Test Bench: The LT2 Ribbon Transducer and Waveguide from Radian Audio Engineering

November 12 2020, 04:10
This month, I characterized a driver from Radian Audio Engineering, a pro sound driver OEM well-known for its great sounding compression and coax drivers. Several of the drivers have been featured in Test Bench over the last several years, including the 1.4-inch 835PB aluminum diaphragm compression driver featured in the February 2020 issue. However, the sample sent to us this month was neither a compression driver nor a coax. It was one of Radian’s new waveguide planar transducers, the LT2 shown in Photo 1. This is effectively a smaller version of the LT6 ribbon transducer reviewed last month in the July issue of Voice Coil.
 
Photo 1: The LT2 is Radian’s new waveguide planar transducers.
Figure 1: Radian LT2 free-air impedance plot.
Figure 2: Radian LT2 on-axis frequency response.
Figure 3: Radian LT2 horizontal on- and off-axis frequency response (0° = black; 15° = blue; 30° = green; 45° = purple; 60° = blue).
Figure 4: Radian LT2 horizontal on- and off-axis frequency response (0° = black; 15° = blue; 30° = green; 45° = purple; 60° = blue).

Designed to be a more hi-fi substitute for small 0.5 to 0.75" compression drivers, the LT2 and all of the other new Radian ribbon high-frequency devices were designed by 35-year loudspeaker engineering veteran Igor Levitsky. Levitsky began designing planar ribbon drivers and speakers in the former Soviet Union in 1986 as a young engineer after graduating from Kiev Technical University in electroacoustics. Beginning in 1995, Levitsky worked with Hi-Vi Research, where he developed a complete production process for manufacturing ribbon diaphragms. Over the next 20 years, Levitsky developed planar ribbon drivers and speaker systems for the consumer market for BG Radia (acquired by Christie Digital in 2014) and for the pro market with SLS Audio (acquired by Dolby in 2014).

Levitsky also worked with OPPO Digital and Dai-ichi Electronics Manufacturing Corp. on the development of the award-winning OPPO PM1 and PM3 planar magnetic headphones. He joined Radian in 2018 with the goal of bringing the latest planar ribbon technology innovations to the pro sound and residential markets. Levitsky holds a total of six patents on planar ribbon drivers and speaker design.

Features for the LT2 waveguide ribbon transducer include a FEA-optimized neodymium magnet motor structure, a Kapton diaphragm, a continuous power handling of 15W AES with a 50W short-term IEC power handing, a 2kHz to 2.5kHz recommended crossover frequency (with a minimum 12dB/octave high-pass network), and a 1W/1m 95dB sensitivity (average output across the operational frequency range with the LT2 waveguide). The LT2 waveguide directivity is 150° × 60° and is optimized for line arrays.
 
Figure 5: Radian LT2 0°-180° horizontal plane polar plot (in 10° increments).
Figure 6: Radian LT2 vertical on- and off-axis frequency response (0° = black; 15° = blue; 30° = green; 45° = purple; 60° = blue).
Figure 7: Radian LT2 normalized vertical on- and off-axis frequency response (0° = black; 15° = blue; 30° = green; 45° = purple; 60° = blue).
Figure 8: Radian LT2 0° to 180° vertical plane polar plot (in 10° increments).

To begin testing, I used the LinearX LMS analyzer to produce the 300-point stepped sine wave impedance plot shown in Figure 1, which represents the LT2 mounted on the optional waveguide. With a nominal 6 Ω impedance, the LT2 has a 4.6Ω DCR, with minimum impedance mounted on the waveguide of 4.63Ω at 3.79kHz. The very benign resonance of this device occurs at 1.69kHz.

For the next set of SPL measurements, I mounted the LT2 ribbon/waveguide combination in an enclosure that had a 14" × 6" baffle. Then, I measured both the horizontal and vertical on- and off-axis at 2.0 V/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 (supplied courtesy of LoudSoft and GRAS Sound & Vibration).

Figure 2 displays the on-axis frequency response of the LT2 ribbon/waveguide combination, which is ±3.5 dB with no major anomalies from the 2kHz recommended crossover frequency to about 1.6kHz, beginning its second-order lowpass roll-off at 1.8kHz.

Figure 3 shows the 0° to 60° on- and off-axis response in the horizontal plane. Figure 4 displays the normalized horizontal plane response. Figure 5 shows the 180° horizontal polar plot (in 10° increments with 1/3 octave smoothing applied), which was generated by the CLIO Pocket analyzer and accompanying microphone (courtesy of Audiomatica SRL).

For the vertical plane of the LT2 waveguide, Figure 6 gives the 0° to 60° response, normalized in Figure 7. Figure 8 shows the CLIO-generated power plot. Last, Figure 9 shows the two-sample SPL comparison showing the two Radian LT2 waveguide driver samples to be matched within less than 1dB to 1.5dB throughout the transducer’s entire operating range.
 
Figure 9: Radian LT2 two-sample SPL comparison.
Figure 10: Radian LT2 SoundCheck distortion plots.
Figure 11: Radian LT2 SoundCheck CSD waterfall plot.
Figure 12: Radian LT2 SoundCheck Short-Time Fourier Transform (STFT) plot.

For the remaining tests, I set up the Listen, Inc. AudioConnect analyzer and 1/4" SCM microphone (provided by Listen, Inc.) to measure distortion and generate time-frequency plots. For the distortion measurement, I mounted the LT2 waveguide combination in the same enclosure as was used for the frequency response measurements. I set the SPL to 94dB at 1m (2.1V determined by using a pink noise stimulus generator and internal SLM in the SoundCheck 17 software). Then, I measured the distortion with the Listen microphone placed 10cm from the mouth of the waveguide. This produced the distortion curves shown in Figure 10. Note that as with the Radian LT6, the LT2 has an extremely low third-harmonic level.

I then set up SoundCheck 17 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 11 shows the resulting cumulative spectral decay (CSD) waterfall plot. Figure 12 shows the Short-Time Fourier Transform (STFT) plot.

After reviewing the data, the Radian Audio Engineering LT2, like the LT6 waveguide driver, is as intended — an interesting substitute for a compression driver (0.5" to 0.75")—and given the reputation of ribbon transducers for detail and clarity, a great alternative to consider. Combine that with excellent build quality, this driver would provide a viable new choice for studio monitors, PA speakers, and PA arrays. For more information, visit Radian Audio Engineering’s website at www.radianaudio.com. VC

This article was originally published in Voice Coil, August 2020
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