UPDATED 9/15/21. I wrote this manual in response to requests from manufacturers who want to do CTA-2010 (formerly CEA-2010) measurements but are confused by the variety of methods and reporting practices used. I hope this manual helps to clear things up and to expand the use of bass output measurements, without which the performance of subwoofers and large speakers cannot adequately be evaluated and classified.
As of this revision , I have been doing these measurements for about 11 years, and have measured more than 300 products using this technique, including traditional subwoofers, passive loudspeakers, subwoofers included in 2.1 soundbar systems, and even 2.0 soundbars and portable/compact audio systems such as AirPlay and Bluetooth speakers. Here's a chart showing results from some of the subwoofers I've measured.
In my earlier days of doing this measurement, I often found it difficult to correlate the results of my CTA-2010 measurements with other technicians’ measurements. Since then, many of the reviewers and manufacturers doing CTA-2010 -- including Don Keele, the audio engineer on whose work much of CTA-2010 is based -- have collaborated to try to ensure their measurements are in the same ballpark. While acoustical measurements always have some amount of inaccuracy, especially outside an anechoic chamber, I think most of the leading technicians now doing this measurement would achieve results within about +/-1 dB with the same sample of the same sub. In fact, I'm proud to say Audioholics now uses my results for their subwoofer ranking system for subwoofers they haven't had a chance to measure.
However, as more manufacturers and reviewers adopt this method, I expect some will get results that do not square with others'. I have found many reasons why CTA-2010 results often disagree. These include:
1) Some people do not seem to have read the standard. However, I can sympathize because the price for the latest version of the standard, CTA-2010B, is now up to $92 on Techstreet, which seems pricey for an e-mailed 11-page PDF.
2) The standard is incomplete in places, and in the absence of specific instructions, technicians employ what they consider to be reasonable and appropriate practices. But of course, these practices may vary from person to person.
3) The reporting standards specified in CTA-2010 conflict with legacy reporting standards.
4) Unless they’re done in an anechoic chamber, acoustical measurements are inherently imprecise, especially when done outdoors (as CTA-2010 measurements almost always are), and especially when the devices under test are being pushed to and past their limits and are may be subject to performance inconsistencies due to thermal fluctuations and other issues. Sample-to-sample variations may occur as well. Thus, minor inconsistencies among measurements done by different technicians should be expected even under perfect conditions.
In this manual, I will do my best to reconcile the recommendations of the CEA-2010A document with the practices currently in use. My goal here is not to advocate any specific measurement or reporting practices, merely to come up with methods that conform as best as possible to the published CTA-2010 standard, to common bass output measurement practice and to good basic audio testing practices. Since I wrote the original version of this manual, CTA-2010B has been released. The changes involve more notes for testing of passive subwoofers; adding 80 to 160 Hz to the test frequencies; and a modification of some of the distortion thresholds. The former two are mostly irrelevant for home subwoofers, and I doubt most people will adopt new thresholds, since no one I know even adopted the changes specified in CEA-2010A.
If you have any questions or suggestions regarding this manual, please contact me at the e-mail address found on this website. Except as noted, all material presented here ©2021 Brent Butterworth. It is free for all to access and use, but none of it may be replicated for any purpose without my permission.
The CTA-2010 process uses a ground plane measurement technique, placing the subwoofer under test on the ground and placing the test microphone on the ground at a certain distance from the subwoofer.
A series of 6.5-cycle tone bursts centered at 63, 50, 40, 31.5, 25 and 20 Hz (and sometimes additional frequencies) is played through the subwoofer. These tone bursts simulate the signals that a subwoofer would receive when playing music or movies. They are short enough in duration that they are unlikely to damage a subwoofer due to thermal overload (i.e., burning out the voice coil or the amplifier output transistors) You can find these tones in 256 kbps MP3 form on the Tech page of this website. Here’s the waveform of the 63 Hz tone burst. Note that the peak level of the tone burst is about -1 dBFS.
