Horn No.1931 -- ES Front Horn for GRS PT6825 Planar Tweeter --- Test & Review

Horn No.1931 -- ES Front Horn for GRS PT6825 Planar Tweeter --- Test & Review


The ES Horn No.1931 is a 500Hz Fc horn specifically designed for the GRS PT6825 Planar Transducer. 

This horn has been designed as a no compromise design specifically for use in 3D printing. The design uses the full wrap-around ES curvature for both axis, as well as a vented rear chamber. The horn features phased slots in the throat to smooth out the frequency response. 

The rear chamber is vented and should be filled with polyfill (pillow stuffing). This has been found to provide the best sound quality.  You can cover the vents internally with bug screen. 

The backside of the horn fully locates the driver so there is no guessing when mounting to the driver. 

  • 3D CAD files for horn and rear cover (.STL format) 
  • Assembly drawings (.PDF format) 
  • 500Hz Cutoff Frequency (Fc) 
  • Frequency Response 500Hz-20kHz 
  • Dimensions (Mouth): 334mm wide x 451mm tall (13.14" x 17.75")


I began by measuring the impedance sweep which shows the fundamental resonance (FS) at 300Hz. There is another smaller peak at 1kHz with virtually no inductive rise in the upper treble. Note the vertical scale is only 10ohm. The standards vertical scale is 50ohm, so this impedance curve is very flat relative to other drivers.


Frequency Response 

Below is the 1m on-axis frequency response gated to 100Hz. We can see that the horn loads down to 400Hz. The response is generally within ±3dB across the spectrum with treble extension to 19kHz (-3dB).  

Below is the frequency response at 0,15,30, & 45 degrees off-axis. We see that the off-axis is very consistent with the on-axis, but we do see a narrowing of directivity 30 degrees off-axis starting at 12kHz. Personally, my hearing goes to 14kHz so I'm not sure if this is a significant issue. 

With measurements taken at 5 degree increments off-axis I was able to create a polar map out to 90 degrees off-axis. I've gated the results at 3.9ms and applied 12dB/octave smoothing.


  • 5kHz region shows very wide and controlled coverage (120 degree listening window) 
  • Generally, the off-axis behavior is very consistent with a gradual narrowing of directivity, but maintaining wide coverage in the treble 
  • Treble coverage is 100 degrees up to 12kHz which is excellent 
  • Consistent broad spectrum coverage is difficult (500Hz-20kHz) but this horn does admirably. 

Time Domain 

Below is the burst decay which shows some stored energy in the treble region but it is well damped by 12 periods. Generally I find anything beyond 12 periods starts to become audible. 


The CSD plot shows the mechanical resonance (FS) at 300Hz and it seems well damped. The small peak at 1kHz that we saw in the impedance sweep shows as as some minor stored energy, but it doesn't seem to be anything to worry about. The rest of the spectrum is relatively clean showing fast decay. 


I started by measuring harmonic distortion at 1m for the 85 and 95dB test signal levels. We can see that distortion does start to rise below the Fc of the horn (500Hz) so it would be wise to include a steep high pass filter in this region. However things clean up considerably at 800Hz where H2 is only 0.03%. Distortion remains extremely low across the rest of the spectrum, barely exceeding 0.10% (-60dB) 

At the 95dB test signal level we see H2 rise with the higher orders remaining very low. H2 for 800Hz has risen from 0.03% to 0.12%, still extremely low. 


 Intermodulation Distortion 

I began by running test tones from 500Hz-20kHz with 12 tones every octave. This produced the following spectrum of noise. We can see modulating products (dark area at bottom of graph). For mid-frequency tones we have -65dB of dynamic range reducing to -60dB for the treble. 

For the 95dB test signal we see -65dB for the midrange and -50dB for the treble.

If we change the test tones to cover from 1kHz-20kHz instead of starting at 500Hz, we get the following. IMD in the midrange has improved from -65dB to -80dB with treble frequencies unchanged at -60dB.  

The 95dB test shows IMD at -70dB for the midrange and unchanged at -50dB for the treble. 

For comparison this result is similar to the RCF ND950 4" voice coil compression driver tested here. The advantage here is less cost and cleaner upper treble in terms of the burst decay. 

Gedlee Distortion (Gm)

Below is the Gm distortion as an overlay between the 85dB and 95dB test signal levels. Treble shows 0.0014 and 0.0031 respectively. This beats out the RCF ND950 which had 0.007 for the 95dB test signal. It also beats out the 18Sound ND1460 which had 0.0118 for the same test signal level. 


Subjective Listening Impressions 

Below is a summary of my listening impressions. 

  • Smooth upper treble
  • Excellent soundstage depth 
  • Great output capability (High SPL) 
  • More dynamic than an average bookshelf speaker but not as good as a compression driver (But smoother sounding than most compression drivers)
  • Great coherency through vocal range 
  • Needs some minor EQ at 1kHz otherwise comes across a tiny bit forward 
  • No horn coloration detected 
Back to blog