In this blog post I test Horn No.1906. This is a departure from my usual horn design in that I wanted to see if I could successfully widen the off-axis coverage by use of a phase plug directly in front of the tweeter. I've never attempted something like this before. This horn rides on from previous horn revisions found in a previous blog post.
The phase plug basically splits the sound into two pathways essentially creating two horns flanked side by side. My hope is that this widens the off-axis coverage without detriment to anything else. I also wanted to see if I could flatten the peak in the frequency response around 10kHz which corresponds to where the driver becomes very directional. My goal is the spread this energy and kill two birds with one stone (wide coverage + flatten response).
Using my ES curvature for both the phase plug and the outside horn flare is shown below.
I had my local guy 3D print me the prototype. I also had him print me a rear cover so that the rear wave is fully absorbed and doesn't effect my on-axis measurements.
The precision of the 3D printing is quite amazing. The openings in the driver perfectly match up to the vanes in the horn.
I started by measuring the on-axis frequency response which is shown below.
For comparison here is the previous Horn No.1905 shown below. My levels are not matched so please don't compare overall sensitivity.
I then produced an off-axis colored polar map to see if I achieved my goal of widening the coverage in the upper treble.
There seems to be quite an anomaly at 16kHz and so I changed the polar map to only extend to 13kHz which is shown below.
We can see that the horn acts like a regular horn up to around 4600Hz where the phase plug takes over and really widens the coverage. The phase plug depth is 40mm and this works out to half the wavelength of 4600Hz as shown below.
Subjective Listening Impressions
The coverage above 4600Hz has significantly increased to 180 degrees! Walking around the room the treble does not change at all, even 90 degrees off-axis. Since I only made one horn I cannot evaluate the stereo soundstage, however even with one horn it is providing great perceived soundstage width across the width of the room.
It appears that this horn is usable from 4kHz upwards. If I want usable coverage down to 2kHz then the horn needs to be twice the depth (80mm).
Harmonic Distortion 85db SPL at 1m
Perhaps the phase plug is too large and only needs to be sized in order to diffuse frequencies in the 10kHz region. However I am intrigued by the ultra-wide coverage pattern provided by this particular horn and it's subjective listening implications. For those who put a high priority on perceived soundstage width (PSW), then this is something very interesting and worth further investigation.
I am currently looking for beta testers who will print the horn themselves and evaluate as a stereo pair. Please contact me if you would like the 3D CAD file to test and evaluate for yourself. email@example.com