Soon after acquiring the Epik Empire to test, we received an email from the owner of Epik Subwoofers wishing to speak 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. His concerns were already on the radar so to speak and there had already been some talk with other subwoofer designers about how to handle it.
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 two radiating from a single surface that can be aimed directly at the microphone.
If you take two 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 two 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 if you have the same two drivers, amplifier, enclosure volume and enclosure alignment, the SPL delivered to a single measurement point will be less than a subwoofer that is allowed to aim all or 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 360degree 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 quite a few emails back and forth with various people over the best course of action to investigate this issue, a decision was reached. What followed was a large amount of work that eventually lead to a convincing argument for the development of a compensation file for the Empire and any other future dual opposed subwoofers in order for them to get a fair shake in the maximum output testing.
What was finally decided upon was to take the usual 2 meter outdoor ground plane response measurement with a simple front firing configuration and to take measurements with the subwoofer in different orientations. We then used that data to gauge how much SPL is being lost at the microphone (by having one or more drivers not facing the microphone position) and developed a compensation file. 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 at the time was very close to the same size and shape as the Empire and uses a 15" driver as well. 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 shown on the right. 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 360deg around the subwoofer.
Note in the orientation 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 a pretty good agreement given that both are dissimilar designs and of different shape and size. The two were simply averaged and some smoothing was applied 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 add compensation to a dual opposed subwoofer. The reasoning being that when you sit and listen to a subwoofer, you are at a single point in space from it (or two if you count each ear as a different position.) So, 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 two 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 average 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 then it would not make sense to add it.
In Room Testing
At this point, we decided to do some extra tests to see how the Empire reacted when placed in room, versus outdoors away from boundaries. We compared that to a similarly sized and shaped front firing subwoofer, the Rythmik again, measured in the same way. The drive level used during the outdoor orientation tests of the subwoofers was recorded and this exact same drive level was used in room. This allowed us to know how much effect the room was having on the two subwoofers’ responses and compare them.
We placed the subwoofers in the back corner of a 4000cu ft room and positioned three microphones around the room, roughly 4 to 6 meters from the subwoofer. The subwoofers’ responses were then measured at 45 degree points of rotation, resulting in eight measurements for the front firing Rythmik and four for the Empire (its dual opposed design makes four of the orientations redundant). We repeated this for each sub at each of the three microphone positions.
The Empire has a very uniform output at all three listening positions regardless of its own orientation.
The results of those measurements are shown on the right. You can immediately notice that the Empire has a very uniform output at all three listening positions regardless of its own orientation. The Rythmik has much larger variations in response depending upon its orientation.
The measurement results for the various orientations at each microphone location were then averaged and compared to the base line outdoor measurement to determine the average amount of gain or effect that the room placement was enacting on the subwoofers’ response. That is composed of 24 measurements for the Rythmik and the 12 needed for the Empire.
As can be seen to the right, the Empire does indeed receive a larger amount of average boost from in room placement as compared to the front firing Rythmik.
The in room results indicated that the Empire's outdoor SPL measurements would indeed be slightly under representative.
The second to last graph shows the extra in room boost that the Empire received compared to the SPL loss measured in the outdoor orientation test using the ED and Rythmik subwoofers. Notice how similar in shape and magnitude they are. There is less than a 1.5db difference between the two at any frequency from 10-125hz. This was the final convincing needed to provide a compensation for the Empire or other dual or triple opposed subwoofers’ maximum output SPL tests to keep the comparison to other subs relative. The in room results indicated that the Empire's outdoor SPL measurements would indeed be slightly under representative. In the end, it was decided to average the outdoor orientation SPL loss with the indoor room gain results to split the difference. That seemed like the best compromise.
So, that is how and why we arrived at providing compensation for the Empire / dual opposed subwoofer for the outdoor maximum SPL tests. It is certainly not perfect, but we feel that it is a reasonable and practical approach to the matter without making the workload excessive. The final graph to the right shows the compensation that was used for the Empire. Hopefully, there will not be too many more subwoofers of this particular configuration tested to avoid this issue altogether.