ES-1200 Biradial Wood Horn

The ES-1200 Biradial is the smaller brother to the ES-800.  It loads down to 1200Hz and features very similar horn flare geometry to it's bigger brother.

This blog post will feature the acoustical measurements of the ES-1200 Horn.  Please see the previous post for measurements on the ES-800 which can be found here.

CAD Rendering 

Time Lapse CNC Machining ES-1200 Biradial Wood Horn



Design 

To get a good sense of the the design here's a quick video showing the rotated model in my 3D CAD system.  


This horn is unique in that it has a wood rear cover that replaces the compression driver's aluminum rear cover.  This increases the rear internal volume to help reduce the resonances to below the passband frequency.  The rear chamber is elliptical in shape and is stuffed with alpaca wool
Measurement test setup in my living room.  I managed to find a better stand for the horn!
I was asked to include measurements comparing the wood rear chamber against the compression driver's factory cover.  So I've included those measurements in this blog post.  This horn will accommodate two compression drivers in it's current design.  The B&C DE400TN and the RCF ND-350.  Both compression drivers use neodymium magnets and copper shorting rings for low distortion. 

For these measurements I will be using the B&C DE400TN however I've opted to not use the factory diaphragms.  Instead I've replaced the diaphragms with non-stock mylar units that I've modified in-house to lower the fundamental resonance (FS).  This effectively increases the bandwidth by allowing the compression driver to play cleanly down to 1kHz with no phase or stored energy anomalies.   I will also show in my measurements the stock titanium diaphragm and my own modified diaphragm for comparison.

Measurements 

To start let's look the the stock B&C DE400TN without the wood rear cover and the stock titanium diaphragm. 
B&C DE400TN shown with stock Titanium Diaphragm
B&C DE400TN mounted for testing.  Don't worry, the driver was still perfectly aligned despite the duct tape!
Frequency Response of ES-1200 with stock B&C DE400TN 
Here is the spectral waterfall decay plot to begin the comparison...
Spectral Waterfall plot of ES-1200 with stock B&C DE400TN
As you can see from the results there is stored energy in the 1.5kHz region which also is correlated in the frequency response as a hump.   The stored energy above 10kHz is showing as sharp dips in the frequency response as well.  For me this types of anomalies would rule out this driver for audiophile applications.  But stay tuned because things get better.

Replacing Titanium Diaphragm with Modified Mylar

I replaced the factory titanium diaphragm with Mylar versions that I've modified in-house to help drive the fundamental diaphragm resonance to below 1kHz which results in the following measurements...

Testing again with Mylar Diaphragms 
B&C DE400TN shown with Mylar Diaphragm (Modified) 

Frequency Response of ES-1200 with B&C DE400TN with modified non-stock Mylar Diaphragms with factory rear cover on the compression driver. 

Spectral Waterfall Plot for the ES-1200 with B&C DE400TN with modified non-stock Mylar Diaphragms with factory rear cover on the compression driver

Replacing the compression driver's rear cover with wood rear cover...

Here are the measurement results of the wood rear cover. 

Adding the wood rear sealed chamber that replaces the compression driver's rear cover plate.
Frequency Response of ES-1200 with B&C DE400TN with modified non-stock Mylar Diaphragms with wood Rear Cover
Spectral Waterfall Plot for the DE400TN, Modified Mylar, Wood Rear Cover 
We finally get to a point where the frequency response is very linear and there is a correlated improvment in the spectral waterfall plot.   Please note that the vertical scale for all waterfall plots are set to 35dB instead of the standard 25dB in order to highlight the improvements.  

Off-axis Response Measurements 

I conducted full off-axis measurements of the horn.  If you are not familiar with these measurements I suggest you read up on their significance.  A well behave off-axis response is a major contributor to the overall sound quality.
Horizontal coverage pattern for ES-1200 Biradial wood horn with 12dB smoothing.  This is the least amount of smoothing that ARTA will allow. 
I had a little chuckle after seeing these results as they are basically perfect.  I personally have not seen a better polar map.  
Vertical coverage pattern for ES-1200 Biradial wood horn with 12dB smoothing.
The vertical coverage has a few small irregularities around 17kHz but still a very respectable result. 

Conclusion

I'm thrilled with the performance of this horn.  This horn achieves a new benchmark for me personally in terms of sound quality.  

Additionally...

Before I go I wanted to include the impedance plots for those interested.  
Impedance Curve for Stock B&C DE400TN
Impedance Curve for B&C DE400TN, Mylar, and Wood Rear Cover