In this blog post I test the BMS 4524 on a custom designed horn (No.2974). This compression driver retails for $150 USD however some sites offer this at 40% off.
The horn can printed by purchasing the STL files here.
Horn No.2974 Features:
- 2.2kHz Fc
- 13° exist angle to perfectly match driver
- ES Horn flare curvature
- 1" Throat
- Support Stand
- Mounts using M5 screws
- 3D printed
- Dimensions 124mm wide x 90mm tall x 40mm deep.
- Unique Patented Design
- 113 dB sensitivity 1 W / 1 m
- 1.9 kHz Crossover
- Very small Size and low Weight
- 25.4 mm Voice Coil
- 8 Ohm
The 4524 shows a nice linear response even into the upper treble.
The ring radiator is a direct radiating type which is appealing to me. This is in contrast to some compression drivers which force the sound wave 180° such as the B&C DE360.
I disassembled the tweeter to see more of the design features. The tweeter uses a radial phase plug that captures both sides of the voice coil. There should be great symmetry between the inner and outer path lengths. This should result in excellent time alignment between the two vanes.
The sound conjugates into a conical expansion which measures 13° as the half angle (26° included). The driver accepts M5 bolts from the front or M4 from the rear.
The center of the diaphragm is secured with a small foam cylinder. This is an interesting solution rather than using a bolt. The B&C DCM50, for example, uses an M6 bolt to secure the center of the diaphragm.
The voice coil gap doesn't have any copper shorting elements but we see tapered cap and pole piece which follows the diaphragm shape. We also see a black felt ring which presses against the diaphragm when assembled. There is also some green polyfill deep inside the voice coil gap for back wave absorption.
Measurements
I start by showing the unsmoothed frequency response. The response graph is not adjusted for sensitivity.
I then measured at 0, 15, & 30 off-axis. The 4524 is providing loading down to 2kHz however pattern control doesn't start until 6kHz. The directivity index is quit flat from 6kHz to 15kHz. The response graph is not adjusted for actual sensitivity.
Time Domain
Below is the CSD plot in either waterfall or sonogram display method. I also show either 20dB or 35dB vertical scale.
Below is the Burst Decay (BD) in either waterfall or sonogram display method. I show either 25dB or 35dB vertical scale.
Conclusion
- Excellent frequency response linearity in the 6kHz-15kHz range
- Main diaphragm breakup occurs at 19kHz (Great result)
- Horn No.4524 provides good off-axis behavior indicating low horn coloration.