Axiom Audio EP500 Subwoofer Review

Lastly, there is a very cool "Trim" feature on this sub, which serves like a subwoofer EQ to tune the frequency response of the sub to your room. Options include: Flat, Half, Full, Load and Remote. Flat does what it implies and provides the flattest frequency response for the sub while Half and Full provide an extra boost above 33Hz (which I found useful in combating a room null where I chose to locate the subwoofer). The Load setting is meant for direct access to the subs DSP and should not be used during normal operation. The Remote setting is a feature to be used with future Axiom electronics (boy I can't wait)!

As you can see the green trace is the "Flat" setting, the yellow trace is the "Half" setting (applying about 6dB of boost centered at 44Hz), and the purple trace is the "Full" setting (applying about 10dB of boost centered at 44Hz).

But there is more to this sub than its phenomenal amplifier. The slightly more manageable box size of the EP500 measures in at 20 x 15 x 20 inches and plays down flat to 20Hz +/- 1.5dB with about 2-3dB less output than its bigger EP600 sibling. This is quite an impressive feat for a subwoofer of this box size and single 12" driver compliment. To get an understanding as to how this is done, a closer look into the mechanics of the driver, enclosure, and DSP processing is in order.

This is one heavy duty high-excursion subwoofer driver with a huge motor structure, 3" diameter dual voice coil, bumped back plate, and cast frame (no Chinese stamped woofers here). This is the type of driver typically reserved for high output car audio systems, except the cone stiffness is much higher than many of those designs, enabling a higher degree of linearity and improved transient response. This is the same driver found on the EP600 (the refrigerator sized flagship subwoofer from Axiom). Thanks to the benefits of lower cost with high volume, Axiom was able to incorporate this same driver design in all of the 12" subwoofer systems including the EP350. I was mildly disappointed to discover the driver was not magnetically shielded. Though, it's understandable since it would add significant weight and size to this already massive driver. My advice is to place this subwoofer away from CRT based displays and don't sit to closely to this monster if you or grandma sports a pace maker :-)

The EP500 cabinet is constructed from ¾ inch thick MDF with a cross bracing adjoining three walls. I would have preferred to see an additional cross sectional brace in the middle of the cabinet adjoining all four walls, and incorporate kicker bracing at each mitered corner to add additional support and further reduce resonance and the mechanical noise floor. However, there are often tradeoffs in adding more bracing such as using up more cabinet volume and complexity in assembling, thus designers must determine what works best for a particular application with the least amount of compromises. Its slot-loaded porting system measures almost 12 inches wide by 2 inches in height and extends over 19 inches into the cabinet. The advantages of having such a large port opening are to minimize port noise and chuffing during high output. It also enables a low port tuning to take advantage of the driver's extension and high-output capabilities.

The port system of the EP500 in conjunction with the DSP processing is how the EP500 works its magic of providing unprecedented extension, slam, and output levels. Let's take a closer look at the porting system to gain a better understanding.

The relative similar amplitude peaks of FL and FH indicate good port tuning while Fm and Fb being only 1.8Hz apart is one attribute that demonstrates it to be a well-executed and linearly-tuned system.

For those who like to get down to the inner details of subwoofer designs, Mark Sanfilipo (our resident loudspeaker engineer) has provided the following equations in the editorial note below to determine a good first-order approximation of the box tuning based on the physical port dimensions we furnished to him on the design of the EP500.

Editorial Note on Port Tuning by Mark Sanfilipo (updated June 23, 2005 )

Driver Branch                                                      Enclosure Branch

First approximation electrical equivalent          Simplified driver/vented box circuit 

Driver Branch
Revc = voice coil DC resistance
Levc = voice coil inductance
Res = resistive electrical equivalent of suspension losses
Cmes = capacitive electrical equivalent due to the cone's moving mass
Lces = inductive electrical equivalent due to suspension compliance

Enclosure Branch 
Lceb = inductive electrical equivalent of the acoustic compliance of air in the enclosure
Rel = resistive electrical equivalent of acoustic resistance of enclosure losses caused by leakage
Cmep = capacitive electrical equivalent of the acoustic mass of the port or vent, including air load.

The vented loudspeaker enclosure is a Helmholtz resonator. Ignoring for a moment the presence of the driver, this resonator is modeled here by the 2nd-order Cmep/Lceb/Rel series tuned-resonance circuit seen at the right side of the circuit pictured above. It's a very simplified electrical equivalent of a Helmholtz resonator.

