ES-600 Biradial Horn

ES-600 Biradial Horn

This blog post features the latest horn that I've developed.  I will show the ES-600's measured performance and provide details on the crossover. This biradial is a Joseph Crowe exclusive design which has some very unique features not seen in any other horn design.  So far I've featured the 1200Hz and 800Hz versions which have been a huge success for me in terms of measured performance and overall sound quality.  The ES-600 Biradial continues in that success.  I will be introducing the ES-290 over the next few months as well which is effectively double the size of the ES-600. 

Customer's ES-600 Horns 
Below are some pictures of my customer's horns finished with a walnut stripe and veneered top and bottom.  He is now eagerly awaiting his bass cabinets and drivers to complete his system.  He has a beautiful hand built 300B tube amp monoblocks that should mate perfectly with the ES-600 Horn.

To start the ES-600 uses my ES flare geometry which includes a full 270 degree wrap around horn mouth which is a first for any biradial that I know of.  This reduces edge diffraction off the horn mouth and minimizes acoustical resonance to very low levels.

The height and width adheres to the golden ratio for an aesthetically pleasing shape.  Also of note is the golden ratio is  closely tied to the Exponential Spiral geometry!   I plan on writing a white paper on the overall design that covers the design elements more closely.  For now I wanted to publish the measured performance.


  • Frequency Response 600Hz - 20kHz 
  • 417mm Wide x 203mm High x 198mm Deep 
  • 1.4” Throat 
  • Suitable drivers
  1.  RCF ND-650 
  2. TAD TD-4003
  3. B£C DE780TN 
Compression Driver
For my testing I used the RCF ND-650 which has the following specifications...
  • 2.5-inch Diaphragm, 1.4-inch Exit Throat / Pure Titanium Compression Driver
  • 180W continuous program power handling
  • Frequency range: 700Hz - 20kHz
Compression drivers that can handle a frequency range from 600Hz-20kHz are very uncommon.  Finding a driver that is commercially available is even more difficult.  Modern compression drivers for pro sound often have a very stiff suspension for power handling purposes.  Even the Radian 745neo only extends down to 1kHz and has a 3" voice coil.  I decided to use the RCF ND-650 because it has a fundamental resonance (FS) of 600Hz as indicated on the published impedance curve.  I was confident that a custom rear cover with increase internal volume would lower the FS enough to accommodate a large 600Hz horn.   As it turns out the rear chamber is VERY small on the ND-650.  Increasing the rear volume with my custom wood rear cover dropped the FS to a meaningful 500Hz.

section view of rear cover for RCF ND-650 Compression Driver.  Rear cover is filled with 30ppi open cell foam and left to vent out the rear of the cover. 

Crossover Schematic for ES-600/RCF ND-650 Horn 

I spend considerable time developing this crossover so that it integrates nicely with a woofer of choice.  I also included a BBC curve which to my ears sounds more pleasant.  This crossover plays a critical role in damping resonances in  the 1kHz region.  I tried various amplifiers with this crossover and I found that my Class A solidstate amplifier was the worst at damping resonances in this region.  The 5.6ohm and 10ohm resistors play a critical role in damping the system so that resonances are damped to below the noise floor of other mechanical and acoustical resonances in the system.  Because of this I don't think you could simply connect a amplifier directly to the compression driver as you would in an active system since the crossover plays a vital role in the overal sound quality.  Let me be cannot use this horn in a active setup where the driver is connected directly to the amplifier.  The sound quality is true audiophile quality and this is only obtained by implementing the crossover that I've designed.  To be even more clear...implementing this in an active system will not deal with the mechanical resonances observed and the resulting sound will be 'average'.   Many active systems fail because they don't observe the time domain aspect to a speaker's performance and the vital role a speaker level passive crossover plays in dealing with these time domain anomolies.  For audiophile level quality in an active system the drivers need to be extremely high performance or operate within the very cleanest part of it's bandwidth.  However there are benefits to covering 600Hz-20kHz with one driver which can be fully appreciated by many audiophiles.

