Test Bench: The ND2T 1.25” Neodymium Compression Driver from Eighteen Sound

This month we characterized the ND2T, a new neodymium 1.25” compression driver from Eighteen Sound. Eighteen Sound was founded in 1997, when AEB Technologies' directors Ivan Paterlini and Vincenzo Baroni made the decision to diversify their electronic automotive control systems business by branching out into the professional loudspeaker market. By early 1998, the team had been assembled at AEB’s existing plant in Cavriago, near Reggio Emilia, in North Italy’s high precision industrial mechanics heartland, and then the first cone driver unit came off the production line. For a bit of background and the history of Eighteen Sound read this article. After a long successful run, Eighteen Sound (and Ciare) was acquired by B&C Speakers on January 3, 2018, but Eighteen Sound continues as a discrete brand separate from B&C Speakers.
  Voice Coil magazine recently featured two Eighteen Sound compression drivers, the ND1TP and the XT120 horn in this year’s April issue; and the NSD1424BTN and the XT1464 horn in the May 2021 issue. For this Test Bench, Eighteen Sound sent another of its latest 1.4” compression driver designs, the ND2T, which I combined with the Eighteen Sound XT1464 elliptical-shaped horn.

Designed for use with 1.4” throat horns, the ultra compact footprint (104mm diameter) Eighteen Sound ND2T compression driver has a 1.4” (36mm) throat diameter driven by a 61mm (2.4”) diameter edge-wound voice coil wound with aluminum wire on a non-conducting Nomex former with a proprietary treatment. This specially treated Nomex former shows a 30% higher value of tensile elongation at operating temperatures (200°C) when compared to Kapton.

This assembly drives the field-replaceable pure titanium diaphragm (Photo 1). The ND2T also incorporates a proprietary tangerine radial phase plug design. Other features include a computer-optimized neodymium ring magnet motor structure, copper pole piece shorting ring (Faraday shield), a continuous power handling of 160W with a 80W nominal power handling rating, a 1.2kHz recommended crossover frequency (with a minimum 12dB/octave high-pass network), corrosion-resistant coating on the magnets and plates, black emissive coating on the rear cover, and a 2.83V/1m 110.0dB sensitivity, plus standard solderable terminals.

Along with the ND2T, I used the Eighteen Sound 1.4” XT1464 60° x 50° elliptical constant directivity horn with a 500Hz cut-off frequency. FEA-computer designed and made from high pressure die cast polyurethane foam, the XT1464 has a smooth flare rate that provides a low distortion spherical wave-front and constant directivity from 1.5kHz. This horn has been featured in previous Test Bench explications, so I will dispense with the vertical off-axis and polar pots; however the horizontal and vertical contour spectrogram color plots are provided.

I began testing using the LinearX LMS analyzer to produce the 300-point stepped sine wave impedance plot shown in Figure 1. The solid black curve was taken with the ND2T mounted on the XT1464 horn, and the dashed blue curve represents the compression driver without the horn. With nominal 8Ω impedance, the ND2T has a 6.10Ω DCR, with minimum impedance mounted on the XT1464 horn of 7.73Ω and at 6.24kHz.
 
For the first group of SPL measurements, I free-air mounted the ND2T/XT1164 combination without an enclosure and measured both the horizontal on- and off-axis at 2.0V/0.5m (normalized to 2.83V/1m) from 0° on-axis to 60° off-axis using the LoudSoft FINE R+D analyzer and GRAS 46BE microphone. Figure 2 displays the on-axis frequency response of the compression driver/horn, which is up 4dB from its projected 1.2kHz crossover to about 2.5kHz, but has a fairly smooth ±2dB response from 2.6kHz to 14kHz with no major anomalies.

Figure 3 depicts the 0° to 60° on-and off-axis response in the horizontal plane. Figure 4 illustrates the normalized horizontal plane response. Figure 5 shows the CLIO Pocket analyzer and accompanying microphone (courtesy of Audiomatica SRL) generated 180° horizontal polar plot (in 10° increments with1/3 octave smoothing applied). As I mentioned, since this Eighteen Sound horn has been previously featured, I am dispensing with the vertical axis measurements, since that is mostly horn related. However, the horizontal and vertical axis contour (directivity spectrogram) for the XT1464 elliptical flare horn is given in Figures 6 and 7, respectively.
 
Last, Figure 8 depicts the two-sample SPL comparison showing the two Eighteen Sound ND2T compression driver samples to be closely matched within 0.5dB to 1.5dB or less throughout the drivers operating range.
 
For the remaining sequence of tests, I set up the Listen, Inc. AudioConnect analyzer and 1/4” SCM microphone (provided courtesy of Listen, Inc.) to measure distortion and generate time frequency plots. For the distortion measurement, the Eighteen Sound ND2T/XT1624 combination was again mounted in free-air in the same manner as was used for the frequency response measurements, and the SPL set to 104dB at 1m (2.23V) determined by using a pink noise stimulus generator and internal SLM in the V18 SoundCheck software).

Then, I measured the distortion with the Listen microphone placed 10cm from the mouth of the horn. This produced the distortion curves shown in Figure 9, exhibiting very low third-harmonic content.
 
After completing this test protocol, I then set up SoundCheck 18 to generate a 2.83V/1m impulse response curve for this driver/horn and imported the data into Listen’s SoundMap Time/Frequency software. Figure 10 shows the resulting cumulative spectral decay (CSD) waterfall plot. Figure 11 shows the Short Time Fourier Transform (STFT) plot.
 
Taking into consideration all the objective measurement data, the Eighteen Sound ND2N compression driver is clearly a well engineered 1.4” compression driver, exhibiting good performance, outstanding build quality, and featuring a number of proprietary features, all typical of a veteran pro sound OEM such as Eighteen Sound. For more informaiton about this driver and other Eighteen Sound OEM pro sound products, visit www.eighteensound.com. VC

This article was originally published in Voice Coil, October 2021