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.

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.