Myna-MDT (Seas U18RNX with Morel MDT29)

18L Bass Reflex Bookshelf

The Myna-MDT is an update on the first Myna. In that version, I had a severe notch with the Seas U18RNX Frequency Response shown in Fig 1. 

Fig 1 – Seas U18RNX RAW Frequency Response • Baffle Width=9-1/2″

Fig 2 – Seas U18RNX NEW RAW Frequency Response

Believe it or not, that notch is caused by using 2 ports. When I reverted back to a single port, the notch disappeared (Fig 2 ). Now that I got that sorted out, I can proceed with re-working the crossover.

Fig 3 – Blue plot=U18RNX with Low Pass network

The Blue plot (Fig 3) is the response of the U18RNX with a new low pass network. I tuned the woofer such that the midrange is at the same level as the bass. Measurements below 500Hz include room reflections. Ignore that deep notch at 150Hz. That’s a floor bounce. 

Fig 4 – Red plot=Morel MDT29 with High Pass network (Surface mounted)

The Red plot in Fig 4 is the Morel MDT29 tweeter with her high pass network. This is with the tweeter surface mounted. I found that flush mounting the MDT29 is worse. 

Fig 5 – Black plot=Myna-MDT Passband

The Black plot in Fig 5 is the summation of the two drivers. The two drivers appear to be crossing properly as no cancellations are seen in the crossover passband.

Fig 6 – Myna-MDT Frequency Response

Fig 6 is the final frequency response of the Myna-MDT. There is a slight emphasis in the midrange. This will result in the midrange sounding slightly forward. Just the way I like it.

Fig 7 – Myna-MDT Null

When I flipped the tweeter wires around, it resulted in a broad null centered at 2kHz (Fig 7). It is not as deep as I would like but it’s good enough. The symmetry in the null indicates the roll-off slopes of the two drivers are similar.

Fig 8 – Step Response

The step response (Fig 8) of the Myna-MDT shows a sharp tip at the bottom. This indicates the phase of the two drivers at the crossover is spot on. 

Fig 9 – Waterfall

Fig 10 – Toneburst Energy Storage

Fig 11 – Spectrogram

The Waterfall (Fig 9) and Toneburst (Fig 10) did not flag any trouble in the treble. The Spectrogram (Fig 11) recorded a streak at 1kHz but it soon dissipated by 6 msec.

Fig 12- Excess Group Delay

The Excess Group Delay in the Myna-MDT registered 3.94 msec at 45Hz. During playback, I did not notice any displaced bass. 

Bass Reflex Alignment

Fig 13 – U18RNX box modelling

The Seas U18RNX is one of those woofers that sounds better with an over-damped alignment (Fig 13). This tightens up the bass considerably and speeds up the attack.

Sound of Myna-MDT

All the issues in the Myna are sorted out with this Myna-MDT. Apart from eliminating the 9kHz notch in the Seas U18RNX, I took the opportunity to refine the crossover. The two drivers are now more aligned at the crossover frequency than before. As a reminder, the Morel MDT29 is surface mounted.

Regarding the sound of the Myna-MDT, I just love this Seas U18RNX. She’s lively, dynamic and has a tight bass punch. Midrange is superb. The same kind of quality as the Seas ER18RNX. These two Seas woofers are my favorites. They are in a class of their own.

The Myna-MDT is one of the best 2-ways I’ve ever done. Highly recommended.

Crossover is available on request. Free for DIY. Not for Commercial use.
Unless otherwise stated, all measurements were made in Full Space (4 pi) with the mic at 36 ins, tweeter axis. Impulse Window=5ms. No smoothing applied.