The driver (in combination with the enclosure) forms the basic system. It's modeled here by the 2nd-order Res/Cmes/Lces parallel resonance circuit, along with Revc & Levc, as seen in the center and left portion of the circuit pictured above. It too is very simplified.

The acoustical inertance provided by the air within and very near both entrances of the port, along with the compliance of the air enclosed by the cabinet, together form a resonant system. The system's driver forms a second resonant system. Together, they form the 4th-order vented box loudspeaker.

At their respective resonance frequencies, the driver branch hits an impedance peak and the enclosure branch hits a minimum. The two impedance peaks typically seen in the usual vented box Z-curve result from the enclosure branch's shunting the driver branch's impedance. The amplitude and shape of the peaks depend upon, among other things, both enclosure and driver losses.

When a given vented cabinet is tuned by altering port dimensions, the values of Cmep & Rel are altered. This, in turn, alters the impedance minimum of the enclosure branch that, running in parallel with the driver's parallel resonance tank, results in an impedance minimum unique to the interaction of the two branches. For most vented box systems, the enclosure branch resonance frequency resides (by design) in general proximity to the driver branch resonance frequency.

In the context of loudspeaker design, when reference is made to "fb" it's used to describe not the resonance frequency of the box as just a simple Helmholtz resonator, but the frequency to which the system is tuned. Though it seems counterintuitive, the frequency to which the system is tuned and the frequency where the inter-peak impedance minimum, fm, occurs by the driver & enclosure circuit branches running in parallel are not necessarily the same. Large voice coil inductance values can result in an fb higher in frequency than fm, for example.

Helmholtz Resonator Equation
fb = resonant frequency
C = speed of sound in air
r = radius of neck
a = area of neck
L = length of neck
L' = effective length of neck

k = correction factor, port with 1 end flanged, the other, not flanged. k = 1.4488 - 1.464 (common range of values)

L' = L + 1.4488r (outer end unflanged)
vb = volume of enclosure

From Kinser & Frey's " Fundamentals of Acoustics "
fb = (c/2*pi) * [a/(L' * v)]^.5
= (179.845) * [a/(L' * v)]^.5

EP500 Approximates
@ port depth = 19"

a = .1191 ft^2
r = 0.1947 ft
l = 1.58 ft
l' = 1.8622 ft
vb = 3.47 ft^3

fb = (s/2*pi) * [a/(L' * v)]^.5
= (179.845) * [a/(L' * v)]^.5
= (179.845) * [.1191/(1.8622 * 3.47)]^.5
= (179.845) * [0.1357]

fb = 24.42 Hz @ L = 19"

I found the EP500 easier to place than most subs. Both myself and Ray Adkins are finding that the more linear the frequency response and bass extension a sub exhibits, the less fussy it is about placement to some extent. The EP500 makes it even easier with its variable trim settings. I settled on rear placement of the EP500 directly behind my listening position and found the "half" trim setting most appropriate to combat a room mode with the subwoofer placed in this location as indicated in the graph below.

By engaging the "Half" setting I minimized the frequency response dip between 42Hz to 50Hz at the primary listening position. Though it did provide more output from 35-40Hz, the overall summed response from all three subwoofers in my system helped to smooth this out. Using a PEQ to flatten the excessive output in the 35-40Hz region is certainly a valid measure to take for achieving the most accurate response.

Upon power up, I did notice an unusual amount of hum, which prompted the words "ground loop" to immediately pop into my head. Just when I was about to start tracing the source of the ground loop, I consulted the handy Axiom manual. It recommended to unscrew the little back panel screw for instances such as these. This physically floats the chassis ground and eliminates a current path between earth ground and the return line. I obliged and my ground loop problem was immediately resolved. Now, if only all of our daily troubles could be resolved this easily, the world would be a much happier place.

After several minutes of idle, I noted the EP500 back panel light remained illuminated green, indicating amplifier power was present. I questioned Axiom about this and they informed me there is no Auto On/Off feature on this sub, unless you utilize the 12V trigger connection. This was a concern to me at first until Axiom informed me the sub goes into auto mute mode when no signal is present, thus the power consumption is negligible since it's a Class D amplifier design. I opted not to use the 12V trigger since I didn't have one pre-wired for the location where I was testing this subwoofer.

