Audax HM130C0 Carbon Fiber Cone with Heil AMT

AUDAX HM130C0 HEIL AMT

Audax HM130C0 – 5.25″ Carbon Fiber Cone Midwoofer
Active Bi-amped

The Audax HM130C0 is the most expensive midwoofer that I have. It cost close to $100 each which to me is a princely sum for a 5.25″ midwoofer.

According to Audax spec sheet, the HM130C0 is their Reference Series. Does it live up to the hype? Lets see what the HM130C0 is all about.

I made a compact bass reflex box of 7 liters for the HM130C0. Baffle width is 6.75″. For the treble, I decided to use a vintage Heil Air Motion Transformer (AMT) tweeter.

Audax HM130C0 RAW Frequency ResponseAUDAX HM130C0 FREQUENCY RESPONSE

Fig 1 – Audax HM130C0 RAW Frequency Response. Measurements below 500Hz in Nearfield.

The Black plot in Fig 1 is the RAW response of the HM130C0. No crossovers are used in this measurement. What I recorded is very similar to Audax’s datasheet. The frequency response is virtually flat all the way to almost 10kHz. This is quite remarkable.

Audax HM130C0 @ 3.8kHz 24dB/octAUDAX HM130C0 RAW 3K8 FREQUENCY RESPONSE

Fig 2 – Black plot = RAW. Blue plot = 3.8kHz (24dB/oct) LPF

The Blue plot in Fig 2 is the HM130C0 crossed at 3.8kHz (24dB/oct). The roll-off is like a text book electrical filter. It is -6dB at 3.8kHz and continues downwards linearly. At 8kHz, it registered -24dB. All without having to install a Zobel network. Impressive.  

Toneburst Energy StorageAUDAX HM130C0 RAW TONEBURST ENERGY STORAGE

Fig 3 – RAW Response of the Audax HM130C0

A frequency response sweep doesn’t tell the whole story. For a more detailed look at what is happening, a Toneburst Energy Storage sweep is one of the measurements I use.  

Fig 3 is the Toneburst Energy Storage of the HM130C0. In a perfect woofer, there are no light blue slices. In the sweep above, we can see a bunch of them from 5kHz~10kHz. So it’s the cone ringing that extends the HM130C0 from 5kHz~10kHz. 

AUDAX HM130C0 TONEBURST 3K8

Fig 4 – Audax HM130C0 @ 3.8kHz (24dB/oct) LPF

Fig 4 is the response of the HM130C0 with a 3.8kHz (24dB/oct) Low Pass Filter. All the excess energy in the previous plot have completely disappeared. 

REW Cumulative Spectral DecayAUDAX HM130C0 RAW WATERFALL PLOT

Fig 5 – Waterfall plot of HM130C0 (RAW)
Rise Time = 0.10 msec. Window = 1.3 msec ( 769Hz ). Time Range = 1.0 msec

Fig 5 is the Waterfall plot of the HM130C0 without a crossover. The decay in the artifacts from 6kHz~10kHz is clearly seen. This is equivalent to the Stored Energy seen in Fig 3.

AUDAX HM130C0 WATERFALL 3K8Fig 6 – Waterfall plot of HM130C0 @ 3.8kHz (24dB/oct) LPF.
Rise Time = 0.10 msec. Window = 1.3 msec ( 769Hz ). Time Range = 1.0 msec

The Waterfall plot in Fig 6 is the HM130C0 with a 3.8kHz (24dB/oct) Low Pass Filter. The cone ringing has disappeared. This confirms the Toneburst plot in Fig 4. Now that the Audax HM130C0 is sorted out, I’ll have a look at the Heil AMT. The tweeter I’m using is from the ESS Performance Series PS-920 and the PS-820. 

Heil AMT Frequency ResponseHEIL AMT RAW 3K8 FREQUENCY RESPONSE

Fig 7 – Black plot = RAW. Red plot = 3.8kHz (24dB/oct) HPF

The Black plot in Fig 7 is the RAW response of the Heil AMT tweeter. Frankly, I never expected this. I thought perhaps my Heil was faulty. I took another Heil and true enough, it measured exactly the same. So this is what a Heil AMT response looks like. 

