve followed my articles about specifications here - what’s important, what’s not, and what’s missing from the published spec sheet - I think you’ll agree that the proverb fits well. While manufacturers rarely publish a specification that a product can’t meet by hook or crook, measurement methods or missing tolerances can make something look better on paper than it actually is.

In this occasional series, I’ll explain how to make measurements on your own gear using free or inexpensive software and tools you might already own. This is more “proofing” than “testing” in that you won’t get lab accuracy with the methods I’m describing here, but you’ll be able to compare its performance against its published specs, fill in the missing pieces, and have a better understanding of what it can do, and how well. We’ll be measuring voltage, current, and resistance or impedance, distortion, frequency response, delay (latency), dynamic response, and maybe some other characteristics. There’s plenty to learn, and much fun to be had in the process.

Some Basic Concepts of Test and Measurement

Most testing involves a stimulus-response setup. You connect a signal source (the stimulus) to the input of the device under test (DUT) and measure the output (the response).

There are absolute and relative measurements. A preamp’s maximum output level is an absolute measurement, one that needs to be measured with a calibrated voltmeter. The characteristics of the preamp’s low-cut filter, however, involve relative measurements. We’re not interested in the actual output voltage of the preamp, but rather, the change in output voltage over the frequency range where the filter is doing its job.

It’s important to understand the difference between accuracy and resolution. A digital voltmeter will measure a fresh 1.5v alkaline battery at around1.566 volts. The meter’s resolution is to 3 decimal places or 1/1000th of a volt (1 mV). However, if the meter’s specified accuracy is ±2%, the actual voltage could be anywhere between about 1.535 and 1.597 volts. Don’t be bamboozled by a measurement with lots of decimal places (resolution) without knowing the measuring accuracy. Digital voltmeters, from where these “to three decimal places” numbers come, have their own set of potholes, but I’ll address them in another installment.

Before checking out a piece of gear, decide what to measure and have a realistic expectation of the results. A mic preamp’s frequency response will be very flat, but if it has a low-cut filter, you’ll want to measure its frequency response with the filter switched both in and out. If you’re investigating a preamp’s “character,” you’ll want to look at distortion both with a test signal that’s comfortably below the maximum input level and with it driven into distortion. And you’ll want to look both at distortion with the input overdriven, and with the output overdriven. There’s a difference. If your measurements differ substantially from your expectations, you might be measuring incorrectly, measuring the wrong thing, the device might be broken, or you might just be surprised!

Lastly, document your testing so you can understand your results.

Now, let’s measure some stuff.

We’re Off To See The Wizard

Room EQ Wizard, that is, REW for short. It’s a free (donations cheerfully accepted) program with versions that support Windows, Mac OS-x, and Linux. As its name suggests, REW is designed for measuring room acoustics, but we’re not going use it in that manner (though you certainly can, for other measurements). For its intended application, REW generates a test signal that’s sent out to the room’s speakers via your computer’s audio interface. A calibrated connected to the interface’s input picks up the room sound, and the difference between the test signal and the microphone’s output is analyzed and displayed.

For the measurements discussed here, we’ll replace the speakers, room, and microphone with the DUT, with REW both providing the stimulus (test signal) and measuring the device’s response. It’s a cool tool, but there are some caveats.

While the accuracy of your measurements is limited by the linearity of your interface, most contemporary recording interfaces are sufficiently accurate and linear for making most audio measurements. Connections between the DUT and interface are mostly straightforward, but be aware that there’s no single best way to mix balanced and unbalanced interconnections – grounding or floating the wrong lead might degrade your test results. Remember, too, that every connection is a potential noise source, so avoid stringing adapters together and take the time to make or modify cables for the most direct connections.

It’s also important to understand headroom and manage gain and signal levels appropriately. REW, by default, expects an input level from the DUT that allows about 12 dB of digital headroom, and will complain if it detects clipping (0 dBFS). You probably allow more headroom than this when recording with your DAW, so treat your REW setup with the same care. Driving a mic preamp into the “warm- and-phat” range from the interface’s output is easy whether the preamp’s input has a transformer or is transformerless, but your interface’s maximum output level may be incapable of driving a input transformer into saturation.

Many of today’s interfaces offer “zero latency” input monitoring that sends the input signal directly to the output, avoiding a trip through the computer. For our use here, an interface’s direct input monitoring must be turned off. REW needs to see only the test signal coming from the DUT, and engaging direct monitoring can cause feedback.

Lastly, recognize that you’re in Computer Wonderland here, a place where no two computers are alike, and sometimes differences can cause unplanned behavior. I’ve only used REW on Windows XP and Windows 7. It’s fair to assume that the other OS versions work similarly, but one never knows. I have some tips here for setting up and using REQ for measuring your audio gear, but this is intended neither as a tutorial nor a review of the program.

