Not soon after I mentioned that I would be testing the Epik Empire I received a pm from Chad the founder of Epik wishing to speak with me about what would be taking place. He was justifiably concerned about the Empire getting a fair shake in the tests due to the dual opposed design of the subwoofer. I already had some concerns about this myself and had been talking with some subwoofer designers about how to handle it.
Let me try to explain the issue as simply as possible in a general manner. A dual opposed subwoofer will always have one or both of the drivers further away from the microphone than a more standard configuration subwoofer, which usually has a single driver and maybe a port or 2 radiating from a single surface that can be aimed directly at the microphone. If you take 2 drivers and mount both on a single face of the subwoofer you can then aim that face at the microphone and the drivers end up roughly 2 meters (In our case) away from the microphone. Now take the same 2 drivers and place one of them facing the other direction on the back of the enclosure and some distance behind the other. Its contribution at the microphone will be diminished. Even though you have the same 2 drivers, amplifier, and enclosure volume and enclosure alignment, the SPL delivered to the single measurement point will be less than the subwoofer that is allowed to aim all or at least most of its radiation at the microphone with a minimum distance between. If you have both drivers side firing so that they are equidistant from the microphone you will have the same issue still with advantage maintained by the subwoofer that can aim most of its radiation directly at the microphone. Now this holds true, if you take a single point measurement. However if you take many measurements at various points around the subwoofer and average the results, they will even back out as the dual opposed subwoofer will have more uniform 360deg power distribution. Unfortunately it is not very practical to do a full polar response work up on every subwoofer tested. That would be ideal but is a lot of additional work. Any subwoofer with radiation from multiple faces is subject to this issue. However ports or passive radiators contribute most of their output over a fairly narrow range, while a dual opposed sub has the output from both radiators covering the entire useful range of the device, so this is a much larger concern in that case.
After many emails back and forth with various people I decided on a course of action to investigate the issue and if need be develop a compensation factor for the Empire and other dual opposed subwoofers. What followed was a large amount of work that eventually lead to me becoming convinced of the necessity of and the development of a compensation file for the Empire and any other future dual opposed subwoofers in order for it to get a fair shake in the maximum output testing.
What was finally decided upon was to simply take the usual 2 meter outdoor ground plane response measurement of a subwoofer with a simple front firing configuration and take some measurements with the subwoofer in different orientations and to use that to gauge how much spl is being lost at the microphone by the dual opposed subwoofer by having one or more drivers not facing the microphone position and subsequently from there to develop a sort of compensation file with the data. It would be best to have a subwoofer of the same size and shape for this purpose. Fortunately the Rythmik FV15-HP that was on hand is very close to the same size and shape as the Empire and uses a 15” driver. Additionally an Elemental Designs A7S-450 was also used for redundancy and to check for basic agreement in the results.
The results for the orientation tests are below. You can see that from turning either the Rythmik or ED subs 90 degrees from the microphone causes a rather substantial drop in recorded output. These are similar sized enclosures and drivers to what is in the Empire. The premise is that this is the amount of output loss at the microphone position experienced by the Empire when its drivers are both side firing versus what it would be if both could be aimed at the microphone. Also notice how the Empire response changes little with either orientation. This confirms it’s more uniform output coverage.
Note in the below graph that the drop off in spl seen at 90 degrees with both the ED and Rythmik are within a db of each other throughout the entire range of concern. This is pretty good agreement given that both are dissimilar designs and of different shape and size. I averaged the 2 and applied some smoothing to come up with the red trace which would be the generic outdoor compensation for a dual opposed sub of the Empire’s size.
The above is a simple thing to do, but initially it did not feel right to me to add compensation to a dual opposed subwoofer. I reasoned that when you sit and listen to a subwoofer that you are at a single point in space from it (Or 2 if you count each ear as a different position.) and why should you not optimize the unit under use for that one position? If you place a subwoofer in an empty parking lot, or in a room, you typically have a listening position somewhere and also a position where the subwoofer will be placed. You may even have a couple of options for subwoofer placement. What you would not do typically is use one subwoofer, switch it out for another and proceed to move the listening position closer to it because the radiators on the new one are placed on the enclosure such that they are farther away. Most people will have the available placement locations in their room and their listening position locked in. Most would simply place the subwoofer in a spot that is available for it and utilize it there. Would you move the subwoofer closer or move the couch closer to one sub versus another, because of the design? Perhaps one might because of large size or shape differences, but not if both are the typical 20” black cube. No matter how you place yourself or a microphone relative to a dual opposed subwoofer at a fixed distance outdoors away from boundaries, you will not be able to get an output level as high as if the 2 drivers of the same subwoofer were both facing directly at you or the microphone. So why should a dual opposed subwoofer receive compensation? Because of the boundaries and objects in a typical room. If a dual opposed subwoofer were to somehow translate its less focused output from separated radiation points into more even room coverage or to leverage better room gain to the listening positions such that it recovers the output lost to a single point outdoor or anechoic measurement, then it would make sense to add compensation. If it does not translate in room, it would not make sense to add it in my opinion.
At this point I decided to do some extra tests to see how the Empire reacted when placed in room, versus outdoors away from boundaries and compare that to a similarly sized and shaped front firing subwoofer, the Rythmik again, measured in the same way. I recorded the drive level used during the outdoor orientation tests of the subwoofers and used this exact same drive level in room. This would allow me to know how much effect the room was having on the 2 subwoofers response and be able to compare them. I used an available back corner in a room of a little over 4000cu ft to place the subwoofers. I decided to use 3 different microphone positions that were well spaced around the room to measure from. They varied from about 4 to 6 meters from the subwoofer placement. The subwoofers then had their response measured in the corner of placement at every 45 degree point of rotation. This results in 8 measurements for the front firing Rythmik and 4 for the Empire since its dual opposed design makes 4 of the orientations redundant. This was repeated for each sub at each of the 3 microphone positions. The results of those measurements are below. You can immediately notice that the Empire has a very uniform output at all 3 listening positions regardless of its own orientation. The Rythmik has much larger variations in response depending upon its orientation.
FV15HP room orientation results
Empire room orientation results