Editorial Note on Auto Off (update 07/14/05)
In our investigative studies we actually discovered that there really isn't a true auto off on any powered subwoofer which doesn't have an independent DC trigger line. In reality most subs simply remove the connection between the power output devices and the subwoofer driver during the auto off state. But the power transformer and associated amplifier electronics are still active.

 

Blue Man Group "The Complex and Graham Nash" Songs for Survivors are two of my favorite multi-channel reference discs (if you haven't already gathered this from my mention of them in various reviews). The Dave Matthews track "Sing Along" from the Blue Man Group disc has excellent LFE content and the EP500 really showed it off. The bass was well-extended with oodles of punch and slam, especially towards the middle of the song with the mini drum solo. You could hear clear delineation of the bass notes, never sounding boomy or one notey as do some high-excursion subs. The DTS 96/24 percussion track really sounded wonderful on my reference system, even more so with the EP500 pumping out the LFE track. Adding the EP500 to my system really helped smooth out the bass response in my room while also providing significantly more low end extension and tactile response.

I rediscovered track #4, "Chelsea Hotel" from the Graham Nash DVD-Audio disc with the EP500. On my reference subs the bass was authorative and well-tempered, but with the EP500 thrown in the mix, slam and "felt" response was elevated to a new level. I enjoyed watching the EP500 rattle the plastic screen on my RPTV while it also rumbled my couch with authority. There were times I felt as if someone had installed tactile transducers under my couch and I actually had to convince myself otherwise. Track #9 "Liars Nightmare" blew me away the first time I heard it as I remember saying to myself "wow, that's really deep bass, atypical of most recordings". With the EP500 engaged, the hairs stood up on my arms as I felt the adrenaline rush from the power of the mighty sub. The EP500 handled multi-channel music with aplomb, never faltering or whimpering when pushed to levels loud enough to implode small furry animals.

 

Let's face it, the main driving force behind owning a subwoofer is for the movie watching experience and wow effect. An action movie is sterile without surround sound, and possibly worse if devoid of deep and powerful bass. I can't count how many times I have walked away from a local Cineplex disappointed from the movie watching experience because the sound system had anemic bass response and/or highly compressed and distorted bass on pivotal action scenes. Terminator 3 is a prime example of a movie that was much more enjoyable when viewed from home on my reference system than it was at any of the movie theaters I originally watched it at. There are too many good action scenes from T3 to discuss in this review but I will say the truck chase and bathroom fighting scenes were the two most memorable for me. The EP500 added much more realism and envelopment to the experience. At times I actually though I could hear Arnold's thoughts emanate from the EP500: "Gene, this is NO girlie man sub. It is very pumped." Who am I to argue with the Terminator.

Just for kicks I popped in a DVD I haven't watched for some time to see if the EP500 could further enhance my enjoyment of the movie. In Toy Story 2.I jumped to the opening scene where Buzz Lightyear was blasting his way to a planet tracking down Zurg. The first few minutes of this movie are simply stunning, both visually and audibly. The bass from Buzz's rockets and the smashing of Zurg's evil robot minions is nothing short of exhilarating. With the EP500 fired up, I proceeded. I found that my butt got a rather vigorous workout while the EP500 shook my couch to the sound of Buzz's rocket thrusters. I felt every bone crushing head decapitation blow Buzz dealt Zurg's evil robots with his laser. Even the music from the 2001 Space Odyssey while Buzz was walking through Zurg's fortress carried more weight and feel to it than I have previously heard. While my intention was to just skip through a few scenes in this movie to assess the EP500's performance, before I knew it I watched the movie in its entirety.

No matter what movie or music disc I threw at the Axiom EP500, it provided a newfound sense of realism and enjoyment that I didn't even know was missing.

It's a bit difficult to find faults in such a high performance, high value product, but as Audioholics, we do our best to be overly critical. With that, here are a few bullet items I would like Axiom to consider to make their product even better:

It's a short laundry list, but it gives us something to look forward to checking out on future Axiom subwoofer products.

The EP500 is one truly powerful subwoofer, capable of very high SPL levels, especially in smaller rooms. We recommend securing any expensive china or knickknacks before firing this bad boy up for the first time. Due to its very linear low frequency extension, output capabilities, and EQ options, the less fussy placement nature of this subwoofer is quite invaluable. However, experimentation is always key. Following the guidelines we have established in our Loudspeaker Setup Tips Pages is certainly a good start. The manufacturer's guidelines and Axiom Audio tech articles available at their site are also great resources to tap into for ensuring you squeeze out maximum performance of this product.

