Test Bench: Celestion CDX1-1742 1” Exit Pro Sound Compression Driver

July 31 2018, 04:00
This month I examined the new 1” exit pro sound compression driver from legendary UK Pro Sound OEM manufacturer Celestion, model CDX1-1742. This driver joins the CDX family of ferrite motor compression drivers, now 15 models in total. Designed for use with 1” throat horns, the CDX1-1742 has a 25.4 mm (1”) throat diameter and a 44.5 mm (1.75”) diameter voice coil wound with edgewound copper-clad aluminum wire (CCAW) on a high-temperature fiberglass non-conducting former, driving a one piece polyimide diaphragm and surround (see Photo 1).
Photo 1: Celestion’s CDX1-1742 compression driver.
Figure 1: Celestion CDX1-1742 free-air impedance plot.
Figure 2: Celestion CDX1-1742 on-axis frequency response.
Figure 3: Celestion CDX1-1742 horizontal on- and off-axis frequency response (0° = solid; 15° = dot; 30° = dash; 45° = dash/dot; 60° = dash).
Figure 4: Celestion CDX1-1742 normalized horizontal on- and
off-axis frequency response (0° = solid; 15° = dot; 30° = dash; 45° = dash/dot; 60° = dash).

Other features include a FEA-optimized ferrite magnet motor structure, a proprietary next-generation “Sound Castle” soft diaphragm clamping assembly that reduces diaphragm mechanical stress and distortion, nominal 50 W (AES) rated power handling, a 2 kHz recommended crossover frequency, and 1 W/1 m 107 dB sensitivity (measured 2 π on a typical horn). Celestion also supplied me with the H1-9040P horn. This exponential flare horn is a 1” exit bolt-on reinforced polymer type with a 90° × 40° coverage pattern.

To begin testing, I used the LinearX LMS analyzer to produce the 300-point stepped sine wave impedance plot shown in Figure 1. The solid black curve is with the CDX1-1742 mounted on the H1 horn and the dashed blue curve represents the compression driver without the horn. With a 5.76 Ω DCR, the minimum impedance of the CDX1-1742/H1 was 7.75 Ω and at 3.28 kHz. For the next set of sound pressure level (SPL) measurements, I free-air mounted the Celestion CDX1-1742/H1 combination without an enclosure and measured both the horizontal and vertical on and off axis at 2.83 V/1 m from 0° on axis to 60° on and off axis, using the LOUDSOFT FINE R+D analyzer and the G.R.A.S. 46BE microphone (supplied courtesy of LOUDSOFT and G.R.A.S. Sound & Vibration).

Figure 2 displays the on-axis frequency response of the compression driver/horn, which is ±1 dB from 1 kHz to 6 kHz with no major anomalies. The response is smooth and even from 6 kHz, where the low-pass horn related roll-off begins, to 20 kHz. Figure 3 shows the 0° to 60° on- and off-axis response in the horizontal plane. Figure 4 displays the normalized horizontal plane response. Figure 5 shows the CLIO Pocket analyzer-generated horizontal polar plot (in 10° increments with 1/3 octave smoothing applied). Figure 6 gives the on- and off-axis response in the vertical plane. Figure 7 depicts the normalized vertical plane response. Figure 8 shows the CLIO Pocket-generated polar plot (in 10° increments with 1/3 octave smoothing applied). Last, Figure 9 illustrates the two-sample SPL comparison showing the two Celestion CDX1-1742 compression driver samples to be closely matched within 1 dB or less above the recommended crossover frequency of 2 kHz.
Figure 5: Celestion CDX1-1742 0° to 180° horizontal plane polar plot (in 10° increments).
Figure 6: Celestion CDX1-1742 vertical on- and off-axis frequency response (0° = solid; 15° = dot; 30° = dash; 45° = dash/dot; 60° = dash).
Figure 7: Celestion CDX1-1742 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 8: Celestion CDX1-1742 0° to 180° vertical plane polar plot (in 10° increments).

For the remaining series of tests, I set up the Listen, Inc. AudioConnect analyzer and 1/4” SCM microphone (provided by Listen, Inc.) to measure distortion and generate time-frequency plots. For the distortion measurement, I again mounted the Celestion CDX1-1742/H1combination in free-air in the same manner as was used for the frequency response measurements, and set the SPL to 104 dB at 1 m (1.94 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 10.

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 and imported the data into Listen’s SoundMap Time/Frequency software. Figure 11 shows the resulting cumulative spectral decay (CSD) waterfall plot. Figure 12 shows the Short Time Fourier Transform (STFT) plot. 
Figure 9: Celestion CDX1-1742 two-sample SPL comparison.
Figure 10: Celestion CDX1-1742 SoundCheck distortion plots.
Figure 11: Celestion CDX1-1742 SoundCheck CSD waterfall plot.
Figure 12: Celestion CDX1-1742 SoundCheck Short Time Fourier Transform (STFT) plot.

Looking at all the data acquired, the UK-manufactured Celestion CDX1-1742 is a well-engineered 1” compression driver, exhibiting good performance, with Celestion’s usual excellent build quality. For more about this and other Celestion OEM pro sound products, visit the company’s website at www.celestion.com.  VC

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