This month, Eastech sent us the tweeter from its magnesium-lithium product line, which includes the AUGDL0003-JN03 tweeter plus 4” thru 8” magnesium-lithium alloy cone woofers. Features for this transducer include a 1” magnesium-lithium foil dome diaphragm and coated cloth surround, a 79 mm × 15 mm ferrite magnet motor, a 110 mm diameter four-screw-cast aluminum faceplate anodized semi-flat black, a two-layer 25 mm diameter CCAW voice coil wound on an aluminum former, a felt damped hollow pole piece (see Photo 1), and copper cap shorting ring (Faraday shield). The tweeter has 89 dB sensitivity and a nominal 4 Ω impedance.
I commenced testing using the LinearX LMS analyzer to produce the 300-point impedance sweep illustrated in Figure 1. The resonance occurs at a fairly low 800.6 Hz (factory spec is 725.3 Hz). With a 3.31Ω DCR (Re), the minimum impedance for this tweeter is 3.78Ω at 2.57 kHz. Following the impedance test, I recess-mounted the Punktkilde tweeter in an enclosure that had a baffle area of 12” × 8” and measured the on- and off-axis frequency response using the Loudsoft FINE R+D analyzer (courtesy of Loudsoft) and the GRAS 46BE 1/4” microphone (graciously provided by GRAS Sound & Vibration), which 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/1 m. Data was then acquired with sweeps at 0°, 15°, 30° and 45°. Figure 2 shows the on-axis response of the AUGDL0003-JN03, which measured ±4.5 dB from 1.0 kHz to 20 kHz, with the magnesium-lithium breakup mode peaking at about 24 kHz.
Figure 3 gives the AUGDL0003-JN03’s on- and off-axis response. 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). Figure 6 shows the two-sample SPL comparison, indicating the two samples were closely matched to within less than 0.5 dB throughout its operating range.
Finally, I again initialized the Listen SoundCheck AudioConnect analyzer along with the Listen SCM-2 1/4” microphone (all provided courtesy of Listen, Inc.) 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 cumulative spectral decay plot (CSD) waterfall plot given in Figure 7.
Figure 8 shows the Short Time Fourier Transform (STFT) displayed as a surface plot. For the last SoundCheck test protocol, I used a noise stimulus to set the 1 m SPL to 94 dB (4.37 V) and measured the second and third harmonic distortion with the microphone placed 10 cm from the dome diaphragm (see Figure 9). Note that the distortion is dominated by the second harmonic distortion (a good thing), and as Eastech claims, is less than 1% throughout the driver’s entire operating range.
For more information, visit www.eastech.com/punktkilde. VC
This article was originally published in Voice Coil, November 2019