ES-600 Biradial with TAD TD-2001

In this blog I feature the TAD TD-2001 compression driver mounted to the ES-600 Biradial wood horn.  The horn featured in the post was custom made for a client which uses a 1" throat instead of the 1.4" that would be normal for this horn.  As a result the horn provides even better off-axis coverage in the treble.  The client will be using the TAD TD-2002 however the TD-2001 is very similar and features the same throat exit angle.  The horn flare geometry has been completely changed to accommodate the TAD driver so that the exit angles are perfectly matched in both the vertical and horizontal axis.

Every effort was made to ensure extreme accuracy during the CNC process.


Custom 1" Throat ES-600 with TAD TD-2001


TAD TD-2001 under test 

Near field distortion measurement for TAD TD-2001


I began the testing by conducting 1 meter on-axis frequency response measurements with the following results... 

On-axis Frequency Response TAD-2001 with ES-600 Biradial 


This combination provides a very linear frequency response.  Response rises 12dB per octave from 500Hz-1kHz.  Response varies ±0.50dB from 1kHz - 5kHz.  We then see a 1dB rise starting a 5kHz which extends to 8.5kHz.  Beyond that there is a gentle 3dB/octave falling response where it is down -2.7dB at 15kHz.


Impedance Data 

TAD TD-2001 with ES-600 impedance curve 

TAD TD-2001 Impedance zoomed in

The TAD TD-2001 has a 300Hz fundamental resonance (FS) however even on a 600Hz horn it struggles to produce strong output below 1kHz.  This would be typical for a compression driver in this size format.  The significant aspect to this measurement is the lack of mechanical diaphragm breakup in the region from 10kHz-20kHz. This is due to the beryllium diaphragm.  For comparison sake here is the impedance curve of a similar driver using a polyester diaphragm.


Impedance Sweep for RCF ND-350 compression driver using Polyester diaphragm

For comparison's sake you can see the breakup on the RCF ND-350 at 8.5kHz, 14kHz, and 17.3kHz.  Relatively speaking this is still a respectably smooth impedance curve considering the polyester material and cost of this driver is 10% that of the TD-2001.

Off-Axis Performance

I began measuring the off-axis performance with a fixed mic location and rotating the horn on my rotary table in 5 degree increments out to 90 degrees off-axis.  As a side note this is the first time that I've used a 1" throat on a 600Hz horn.  Generally speaking the off axis coverage would be wider with the smaller throat.


Starting with the off-axis coloured polar map out to 90 degrees off-axis provided the following results...

Normalized off-axis coloured polar map for ES-600 Biradial with 1" throat (12dB smoothing)


To interpret this results, the listening window is described as the -6dB downpoint for a given frequency.  So at 10kHz we have a 90 degree listening window.  At 15kHz this narrows slightly to 80 degrees.  A few other attributes are worth noting...


  • Pattern control is still strong even down to 1kHz.  
  • There is a gradual narrowing of coverage across the bandwidth which translates into excellent power response into the room 
  • The horn provides consistent even coverage even out to 20kHz 

Showing the response in a different ways highlights the off-axis performance...

Normalized off-axis waterfall 12dB smoothing for ES-600 Biradial using 1" throat 

Step Response 


The step response settles nicely after only 1.50 milliseconds.

TAD TD-2001 on ES-600 Biradial Step Response

Comparing this to the RCF ND-350 on the same horn yields the following result...


RCF ND-350 on ES-600 Biradial Step Response

The RCF ND-350 takes about 20% longer to settle (1.50ms versus 1.80ms).


Spectral Burst Decay 

Burst decay was done at 1 meter and gated to eliminate room reflections below 1kHz.  I've shown both 25dB and 35dB vertical scale to highlight the low noise floor performance.


Burst Decay for TAD TD-2001 with ES-600 Biradial (35dB Vertical Scale) 

Burst Decay for TAD TD-2001 with ES-600 Biradial (25dB Vertical Scale)

Subjective Listening Impressions 

I conducted my music listening evaluation in my small listening room with the Autumn Series bass cabinet which uses a 50liter onken with the Fostex FW208HS woofer.  I used the Xkitz active crossover which was set to 1.25kHz using a 24db/octave slope.  I was able to use this simple but effective active crossover since the raw frequency response met my target curves for flatness.  Source was my Chord Mojo DAC with Chord Poly digital player using Roon to access my music library.


Below is a ranking for my subjective impressions on the sound quality...


Soundstage Depth --- 9/10

Soundstage Width --- 8/10

Smoothness --- 10/10 (similar to Harbeth)

Coherence between midrange and treble --- 8/10

Coherence between mid-bass and midrange --- 6/10 (due to high crossover point and being 2-way)

Vocal Clarity Male --- 7/10 (male vocals mainly produced by 8" woofer)

Vocal Clarity Female --- 9/10 (female vocals are mainly produced by horn)

Accurate Musical Instrument timbre --- 9/10 (particularly stringed instruments)

Sense of Dynamic Range: --- 9/10 (for horn only)



The TAD TD-2001 in combination with this horn produces some of the best clarity that I've heard.  During my listening sessions it was a little upsetting that I personally don't own either the TAD TD-2001 or the ES-600 horn using the 1" throat.  Is it bad that I'm jealous of my customers?   


The ES-600 Biradial is available as a custom built-to-order product on my website.


For inquiries please contact