University Sound MM-2
A paging horn built to survive live steam, salt spray, and the inside of a boiler room — and to still be intelligible when it gets there.
Overview
The MM-2 is not a hi-fi speaker, and anyone who buys one expecting to listen to records through it is going to be disappointed fast. It is a submergence-proof paging and public-address horn — University Sound's own literature calls it a "conservatively rated 25-watt" unit — designed for wall, ceiling, or bulkhead mounting and sold into exactly the environments where ordinary speakers die. The housing is a heavy-gauge spun aluminum case with a die-cast aluminum center assembly, all metal parts anodized and finished with baked-on acrylic. The diaphragm is phenolic impregnated linen-base, the voice coil is 2.0 inches across, and the magnet is a "W"-shaped Alnico V structure weighing 0.93 kg with plates.
What makes the MM-2 interesting isn't subtlety — it's the sheer stubbornness of the design. The speaker is self-draining, treated with an anti-fungicide (the source material spells it "anit-fungicide," a typo that has followed the spec sheet around for decades), and rated to withstand fungus, dust, salt spray, live steam, and gases. That's not marketing bravado; it's a U.S. Navy specification the thing was engineered to meet. The voice-coil/diaphragm assembly can be reached for cleaning by pulling the die-cast reflector off the front, which is the kind of serviceability you only design in when you expect the unit to be filthy.
The electrical numbers are pure commercial-sound territory. Frequency response runs 800–5,000 Hz ±5 dB, with a usable low-frequency limit of 600 Hz — this is a speech band device, full stop. Impedance is 16 ohms. Sensitivity is quoted at 104 dB at one meter with one watt, measured with pink noise band-limited from 800 to 5,000 Hz, which is the sort of efficiency that lets a small amplifier throw voice across a noisy room. Beamwidth is 82° horizontal and vertical at 2 kHz, and the directivity factor is 10.25 at the same frequency. Power handling is stated as 25 watts over eight hours at a 6-dB crest factor, tested with 500–5,000 Hz pink noise.
The mounting hardware differs by variant, and buyers should know which one they're getting. The base MM-2 uses a serrated swivel bracket with a locking nut, giving at least 90° of vertical adjustment, with three mounting holes in the base of the mount. The MM2F — a separate model that gets conflated with the base unit constantly — is the flush-mount version, with six mounting holes on a 6.625-inch circle around the flange (the manufacturer document gives conflicting hole diameters of 7/32 inch and 1/2 inch), dimensioned to sit behind a 6-1/4-inch hole. The MM-2 itself measures 15.2 cm in diameter by 12.8 cm deep and weighs in at 2.4 kg net; the MM2F is larger at 18.4 cm diameter and shallower at 9.5 cm, at 2.8 kg. The manual's figures are the ones to trust here — third-party listings disagree on dimensions and weight, and the manufacturer document names the model explicitly.
A cable entrance sits on the bottom side of the housing, and there's room inside for a matching transformer — University Sound's own model 5030, a 30-watt unit — for 70-volt line distribution. As shipped the MM-2 is transformerless, and the manual is direct about why that matters: feeding frequencies below the horn's cutoff draws excessive current through the driver, so a series capacitor is recommended for protection. For a 16-ohm driver on a 25-volt line the manual calls for 100 mf at 150 V DC (or 150 V non-polarized electrolytic, or two 150 V electrolytics of double the value back-to-back for 70-volt lines). That's the sort of detail that separates a working installation from a burned coil.
Key Features
Submergence-Proof Housing to U.S. Navy Specification
The case is spun aluminum, the center assembly die-cast aluminum, everything anodized and coated with baked-on acrylic. The design is self-draining — water that gets in has a path out — and the whole assembly is built to shrug off salt spray, live steam, fungus, dust, and gases. This is the feature that defines the model. A paging horn that lives above a ship's engine room or in a mine shaft has to survive conditions that would destroy a conventional driver in a season, and the MM-2 was specified to do exactly that. The anti-fungicide treatment on the voice-coil/diaphragm assembly is part of the same package.
Phenolic Linen-Base Diaphragm and 2.0-Inch Voice Coil
The diaphragm material — phenolic impregnated linen-base — is chosen for stiffness and moisture resistance rather than exotic resonance behavior. Combined with the 2.0-inch voice coil and the "W"-shaped Alnico V magnet structure, it's a driver built for durability and output rather than extension. The magnet weighs 0.93 kg with plates per the manufacturer's specification sheet. The front reflector removes for access to the diaphragm assembly, which is the maintenance path owners will actually use.
104 dB Sensitivity and 82° Coverage
At 104 dB at one meter per watt, the MM-2 needs very little amplifier to make itself heard — which is the point in a paging system where a single small amp may be driving a whole trunk line of horns. The 82° horizontal and vertical beamwidth at 2 kHz gives a roughly conical coverage pattern, and the 10.25 directivity factor at the same frequency tells the installer how much of the output is going where it's aimed versus spilling. The manual's own design guidance is worth repeating: coverage patterns from multiple units should overlap only slightly, so each section of the audience is covered by a single speaker, and nothing gets aimed at a reflective wall.
Serrated Swivel Bracket with 90° Adjustment
The base MM-2 mounts on a serrated swivel bracket with a locking nut, adjustable through at least 90° of vertical arc, with three holes in the base of the mount. The serrations hold the aim once it's set — a small thing until you've watched a smooth-friction bracket drift out of position over a year of vibration. Flush-mount installations use the MM2F instead, which is a different model with a different flange and should not be confused with the bracket version.
Transformer Provision for 70-Volt Lines
The housing is prepped for a University Sound 5030 (30 W) matching transformer, though the MM-2 ships without one. For distributed paging systems this is the difference between a speaker you can hang on a constant-voltage line and one you can't. The manual's capacitor recommendation for low-frequency protection applies to transformerless drivers and is not optional advice — below 600 Hz the horn unloads and the driver starts drawing current it can't dissipate.
Historical Context
The MM-2 belongs to a family of University Sound submergence-proof speakers — the MM2, MM2F, and MM2TC are documented as a series designed for ships and emergency vehicles for intercom and voice paging. The design brief is written into the spec sheet: built to penetrate high noise levels in boiler rooms, mines, and railroads, and ideal for use near water or spray. The U.S. Navy specification behind the submergence-proof rating places the design in the shipboard and industrial market rather than the consumer one. No production years are documented for the model, and the archive has no confirmed original retail price.
Collectibility & Value
This is a working commercial part, not a collector's item in the usual sense, and the market reflects that. Listings on eBay describe units as "VINTAGE UNIVERSITY SOUND SPEAKER - SUBMERGENCE PROOF - MODEL MM-2 - Untested" and "University sound MM-2 submergence-proof speaker 16OHMS 25W" — asking prices only, with no confirmed sold prices for the base MM-2 available to the archive. One retailer lists the model at $486.00 each with stock marked "Limited," though that listing carries no date and may reflect new-old-stock or current production rather than the vintage market. A separate MM2F listing shows an offer of $70.00 plus $8.37 delivery, sold untested in original packaging — that's the flush-mount variant, not the base MM-2, and shouldn't be read as a comparable.
Buyers should assume any unit found in the wild is untested and price accordingly. No common failures, maintenance procedures, or spare-parts availability are documented, and no owner reviews of sound quality turned up in the research — the archive has listings and spec sheets, not listening impressions. The diaphragm assembly is accessible for cleaning, which is the one service item the literature confirms. Beyond that, condition of the phenolic diaphragm and the integrity of the housing seals are the things to inspect, and there's no published guidance on what failure looks like.
eBay Listings
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