The technician raises the level for the tone bursts until the distortion exceeds any of the thresholds set by CTA-2010. For example, the threshold for the 2nd harmonic (126 Hz for a 63 Hz tone burst) is -10 dB below the peak level of the fundamental tone. So if the peak level of the fundamental is 120 dB SPL, the level of the 2nd harmonic cannot exceed 110 dB SPL. Below is the list of thresholds used in the original CEA-2010 document, and still in most common use.:
2nd harmonic: -10 dB
3rd harmonic: -15 dB
4th and 5th harmonics: -20 dB
6th, 7th, 8th harmonics: -30 dB
9th and higher harmonics: -40 dB
Note that in CEA-2010A, the threshold is -25 dB for the 4th and 5th harmonics; -35 dB for the 6th, 7th and 8th harmonics; and -45 dB for 9th and higher harmonics. In CTA-2010B, the thresholds change with frequency. However, almost all of the audio pros I know who do CTA-2010 use the original thresholds.
You can gauge the level of the fundamental and harmonics by viewing them on a real-time audio spectrum analyzer or by using various CTA-2010 measurement programs. Turn the level down until none of the harmonics exceed the CTA-2010 threshold. Then you note the peak level of the fundamental. That’s your CTA-2010 result. Repeat the process for all six test frequencies. (Some technicians measure at lower and/or higher frequencies, too, but in my opinion the six test frequencies from 20 to 63 Hz give a complete picture of a subwoofer’s performance.) Now that you understand the basic procedure, let’s get into the details.
The basic hardware required for CTA-2010 measurements is a computer, a USB stereo audio interface, a measurement microphone and a microphone calibrator.
The USB audio interface isn’t critical as long as it’s a reasonably good one. It must have a microphone input with 48-volt phantom power output, separately adjustable left and right recording level, and preferably a headphone output. Most pro-style USB interfaces will work. I use the M-Audio Mobile Pre, which is no longer available, and have also used the Focusrite Scarlett 2i2. Other USB interfaces that should work for CTA-2010 (although I have not tried them) are the PreSonus AudioBox USB and Sound Devices USBPre. The USBPre has been used for CTA-2010 by at least one manufacturer.
The microphone should be an electret condenser measurement microphone. I cannot find a specification for the microphone in the CTA-2010 document, but any electret condenser measurement mic should work. To my knowledge, all of these microphones have flat response below 1 kHz (adequate for CTA-2010 because the 9th harmonic of 63 Hz, the highest standard test freqeuncy, is 567 Hz). As a few subwoofers are capable of delivering more than 125 dB at 2 meters, it seems to me that a microphone rated to 130 dB would be the best choice. Everyone I know who does CTA-2010 tests uses the Earthworks M30 microphone, and that’s why I use it, but there’s no reason a different measurement mic couldn’t work.
The calibrator can be any standard microphone calibrator that accepts the size of measurement microphone you use. The M30 is a 1/4-inch mic, but Earthworks sells an adapter for use with 1/2-inch calibrators. (Unfortunately the adapter, which is just a little plastic cylinder, cost $30 last time I checked.) My preference is the Reed SC-05 but any calibrator that fits your microphone should work.
If you’re using an RTA or Room EQ Wizard to do CTA-2010, the connections are straightforward. Connect the microphone to one of the inputs on the USB interface. Connect the output of the USB interface to the subwoofer. Set the input level for the microphone to minimum or near minimum, because the incoming signal will be high. Calibrate the microphone using whatever features your RTA software offers for mic calibration. In TrueRTA this is the “SPL Calibration” function under the Audio I/O pull-down menu. In REW, it's under the Calibrate button if you click on the SPL meter function.
If you’re using the freeware program with Igor Pro, the connection/calibration process is somewhat unintuitive. Connect the microphone to input 1 of the USB interface and set the input level to minimum. Connect input 2 of the USB interface to output 2, so it forms a loop, and set the input level to maximum. (I use the rear-panel line input/output jacks on the M-Audio Mobile Pre for this connection; that’s the blue cable toward the top of the photo below.) You can see what the connections look like here. Note that I’ve taped “reminder labels” on the Mobile Pre so I’ll remember what the correct settings are.
