This blog features a full set of measurement on the Altec 811B biradial horn. My plan was to test this horn with a pair of Altec 806-8A compression drivers however the measurements were so poor that I didn't think the off-axis polars would give the horn a fair chance at performing it's best. So instead I used the TAD TD-2001 so that no argument could be made that there was something wrong with the driver.
The Altec 811B is a cast aluminum horn measures 476mm wide x 219mm high x 343mm deep. The horn uses an exponential expansion to provide a specified 90 x 40 degree coverage pattern.
The reason for the test is to compare this horn's measured performance against my own horns which I build in my shop using my CNC machine.
For information on the my products please visit my website.
ES-600 Biradial Horn (Most similar to the Altec 811B)
The Altec 811B rings like a bell if it's not securely mounted to a bass enclosure. To mitigate the vibration I had to securely wrap the horn in a blanket using pre-stretched stretch wrap. The horn continued ring during my knuckle wrap test at the horn mouth (Lip) so I added additional damping by clamping a sock sandwiched between wood blocks, which was C-clamped in place. Finally the horn was dead sounding to the knuckle wrap test. I measured the horn at 1.5 meter mic distance using my rotary table, careful to keep the mouth of the horn along the pivot axis.
For a direct comparison you can see how the TAD TD-2001 compression driver performs on my ES-600 biradial by clicking HERE.
On-Axis Frequency Response
Add captionAltec 811B with TAD TD-2001, 1.7ms gate, 1.5m mic distance
Off-Axis Coloured Polar Map
Altec 811B with TAD TD-2001 Off-Axis Coloured Polar Map (12dB smoothing, 4ms gate)
Altec 811B with TAD TD-2001 Off-Axis Waterfall Map (12dB smoothing, 4ms gate)
The step response takes an unusually long time to settle at 9.5 milliseconds.
For convenience I've done a side by side comparison below with the Altec 811B on the left column and the ES-600 Biradial on the right.
There isn't really much to conclude other than observing the measured performance. The Altec provides very wide coverage making it suitable for it's original application of sound reinforcement. The severe edge diffraction effects the off-axis consistency as well as all time domain aspects (step response, burst decay) which would result in a blurring of transient detail across it's bandwidth.