Introduction
In this blog post I test a new waveguide design that I came up with for the Bliesma T34B-4 Beryllium dome tweeter. The specification page for the T34B-4 can be found here. The tweeter retails for around $500 USD. I directly compare this new waveguide/tweeter combo against the RCF CD350 compression driver with the ES-800 Biradial. I chose this comparison because the RCF/ES-800 combo represents a pretty standard setup, and a good all-around performer. The RCF retails for $90 USD, and so there "should" be a considerable step up in performance with the Bliesma.
Waveguide Design Elements
I wanted a waveguide that was based on my ES-Biradial horn flare geometry so that I could control vertical and horizontal directivity independently. The biradial shape also provides closer driver spacing to the low frequency driver.
The horn is 3D printed in two components. The first component is the throat piece which closely follows the shape of surround. This requires high precision printing which is why this is a separate piece. The second piece is the horn mouth which is printed in a different axis than the first piece.
Measurements
I began by measuring the raw 2439 frequency response at 1m (see red below). I then installed a capacitor to flatten the response for further testing. I found that a 6.8uF capacitor was ideal (see red response below). The raw impedance sweep is also shown as an overlay in black.
I then conducted the same response with the RCF CD350/ES-800 Biradial combo. A 3.3uF capacitor was enough to provide a reasonably flat response.
I then measured the horizontal and vertical directivity of the T34B/2439 combo.
Horizontal coverage is characterized as a consistent and wide coverage of 120 degrees however it narrows somewhat starting at 8kHz where we see directivity at 70 degrees for the 11kHz region. Pattern control is maintained as low as 1.2kHz.
The vertical directivity is characterized as a gradual narrowing starting at 2kHz narrowing to 40 degrees at 11kHz. Overall behavior is devoid of any diffraction or spurious resonances.
I then measured the RFC CD350/ES-800 Biradial combo which provides constant directivity across the operating range. Coverage starts out at 120 degrees at 1kHz and ends at 100 degrees for the 11kHz region.
The vertical is characterized as a gradual narrowing starting at 2kHz and ending at 60 degrees coverage for the 11kHz region.
Generally we see a more optimal polar map for the ES-800 biradial. One factor contributing to this is the larger throat of the bliesma at 1.33" versus 1".
Time Domain
I then measured the Burst Decay for the Bliesma. We see a perfectly clean result within the audible band.
The CSD plot also is very commendable.
I then measured the RCF CD350/ES-800 Biradial combo. The burst decay shows some ringing in the 10kHz region out to 15 periods. Personally I like to see a clean burst decay to within 18 periods. So this is acceptable.
The CSD plot is shown as well for reference.
Distortion
I then looked at distortion using a 95dB test signal level referenced at 1m. The mic was placed at 20cm from the horn mouth. H3 and H4 measure at around 0.008% for the 2kHz region.
If we change the vertical scale to dB instead of percent, this translates into -82dB.
I then measured the RCF CD350/ES-800 Biradial combo under the same conditions and achieved 0.007% for H3 and 0.014% for H4.
Again, changing the vertical scale to dB results in H3 at -83dB and H4 at -77dB.
Conclusion on Harmonic
Both the RCF CD350 and T34B have similar harmonic distortion profiles for H3 and H4.
Intermodulation Distortion using Multiband Multitone
I then measured the IMD of the Bliesma at the same 95dB test SPL that we used for the Harmonic test. IMD is at -60dB for the 1.5kHz region increasing to -70dB for the 5kHz region. Upper treble is around -65dB.
I then tested the RCF CD350/ES-800 Biradial combo. We -64dB for the 1.5kHz region -61dB for the 5kHz region and -55dB for the upper treble.
The Bliesma seems to outperform the RCF marginally on the IMD by about +5 to +8dB dB. This may be more academic since both results are far below the threshold of audibility.
Gedlee Distortion (Gm)
Below is direct comparion using Gedlee Distortion for the 95dB test SPL. I'm using Virtins Multi-Instrument Software to conduct this test. The RCF is shown in red with the Bliesma in Blue.
Conclusions
The RCF CD350/ES-800 Biradial combo is a tough act to follow but the Bliesma pulls it off by offering a smoother response, lower distortion, and a cleaner time domain. However the larger diameter of the Bliesa introduces some narrowing of directivity in the critical upper treble where soundstage width could be compromised. The Bliesma has no trouble digging down deep into the lower midrange without sounding stressed or resonate, which is often the case when mounting a dome tweeter in a waveguide.
My subjective evaluation was brief and conducted in mono. Overall I would say that the Bliesma sounded excellent, with just a slight lack of upper treble energy compared to the RCF. Directly comparing the two revealed that the compression driver is more expressive and colorful, highlighting dynamic contrast more readily. Both had similar levels of clarity through the midrange and treble.