Don Keele suggests using the headphone output of the USB interface to drive the subwoofer, which is a good idea no matter what software you're using, because it allows the output level driving the subwoofer to be adjusted using the headphone volume control. However, because the maximum output of the headphone jack is typically much higher, this can overdrive the sub input. (On my M-Audio Mobile Pre, max output from the line outs is 1.8 volts RMS, and from the headphone output it’s 3.6 volts RMS.) I recommend testing the headphone output of your USB interface with a -1 dBFS sine wave (or -3 dBFS if you're using REW, which is the highest it will go), noting the headphone level setting required to get a 2 volts RMS output, and not exceeding this level during testing. A subwoofer should be able to reach maximum output with a 2 volts RMS signal. Unfortunately, maximum test signal level is not specified in CTA-2010. In a future revision of the standard, I hope that either a maximum test signal level is specified, or the maximum output level for the USB interface with a 0 dBFS signal is specified.
I set the levels within the Windows Control Panel at 50 for input level, 98 for output level, and I make sure the system volume control (the one accessible at the lower right in Windows) is turned all the way up.
Every time you set up for a CTA-2010 test, you should calibrate the microphone. To do this in the freeware program, click the Calibrate Mic button in the Calibration window in the software. I calibrate at 114 dB SPL, because that is closer to typical subwoofer measurement levels than the usual 94 dB. (Technically this shouldn’t make a difference, though, and if your calibrator can output only one level, just calibrate the software to that.)
With the Igor Pro freeware program, turn on the calibrator, hit the Start Data Gathering button and let it run until the calibration level stabilizes (usually 10 or 15 seconds). Hit the Stop Data Gathering button, then the OK button to complete calibration. Note that the Counts Per Pascal readout shouldn’t change much from your last calibration. You can double-check your calibration by changing the Burst Frequency setting to 1,000 Hz in the Operation window, then hit Do Single Burst Test in the Operation Window. In the Acoustic Data window, the Peak SPL, dB result should be 117 dB if you calibrated at 114 dB RMS.
With REW, click the Calibrate button on the SPL meter screen, then select "Use an external signal" under the drop-down window. Play the tone from your calibrator, set the level in the REW screen to 114 dB, then click Finished.
CTA-2010 testing is best done in a large open space, so reflecting objects cannot significantly affect the measured sound waves. Typically the measurements are done in a parking lot, because many parking lots have nearby AC outlets to power the subwoofer under test. There are, however, alternatives for those who do not have access to such a space.
The CTA-2010 document mandates that the minimum distance from the microphone and the subwoofer to the nearest reflecting object be 0.75 times the wavelength of the minimum measurement frequency. For 20 Hz, this distance is 11.8 meters or 38.7 feet. When you incorporate the mic-to-subwoofer distance, this means you need a cleared, obstruction-free area about 26 meters or 84 feet in circumference. The CTA-2010 document also mandates that the maximum ambient noise level be no higher than 50 dB RMS SPL at any frequency, although by averaging multiple test bursts, which is allowed in the CTA-2010 software, you can increase the signal-to-noise ratio — i.e., averaging four test bursts improves S/N ratio by +6 dB — and thus do your measurements in a noisier environment.
For those who do not have access to a large, relatively quiet space with an AC power source, CTA-2010 permits the use of a room correction factor to compensate for the effects of nearby reflecting objects. This allows CTA-2010 measurements to be done in a room or in a relatively small outdoor space. (I use my backyard.)
To create a room correction factor, you must first measure the ground-plane frequency response of a sealed-box subwoofer in a test environment that meets the standard cited above, or in a calibrated anechoic chamber. I built a sealed-box subwoofer for this purpose, using a 15-inch driver. The design of the subwoofer is not critical as long as it is a sealed-box design and has reasonable output at 20 Hz. (Mine is around -10 dB at that frequency.) Because I use a battery-powered amplifier to drive the subwoofer for these measurements, I was able to do them on a nearby soccer field that does not have available AC power. I recommend you run these measurements at 1, 2 and 3 meters.
Now run the same frequency response measurements in your testing environment, with the subwoofer and the microphone at the exact positions you intend to use for testing. If you display the open-space frequency response measurement on the same screen as the measurement taken in your test environment, you can gauge the room correction that is required at each frequency. Here is the result comparing the measurement in the soccer field (purple trace) and the measurement in my backyard (green trace).
