Test Bench: B&C Speakers’ DE640-16 Compression Driver

December 18 2024, 17:00
The B&C Speakers DE640-16 Compression Driver is a 1.4" exit design, which along with the recent DCM414 midrange and the DE360 models represent a line of two-point suspension ring radiator type compression diaphragms. The advantages of the two-point ring radiator were already recognized and explored in the very unique Advent hifi speaker designed by Henry Kloss and introduced in 1970. But this is a pro audio-oriented compression driver, well suited for stage monitors, PA speakers, and sound reinforcement applications in general.
 
Photo 1: B&C Speakers DE640-16 ring radiator compression driver is shown with the Eighteen Sound XT1464 horn.
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Photo 2: This is a close-up view of the front and back of the DE640 compression driver.

The DE640-16 is shown in Photos 1-3 along with the Eighteen Sound XT1464 60°×50° coverage horn. The DE640, along with the new B&C Speakers DCM414 midrange compression driver and the DE360 (discussed in Voice Coil, May 2018 and available here) represent a line of two-point suspension (the normal outside perimeter that interfaces with the voice coil, and at the center of the diaphragm) ring radiator type compression diaphragms. One of the major advantages of the two-point ring radiator suspension is suppression of rocking modes. The Advent hifi speaker used one of the first ring radiator high-frequency devices in audio.
 
Photo 3: Here is an inside look at the DE640 ring radiator diaphragm and motor structure.
In terms of features, the B&C Speakers DE640-16 is designed for use with 1.4” throat horns, which means it has a 35.6mm (1.4”) throat exit diameter. With a 110mm diameter and 49mm depth motor structure, this device incorporates a 65mm (2.6”) high-temperature Polymer diaphragm driven by a 65mm diameter voice coil wound with copper-clad aluminum wire (CCAW) on a high Qm non-conducting Kapton former driven by a neodymium ring magnet. Additional features include a 1kHz recommended crossover frequency (second-order or higher high-pass filter), 1kHz to 18kHz range, 111dB at 2.83V/1m sensitivity (16Ω version) and a 140W continuous power rating, making this compression driver well suited for applications such as stage monitors and PA speakers.

The horn I used in conjunction with the DE640-16 was from B&C Speakers’ sister company Eighteen Sound, the Eighteen Sound XT1464. This elliptical flare horn has a 1.4” exit bolt-on type mounting flange and is made from injection-molded polyurethane. The XT1464 provides a 60°×50° constant directivity coverage pattern and a 800Hz cut-off frequency (constant directivity from 1.5kHz upward), making it a good match for the DE640 compression driver. Figure 1 and Figure 2 give the horizontal and vertical directivity maps for the Eighteen Sound XT1464.
 
Figure 1: Eighteen Sound XT1464 horn horizontal directivity map.
Figure 2: Eighteen Sound XT1464 horn vertical directivity map.

Testing began using the LinearX LMS analyzer to produce the 300-point stepped sine wave impedance plot shown in Figure 3, with the solid black curve representing the B&C Speakers DE640-16 mounted on the Eighteen Sound XT1464 horn and the dashed blue curve representing the compression driver without the horn. With an 8.75Ω DCR (Re), the minimum impedance of the DE640/XT1464 was 11.25Ω and at 6.4kHz.
 
Figure 3: B&C Speakers DE640-16 free-air impedance plot.

For the next set of SPL measurements, I free-air mounted the B&C Speakers DE640/XT1464 combination without an enclosure and measured both the horizontal and vertical 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 the GRAS 46BE microphone (supplied courtesy of Loudsoft and GRAS Sound & Vibration).

Figure 4 displays the on-axis frequency response of the DE640/XT1464 combination, which is ±2.5dB from 800Hz to 17kHz with no major anomalies throughout its operating range from 1kHz recommended crossover frequency to about 18kHz.
 
Figure 4: B&C Speakers DE640-16 on-axis frequency response.

Figure 5 shows the horizontal SPL data from 0° on-axis to 60° off-axis, with the normalized version depicted in Figure 6. Note, since Eighteen Sound supplied the horizontal and vertical directivity maps for the XT1464, I chose not to do the vertical SPL measurements, and just provided the horizontal polar plot using the CLIO Pocket analyzer (Figure 7). Last, Figure 8 illustrates the two-sample SPL comparison showing the two B&C Speakers DE640-16 compression driver samples to be very closely matched ≤0.5dB throughout the operating range of the transducer up to 10kHz.
 
Figure 5: B&C Speakers DE640-16 horizontal on and off-axis frequency response (0°=black; 15°=blue; 30°=green; 45°=purple; 60°=blue).
Figure 6: B&C Speakers DE640-16 normalized horizontal on- and off-axis frequency response (0°=black; 15°=blue; 30°=green; 45°=purple; 60°=blue).
Figure 7: B&C Speakers DE640-16 horizontal polar plot.
Figure 8: B&C Speakers DE640-16 two-sample SPL comparison.

For the remaining series of tests, I set up the Listen AudioConnect analyzer, SoundCheck 21 software, and the Listen ¼” SCM microphone to measure distortion and generate time-frequency plots (courtesy of Listen, Inc.). For the distortion measurement, the DE640/XT1464 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 (1.65V determined by using a pink noise stimulus generator and internal SLM in the SC21 SoundCheck software). I then measured the distortion with the Listen ¼” measurement microphone located 10cm from the mouth of the horn. This produced the distortion curves shown in Figure 9 (red curve=second harmonic, blue curve=third harmonic). Note the very low third harmonic, in fact, it’s so low, the second harmonic totally tracks the THD curve.
 
Figure 9: B&C Speakers DE640-16 SoundCheck distortion plots.

Next, I set up SoundCheck 21 to generate a 2.83V/1m impulse response for this driver/horn combination and imported the data into Listen’s SoundMap Time/Frequency software. Figure 10 shows the resulting cumulative spectral decay (CSD) waterfall plot, and Figure 11 shows the Short Time Fourier Transform (STFT) plot.
 
Figure 10: B&C Speakers DE640-16 SoundCheck CSD waterfall plot.
Figure 11: B&C Speakers DE640-16 SoundCheck Short Time Fourier Transform (STFT) plot.

The DE640-16 ring radiator displays excellent performance and is definitely an interesting new addition to B&C Speakers’ extensive line-up of neo motor 1.4” compression drivers. For more information about this new driver and other pro sound products from B&C, contact B&C Speakers. E-mail Bennett Prescott, or visit www.bcspeakers.com. VC

This article was originally published in Voice Coil, September 2024.

 
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About Vance Dickason
Vance Dickason has been working as a professional in the loudspeaker industry since 1974. A contributing editor to Speaker Builder magazine (now audioXpress) since 1986, in November 1987 he became editor of Voice Coil, the monthly Periodical for the Loudspeake... Read more

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