The Heil is anything but flat. There is a huge notch at 2kHz followed by a second one at 6.5kHz. Such a response is common when a compression driver and a horn is not well matched. Even a cheap $20 modern tweeter is flatter.

The Red plot is the Heil AMT with a 3.8kHz (24dB/oct) High Pass Filter (HPF). That took care of the notch at 2kHz. I’ll just have to live with the one at 6.5kHz. 

Toneburst Energy StorageHEIL AMT RAW TONEBURST ENERGY STORAGE

Fig 8 – Heil AMT (RAW)

Fig 8 is the Toneburst Energy Storage of the Heil AMT without any crossover. Some excess energy (light blue slices) is seen from 6kHz onwards. It seems that the notch is causing the diaphragm to ring.

REW Cumulative Spectral DecayHEIL AMT RAW WATERFALL

Fig 9 – Waterfall plot of Heil AMT (RAW)
Rise Time = 0.10 msec. Window = 1.3 msec ( 769Hz ). Time Range = 1.0 msec

The Waterfall plot in Fig 9 reveals the decay from 6.5kHz upwards. There’s a strip at 15kHz and a thicker one from 7kHz~10kHz. This is consistent with the Toneburst Energy Storage sweep in Fig 8. 

Toneburst Energy StorageHEIL AMT 3K8 TONEBURST ENERGY STORAGE

Fig 10 – Heil AMT @ 3.8kHz (24dB/oct) HPF

The sweep in Fig 10 is of the Heil AMT with a High Pass Filter at 3.8kHz (24dB/oct). The excess energy from 1kHz~2kHz has disappeared. However, there’s no change in the ringing from 6kHz upwards. 

REW Cumulative Spectral DecayHEIL AMT 3K8 WATERFALL

Fig 11 – Waterfall plot of Heil AMT @ 3.8kHz (24dB/oct) HPF
Rise Time = 0.10 msec. Window = 1.3 msec ( 769Hz ). Time Range = 1.0 msec

Fig 11 is the Waterfall plot of the Heil AMT at 3.8kHz (24dB/oct). Comparing this plot with Fig 9, we can see the difference below 3kHz. The decay is almost gone.

Harmonic DistortionHEIL AMT HARMONIC DISTORTION

Fig 12 – Distortion of Heil AMT tweeter. Red plot=2nd Harmonic. Violet plot=3rd Harmonic

Even though the Heil AMT tweeter rings at the higher treble, no offensive distortion is recorded. In fact, distortion is slightly lower that normal, coming in at -60dB below the fundamental. That’s a relief.

Crossing the Audax HM130C0 with Heil AMTAUDAX HM130C0 HEIL AMT 3K8

Fig 13 – Blue plot = HM130C0 with LPF 3.8kHz (24dB/oct). Red plot = Heil AMT with HPF 2.6kHz (24dB/oct)

Fig 13 shows the acoustic crossover of the HM130C0 and the Heil AMT tweeter. Both are crossed electronically at 3.8kHz (24dB/oct) Linkwitz Riley. Acoustically, the two drivers intersect at about 3.2kHz.

Summed Response of HM130C0 and Heil AMTAUDAX HM130C0 HEIL AMT 3K8 HP LP SUM

Fig 14 – Black plot = Summed Response of HM130C0 and Heil AMT

The Black plot in Fig 14 is the summed response of the HM130C0 with the Heil AMT tweeter. There are no cancellations on either side of 3.2kHz. This indicates good summing of the two drivers.

Frequency Response of HM130C0 with Heil AMTAUDAX HM130C0 HEIL AMT 3K8 FINAL SUMMED RESPONSE

Fig 15 – Final Frequency Response

Fig 15 is the final frequency response of the HM130C0 with the Heil AMT. The response is slightly tilted towards the midrange and the treble. This is deliberate because when I integrate in my subwoofer, it will sound just right.

Crossover NullAUDAX HM130C0 HEIL AMT 3K8 CROSSOVER NULL

Fig 16 – Crossover Null with Heil AMT wired in Reversed Phase

In Fig 16, I re-wired the Heil AMT in Reversed Phase. It resulted in a beautiful deep notch at the crossover frequency. This indicates the two drivers are very close to being Time Aligned.

