Electro-Voice HP940
A constant-directivity horn that spent decades as the default high-frequency load in E-V's biggest systems - and the reason a whole shelf of equalizer modules exists.
Overview
The HP940 is a horn. It is not a speaker system or an amplifier, and you don't plug a guitar into it. It is the high-frequency flare Electro-Voice built to sit on top of a compression driver. The historical record keeps turning it up in the same places: arena clusters, theatre systems, and the spec sheets of E-V's own crossovers. Nobody bought one on its own and took it home. It was a component, and it earned its reputation by being the component everyone specified.
The clearest documentation of the horn is Electro-Voice's own equalization literature. The company's XEQ-3 crossover shipped with a table of EQ modules, each matched to a specific horn-and-driver pairing, and the HP940 shows up repeatedly in that table. The EQM module is listed for the HP940 driven by DH1, DH1A, or DH2 drivers. The EQR module is listed for the HP940 with the DH1A. So the HP940 was designed around the DH1 family of compression drivers, and Electro-Voice considered the match important enough to build dedicated corrective EQ for it rather than trust a flat electrical response. The manual says why - the equalization circuits exist to "accurately equalize high-performance compression drivers used with constant-directivity horns." The HP940 is one of those horns, and the EQM and EQR modules are the correction curves that go with it.
A constant-directivity horn is a compromise. It trades some on-axis smoothness for the ability to hold its coverage pattern as frequency rises, so the people at the back of the room hear the same balance as the people at the front. The price is a response curve that needs help. Electro-Voice published the help as a plug-in module, which means an HP940 in good working order is only half the story. The other half is whatever EQ is feeding it.
The HP940's second life is in installed sound. Period coverage of the horn itself is thin, but the systems built around it are well documented. Sound & Communication (1990, p.53) described an arena installation in which twenty-six suspended satellite clusters were each made up of three E-V HP940 horns powered by DH1A drivers, with low frequency supplied separately. That is the job the horn was designed for: arrays covering a large space with a controlled pattern, three deep per cluster. The same magazine's March 1986 issue (p.38) traced the constant-directivity horn concept back to Electro-Voice's mid-1970s work, which is the design lineage the HP940 belongs to.
In the surviving record, the HP940 is not a collector's trophy the way a Western Electric or an Altec horn is. It has a working following, not a romantic one. The people who know it maintain installed systems, rebuild arena clusters, or assemble period-correct E-V rigs from the DH1 era, and they know it by its part number and its EQ module. The horn appears in the XEQ-3 manual's tables, in the MTS-1 announcement, and in the arena cluster description - three separate contexts, all professional, all consistent. For a component that was never sold as a standalone product, that is a lot of documentation.
The practical caution is the one that applies to every constant-directivity horn from this era: the horn is only as good as the driver behind it and the correction in front of it. An HP940 with a mismatched driver or no EQ module will not sound like the systems in the period coverage. Electro-Voice's own tables make the pairings explicit, so check which module the horn was originally run with before assuming a bare horn is a drop-in replacement. Beyond that, the record is thin. There is no documented production run, no published price, and no independent bench test of the horn in isolation in the material available here.
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