If you're using the Igor Pro freeware program, write down the difference between the free-space measurement and the test environment for frequencies in 1/3rd octave steps starting at 10 Hz and going up to 2 kHz. (The Enter Chamber Correction Data screen in the CTA-2010 software goes up to 10 kHz, but I have filled in the data only to 2 kHz, because higher frequencies are rarely if ever significant in CEA-2010.) If the test environment attenuated the audio from the device under test, write the result as a negative number (i.e., -2.3 dB at 40 Hz). Transfer these values to the Enter Chamber Correction Data screen and hit OK when finished.Note that if you intend to use different mic-to-subwoofer test distances (we’ll get to that next), you will need to create chamber correction data for each measurement distance, then save each routine as a separate Igor Pro project, such as “CTA-2010 2M” for a 2-meter distance. Then when you test with a 2-meter mic-to-subwoofer distance, open this project and your settings will be correct.
If you're using the CTA-2010 routine in Room EQ Wizard, you'll need to divide the measurement taken in your test environment by the measurement taken in free space. This should give you a correction curve that hovers above and below the 0 dB line. Open the Preferences window in REW and enter this as your correction file.
The distance from the microphone to the subwoofer under test has been the subject of some controversy. The original CEA-2010 document specified a distance of 1 meter or greater, but most of the people doing subwoofer output measurements were measuring at 2 meters, to incorporate a better blend of the contributions of all radiating devices (drivers, ports, passive radiators). The CEA-2010A standard was changed to accommodate this concern, but the new document specifies a minimum test distance of 3 meters. In my opinion, 3 meters seems unnecessarily far because it is unlikely to significantly improve the incorporation of all radiating elements, while it reduces the signal-to-noise ratio of the measurement. I don’t know of anyone doing CTA-2010 who is using 3 meters as the measuring distance.
Another problem is that with devices that have relatively low bass output, such as wireless speakers and the subwoofers that come with 2.1 soundbars, a measurement distance longer than 1 meter reduces the signal-to-noise ratio to the point where it is often impossible to get CTA-2010 results at frequencies below 31.5 Hz. Also, for such small devices, a 1 meter measurement distance seems adequate to incorporate the contributions of all radiating elements.
My recommendations for test distances are as follows:
Powered subwoofers with drivers 8 inches or larger: 2 meters
Powered subwoofers with drivers 7 inches or smaller, wireless speakers, 2.0-channel soundbars: 1 meter
Of course, subwoofers come in many configurations, with drivers, ports and passive radiators in different positions. CTA-2010 mandates that the device under test be oriented so that the measurement microphone is equidistant from the major radiating elements. In the case of a subwoofer with a driver on one side and a passive radiator or port on an adjacent side, the subwoofer should be angled 45 degrees. If the subwoofer’s radiating elements are on opposite sides, the subwoofer should be turned so its radiating elements point 90 degrees away from the microphone, as seen below. In the case of a subwoofer with radiating elements on three sides (usually a driver on one side and passive radiators on two sides), I point the driver directly at the microphone. The measurement distance is considered to be the distance between the microphone and the closest point on the device under test.
A complication arises with subwoofers that have active drivers on both ends. With these subs, the radiating elements (drivers) are further away from the microphone than they would be with a front-firing subwoofer. There are various arguments one could make for and against awarding extra dB to subs of this design (and not to subs with a front-firing driver and rear-firing port, which according to CTA-2010 requirements are also measured from the side and not the front). One could argue, as the website Data-Bass did (although not without reservations), that a compensation curve could be used that incorporates the differences in room gain among subs of differing designs. I've tested only one large sub with this configuration, and in this case, I calculated the theoretical reduction in dB that the additional driver-to-mic distance would cause, and incorporated it into my results.
I can find no specification in the CTA-2010 document for the settings on the subwoofer or other device under test. In the absence of a standard, and in the interest of simplicity, I do CTA-2010 measurements with the device under test set to full volume. If the subwoofer has a built-in crossover, I set it to bypass or to the maximum possible crossover frequency. I deactivate all EQ modes and set the subwoofer or device under test for the flattest possible response.
If you're using the Igor Pro freeware program, select your test frequency in the Burst Frequency, Hz window, select whatever number of bursts you want to average. The more bursts you average, the better the signal-to-noise ratio (and consistency and accuracy) of your measurement, but the longer it will take; I usually select 4 because my test environment is reasonably quiet. Click the Auto Burst Locate and 1/12th-Octave Smoothing windows (plus the Use Chamber Correction window if you're using a correction curve), then click Do Single Burst Test.
