Line Magnetic 555 on the ES-450 Biradial Horn
Introduction
For this test I evaluated the Line Magnetic 555 compression driver on the ES-450 biradial horn. This driver is inspired by Western Electric’s 555 field coil unit but updated for modern manufacturing. It features a large-format diaphragm and phenolic suspension, optimized for mid-frequency coverage with a smooth roll-off and vintage voicing.
The ES-450 is a biradial horn with a 450 Hz cut-off, chosen for its low coloration, consistent directivity and ability to load the driver well into the low midrange.
More information on the ES-450 biradial can be found here.
Frequency Response + Impedance
We see a gradual rise into the upper midrange peaking at around 7kHz. Impedance shows a smooth curve with a peak at 190 Hz corresponding to the horn/driver resonance. Above 300 Hz the impedance stabilizes. 
If we compare the frequency response with the A23 Mid Frequency Horn (featured here) we can see there is a large difference between the two horns. The A23 horn loads much lower into the midrange. The A23 horn taps out at around 2kHz while the ES-450 Biradial extends to 8kHz.
Contour Circuit
I also tried a contour circuit (1.50 mH and 22 Ω) to tame the upper mid bump:
This shows the response (blue) flattening nicely above 5 kHz compared to the stock curve (red). I used the contoured response for all proceeding measurements.
Off-Axis
Line Magnetic 555 on ES-450 Biradial 0,15,30, & 45 Degrees Off-Axis
Distortion and IMD
At 85 dB, the response is very linear through the midband. We see a gradual rise into the upper midrange peaking around 110 dB, with clean output up past 10 kHz. Distortion components remain below 0.2% in the 1–5 kHz region.
At 95 dB, the driver maintains composure. The second harmonic increases slightly (0.57% at ~1 kHz) but still remains low for a driver of this type.
Intermodulation distortion at 85dB with a 500 Hz/3 kHz test tone pair is measured showing a RMS noise floor is -45dBFS.
Please ignore the 69.77% distortion figure in red at the bottom of the graph. ARTA software does not show proper SMPTE distortion if a 10kHz F2 signal is changed to a different frequency.
At 95 dB, RMS noise floor rises as expected to -35.5dBFS.
Time Domain (Burst Decay & CSD)
The burst decay plot shows quick settling of energy in the midband. Some stored energy is visible above 10 kHz, typical of large-format drivers.
The Cumulative Spectral Decay plot confirms this: clean decay through 1–5 kHz with a few ridges in the upper treble.
Conclusion
The Line Magnetic 555 on the ES-450 biradial horn delivers a very authentic, vintage-style sound with excellent midrange clarity. The driver stays composed at higher output levels and can be easily equalized with a simple contour network to smooth the upper response.
For those chasing the Western Electric sound but wanting modern off-axis consistency with low horn coloration, this combination is an impressive option.
Additional SMPTE test data for your perusal. I tested using a 10kHz F2 test tone, using a 1kHz or 500Hz F1 tone, at both 85dB and 95dB test SPL. (referenced at 1m).