REW Setup and Calibration Setting up REW to work with your interface is essentially the same as setting up a DAW. Open the Preferences window, click the Soundcard tab, and choose the input and output to which you’ll use for connecting the DUT. REW is a single channel processor, so you’ll need two passes to measure a stereo device. Select the ASIO driver if your interface has one, otherwise, for Windows, select Java, which uses the standard Microsoft sound card driver. The Mac version uses the OS-X Core Audio device selection.

44.1 kHz sample rate is adequate for measuring across the conventional audio bandwidth, but with a higher sample rate, you can measure at above 20 kHz. The rest of the settings can be left at their defaults for now.

The next step is to calibrate the interface. First, connect a cable between the interface’s output and input. If your interface has input gain and output level controls, set them to their normal working positions to approximate unity gain. Click the Check Levels button and, if necessary, fiddle with the interface’s input and/or output controls to make REW’s meters match within a couple of dB. If the interface has a Clip indicator, be sure it remains off. Your goal is for both the Output and Input meters on the Calibrate screen to indicate close to –12 dB.

When you get the levels set properly, click the Calibrate button, and instructions at the bottom of the page will walk you through the procedure. This measures the frequency response of your interface and creates a correction table so you’re your measurements will be corrected for any irregularities in your interface’s frequency response.

Since the calibration sweep starts at 0 Hz, low frequency roll-off is normal, as is the sharp drop-off above the Nyquist frequency (half the sample rate). If all is well, the frequency response will be flat within a few tenths of a dB over the normal audio range. Finally, click the Make Cal button (that’s short for “use this as the calibrated interface”), name and save it, and your test system is ready to go.

Measurements during a session are stacked at the left of the screen. REQ’s test signal provides data for several different measurements in one shot. Tabs above the plot area of the measurement screen select what’s displayed on the graph. “SPL & ” shows the most recent measurement or one you highlight in the stack. “All SPL” superimposes the frequency response of all of the measurements on a single graph. Check boxes at the bottom the screen allow you to choose which measurements are included in the plot. The graph here shows six frequency response measurements of a graphic equalizer with a different setting of the sliders on each test run. The camera icon at the left takes a screenshot to go into your test report. You can save your measurement data in a file so you can view measurements later or look at other data from the same test run.

Real Measurements

Before writing this article, I measured everything in sight with REW and chose a few examples to show some interesting things you can learn about your gear. We’ll measure frequency response of a parametric equalizer at different bandwidth settings, and an overdriven mic preamp with some (ahem!) character to illustrate distortion characteristics.

Frequency Response

An equalizer’s job is to put humps and dips in its frequency response so it’s an ideal test subject. Here’s a series of frequency response plots of one section of a parametric equalizer, with the goal being to determine the actual range of its bandwidth knob, which is uncalibrated, with only a “wide” symbol at one end of the knob’s rotation and a “narrow: symbol on the other, with no mention of bandwidth in its spec sheet.

With the center frequency fixed at 400 Hz and the gain at maximum boost, I measured frequency response at three different bandwidth (Q) settings - widest, narrowest, and in the middle, and then repeated those measurements with maximum cut. Using those measurements, we can calculate some real “spec” numbers for the equalizer. With the medium bandwidth setting, the boost (yellow) curve drops 3 dB below the peak at 270 and 540 Hz. Bandwidth is defined as the frequency difference between the 3 dB limits, which in this case equals 270 Hz. Q equals the center frequency divided by the bandwidth: 400 ∕ 270 ≈ 1.5. Work out Q for the other bandwidth settings. There will not be a test.

When measuring anything with more than 12 dB off flat frequency response (REW’s nominal headroom), it will be necessary to do some level adjusting in order to avoid clipping. In this case, the equalizer’s bandpass section that we’re testing has a maximum boost of 15 dB boost at its center frequency, we’d be exceeding REW’s nominal 12 dB of headroom before clipping. An easy fix is to reduce the test signal level in the Measurement screen from –12 dBFS as shown here to -20 dBFS. You could also use level controls on your interface or the equalizer itself if you have them.

Distortion

Normally we expect as little distortion as possible, though sometimes we want to add some color to our sound. One way to do that is to “overdrive” a device in order to increase its distortion above its designed amount. In practice, you’d judge the right amount of “overdrive” by ear, but knowing a device’s distortion characteristics can help you decide which are good candidates for “color” and which just sound bad when overdriven.

