Audioholics 2010 Subwoofer Shootout Measurement Methods

The Ground Plane Testing Method

All of the subwoofers in the review were placed on the ground, in a large parking lot free from obstructions or objects from which sound would be reflected save for the ground. 

While this is the method shown in the CEA standard, I imposed my own "peculiar" take on this based on 20 plus years of experience in measuring vented sub-woofers in real world rooms.  This method as shown will emphasize the output of the vent relative to the driver output because of proximity errors, (in other words the mike is closer to the port than it is to the speaker).  Another source of error will occur as differences in the size of the speaker vs the port are common.  (Because the port is smaller, it contributes more at very close distance from the mike than the larger surface area of the sub, even if far far away, their contribution would be equal). My solution was simple, if not perfect.  I turned the box on its side, and measured the microphone relative to a line midway between the center of the ports, and the center of the driver.  This would tend to give me greater accuracy than the method as pictured above, and all subwoofers with this configuration were tested on their sides for this reason.  This is something we hope CEA will notate in their next revision of the standard.

Subwoofer Box orientation with ports and driver both against the ground

This is how I tested the Rythmik sub in my parking lot.  Notice the ports are to the side of the driver, not underneath as you would place it at home.

The area where the subwoofer was placed was on a carpet to protect the cabinets and microphone, on the ground as per the guidelines shown in the CEA 2010 standard.   This mike was placed on the ground 39.37 inches (that's a meter to you mate) from the front of the baffle (no grilles were used for front firing woofers).  This technique, while reasonable does place larger woofers, and systems with side firing woofers and/or ports at a disadvantage in large part because it still requires the meter be measured to the front of the cabinet instead of the centerline between speaker and ports or passive.  In my opinion, the standard groundplane measurement as we previously illustrated deals with those configurations in an unfair way.   This may be in large part due to many years of precedent in calling for measurements to be made relative to the front of cabinets instead of the acoustic center of the radiator(s).  That is a simplification which is reasonable, even if an additional source of error.

CEA 2010 Proposed Alternative Groundplane Measurement Technique
for Alternate Driver Port Configurations

This tradition of measuring the SPL at 1 meter from the cabinet face (or side) may be in large part due to many years of precedent in calling for measurements to be made relative to the front of cabinets instead of the acoustic center of the radiator(s).  That is a simplification which is reasonable, even if an additional source of error.  (One could point out that a deep woofer actually mounted to the front panel, has an acoustic center farther away than the shallow woofer mounted in the same box.  At some point, we have to pick a convention and stick with it, regardless of its imperfection.)  One way to make this issue less bothersome is by increasing the measurement distance to 2 or 3 meters.  The problem with that solution is that the further away from the speaker you get, the lower the ratio of direct sound versus the reflected sound which is essentially a function of the acoustic space in which you measure, and not the speaker itself.  

Testing subwoofers accurately and without the response of the room added to your measurement requires either an outdoor measurement setup, or a very very large anechoic chamber, the kind of which few in the world exist large enough to be usable down to the lowest frequencies we can hear (16Hz).   I being a poor soul, devoid of either a chamber or a 90 ft pole with a platform and ladder, allowing me to do true “free air” testing, opted for the outdoor method.  It is common knowledge among those of us who measure sound frequently in real rooms that once your mike is more than a cm or two away from the cone, most of what you are measuring is room reflection, not speaker output.  So, how do we get around this problem without a multi-million dollar room that will eat all reflections? (An anechoic room).  We measure outside.  But what about ground reflections?   Are they not also a source of errors?  Yes, unless you place the microphone directly on the ground so the “reflection” is actually in phase with the direct sound!  This method is known in the industry as “ground plane” measurements. 

Is this the best way to measure the speaker?  Shouldn't it be up in the air, pointing directly at the center of the subwoofer?  Those are all good questions, so let's take a minute to consider how sound forms around the speaker box at very low frequencies. 

