Design Study— Ultra-Wide Coverage Horn No.2275 with RCF ND350 16ohm

Design Study— Ultra-Wide Coverage Horn No.2275 with RCF ND350 16ohm

In this blog post I test a new horn design that I've come up with. This is a blend of omnidirectional, biradial, and Western Electric 35A.


Below is the Duevel Sirius omni-directional speaker which radiates sound 360°.

Bent Horn 

The Western Electric 32A tilts the horn axis as shown below. 


The biradial along with other exponential horns are characterized by a wide but gradually narrowing coverage as you move into the upper treble. An example is shown below. Coverage starts out wide at 2kHz and then narrows to 80° at 10kHz. 


New Design Concept

Horn No.2275 tilts the horn axis and relies on an upward pointing conical type of structure to distribute the sound wider than a standard ES biradial horn. Modelling the horn took considerable time due to the complex geometry, however I was still able to fully utilize the ES curvature throughout. 

Design Goals 

  • Provide constant directivity 
  • Provide ultra-wide coverage (120 degree listening window)
  • Low coloration, low resonance design 

I decided to push the coverage as wide as possible with the early prototype. I could then reduce the coverage as needed in future designs, depending on the application or customer preference. This early prototype aims for a 180° listening window. If coverage is evenly distributed across the listening window then the concept is fully validated for future designs requiring narrower coverage. 


We saw earlier that the exponential horn's directivity narrows to 80° at 10kHz. Many enthusiasts prefer this coverage pattern so that early side wall reflections are minimized getting more direct sound, and less interaction with the room. Generally speaking, this offers better clarity at the expense of perceived source width (PSW). On the opposite end of the spectrum of preference are those that prefer wider coverage which provides a wider PSW as well as perceived source envelopment (PSE) which is provided by stronger early side wall reflections among other early boundary images. This generally comes at a loss of on-axis clarity however if the room is properly treated with plenty of diffusion then this can be almost entirely mitigated. Most people don't have acoustically ideal rooms and so the narrower 80° listening window is the best option. 

However some enthusiasts are blessed with a room that is heavily treated with proper diffusion which eliminates pesky flutter echoes but maintains a high level of liveliness. In this instance it would be desirable to have a loudspeaker coverage pattern which is ultra-wide giving that elusive wall of sound or a feeling of envelopment. 

Version 1 Prototype Horn No.2237

Physical Dimensions 


Horn No.2275 measures 250mm wide x 116mm tall and accommodates a 1" compression driver. 

I had the horn 3D printed using 50% fill for testing. The prototype horn does not have mounting features for the compression driver and so used tape to hold it in place. 


With the mic placed at 1m I conducted an on-axis frequency response. The driver used is the RCF ND350 16ohm version. I've overlaid the impedance sweep in green. 

I then decided to add a 2.2uF capacitor to flatten and high pass filter the response. Below is an overlay on the affect of the capacitor. 


I then decided to measure the off-axis frequency response at 0,15,30, & 45° increments. The response is virtually identical until 45° off-axis where we are only down -2dB evenly across the spectrum.

I then measured off-axis in 5° increments up to 90° off-axis to produce the follow colored polar map. 

Based on the result, I decided to continue measuring further off-axis. Below is the polar map out to 120° off-axis. 

Here is the same data displayed as a waterfall 1 graph. 

Below is the same data displayed as a waterfall 2 graph. 

Time Domain 

Below is the CSD plot.

Below is the burst decay plot. 


Distortion is not really the scope of this review but I will show harmonic anyways at 85dB for 1m. The amplifier used is the Nelson Pass ACA Mini. 


I decided to measure the same compression driver on a 1.5kHz circular ES horn just to see how the driver's response looks on a more familiar horn. 

Below is the frequency response of the RCF ND350 16ohm on the 1.5kHz circular ES Horn. I've also shown the affect of the 2.2uF capacitor in the signal path. 

I then decided to compare the response of Horn No.2275 against the 1.5kHz ES Circular Horn. Below is an overlay comparing the two horns with the same compression driver. Both responses shown is with the 2.2uF capacitor. 

We can see that Horn No.2275 does nothing to harm the frequency response smoothness. It seems to drastically widen the off-axis coverage without creating other issues in the response. The upper treble energy is distributed to the off-axis which is why we see a drop compared to the circular horn. Both horns would have the same power response into the room. 

Subjective Listening 

Horn No.2275 sounds distinctly different in that it broadcasts the sound very strongly through the room. It was easy to perceive the additional room energy creating a sense of spaciousness as a backdrop to the direct on-axis sound. I did not detect any reduction in clarity when directly comparing against the circular ES horn. Overall it seems as though my design efforts have succeeded in achieving my target goals. The next phase will be to print another horn and listen as a stereo pair. Stay tuned! 


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