ES-800 sitting on top of ES-600 Biradial Horn 

Custom wood cover replaces stock cover on RCF ND-650 to lower fundamental resonance (FS) from 600Hz to 500Hz 

ES-600 Biradial horn throat carefully matched to the 1.4" throat of the RCF ND-650

Measurement setup in ARTA (see results below)

RAW Frequency Response (no EQ) for ES-600/RCF ND-650
Frequency Response measurement with custom crossover (ARTA) 

Frequency Response measurement with custom crossover (REW)
Burst Decay in ARTA 

Spectrogram settings in REW Measurement Software (see results below) 

Spectrogram burst decay results in REW 

Off-axis coloured polar map for ES-600 Biradial 
Listening Impressions
I am very happy with the sound of the horn.  When I'm listening I often ask myself if this would impress somebody that's new to audio.  When I hear this horn I think it would impress pretty much anybody.  There's massive scale to the sound combined with very coherent sound stage depth.   This is a very special sounding horn.  There is no lack of upper treble detail and the horn sounds balanced even when you move about the room.  I'm very happy with the integration of the Fostex FW208HS woofer.  They have very close rhythmic timing even on complex harmonics such as acoustic guitar.   I could go on further about the overall sound quality but my hope is that the measurements speak for themselves. 

General comments on sound quality
It is always tough for me to write about my listening impressions especially since I now have my own business selling audio components including this horn.  That is why I rely so heavily on the measured performance so that you can decide for yourself if this type of product would be a good fit for you.  However I also know that many music lovers or even audio enthusiasts don't fully understand what these measurements mean.  And even if they did know what they meant you also need something to compare against in relative terms.  How would this horn perform against say...a Tannoy MG15 coaxial?  It's unfortunate that so many manufacturers don't publish any performance data, or if they do can it be trusted?  For example the Radian 745Neo publishes a very flat frequency response however AudioXpress Test Bench shows that it is simply not the case.
Factory published frequency response for Radian 745Neo mounted on horn described as "Driver mounted on horn with 120°x40° nominal coverage and following dimensions: 356 mm (14”) mouth width, 152mm (6”) mouth height, 165mm (6.5”) horn depth"

Measured frequency response AudioXpress Test Bench for Radian 745Neo on B&C ME90 horn. 

 How does someone make a decision other than simply relying on other people's opinions?  It's practically impossible to listen to horn speaker systems in person unless you attend audio shows, and even then the environment is often very compromised.  I've been having a lot of discussions with folks that inquire about my services.  They are left in the dark basically trying to navigate this world of audio and there left relying on opinions.  They often will take a chance on a particular product only to find out it wasn't all that great after all.  They will often lose money along the way having to sell at a loss.   I firmly believe that measurements do tell a very clear picture of what the overal sound is going to be like, specifically time domain measurements as they relate so closely to the overall frequency response.  The burst decay and spectrogram is my personal guiding light as to specific design directions.  The off-axis response is related to these measurements in that they will also closely correlate to the perceived frequency response in your room. 

To emphasis this point I recently had opportunity to measure a horn speaker that I purchased because I needed the TAD TD-2001 compression drivers from the cabinets.  I decided for interest's sake to measure the speakers and see how they perform.   I know practically nothing about the speakers other than they have a nice smooth oblate spheroidal horn, very heavy baltic birch cabinet, TAD 12" mid-woofer.   For reference I call this speaker the "OS Speaker".  I had to take the off-axis measurments twice because I thought that I had done something seriously wrong...nope... I did not. 

OS Speaker recently purchased used.  

OS Speaker Burst Decay showing strong resonance at 4kHz and 8kHz.

OS Speaker Off-axis coloured polar map 

OS Speaker Frequency Response 
I attempted to EQ this speaker to sound better but there was no hope.  The sound was very unbalanced no matter what I tried.  The sound was also very forward and aggressive, even when powered by my 15 watt Class A solid state amp. 

If you are interested in a horn like this then please contact me.  I make these horns in my basement workshop.  Each horn is custom made to order.  I can also work with you to develop a finished speaker.  Contact
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