SEAS Prestige 27TBFCG Review

SEAS Prestige 27TBFCG Review

In this blog post I test the SEAS Prestige 27TBFCG (H1212)  mounted on Horn No.2007.

I will test the tweeter flush mounted in a regular stand mount speaker to compare directly against the 2007 horn. I will do a series of modifications to the tweeter to see if the performance can be improved further. 

I have been on the look out for an aluminum dome tweeter after testing a series of soft domes. Although the soft domes sound great, they lack that last little bit of dynamics especially when comparing to a compression driver. 

I was looking for an aluminum dome with a lower FS so that it could potential be used in Horn No.2007 which loads down to 1kHz. I've been wanting to see if a 25mm dome could be successfully loaded down to 1kHz if mounted in an appropriate horn assuming that the FS was well below the cutt-off frequency of the horn. 

SEAS Prestige 27TBFCG 

  • Aluminium/magnesium alloy diaphragm 
  • Sonomax surround for low resonance
  • Voice coil windings immersed in magnetic fluid
  • Stiff and stable rear chamber 
  • Free Air Resonance 550 Hz
  • Sensitivity (2.83V, 1m) 91.5 dB
I began by testing in my Signet SL165 stand mount speakers by replacing the existing tweeter which also happens to be a SEAS (now discontinued).
Below is the impedance sweep for the SEAS Prestige 27TBFCG, we see the FS at 660Hz which is 110Hz higher than published. 
Below is the on-axis frequency response for the 27TBFCG. We see a reasonably flat frequency response with a gradual roll-off starting at 15kHz. Mechanical breakup is very high at 27.6kHz. 
Horizontal off-axis is shown below at 0,15,30, & 45 degrees. 
Below is the CSD plot which shows extremely fast decay above 2kHz. 
Extending the CSD plot out to 50kHz is shown below. This test requires re-measuring at 192kHz sampling rate. There's no surprise here that the mechanical breakup at 27.6kHz is dominate. But we do see a small resonance at 17kHz was well which is still outside the audible band. 
The burst decay is shown below. We see a very clean result on par with the best available. 
Intermodulation Distortion
This is an area I was curious about since this is a rather small dome tweeter and so it is susceptible to modulation artifacts compared to a tweeter with more radiating surface area. But we can clearly see from the above test results that the small size is providing exemplary results. I was also curious as the weather the breakup at 27kHz would modulate back down into the audible band. To investigate this I decided to test at both 20kHz and 50kHz test tone upper limits. 
Below is the IMD result at 85dB 1m for test tones ranging from 1kHz to 20kHz. We see IMD at -52dB for 1kHz improving to -62dB for 3kHz reducing again to -55dB at 10kHz. My own "in-house" target is -70dB for a listening level of 85dB 1m. This result is shy of this target by about 12dB. 
Re-testing with the test tones extending out to 50kHz is shown below. ARTA does a peculiar thing where the test tone output level falls -3dB when adding more test tones in this manner. We see 1kHz and 3kHz unchanged but we actually see an improvement at 10kHz from -55dB to -58dB, however this is due to the test signal level reducing from 85dB to 82dB as mentioned earlier. The conclusion here is that we are not seeing the breakup mode at 27kHz modulate back down into the audible band. 
Increasing the test signal to 95dB is shown below. Distortion for the 1kHz region is -50dB, 3kHz is also -50dB, and 10kHz is -44dB. So we see this tweeter is not working miracles and prefers the lower listening levels. 
For interests sake below is the 95dB test signal shown out to 50kHz. 
Mounting the Tweeter to Horn No.2007
When I had purchased the tweeter I was hoping that I could remove the 100mm diameter bezel to mount the bare tweeter to my horn. However when I received the tweeter I removed the bezel to discover that the diaphragm and bezel are a permanent assembly including the steel protective mesh.
So my only option was to mount the entire tweeter assembly to the horn including the perforated mesh. I applied thin foam to form a seal (see above photo). This resulted in a gap in the horn throat around the outside perimeter of the surround. My suspicion was that this would introduce a cavity resonance in the 10kHz region due to improper loading. This was confirmed when I conducted a frequency response measurement as shown below.  We can see the large cancelation at 11kHz as a result of the imperfect physical geometry in the horn throat. 
I was curious if the protective mesh could be removed. It appeared to be permanently glued in place. Still determined, I attempted to remove the grill with side cutters. To my surprise it came off with a forceful pull. 
I was then able to apply a thin foam strip just outside the surround perimeter to completely fill the void present when the mesh was in place. 
With the tweeter's protective mesh removed, I then re-mounted it to the horn and measured the response. 
We can see that the dip at 11kHz is completely gone and we are left with a flat response!  
I then added a 4.7uF capacitor to flatten the response for further testing. The resulting response is shown below. 
I then conducted horizontal off-axis measurements at 0, 15, 30, & 45 degrees off-axis. 
Showing the tweeter on the Signet again is shown below for quick comparison.
When I conducted the CSD and Burst Decay tests with the tweeter mounted in the Signet SL16 cabinet, I placed the mic in the near field to capture the tweeter's performance. This meant the mic was about 2cm from the tweeter's diaphragm. This has the effect of ignoring cabinet edge diffraction by focusing so close on the tweeter itself. However if we measure the CSD and Burst Decay using a 60cm mic distance we can capture any artifacts generated by cabinet edge diffraction. Below are the CSD and BD results of the Signet SL16 cabinet using a 60cm mic distance. I've fully gated the result so that we only seeing the cabinet and not any reflections from the room. 