There is a school of thought that says, ground-plane measurements are too, a source of less than perfect information, so they prefer to climb a tall pole and hoist their speakers far above the ground.  On a clear, sunny and windless day, this is indeed, the best of all methods (minus that huge anechoic chamber) but this particular writer has another peculiarity, fear of great heights.  So, between that and the CEA standard as cover, I opted for the ground-plane method.  For those unfamiliar with the concept, let me explain it like this.  First of all, we need remember what sound is, a wave motion propagated through particles (air molecules) which end up in tightly or sparsely populated groups with densities greater or lesser than ambient air pressure.  The louder the noise, the more intense the difference between the maximum and minimum density of air molecules.

Softer sounds barely effect the relative pressure at all.  Since the speed of sound is a relative constant (it varies slightly with temperature pressure and humidity), we can calculate the distance between the pressure peaks and minima's in a waveform based on the frequency.  The relationship is simple and stated thusly: C = λ * F (Speed of sound (C) = Wavelength (λ) times frequency (F)).  Since C is a constant, if we know the frequency (F) we can calculate λ (lambda) which is the distance between the peak-to-peak pressure (either negative to negative or positive to positive) in whatever units the speed of sound is expressed in.

2 Full Cycle of Sine Wave – Single Frequency vs Time

Let's do some simple math now.  I want to know how long a 20 Hz wavelength is.  In the graphic above, if the horizontal time scale is equal to 0.100 seconds, and I have two cycles as shown, my frequency would be = (2/0.100 seconds) = 20 Hz.  My normal C is 343 Meters per second = 13,504 inches per second = 1125.3 ft per second.  So expressed in inches, the wavelength at 20 Hz is 13,504/20 = 675.2 inches long, or about 56.27 ft or 17.15 Meters long.  Wavelength is the distance that 20 Hz covers in one cycle and it takes 50 milliseconds to complete.  Another way to say the same thing is the period of 20 Hz is 50 milliseconds. (Period = 1/F)  At 200 Hz, the wavelength is 1/10th as long, and at 2000 Hz, the wavelength is 1/100th as long; and at 20,000 Hz, the wavelength is exactly 1/1000th as long or only 0.675 inches.  Remember, since the speed of sound is essentially  constant, what changes as we change the frequency at which we excite the air, is the number of peak and minimum pressures within a given distance.  We call one cycle of distance the wave-length (Length of the wave, of course!)

The 6.5 Cycle Waveform Used In Don Keele's CEA Testing Program

Above is the graphic which shows the waveform used in the CEA test program.  This waveform has the requisite 1/3^rd octave width needed for testing.  An important note regarding this test is that the program, and the numbers which we will publish in the specific product reviews are all representing the PEAK output of the subwoofer, NOT the RMS (root mean square) value which is so often the gold standard used to quantify amplifier power, or loudspeaker power handling.  This is appropriate because a sine wave has only a 3dB peak to rms ratio.  (Meaning a 100 watt RMS sinewave, has a peak value of 200 watts).  Subwoofers have such a restricted bandwidth, only the least expensive of them ever get destroyed by heat or rms power delivered.  The real test for a subwoofer is in the amount of peak power it can take, and more importantly the peak SPL within reasonable distortion limits it can deliver.

For the CEA measurements, while the harmonics are measured up high, the highest fundamental frequency of importance is 80 Hz, where the wavelength is 168.8 inches long.  Because this wave-length is still large compared to the size of the box, and the distance from the acoustic center to the ground, our measurement error resulting from ground-plane measurements remains small.  (Of course, since we are looking up to the sixth harmonic of the maximum driving frequency center (63 Hz), we might also want to know how big that 378 Hz wavelength is, since we are not on the speakers main axis, and judging its output up that high by looking at the distortion components!)  As the frequency increases, and the box and speaker are no longer small by comparison, the sound no longer envelopes the enclosure or propagates evenly in every direction.  At a high enough frequency, the errors from the ground-plane method become so large as to make the measurements useless.  That is not the case for the subwoofer bandwidth.  Here, the wavelengths are large enveloping bubbles of air that move about equally in all directions.  Unless of course, you are on the ground.  Here, the air stops, and the vibrations move out not as spheres, but hemispheres.  (See graphic below) 

What this means is a number of things.  If the cabinet is small compared to this wavelength, then placing it and the microphone both on the floor is a pretty good way for us to approximate with relative ease what the speaker does at LOW frequencies ONLY, provided we have a big enough parking lot, and a long enough mike cable, and the sound from the speaker is loud relative to the background noise you are subject to. 

