DIAPHONE

     Many of you are familiar at least to some degree with the diaphone foghorn, especially if you are over fifty and have either lived near the ocean or the Great Lakes or had shipping or marine experience. Furthermore, you may have read the article entitled “The Voice of the Lighthouse” in Horn & Whistle issue # 103. In addition to describing its history and operation, that article also featured pictures of Harry Barry’s operating diaphone foghorn overlooking Lake Erie.
      Finding a diaphone foghorn today is very difficult if not virtually impossible. First of all, relatively speaking, there were never that many produced, probably fewer than two thousand. With the advent of more modern electronic aids to navigation and electric fog signals, almost all of the operating diaphone installations were shut down by 1970, and in many cases, the horns were scrapped. A few have made it to private col-lections, and a few more are operated by preserv-ationist groups at actual lighthouses, but these are unavailable for private purchase.
      Even if you were fortunate enough to locate one, making it work would be quite an undertaking. The air requirements of a diaphone foghorn are truly prodigious; A home workshop air system is not sufficient for one of those horns. Considering that a diaphone foghorn requires two input air supplies, one requiring a 1 1/2" supply pipe, and the other a 4" pipe as well as at least one receiver of 1000 gallons capacity and at least a 20HP compressor, you can easily see that the air supply would require a lot of room and power as well.
      However, there is a way to get the advantages that a diaphone has to offer such as a unique and unmistakable sound of unbelievable intensity and long-distance signal propagation ability and to do this without undertaking a fruitless search or needing unlimited funds. Furthermore, it’s possible to employ as little as a 60 gallon receiver and a 2 HP compressor which is typical for many home workshops. I refer of course to the Gamewell type B fire station diaphone.
       For many years, these specialty horns were manufactured and sold by the Gamewell Corp-oration, a well-known Massachusetts maker of municipal fire alarm systems. The Gamewell dia-phone is smaller than a foghorn, and requires much less air. But it is a genuine diaphone and operates exactly the same way as its foghorn counterpart.

       Even though smaller and pitched an octave higher, the “little brother” of the foghorn is never-theless a mighty signal indeed. With a deafening blast that noticeably outperforms many chime RR horns and is louder than most of the typical air raid sirens in common use today, the Gamewell B fire station diaphone is definitely a serious signal.
      The usual expected effective range for a type B is five to six miles in an urban area, and some have been heard well at much greater distances. In many cases, just a single diaphone is all that is required to warn an entire municipality. Unlike the situation with diaphone foghorns, there are still many Gamewell type B diaphones in active service, and it’s possible to find and purchase one with a little research and making a few inquiries.
      For the benefit of those who have not seen the article in H&W #103, let’s go over a few of the diaphone basics and also the operation of these horns and see why they sound as they do and how they can be so loud in proportion to their physical size. As is true with many other loud signals, the diaphone operates on the principle of the modul-ated or “chopped” air stream.
      If you take a blast of compressed air and mod-ulate or “chop” it into a series of regularly recurring puffs or pulses by means of a rapidly opening and closing valve, you will have a very effective pro-ducer of loud sound. Furthermore, if you chop the stream of compressed air into pulses at the small end of a suitable resonator or horn which effect-ively couples the resulting sound to the surrounding air, you will find that you can make a very loud noise, and the physical noisemaker does not have to be very large while still generating a very power-ful sound wave. This is the operating principle of the diaphone horn.
      The valving or air-chopping arrangement in the diaphone consists of a hollow, cup-like piston in a close fitting cylinder. The cylinder has a number of circumferential slots machined in its walls, and the piston likewise has corresponding slots. As the piston moves longitudinally through the cylinder, there will be a point in its travel when the slots in the piston will line up with those of the cylinder. By making the piston oscillate rapidly back and forth in the cylinder, we can alternately open and close the slots.
      Next, if we surround the cylinder with com-pressed air, then we can see that when the slots of the cylinder and piston coincide, the air around the cylinder will pass through the slots and enter the piston. This piston, as stated, is hollow, and also open at one end like a cup.
      The open end of the piston communicates directly into the narrow end of the attached re-sonator or horn. Thus, by visualizing what is hap-pening, we can see that when the slots of the cylinder and piston coincide, the compressed air surrounding the cylinder will blow through the slots, enter the piston, and pass into the throat of the attached resonating horn.
      As the piston moves farther, the slots close, and air will no longer pass through to the resonator. By making the piston oscillate rapidly back and forth, the diaphone generates a rapid series of high-amplitude pulses of air in the narrow end of its attached resonator, and the end result of this action is the generation of a very high-amplitude sound wave.

Figure 1. Against the backdrop of the sky and a radio antenna, a Gamewell type B Diaphone atop its air supply pipe stands ready to blast out a powerful warning to its surrounding community.

Here are two sound clips, both of the same unit. By varying the way in which the air pressure is admitted, you can exert a con-siderable amount of influence over the diaphone's signal, even getting it to produce the so-called "grunt" at the termination of the blast.