Horn No. 1155 for RAAL 70-20XR Pure Ribbon Tweeter

Horn No. 1155 for RAAL 70-20XR Pure Ribbon Tweeter

Horn No.1155 using ES-Biradial Geometry for RAAL Ribbon Tweeter 



This blog post features a custom horn that I designed and built for a customer that uses the RAAL 70-20XR pure ribbon tweeter.

The 3D CAD Files are available to purchase on my site by clicking HERE.  This allows for either 3D printing the horn + adapter plate or CNC machining.  Please note this is for personal use only.  For commercial licencing please inquire. 

This horn is also available as a custom order product from my website. Please click HERE For more details. 

The below comparison can be downloaded as a PDF by clicking HERE.



Horn No.1155 for the RAAL 70-20XR


Test baffle for RAAL 70-20XR flush mounted with the exception of the small area at the top.


Frequency Response

The 1155 horn is designed to provide 9dB of boost starting at 1.5kHz and will have a 3dB per octave falling response above 1.5kHz.  We see that the this naturally sums to a flat response since the RAAL driver on it's own has a 3dB per octave rising response starting at 1.5kHz.  So the horn takes advantage of the natural raw response of the RAAL.

RAAL 70-20XR with and without 1155 Horn


Off-Axis Coloured Polar Map 

The horn provides a nice gradual narrowing of directivity with no edge diffraction artifacts.  The coverage window remains wide at 100 degrees at 13kHz. 

The non-horn test baffle has a very wide coverage window at 6.5kHz of 160 degrees.  However an octave above this that window has narrowed to only 80 degrees.  So the sound power is down -50% between these two octave frequencies.


RAAL 70-20XR Off-Axis Coloured Polar Map


RAAL 70-20XR Direct Radiating (No Horn)



The off-axis waterfall really highlights the difference.  The PDF can be downloaded HERE




For this project I was not able to show any distortion advantage to the horn.  I'm not sure why this was the case.  One thing that seemed very unusual was how quickly distortion set in as loudness increased.  Using a distortion limited SPL test I found that the RAAL 70-20XR had a maximum SPL of only 85dB at 1 meter.  You would have to decide if this is a limitation for you or not based on your listening habits, music style, and room size. Personally I would want more output from the RAAL since you also have to factor in headroom for dynamic peaks in the music.  Please note the distoriton measurements were done with a 7.8uF capacitor to protect the tweeter which is why the frequency response does not look as flat as the raw response.  Click HERE For the PDF version of below.


Distortion incrementally increased by 5dB from 70dB to 85dB where distortion crested 1%









Time lapse of the horn construction and assembly is shown below...



Assembly drawing of 1155


Subjective Listening Impressions 

I listened to the RAAL in both configurations and I much preferred the sound with the horn.  The sound is very smooth with excellent detail.  I didn't notice any distortion when playing at higher listening levels so the 85dB limit may not be a concern in practice.  I tested out 1.5kHz, 2kHz, and 2.5kHz crossover points and I found that 2kHz was a nice spot subjectively with the the SB Acoustics 5" woofer.  Overall the sound was very pleasant that I could not find fault with other than a lack of dynamic range.  This definitely meets my performance target for sound quality, and it looks great too.


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