You should hear the bursts coming out of the subwoofer, and see the RTA readout of the fundamental tone of the test frequency, plus the distortion harmonics at higher frequencies. Raise the test signal level until the Overall window in the Pass/Fail Data screen reads "Fail," then lower the level (using the level control on the USB interface or the onscreen controls in the program) very slightly for each round of test bursts until it reads "Pass." Note the level in the Peak SPL, dB window on the Acoustic Data screen. This is your CTA-2010 result. You may also wish to record the distortion level that appears in the THD, 2nd - 5th, % window. If the Overall window in the Pass/Fail Data screen reads "Pass" even with the test signal level at maximum, note the Peak SPL, dB level on the Acoustic Data screen, and make a note that the sub did not exceed the CTA-2010 distortion threshold at this frequency.
If you're using REW, open the Generator window. You'll see a drop-down window that selects the test signal; it'll probably say "Sine Wave." Select "CEA-2010 Burst," select your first test frequency (probably 63 Hz) and raise the RMS Level dBFS to -3.0, which is the highest it will go. Now open the RTA window, and click on the Settings icon in the upper right. Under Smoothing, select "1/12 smoothing." Under Averages, select whatever number of bursts you want to average. The more bursts you average, the better the signal-to-noise ratio (and consistency and accuracy) of your measurement, but the longer it will take; I usually select 4 because my test environment is reasonably quiet.
Click the red button in the upper right, and you should hear the test bursts play through the subwoofer, then see the fundamental tone and its harmonics appear on the display screen, along with a maximum dB level reading in bold type above the fundamental tone (which you can see if you scroll back up to the REW sample screen, where it reads "116 dB peak"). If the distortion is below the CTA-2010 thresholds, this reading will be in black. Raise the level until it turns red (which means that one of the CTA-2010 distortion thresholds was exceeded), then lower the level very slightly in small steps until the maximum dB level reading turns black again. This is your CTA-2010 result. If the peak dB readout stays black even with the test signal level at maximum, make a note that the sub did not exceed the CTA-2010 distortion threshold at this frequency.
This is another area where practices among those doing CTA-2010 measurements have varied. The CTA-2010 document is specific in saying, “This SPL shall be referenced to a 1 m ground plane level,” and as described above this is a peak level measurement. However, common practice before CTA-2010 was to report the results at 2 meters using RMS values. Many, perhaps most, of the technicians currently doing CTA-2010 are still reporting at 2 meters RMS. This is not a problem for people who understand what these measurements mean, because the 2 meter RMS result can easily be converted to a 1 meter peak result by adding +9 dB. However, this has caused some confusion for some consumers who understandably look at the numbers without checking to see what the numbers are referenced to.
My practice with published reviews, such as this review of the Elac Adante SUB-3070 published on SoundStage, is now to show both results — 1 meter peak and 2 meter RMS — in the hope that this will make consumers aware that there are two different CTA-2010 reporting standards in common use. For my consulting work, I always report 1 meter peak results.
Results for each test frequency should be reported, and the averages of results for each octave (20 to 31.5 Hz, 40 to 63 Hz) should also be presented. These averages must be calculated by converting the results in dB to pascals, averaging the results in pascals, then converting the pascal average back to dB. I have created an Excel sheet that does this calculation for you, which you can download here.
On smaller devices that do not have measurable output at 20 Hz, you can still calculate an average output for 20 to 31.5 Hz by taking the 25 Hz result, subtracting -18 dB, and using that figure for the 20 Hz result in the average calculation.You can see an example of how I report CTA-2010 results on this page. Note that I use an “L” after the dB result to denote measurements on which none of the CTA-2010 distortion thresholds were exceeded.
If you would like to compare your CTA-2010 measurements with some of mine, a spreadsheet of many of the CTA-2010 measurements from my reviews is available here.
I hope this manual has clearly explained CTA-2010 and answered all of the questions you may have about it. If you have any further questions or suggestions for how this manual might be improved, please contact me at the e-mail address found elsewhere on this site.