My Unity Horn Speakers

After building several high efficiency speaker systems for other people, I've come to the conclusion that high efficiency is a very important goal for true to life dynamic sound reproduction. My previous systems required kilowatts of drive, but could only reach a bit over 100dB at the listening position. All systems with moderate efficiency speakers suffer this fate. The 4 meter listening distance reduces the loudspeakers 1 meter maximum output by up to 12dB, depending on the acoustic absorption in the room. Thermal compression takes it's toll at high power levels, robbing you of the last 4 or 5 dB that you so badly needed to allow your system to do it's business.

My system now uses the Lambda Acoustics Unity horn from 300Hz up. 70 to 300 Hz is handled by an Eighteen Sound 15LW1401 per side, in a 75 liter sealed enclosure with a high Q 4th order high pass filter giving the required 4th order L-R high pass acoustic response. The subwoofer, which is crossed at 70Hz uses a pair of 12" Peerless XLS drivers in a 4.4M Tapped Horn.

The Unity horn is the brainchild of Thomas Danley, who also invented the Servodrive subwoofer. It is a multi driver horn, which combines the output from five drivers into a single constant directivity horn. The picture on the right shows how the drivers are arranged on the back of the horn. You can see the four midrange drivers and the compression driver in the middle. My horns use the B&C DE-25/16 compression driver. Sadly Lambda Acoustics went out of business a few years ago. The Unity horns are no longer available to the DIY market.

This horn offers many of the advantages of my old ribbon speakers, (low distortion, low thermal compression and well controlled vertical directivity) but adds a few as well (high efficiency and well controlled horizontal directivity). The efficiency of this horn is very high, at around 100dB/W/M. To keep up with this extreme output, I drive the 15LW1401 with around 400W RMS for each channel for the bass. This gives me over 120dB of maximum output capability at 1 meter, including thermal compression. (which is 2.3 dB @ 350W and 3.4dB @ 700W for this driver) The sub is driven with an Australian Monitor AM1600, one driver per channel.

The bass and mid/high amplifiers are Australian Monitor 1K2s.

Back of the Unity Horn

Benefits and Specifications of the Unity Horn From the Lambda Acoustics web site

This patent-pending technology is based on utilizing a pyramid-shaped, multi-band wave guide, to properly combine the amplitude and phase from a multiple number of drivers into a single coherent acoustic output. It is in effect a 2-way horn, 5 driver system including a simple passive crossover network that effectively acts as a single driver on and off axis. Unlike similar sized enclosures, the 16-inch square horn exit provides an essentially constant 60 x 60 degree pattern down to several hundred Hz. Using this technology, more of the sound is aimed directly to where the cabinet is pointed minimizing side and floor reflections. This benefit assists in controlling sound in highly reverberant rooms or where the maximum preservation of a recording's stereo image is desired.
Clarity Unmatched voice intelligibility and precise music reproduction due to the time-aligned phase coherent design.  This is the only multi-way design to accomplish such in both the on and off axis response.
Controlled Directivity 60 x 60 dispersion pattern provides superb energy time curve response over an extended bandwidth while lowering undesirable side wall and floor reflections from reaching the listener and clouding the sound.
Frequency Response 300hz ~ 22,000Hz +/ - 3dB
Efficiency 100dB @ 1W1m with an 8ohm nominal load
Power Handling 100W RMS (120dB) typical music program assuming 6dB high pass crossover at 300hz
Distortion @ 112dB/1M 300 Hz = .477%
600 Hz = .194%
1200 Hz = .386%
2400 Hz = .967%
4800 Hz = 1.5%
Distortion @ 90dB/1M 300 Hz = .078%
600 Hz = .029%
1200 Hz = .062%
2400 Hz = .110%
4800 Hz = .049%
Unparalleled Performance Very low distortion, very high output.  Consider at even 116dB each 5" mid range driver is receiving only 1W, the power compression and harmonic distortion levels of the speaker rivals those that only amplifiers could previously attain.
Dimensions 16.5" x 16.5" at the mouth outer lip, 14" deep overall, very small size for such high performance.
No "Horn" Sound! Due to the large radiating area of the 4 midbass drivers, the "horn" sound has been eliminated in this advanced patented design.
Drawing of the Unity Horn

This is what my speakers look like with the grills removed. The speakers are covered in a Tasmanian Myrtle timber veneer, with solid timber edgeing. The front panel is Charcoal hammer finish paint. No fasteners are visible for the grills because they are held in place with eight neodymium magnets per grill. The magnets are glued into recesses in the back of the solid timber edging around the front of the enclosure.

The enclosures were constructed with two bass sections to allow the fitment of another pair of 15LW1401's in the future. This would minimise the floor bounce notch problem and increase the sensitivity of the bass section up to 104dB/W/M. This has proved not to be necessary, because the bass driver has extremely high output capability, and only covers a very small part of the spectrum (70 to 300Hz). The notch caused by reflections off the floor is not great enough to be an issue. This picture shows the speakers fitted with a pair of JBL 2226H drivers. I was using these when I initially set up this system. The 18 Sound drivers perform better than the JBLs.

The crossover for the Unity is the latest design by Tom Danley, which replaced the original one as shipped by Lambda Acoustics. This is then crossed actively to the 15LW1401's. The original Unity crossover had a few bugs which have been ironed out in the latest version.
The front of my Unity horn speakers

In the left hand photo the bracing in the two bass sections can be seen. The front panel driver cutouts have not been made at this stage. Note that the horn has not been screwed into the enclosure in this photo. Two pieces of wood were cut on a 30 degree angle and screwed through the sides of the enclosure and the sides of the horn. This mounting arangement proved very simple, and is very secure. These pieces of wood can be seen in the photo at the top of this page.

The right hand photo shows the enclosures just before finishing. There is a lot of wood dust around from the final sanding of the speakers. Both bass sections have been heavily stuffed with Dacron to absorb some of the backwave off the drivers. The stuffing in the upper bass section can be seen, the lower bass section has been treated the same, but the driver hole has not yet been cut. The carcass is made from 25mm MDF except the front and back, which is 32mm MDF. All panels are glued and screwed. The enclosures are very solid and heavy. The veneer is laid up on MDF for a total thickness of 4mm, making the top and sides 29mm thick.
Bracing Front of the Unity Horn

My system
And the completed system with grills attached.
My system
And the completed system with grills removed. This small prototype 25Hz tapped horn has been replaced by one which is almost double the size.
This is the measured response at the listening position of the two channels, no smoothing. The rise below 100Hz is a measurement artefact, the response is flat to below 20Hz.

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