If we then compare this to the result with Horn No.2007 we can see things clean up a bit. 
Intermodulation Distortion 
In the previous test we saw the tweeter fall short of our target by about 12dB, providing only -52dB at 1kHz, -62dB for 3kHz, and -55dB for 10kHz. However with the tweeter mounted on Horn No.2007 we see 1kHZ now at -62dB, 3kHz at -70dB, and 10kHz at -67.  Horn No.2007 is providing a full 10dB improvement at 1kHz, 8dB at 3kHz, and 12dB at 10kHz. 
Increasing the test SPL to 95dB we see distortion rise in a linear fashion. 3kHz is -60dB which achieves our target for sound quality in the context of output level. 
Removal of Ferrofluid
This particular tweeter model has ferrofluid in the voice coil gap. To continue our investigation I wanted to see the effect of removing the ferrofluid. My rationale here is that the fluid is lowering the 'Q' of the resonance which causes the impedance peak to encroach across the 1kHz and 2kHz region. We can see this in the impedance sweep below for when the tweeter is mounted in the Signet SL16 test cabinet. 
The impedance sweep for the 2007 isn't much different as we can see below. 
We see the horn drops the FS from 650Hz down to 584Hz. 
I then removed the ferrofluid from the gap using paper. The ferrofluid looks like dirty engine oil.
I then re-measured the impedance with the fluid removed. We can see that another peak has shown up in the 1.2kHz region. This is likely a new air cavity resonance around the voice coil that the fluid was previous blocking or damping. 
So we really haven't done much to improve things. 
I decided to then re-measure the IMD which is shown below. We see the cavity resonance showing up clearly in the 1.2kHz region. We also see a new problem at 15kHz. ouch! 
I decided to then remove the foam plug in the pole piece and drill a hole out the back of the tweeter. My idea here is to lower the FS enough and perhaps the resonance may go away by allowing everything to escape out the back of the driver. I'm really guessing at this stage!
I then re-measured the impedance with the hole in the rear chamber cap and the foam plug removed from the center pole piece. The FS actually goes up from 589Hz to 607Hz, not what I expected. 
Does the IMD improve at all with the vented rear chamber? We see basically no change in the performance with the vented rear chamber. 
Ferrofluid Conclusions
The ferrofluid plays a vital role in this particular tweeter's design in preventing certain resonances from setting up at 1.2kHz and 15kHz. I will need to add the ferrofluid back in to restore the tweeter's performance. 
Subjective Listening Impressions 
I set the tweeter up in mono with both a 2-way and 3-way setup. The tweeter sounded stressed when crossing at 1kHz so when I increased the crossover point to 2kHz and the tweeter cleaned up perfectly. Objectively we can see this in the impedance sweep where there is a subtle bump in the 1-2kHz region. The CSD plot also indicates this where there is a secondary resonance in that region as well. 
I listened to a variety of music and I can confirm that the tweeter has just the right amount of dynamics when compared to a soft dome. It's exactly what I was looking for in terms of being able to reproduce electric guitar, harmonica, and cymbals with convincing realism. The acoustic guitar came through with perfect realism being able to hear the sometimes shrill squeaks of the fingers sliding on the strings. This is not to say the tweeter was harsh. Only when called for did the tweeter relay the actual musical content and did not simply damp and smooth out the ugliness. Was the tweeter unpleasant in any way? I did find that there is a defined limit to out loud the tweeter will play, coming on as stressed at very elevated levels. But within normal listening levels of 75dB-90dB I found the tweeter entirely enjoyable.  The tweeter seems to bridge the gap on the clarity between a dome tweeter and compression driver, offering more refinement than nearly all compression drivers currently available. When I say clarity, I'm referring to the low level detail retrieval at normal listening levels. The extremely fast decay in the CSD plot confirms what I'm hearing, in that the tweeter has excellent detail. This test has really got me wondering if multiple tweeters can be used to lower IMD to that of a compression driver while maintaining the refinement and detail that I'm hearing. 
Purifi 8" Project 
Horn No.2007 was originally designed for the 8" Purifi project. I was optimistically hoping the SEAS Prestige 27TBFCG could be loaded down to 1kHz cleanly however it really wanted to be be crossed at 2kHz. The cabinets are built for the Purifi 8" woofers and professionally stained with an espresso tint. I have everything ready for assembly and testing. I just need to 3D print another 2007 horn for a stereo pair. After all of this, my thinking is that maybe the Purifi can actually be crossed at 2kHz without much issue. If this is the case then SEAS tweeter might just work to provide and excellent combination. My plan with the 8" Purifi project is to keep them here for future testing so that customers can commsion me to develop a finished speaker using thier choice of tweeter or horn. But I wanted to have a standard design that gets people up and running on something with textbook performance, and any future changes can be benchmarked against the current design. There are some other tweeters that I would like to try such as those from BlieSMa. I also see that Purifi is introducing their own tweeter and waveguide solution which looks promising. So for now, stay tuned for progress on that front, as you should see something in the next two months. 
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