Test Bench: Omega Audio ODT29 Cloth Dome Tweeter

January 7 2021, 14:10
For this month’s Test Bench, I characterized the ODT29 fabric dome tweeter from Omega Audio, an OEM transducer manufacturer new to the pages of Voice Coil. The company, headquartered in Holon, Israel, was founded in 1978 when the founder of Omega Audio, Levi Mordechai left Morel. All of Omega Audio‘s ODT line of tweeters, dome midranges (see Photo 1) are handmade in the facility, from dome coating to coil winding and motor assembly. The woofers are also hand assembled. All of this is done by Levi Mordechai himself, as it has been since 1978, and for this reason Omega Audio is a high-quality, small-scale manufacturer.
 
Photo 1: All of Omega Audio‘s ODT line of tweeters, dome midranges are handmade in Holon, Israel.
Photo 2: Omega Audio’s ODT29 is a 29mm fabric dome tweeter.
Figure 1: Omega Audio ODT29 free-air impedance plot.
Figure 2: Omega Audio ODT29 on-axis frequency response.

The features for Omega Audio’s ODT29 fabric dome tweeter, seen in Photo 2, include a 29mm hand-coated fabric type diaphragm with a fairly narrow 2mm wide surround, a felt damping ring on the top of the pole piece, ferrite motor, a plastic faceplate with a short waveguide, an aluminum voice coil former wound with copper wire, and gold-plated terminals.

Testing commenced using the LinearX LMS analyzer to produce the 300-point impedance sweep illustrated in Figure 1. The tweeter resonance occurs at a moderately low 730Hz (factory spec is 750Hz). With a 5.55Ω DCR (Re), the minimum impedance for this tweeter is 5.91Ω at 1.5kHz. 

Following the impedance test, I recess mounted the Omega Audio tweeter in an enclosure with a baffle area of 15” × 6” and measured the on- and off-axis frequency response using the LoudSoft FINE R+D analyzer (provided by LoudSoft) and the GRAS 46BE 1/4” microphone (courtesy of GRAS Sound & Vibration). The equipment was set up to measure the 200Hz to 40kHz frequency response (using a 192kHz sampling rate) at 2V/0.5m normalized to 2.83V/1m. Sweeps were performed at 0°, 15°, 30°, and 45°.
 
Figure 3: Omega Audio ODT29 horizontal on- and off-axis frequency response (0° = black; 15° = blue; 30° = green; 45° = purple).
Figure 4: Omega Audio ODT29 normalized on- and off-axis frequency response (0° = black; 15° = blue; 30° = green; 45° = purple).
Figure 5: Omega Audio ODT29 180° horizontal plane CLIO polar plot (in 10° increments).
Figure 6: Omega Audio ODT29 two-sample SPL comparison.

Figure 2 shows the on-axis response of the ODT29, which measured ±3 dB from 2 kHz to 19 kHz. Figure 3 shows the ODT29’s on- and off-axis responses, with the off-axis curves. Figure 4 shows the curves normalized to the on-axis response. Figure 5 shows the CLIO 180° polar plot (measured in 10° increments with 1/3 octave smoothing). Figure 6 shows the two-sample SPL comparison of the ODT29, indicating the two samples were closely matched to within 1dB throughout its operating range from 2kHz to 10kHz.

For the final group of tests, I fired up the Listen, Inc. SoundCheck analyzer along with the Listen SCM 2 1/4" microphone (provided courtesy of Listen, Inc.) and measured the impulse response with the tweeter recess mounted on the same 15" × 6" test baffle. Importing this data into the Listen SoundMap software produced the cumulative spectral decay (CSD) waterfall plot shown in Figure 7. Figure 8 shows the Short Time Fourier Transform (STFT) displayed as a surface plot. For the SoundCheck final test procedure, I set the 1m SPL to 94dB (7.31V) using a noise stimulus, and measured the second- and third-harmonic distortion at 10cm, depicted in Figure 9.
 
Figure 7: Omega Audio ODT29 SoundCheck CSD waterfall plot.
Figure 8: Omega Audio ODT29 SoundCheck STFT surface intensity plot.
Figure 9: Omega Audio ODT29 SoundCheck distortion plots.
There are not a lot of small hand-built OEM driver manufacturers left on the planet, so I think Omega Audio is pretty special. The performance and build quality of the ODT29 definitely shows its linage from Morel, which is a good place to be coming from. For more information, visit www.omega-audio.co.il. VC

This article was originally published in Voice Coil, October 2020.
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