REW measures harmonic distortion as a function of frequency, plotting each harmonic (up to 10th if you choose) of the test signal. In addition, it computes and plots total harmonic distortion (THD), which is what you usually find on a spec sheet. Folklore tells us that even harmonics (2nd, 4th, etc.) sound pleasant and odd harmonics (3rd, 5th, etc.) sound nasty. If you want warmth, look for a healthy dose of even harmonics, and odd harmonics if you’re looking for grit.

THD of the preamp measured here, 0.066% at 1 KHz, is barely audible. Here, the 2nd harmonic (red) is predominant, and is probably contributed by its input transformer. The 3rd harmonic (green) is probably from the electronics, and is somewhat lower.

Increasing the input level by only 3 dB from the measurement above shows a substantial increase in THD, to 6.75%. This distortion is clearly audible and might be useful grit. What’s interesting here is that the relative levels of the individual harmonics have changed. The 3rd harmonic (orange) is predominant over most of the frequency range, comprising nearly all of the THD until it drops into the bit bucket at around 7 kHz. This is because at the 44.1 kHz sample rate, the 3rd harmonic of frequencies above about 7 kHz exceeds the Nyquist frequency. The 2nd harmonic is also greater than in the “clean” measurement, but at this level, odd harmonics (3rd and 5th) contribute most of the distortion.

Overdriving the input to this level of distortion puts the preamp’s output level at a level that exceeds the maximum input level my interface. This can’t be fixed by reducing the preamp’s input level because then it wouldn’t distort. Since the preamp doesn’t have an output level control, I inserted a pad between the preamp output and interface input to keep the system happy. You may need to do the same.

No Hardware? No Problem!

I initially had reservations about writing this article since most signal processing is done with software plug-ins these days and you’d assume you have nothing to test. Well, get to work! It’s possible use REW to measure virtual signal processors (plug-ins) just as you can with real hardware. In order to do this, we’ll use a software “virtual cable” with the plug-in installed on a DAW track.

Start up your favorite DAW, create a track, and install the plug-in you want to measure. Instead of assigning the track’s input and output to their normal interface I/O ports, assign them to two virtual cables. Then set up REW’s input and output to use those virtual cables instead of your interface.

As a hardware guy, this seems pretty ethereal to me, but, by golly, it works. I’ve shown the application Virtual Audio Cable (VAC) here, but there are others. They install as drivers and appear to your audio software as available input and output devices. Free trial versions are available. With VAC’s, a voice breaks in occasionally to remind you that you haven’t paid for it yet. If the nag interrupts your measurement, just re-run it. VB-Cable, another virtual cable application gives you only two cables in the free trial version, but that’s sufficient. For Mac users, I’ve been told that Loopback will do the job but being a Windows-only shop, I haven’t been able to test or illustrate it.

Here’s the REW setup with VAC. The two cables are named Line 1 and Line 2. You can use either cable as the Output Device as long as you use the other end of that cable as the Input device for the DAW track, and vice versa. The nomenclature is a bit confusing but you’ll get it sorted out.

Reaper’s Audio Device Settings page, which is where you’d normally point it to your interface’s inputs and outputs, is shown here to the left. Note that Line 1 (the virtual cable) connects the Output of REW to the Input of Reaper and Line 2 connects Reaper’s output to REW’s input.

For illustration, I’ve made some goofy settings on an EQ plug-in and used REW to measure the frequency response of the track. The plug-in is on the left and the REW measurement is on the right. Pretty close?

Depending on your DAW and how it handles monitoring, this may take a little finagling to get working. Try arming the track, and/or enabling Input Monitoring.

Knowledge is Power

Today, even low cost audio gear performs remarkably well, so there’s no compelling need to dig into the details, but it’s interesting, fun, and sometimes useful. You may discover things aren’t as benign as you expect. For example, a popular mixer’s mic preamp with a frequency response specified as: “ 20 Hz – 20 kHz” is essentially flat at low gain, but at full gain, it’s about 1.5 dB down at 20 Hz. (Figure 12) Bug or feature? When recording a quiet source, a little low frequency rolloff might be just right.

Use your newfound knowledge prudently. Don’t accuse a manufacturer of false advertising based on your test results, particularly when you have only a partial specification. And this is why you should care about specifications!

In future installments, we’ll tackle other measurements and other measuring tools and test equipment.

Sources For Software:

Room EQ Wizard info and downloads: https://www.roomeqwizard.com/ Versions for Windows, MacOS, and Linux

An excellent stand-alone (not a DAW plug-in) for Windows: http://vincent.burel.free.fr/download/index.htm

Virtual Audio Cable: http://software.muzychenko.net/eng/vac.htm (Windows) VB-Cable: http://www.vb-audio.com/Cable/index.htm (Windows) Loopback: https://rogueamoeba.com/loopback/ (MacOS-x) Jack, for the courageous, is a standard part of nearly all Linux distributions