In this blog post I test the RCF ND1411-M small format compression driver.
I purchased these drivers because I've had good success with the larger brother (ND350) and I wanted to see if this version could offer more transient detail and smoothness. Based on the published data the driver has no breakup modes until 18kHz.
- 1.5-inch Diaphragm, 1-inch Exit Throat
- Frequency range: 1500Hz - 20kHz
- Optimized geometry radial phase plug
- 0.1mm thick Mylar diaphragm
Below is the raw frequency response. I found it quite odd that the driver had a pronounced peak at 1.6kHz followed by a dip at 2kHz. Was there a problem with the driver?
Below is the impedance sweep which shows a strong resonance at 2kHz.
Zooming in we can see things more clear. One thing worth noting is the absence of breakup in the upper treble.
The 2kHz issue was troubling me and I thought perhaps that I had miss-installed the voice coil when reassembling. I investigated but found no issue. So I decided to test the driver with the rear cover removed. This produced very little difference in the response as you can see below in red.
I decided to see how the response was affected by various capacitors installed. Below is an overlay of 1.0uF, 2.2uF, and 5.6uF.
Even the 1.0uF capacitor did not provide adequate filtering of the midrange frequencies. So I decided to add a 0.33uF inductor creating a 2nd order filter.
This produces a 4kHz high pass filter with a 12dB/octave slope.
Below is the off-axis at 0,15,30,and 45 degrees. Directivity is fairly constant and consistent with narrowing occurring at 17kHz 45 degrees off axis.
Looking at the step response we see and very fast initial impulse but there are some continuing artifacts after the initial sweep making not the cleanest step response I've seen.
The burst decay and CSD below shows no resonant behavior until around 19kHz.
Looking at harmonic distortion at 85dB, 95dB, and 105dB is shown below.
Conducting intermodulation distortion at 85dB, 95dB, and 105dB is shown below. Intermodulation is poor compared to the Viawave SRT7 pure ribbon tweeter tested on horn No.1671. Distortion on the ND1411-M is -55dB where the Viawave is a full -70dB. So the Viawave has -15dB better distortion performance.
I attempted to integrate the ND1411-M into a three-way setup playing above 6kHz. I was disappointed with the overall sound quality. The driver lacked that last bit of smoothness and refinement. Despite my efforts, I could not come to a place where I found myself enjoying the sound. It didn't do anything wrong, it just sounded flat and lifeless. In contrast, the RCF ND350 compression driver, which is the larger brother, sounds full, rich, and detailed.