Recently I was commissioned to design and build a small horn for the LCY K100 ribbon tweeter. The customer provided the tweeter so that I could get accurate physical measurements of the throat transition area along with the mounting hole pattern.
The horn I designed is based on the latest biradial technology using the ES horn flare curvature.
I began testing the stock tweeter by making a test baffle from some scrap walnut I had around. (see below)
I decided to add some bevels to help with edge diffraction. The baffle width is 15cm. The baffle is 5cm thick. I also flush mounted the tweeter.
Below is the acoustical measurements for the stock tweeter. The on-axis frequency response is shown below. We see a reasonably flat response from 6kHz and up.
Burst decay is shown below. There does appear to be a small High-Q resonance in the 9-10kHz region.
The waterfall plot (CSD) is shown below. Again, something happening at 10kHz region.
Step response is shown below. Transient response for the this tweeter is very fast.
Conducting off-axis produces the following colored polar map. We see the dip at 9kHz on-axis being filled in as we move off-axis producing a strong ridge. We see a 90 degree listening window at 15kHz.
I then removed the stock bezel from the tweeter and mounted horn No.1702 using the same four screws that came with the tweeter.
I then re-tested with the frequency response shown below.
Below is a comparison between the two responses. The horn adding sensitivity across the spectrum. Oddly the dip at 9kHz has now become a peak. The horn adds about 7dB sensitivity in the 2kHz-4kHz region. But this is not enough to sum to a flat response with the upper treble.
Still, a contour circuit could be implemented and still retain 90dB sensitivity. However as we will see later, this is not advisable.
Burst decay with Horn No.1702 is shown below. There still seems to be the pesky issue at 10kHz.
The waterfall plot for horn No.1702 is shown below. We definitely see things clean up with the exception of the 9kHz issue.
Step response is shown below. The result is marginally cleaner.
I then conducted off-axis measurements for the No.1702 horn which are shown below.
The off-axis colored polar map shows a more consistent and controlled behavior. The listening window at 15kHz has increased from 90 degrees to 100.
A multitone signal at 90dB SPL at 1 meter is shown below for the stock tweeter using a 2kHz 4th order high pass filter. Distortion products are -40dB below the fundamental tones at 10kHz. For reference, distortion below -60dB is commonplace for many high frequency drivers I test.
Stock Tweeter (Below)
Horn No.1702 (Below)
Comparing with the horn in place shows no meaningful improvement.
Stock Tweeter (Below)
Increasing the crossover point from 2kHz to 4kHz with the stock tweeter at 90dB is shown below. Distortion in the 10kHz region improves marginally with the higher crossover point. (-40dB improves to -45dB)
Horn No.1702 (Below)
Comparing the same with the horn in place is shown below. We do see some improvement in the 5kHz region with the horn in place (-55dB versus -62dB with the horn) This is now approaching our target for sound quality, however only in the 5kHz region.
Where to go from here?
I decided that perhaps I was asking too much from this small ribbon tweeter and so I increased the high pass filter from 4kHz to 8kHz and lowered the test SPL from 90dB to 85dB. The result for the horn is shown below.
-55dB does not meet my performance target and so I decided to look at other changes to the horn that could possibly help. I decided to plug the driver throat with 30ppi open cell foam. (see below)
I then retested the horn with the foam in place. The resulting test data is shown below. Distortion is now -60dB down. This is with a test signal of 85dB along with an 8kHz high pass filter.
I then looked again at the frequency response with the foam in place, which is shown below. I changed from a 8kHz to a 1kHz high pass filter for this result. As we can see, things are much better in the 9kHz region.
It is clear that this small ribbon tweeter has limitation on overall output capability. The sound quality is strictly conditional on the following:
- 8kHz or higher crossover point
- 85dB maximum SPL using multitone test signal at 1 meter
If your specific application warrants, this tweeter can offer razor sharp transients along with very wide off-axis coverage even into the upper treble.