Toneburst Energy StorageAUDAX HM130C0 HEIL AMT 3K8 TONEBURST ENERGY STORAGE

Fig 17 – Toneburst Energy Storage of HM130C0 and Heil AMT

REW Cumulative Spectral Decay

AUDAX HM130C0 HEIL AMT 3K8 WATERFALLFig 18 – Final Frequency Response of HM130c0 with Heil AMT
Rise Time = 0.10 msec. Window = 1.3 msec ( 769Hz ). Time Range = 1.0 msec

Fig 17 and Fig 18 are 3-D representations of the final summed frequency response of Fig 15. As before, there are excess energy at 6kHz and above. That’s from the Heil AMT. Below that, there’s hardly any stored energy. We can deduce from this that the mid-range is very clean. 

My thoughts on the Audax HM130C0 and the Heil AMT

Having worked with the HM130C0 and the Heil AMT, I find you either love them or hate them. When they are good, they are excellent. Conversely, when they are bad, they are horrible.

Case in point, the Audax HM130C0  is totally deficient in bass. It only managed 70Hz (-3dB) in a bass reflex. So, unless you are listening to violins all the time, it’s useless.

Where the HM130C0 really shines is in the midrange. It exhibits a flat frequency response all the way up to 10kHz. Granted that the midrange extension from 6kHz~10kHz is derived from the cone ringing, it is not an issue when crossed at 3.8kHz (24dB/oct).

A huge plus in the HM130C0 that is not apparent is the speed. This midwoofer is fast. By that I mean the transient. I picked this up when I first listened to the HM130C0 without any crossover. This prompted me to measure the Step Response shown in Fig 19.

AUDAX HM130C0 RAW STEP RESPONSEFig 19 – Step Response of RAW Audax HM130C0 (no crossover)

I lost a bit of the transient with a 24dB/oct crossover so it doesn’t sound as fast as it can be. If I want to harness this strength of the HM130C0, I will have to design it with a 1st order network and aim for a transient perfect step like in the Thiel speakers. Obviously that will mean a first order for the tweeter too but that is not an issue. I know which tweeter can do the job.

As for the Heil AMT tweeter, it’s a mixed bag. I have two issues with this tweeter. One is the ragged response which is totally unexpected. The second is the diaphragm ringing. Crossing it at 3.8kHz took care of the notch at 2kHz but there still remains another notch at 6.5kHz. As for the diaphragm ringing, there’s nothing I can do about it. I will just have to bite the bullet and see how it turns out.

how does the HM130C0 with Heil AMT sound like

Before I proceed any further, I should point out that in the final audition, I added the Albatross for the bass. So effectively, it is a Tri-amped Active System with the Albatross Bandpass Sub crossing over to the HM130C0 at 250Hz (24dB/oct). 

Regarding the Audax HM130C0, I can’t find any fault. The midrange is absolutely clean to the point of being clinical. One thing I noticed is it likes being played loud. At low volume, it sounds a bit lifeless. Anyway, it’s quite silly to use a $100 midwoofer for soft background music.

As for the Heil AMT tweeter, it doesn’t sound as bad as the measurements. If I were to compare this Heil AMT with HiVi RT1C planar tweeter, the RT1C sounds “plasticky” whereas the Heil sparkles. The treble with the Heil has more body too. Perhaps it’s due to the diaphragm ringing but I didn’t detect anything offensive during playback.

I threw an assortment of music at the system. It did particularly well in vocals and strings. Male and female voices are reproduced accurately. They don’t sound nasal or chesty.

Am I elated with the Audax and the Heil?

Not really. Actually, I’m a bit disappointed. I have two highly respected drivers and what I got is just above average. On a scale of 1-10, it’s only 7 at most. I should be getting a 9 at the very least. It is not the fault of the drivers. Something is not right with the design. I will replace the Heil with another tweeter in future. I have this nagging feeling that the Heil doesn’t quite match the Audax.

Note: All measurements were made with the mic at 1m, tweeter axis. Impulse Window=5ms. No smoothing applied.