Correction: The Dipole15 is not 7.5mm Xmax. It is 15mm Xmax one way, 30mm p-p. Suspension travel is 25mm one way, 50mm p-p.
In this blog post I test the Acoustic Elegance Dipole 15 in bass cabinet No.1798. This is the fifth pair we've build of this cabinet with the previous four being sold. So this pair with the AE driver + wood phase plug is available to buy.
Wood Phase Plug
I decided to design and make a solid hardwood phase plug for the AE Dipole driver, taking advantage of the removable phase plug on the stock driver. The stock driver has an aluminum bullet style phase plug. When speaking with John from Acoustic Elegance he advised me to upsize the tapped hole in the pole pices from M6 to M8 to account for the added weight of the new hardwood phase plug. So I had john make me a custom pair of AE Dipole 15 drivers with the inlarge tapped (threaded) hole. Below are some Solidworks section views to show how the phase plug is assembled to the driver. I am using a stainless steel wood screw threaded stud.
Below is a time lapse showing CNC machining of the phase plug.
Recently I posted on our local Facebook community page looking for somebody that was willing to share their living room so that we could setup and conduct final crossover tuning on our finished systems. I was lucky enough to have a local audiophile respond to the online post willing to accommodate various speaker systems. Craig has a perfectly sized listening room that is great for the last stage of critical listening and final system tuning. I also helped Craig with some of his room acoustic issues. Stay tuned for more pictures and testing in Craig's room.
Measurements
All measurements are done with the rear cover removed from the bass cabinet allowing open baffle/dipole operation.
I started by measuring the impedance sweep. The AE Dipole 15 has a very low fundamental resonance (FS) of 21.6Hz. The impedance shows an unusually flat impedance sweep indicating zero inductance in the voicecoil (yellow highlighted). This also results in a perfectly flat phase response (green highlighted).
I then measured the frequency response (in-room) at 1m mic distance, using 1/3 octave smoothing. The response is flat to 20Hz with a 2dB/octave rising response that continues in a linear fashion to 500Hz. We see some breakup occurring at 2.5kHz. The low frequency extension comes at a price to overall sensitivity, which is only 88dB for the bass region.
I decided to compare the response with either a tube amplifier (red trace) (push pull KT88) against a solid state class A/B (green trace). It appears as though the tube amplifier has a little less control of the driver at resonance (21Hz).
I've overlaid the impedance (red) with the raw frequency response of the driver/cabinet. 
Distortion
I then measured harmonic distortion using a 50cm mic distance in-room using an 85dB at 1m test SPL level. Distortion is low at only 0.1% from 73Hz - 200Hz. Distortion drops even lower in the 200Hz - 500Hz region.
I then measured intermodulation using a 12 test tone per octave test signal at 85dB 1m ranging from 50Hz-2kHz. We can see that IMD is at -72dB for the 200HZ region and around -65dB in other parts of the frequency spectrum.
I then looked at implementing a passive low pass filter which is shown below. 
Below is the response overlay of the passive filter. The natural drop in the response at 500Hz works to our advantage in providing a target 12dB/octave slope. The resulting in-room response is very linear. 
Comparisons
How does the AE Dipole compare to other drivers I've tested to date?
Below is a summary looking at IMD at 100Hz, for the 85dB test signal.
| Bass Cabinet No.1798 Distortion Comparison | |
| Driver Model | IMD, 100Hz 85dB SPL 1m |
| Dynamic Range (dB) | |
| Acoustic Elegance Dipole 15 | 60 |
| SB Audience 15SW800 | 60 |
| Beyma 15LX60V2 | 53 |
| TAD TL-1601B | 61 |
Subjective Listening
I found that the AE Dipole 15 offered similar sound quality as the other drivers mentioned above. The difference was in the low frequency extension. The AE was able to dig down to 20Hz and this was noticeable on a variety of tracks. It provides a little more palpability to the overall music feel. I did not detect any lack of bass impact which is common with open baffle cabinets.
Conclusion
Pros
- Low distortion
- Bass extension to 20Hz
- Linear frequency response
Cons
- Low sensitivity
- Limited xmax (7.5mm)
- High cost
Although not as costly as the TAD TL1601B ($1,750 USD each) the Dipole 15 comes in at $459 USD each. This is quite a step up compared to the SB Audience 15SW800 at only $295 USD which also has much higher sensitivity at 92.5 for the 50Hz region, and much more xmax at 14.27mm compared to 7.5mm of the Dipole 15.
Overall the Dipole 15 represents a well engineered and great performing woofer.