Orion-RT13 (Audax HM130CO with HiVi RT1.3WE)

Orion-RT13
6 Liters Bass Reflex

The Orion-RT13 is a compact 2-way based on the Audax HM130CO. This is the best 5-1/4″ mid-woofers I have. The vocal reproduction is incredible. None of my 6-1/2″ woofers comes close.

In this project, I am mating her with the HiVi RT1.3WE planar tweeter. The reason I’m using her is because my RT1C-A can’t cross below 4kHz. Hopefully, this RT1.3WE can go lower without straining.

Fig 1 – HiVi RT1.3WE Frequency Response • Baffle Width=6-3/4″

The Black plot in Fig 1 is the RAW response of the RT1.3WE when she’s flush mounted in a box with a baffle width of 6-3/4″. Strangely, her response is smoother when she’s surface mounted (Red plot). I shall be using her surface mounted from here on.

Fig 2 – Audax HM130CO RAW and Low Pass Responses

In Fig 2, the Black plot is of the Audax HM130CO without any crossover. She has an astonishingly flat response extending all the way to 9kHz. The Blue plot is with a low pass network to cross to the RT1.3WE.

Fig 3 – Orion-RT13 Passband

The Black plot in Fig 3 is the crossover passband. No cancellations are recorded, indicating the summation is quite good. 

Fig 4 – Orion-RT13 Frequency Response

The final frequency response of the Orion-RT13 is in Fig 4. I tuned the response such that the midrange is only slightly more than the bass level. Disregard the deep notch at 150Hz. It’s a floor bounce that my mic picked up.

Fig 5 – Orion-RT13 NULL

Fig 5 is the null with the tweeter wires reversed. It centers at 3kHz, exactly where the two drivers are crossing in Fig 3. 

Fig 6 – Orion-RT13 Frequency Response with Nearfield below 500Hz

Fig 6 is with a nearfield response spliced in at 500Hz. It is an approximation of what the Orion-RT13 looks like in an anechoic chamber. Note the deep notch at 150Hz has vanished. This proves that it was a floor bounce.

Fig 7 – Port Resonance

The Brown plot in Fig 7 was made with the microphone at the mouth of the port. It recorded an unusually high peak at 1kHz. That’s the reason why there’s a hump at 1kHz in the Orion-RT13 frequency response. I will have to use another port of a different dimension to avoid the 1kHz peak. Fortunately during playback, I could not pick up the bump. Nonetheless, it should not be there. If the Orion-RT13 is used as satellites with subwoofers, it will be a sealed box. In that case, the bump will not be there. 

Fig 8 – Orion-RT13 Step Response

The step response (Fig 8) of the Orion-RT13 shows a fast and linear transient. The HM130CO hits 100% at 200 microsec. That’s an amazing 200 microsec faster than most 6-1/2″ woofers. I can expect a very lively sound from her. 

Fig 9 – Orion-RT13 Waterfall

Fig 10 – Orion-RT13 Toneburst Energy Storage

The Waterfall and Toneburst plots recorded an exceptionally clean treble. The light blue slices from 5kHz to 15kHz are probably diffraction. 

Fig 11 – Orion-RT13 Spectrogram

The Spectrogram shows some streaking at 1kHz. Fortunately, I did not detect any smearing during playback.

Fig 12 – Orion-RT13 Impedance

The Orion-RT13 would be rated as an 8Ω Nominal Impedance. The lowest she goes to is 5Ω and that is at 20kHz. Port tuning is 65Hz. Electrical phase doesn’t vary much. From 200Hz upwards, she stays within +/- 20°. All in, a rather friendly load for power amplifiers.

Sound of Orion-RT13

The Orion-RT13 is superb for vocals. Diana Krall sounds life-like in Girl in the Other Room. She’s not shouty nor sibilant. No shrillness. In Boyz II Men, I can make out clearly the different singers. With some speakers, it’s just one bloody mess.

In the treble, the HiVi RT1.3WE doesn’t sound harsh. In fact, she sounds like RT1C-A except that I can cross at 3kHz. Even though her frequency response is not as flat as I would like, I didn’t detect any emphasis at 4.5kHz and 15kHz. Maybe it’s partly due to the nature of these planar tweeters. I don’t know if “airy” is the correct term to describe their sound but they are excellent when you want to maintain the focus in the vocals. Notice how some tweeters draw your attention when they come on. No with these planars.

If there’s one weakness with the Orion-RT13 is in Rock music. Her bass is not that loud and doesn’t go deep down to 40Hz. That is not to say there’s no bass. No, there is. Bass is actually tight and fast. The kind of bass I like. I would say the Orion-RT13 shines best with Jazz. 

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.