Test Bench: The SB Acoustics SB29BNC-C000-4 Tweeter

June 5 2019, 05:30
This month I am characterizing the SB29BNC beryllium 29 mm dome tweeter from SB Acoustics. Test Bench has covered a number of SB Acoustics tweeters, including the ferrite motor 29 mm ring dome SB29RDC-C000-4 in the August 2009 issue; it’s neodymium 29mm ring dome version, the SB29RDNC-C000-4 seen in the August 2011 issue; the SB26STCN-C000-4 1” neodymium motor tweeter in the September 2011 issue; and the ferrite version of the SB26STCN, the 1” SB26STAC-C000-4 in the November 2011 issue; the Satori TW29RN in February 2016; and last the Satori TW29BN beryllium tweeter in February 2017. It’s worth repeating that all the Sinar Baja (SB) transducers are developed by former Danish Vifa/Scan-Speak engineers Ulrik Schmidt and Frank Nielsen, co-owners of Danesian Audio (www.danesian.dk).
 
Photo 1: SB Acoustics SB29BNC 29 mm beryllium diaphragm neodymium dome.
Figure 1: SB Acoustics SB29BNC impedance plot.
Figure 2: SB Acoustics SB29BNC on-axis frequency response.
Figure 3: SB Acoustics SB29BNC horizontal on- and off-axis frequency response (0° = black; 15° = blue; 30° = green; 45° = purple).
Figure 4: SB Acoustics SB29BNC normalized on- and off-axis frequency response (0° = black; 15° = blue; 30° = green; 45° = purple).

This month, SB Acoustics sent Voice Coil its second high-end 29 mm beryllium diaphragm neodymium dome, the SB29BNC-C000-4 (see Photo 1). Features for this transducer include a small format 72 mm diameter injection-molded plastic faceplate, an 8 mm wide coated cloth surround, a 29 mm beryllium diaphragm, a copper cap on pole piece for reduced voice coil inductance and minimum phase shift, internal pressure equalization and flow resistor for non-resonant coupling of cavities, copper-clad aluminum wound voice coil wire, 0.25 mm Xmax, 95 dB sensitivity, a foam-mounting gasket, and gold-plated terminals.

Testing commenced using the LinearX LMS analyzer to produce the 300-point impedance sweep illustrated in Figure 1. The magnetic fluid damped resonance occurs at a moderately 919.7 Hz. With a 2.9 Ω DCR (Re), the minimum impedance for this tweeter is 3.16 Ω at 3.58 kHz. Following the impedance test, I recess-mounted the SB tweeter in an enclosure that had a baffle area of 17” × 8” and measured the on- and off-axis frequency response again using the Loudsoft FINE R+D analyzer (courtesy of Loudsoft) and the GRAS 46BE 1/4” were set up to measure the 200 Hz to 40 kHz frequency response (using a 192 kHz sampling rate) at 2 V/0.5 m normalized to 2.83 V/1m. Data was then acquired with sweeps at 0°, 15°, 30°, and 45°. Figure 2 shows the on-axis response of the SB29BNC-C000-4, which measured ±2 dB from 2.0 kHz to 26.1 kHz, with the beryllium breakup mode at peaking at about 38 kHz.

Figure 3 gives the on- and off-axis response of the SB29BNC-C000-4 beryllium tweeter. Figure 4 shows the off-axis curves normalized to the on-axis response. Figure 5 shows the CLIO 180° polar plot (measured in 10° increments with 1/3 octave smoothing). The two-sample SPL comparison is illustrated in Figure 6, indicating the two samples were closely matched to within slightly less than 1 dB throughout its operating range.
 
Figure 5: SB Acoustics SB29BNC 180° horizontal plane CLIO polar plot (in 10° increments).
Figure 6: SB Acoustics SBN29BNC two-sample SPL comparison.
Figure 7: SB Acoustics SB29BNC SoundCheck CSD waterfall plot.
Figure 8: SB Acoustics SB29BNC SoundCheck STFT surface intensity plot.

For the next test procedure, I again initialized the Listen SoundCheck AudioConnect analyzer along with the Listen SCM-2 1/4” microphone to measure the impulse response with the tweeter recess-mounted on the same test baffle. Importing this data into the Listen SoundMap software produced the CSD waterfall plot given in Figure 7. Figure 8 shows the STFT displayed as a surface plot. 

For the last SoundCheck test protocol, I set the 1 m SPL to 94 dB (2.55 V), using a noise stimulus and built in SLM, and measured the second and third harmonic distortion with the microphone placed 10 cm from the dome diaphragm (see Figure 9).
 
Figure 9: SB Acoustics SB29BNC SoundCheck distortion plots.

All things taken together, and since I know that Frank and Ulrik (Danesian Audio) spend a lot of time listening to various iterations of a driver as they go through the development process, I’m guessing that this is a very fine sounding product.
For more information visit www.sbacoustics.com. VC

This article was published in Voice Coil, March 2019.
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