This is the first time I’m loading the Seas U18RNX in a 27 liters bass reflex. I want to see how much difference a larger box will make in the bass. Since my Seas 27TBFC tweeter is already mounted, I might as well use her. Furthermore, sharing the same tweeter will make comparison with the Starling-TBFC more relevant.
Fig 1 – Black plot=U18RNX RAW Response • Baffle Width=9-1/2″
The Black plot in Fig 1 is the RAW response of the Seas U18RNX. She is one of the few woofers with a very graceful roll-off. No ugly cone breakup. The Blue plot is with my Low Pass network tuned to the bass level.
Fig 2 – Blue plot=U18RNX Low Pass • Red plot=Seas 27TBFC High Pass
The Red plot in Fig 2 is the Seas 27TBFC tweeter adjusted for the U18RNX. The two drivers are crossing at about 2kHz.
Fig 3 – Black plot=Myna-TBFC Passband
The Black plot in Fig 3 is the passband of the crossover. There is a slight dip at 2.5kHz and 5kHz. They look like baffle edge diffraction. It’ll be interesting to see whether they’ll go away when I use a waveguide with the 27TBFC.
Fig 4 – Myna-TBFC Frequency Response
Fig 4 is the final frequency response of the Starling-TBFC. There is a bite at 5kHz but it’s something I can live with. I don’t think I can hear anything amiss at that frequency. If it’s at 700Hz~3kHz, that would be problematic.
Fig 5 – Myna-TBFC Null
When I flipped the tweeter wires around, it resulted in a null (Fig 5). It is not that deep but good enough. There’s no point trying to get a deep notch and in the end, destroying the sound. The tip of the null centers at 2.1kHz.
Fig 6 – Step Response
The U18RNX in the Myna-TBFC exhibits a fast transient. At 90%, she’s at 300 microsec. In the last 10%, she slows to hit the apex at 400 microsec.
Fig 7 – Waterfall
Fig 8 – Toneburst Energy Storage
The Waterfall (Fig 7) and Toneburst (Fig 8) plots don’t show any anomalies. There are some excess energy at 1kHz~2kHz which last for 10 cycles.
Fig 9 – Spectrogram
The Spectrogram (Fig 9) hot streak at about 1.2kHz, followed by some delayed spots. They are largely dissipated by 5 msec.
Fig 10 – Excess Group Delay
The Excess Group Delay recorded the Myna-TBFC at 5.66 msec at 45Hz.
Bass Reflex Alignment
Fig 11 – U18RNX box modelling
27 liters seems to be the optimal volume for the Seas U18RNX. This is based on my Thiele & Small parameters derived by DATS. If you’re using Seas spec sheet, it’s only 16 liters. You can use anything from 16 liters to 27 liters if you don’t have the means to extract the T/S from you woofer.
Myna-TBFC vs Starling-TBFC
This is where it gets interesting. Both of these designs use the Seas 27TBFC tweeter. So it boils down to the woofer.
It is in the bass that the U18RNX beats the ER18RNX. She is more dynamic and forceful. This is the only woofer that is “complete”. By that, I mean she does bass and mids perfectly. The only other woofer that can do this is the Scanspeak 15WU.
Myna-TBFC vs Myna-DXT
In this comparison, it’s about the different volumes and the effects on the bass. On paper, the TBFC’s 27 liters is the optimal volume. However, I lost the magic in the U18RNX. The bass in nowhere close to the 15 liters Myna-DXT. There’s no bass texture in this larger box. Neither is the bass as forceful. Personally, I would go for the 15 liters Myna-DXT. Same woofer, different volumes, sounds totally different.
Crossover is available on request. Free for DIY. Not for Commercial use.
February 3, 2021Projects