Ground Plane Measurement  

This relatively useful approximation to true “free space” radiation (sometimes called 4 pi) is called half-space or (2-pi) radiation.  In our case it's technically more correct to refer to it as groundplane since the driver is not flush mounted to the actual ground.  In simpler terms, the bubble forming about the speaker (which is at the center of the radiation) is forming a hemisphere about itself, not a complete sphere.  If the size of the wave-length were near or less than the distance from the acoustic center of the port or speaker, (relatively high frequencies) this approximation would be poor, and the ground plane technique would introduce intolerable errors.  At very low frequencies however, it remains both a practical and well accepted practice among sound professionals for characterizing very low frequency performance of loudspeakers.  You will notice the ground-plane technique produces not a full sphere, but rather a half (hemi) sphere of sound.  Since we are maintaining the same pressure in half the volume, we find our ground plane measurements show an increase (approaching, yet never exceeding) 6dB compared to the true free space 4 pi measurements (measured at the same distance).   Those are the numbers we will show here, and while this results in a source having an effective height double to that made with a free space measurement, the error is acceptably small.  (Unless you went out and bought a 90 ft pole, and a really tall ladder already.)

Mirror Image Equivalent of Ground Plane if Done In Free Space

You can think of ground-plane technique like a free space technique where you have two subwoofers stacked one on top of another.  The ground itself acts a bit like an acoustic mirror, reflecting the wave that hits it.

For more information about 4pi, 2pi and groundplane, please read: Subwoofer Measurement Tactics

Test Gear Used

• The microphone used was a brand new Earthworks M30 omnidirectional laboratory grade measurement microphone (http://www.earthworksaudio.com/our-microphones/m-series/m30/) which came with a calibration.
• Microphone preamp = True Systems P-Solo (http://www.true-systems.com/p-solo.html)
• Echo Mia Midi Soundcard (http://www.echoaudio.com/Products/PCI/MiaMIDI/index.php)
• Dell Optiplex Computer running Windows XP SP3
• Don Keele's CEA 2010 Measurement software (God bless Don Keele!)
• Lots and lots of cables of every kind, mostly 1/4” TRS and/or XLR Canon
• Far too many large diameter extension cords
• B52 Professional Audio's parking lot (when the rain finally let up)

CEA 2010 Subwoofer Definition

Right off the bat, I was less than impressed with this standard when I got to section 2.2, where they define subwoofer as “A speaker designed to reproduce all or a portion of the audio signals below 120 Hertz (Hz).”  Well, by that definition, a mini speaker I use next to my PC which has no bass to speak of, yet a corner frequency of 100Hz (-3dB pt.) is a subwoofer.  Clearly this revision of the standard has not yet been put under a lot of scrutiny, but while on the subject, let me attempt to define subwoofer, a word which I am frequently reminded by my spell-checker does not exist as I write it.

A subwoofer is a speaker which is designed to specifically reproduce the lowest usable frequency range of any given sound system.  It is usually characterized by a relatively large size of both the cabinet and driver, and generally has limited usable frequency response above 200Hz, especially off the main axis.  The term Sub-Woofer (meaning literally below the woofer) is an indication of the intent of its use below (frequency band) a regular woofer, which typically has a frequency range sufficient in bandwidth to cross over to a mid range or high frequency loudspeaker.  How is it we all know that, yet the CEA standard seems to define so poorly the very thing which it displays expertise at measuring?  I will ask them, and see if they will agree to a different definition.  

What's Next?

The actual system testing of course.  We are going to bring the subwoofers outside, and find their maximum output by virtue of the CEA testing standard we touched on above.  The following articles will have more detail about the actual levels you can expect, as well as some simplified physics, so you can all become experts at judging subwoofers in your own right.