Test Bench: Celestion CDX14-2420 1.4” Compression Driver

March 20 2019, 10:35
In this Test Bench I characterized the CDX14-2420 1.4” compression driver from legendary UK Pro Sound OEM manufacturer Celestion (see Photo 1). This transducer joins the CDX family of neodymium motor compression drivers, now totaling 12 models (and the Celestion ferrite line of compression drivers has grown to 16 models).
 
Photo 1: Celestion’s 1.4” CDX14-2420 compression driver.
Figure 1: Celestion CDX14-2420 free-air impedance plot.
Figure 2: Celestion CDX14-2420 on-axis frequency response.
Figure 3: Celestion CDX14-2420 factory plane wave tube and horn measurements.

Designed for use with 1.4” throat horns, the CDX14-2420 has a 35.6 mm (1.4”) throat diameter driven by a 60 mm (2.4”) diameter voice coil wound with edgewound copper-clad aluminum wire (CCAW) on a high-temperature fiberglass non-conducting former, which drives a deep-drawn titanium diaphragm and polyimide surround.

Other features include a FEA-optimized neodymium magnet motor structure, a continuous power handling of 140 W (3 dB greater than the 70 W AES rating), a 1.2 kHz recommended crossover frequency, and 1 W/1 m 106.5 dB sensitivity (measured 2 π on a typical horn). Celestion has four horn models, but they are all 1” throat horns. Given that, I mated the Celestion CDX14-2420 with a B&C Speakers ME90 80° × 60° constant directivity cast aluminum horn with a 900 Hz cutoff frequency.

To begin testing, I used the LinearX LMS analyzer (rest in peace Chris Strahm) to produce the 300-point stepped sine wave impedance plot shown in Figure 1. The solid black curve represents the CDX14-2420 mounted on the B&C Speakers ME90 horn and the dashed blue curve represents the compression driver without the horn. With a nominal 8 Ω impedance, the CDX14-2420 had 5.70 Ω DCR, with minimum impedance, mounted on the ME90 horn, of 7.36 Ω and at 4.6 kHz.
 
Figure 4: Celestion CDX14-2420 normalized on- and off-axis frequency response (0° = solid; 15° = dot; 30° = dash; 45° = dash/dot; 60° = dash).
Figure 5: Celestion CDX14-2420 normalized horizontal on- and off-axis frequency response (0° = solid; 15° = dot; 30° = dash; 45° = dash/dot; 60° = dash).
Figure 6: Celestion CDX14-2420 0° to 180° horizontal plane polar plot (in 10° increments).
Figure 7: Celestion CDX14-2420 vertical on- and off-axis frequency response (0° = solid; 15° = dot; 30° = dash; 45° = dash/dot; 60° = dash).
Figure 8: Celestion CDX14-2420 normalized vertical on- and off-axis frequency response (0° = black solid; 15° = blue dot; 30° = green dash; 45° = purple dash/dot; 60° = blue dash).
Figure 9: Celestion CDX14-2420 0° to 180° vertical plane
polar plot (in 10° increments).

For the next set of SPL measurements, I free-air mounted the Celestion CDX14-2420/ME90 combination without an enclosure and measured both the horizontal and the vertical on and off axis at 2 V/0.5 m (normalized to 2.83 V/1 m) 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 2 displays the on-axis frequency response of the compression driver/horn combination, which is relatively smooth with no major anomalies from the 1.2 kHz recommended crossover frequency to about 12 kHz, with the typical downward sloping response of a constant directivity horn. Figure 3 gives the factory measured plane wave tube frequency response along with an 80° × 60° horn measurement.

Figure 4 shows the 0° to 60° on- and off-axis response in the horizontal plane. Figure 5 displays the normalized horizontal plane response. Figure 6 shows the 180° horizontal polar plot (in 10° increments with 1/3 octave smoothing applied), generated by the CLIO Pocket analyzer and accompanying microphone (courtesy of Audiomatica SRL).

Figure 7 shows the on- and off-axis response in the vertical plane. Figure 8 depicts the normalized vertical plane response. Figure 9 shows the CLIO Pocket-generated 180° vertical plane polar plot (in 10° increments with 1/3 octave smoothing applied).
 
Figure 10: Celestion CDX14-2420 two-sample SPL comparison.
Figure 11: Celestion CDX14-2420 SoundCheck distortion plots.
Figure 12: Celestion CDX14-2420 SoundCheck CSD waterfall plot.
Figure 13: Celestion CDX14-2420 SoundCheck STFT plot.

Last, Figure 10 illustrates the two-sample SPL comparison showing the two Celestion CDX14-2420 compression driver samples to be closely matched within 1 dB or less above the recommended crossover frequency of 1.2 kHz to above 12 kHz.

For the remaining series of tests, I set up the Listen, Inc., AudioConnect analyzer and 1/4” SCM microphone (provided to by Listen, Inc.) to measure distortion and generate time-frequency plots. For the distortion measurement, I mounted the Celestion CDX14-2420/ME90 combination in free-air (in the same manner as was used for the frequency response measurements) and set the SPL to 104 dB at 1m (1.99 V determined by using a pink noise stimulus generator and internal SLM in the SoundCheck 16 software). Then, I measured the distortion with the Listen microphone placed 10 cm from the mouth of the horn. This produced the distortion curves shown in Figure 11.

Following this test sequence, I then set up SoundCheck 16 to generate a 2.83 V/1 m impulse response curve for this driver/horn combination and imported the data into Listen’s SoundMap Time/Frequency software. Figure 12 shows the resulting cumulative spectral decay (CSD) waterfall plot. Figure 13 shows the short-time Fourier transform (STFT) plot.

Considering all the measurements taken together, the UK-manufactured Celestion neodymium CDX14-2420 is obviously a well-engineered 1.4” compression driver, exhibiting good performance, combined with Celestion’s usual excellent build quality. For more information, visit www.celestion.com.  VC

This article was originally published in Voice Coil, December 2018.
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