Although I achieved very good results locating this subwoofer behind the listening position, I would advise caution when attempting this type of configuration. Due to the high output and low extension characteristics of this subwoofer, I found it at times to be localizable, even with the LPF set to 80Hz. What was actually happening was that the enormous tactile effects and sense of pressure waves were distracting me enough to perceive there was a subwoofer somewhere in close proximity to me. I ultimately found pushing the sub back a few additional feet and off-center to the primary listening positions, lowering the output a few dB below reference to the rest of my system, and tweaking the crossover down a tad, helped to alleviate localization issues.

I have now listened to the EP500 in two completely different listening environments and systems. The first was my factory tour to Axiom Audio nearly a year ago, and the most recent was when integrated into my own reference system. In both instances, the EP500 proved to be an outstanding performer regardless of price. It had the ability to command the respect of any bassaholic whether their preferences lean towards home theater or music. Its almost limitless dynamics and tactile response aren't necessarily the defining characteristics that lend the EP500 to be an awesome subwoofer. Its low level linearity and control at high output levels really are the icing on the cake. There are many great subwoofers (especially available online) at this highly contested price point that offer commendable performance and cosmetics, but we feel the EP500 is class leading in performance in this price category. Alas "King of the Deep" is an appropriate title for it. If you are serious about bass, you owe it to yourself to check this subwoofer out. With Axiom's great customer service (one of the best we have seen in this business) and generous 30 day return policy, it's a risk free, no-brainer decision in this humble reviewer's opinion. In fact, if you aren't happy with the performance of this sub, please send it my way, my reference system could always benefit from the addition of an EP500 or two.

The toughest choice about the EP500 isn't debating its performance edge over the competition as much as it is figuring out which of the four standard finishes and two grill colors to choose from. As if that wasn't difficult enough, for a small premium there are even custom finish choices with six different grill color options . Now that's custom!

To go one step further, one must test this frequency response at various power levels to determine power bandwidth uniformity and freedom from excessive compression. With that in mind, we ran some frequency vs SPL tests (borrowed from our First Look Article ) on the EP500 at various output levels to validate Axiom's claim quoted below:

"Our DSP system wants things to go smoothly all the time - no bumps in the road. Using a digital roadmap, the XLF controls every frequency from 17Hz to 100Hz, never deviating more than +-1.5dB through this entire range."

Thus the Axiom Audio claim about maintaining +/-1.5dB 20Hz to 100 Hz for the EP500 through the entire range is in fact true providing that the SPL output is kept to around 80dB (anechoic) which equates to about 91dB in a real room or +/-3dB under 92 dB (anechoic) which equates to about 103dB or so in a real room.

Editorial Notes on 4pi and 2pi Steradians Environments
4pi steradians describes a complete sphere. 2pi steradians describes a half a sphere or hemisphere. Therefore, when we measure outside on a pole we say we're measuring in 4pi. Or if we've got an anechoic chamber with a wire floor so that we are completely surrounded on all six sides by big ole fiberglass wedges we say that's 4pi. If we have a chamber with a hard floor into which we put a baffle and speaker, with the other five surfaces being those fiberglass wedges then that's 2pi. This is one way to measure an in-wall speaker. Similarly, if we go outside, dig a hole and put a speaker-in-a- baffle over that hole and shooting up, then we're measuring in 2pi.

1 meter in room measurement (ground plane technique, unsmoothed)

The green trace represents the EP500 while the purple trace is from my reference sub, the RBH 1010-SEP. Notice the EP500 has significantly more output below 25Hz. You can see the slight dip at the box tuning frequency of 24Hz in the EP500, but then the driver response below tuning is linearized via DSP processing to help extend the low frequency range. I noticed a slight bump of about 3dB centered at 44Hz, the second impedance peak in the open port response. I am uncertain of its cause and could only speculate that it indicates slight system overshoot which isn't uncommon with high mass drivers such as the one used in the EP500.

The purple traces represent the bass response of my dual RBH Sound 1010-SEP subwoofers while the blue trace represents all three subwoofers (dual RBH 1010-SEP's and the Axiom EP500). Notice the dip between 80Hz and 100Hz is virtually eliminated by adding another subwoofer into my system. Ideally four subwoofers strategically placed though out the room would yield bass nirvana for virtually all listeners, but three subs seem to do commendably well also.