TYPES of HORNS
they all look somewhat similar, there are many differences. There
are the public address (PA) horns, usually short and with a rapid flare
rate. The flare rate is a measurement on how rapidly the horn
expands. More on that later. Similar to the PA horns are
those seen on emergency vehicles and also the type used by emergency
service personnel; police and fire officials. These are equipped
with their own battery operated amplifier and usually of low power
output. Admittedly, they operate in a band sufficient for
intelligible voice reproduction but their high acoustical output is a
testimonial to the efficiency of the horn. Another mundane device
is that used by cheerleaders, a conical horn.
Many musical instruments are horns, even the woodwinds are a form of horn. Early phonographs prior to the 1930's were equipped with a horn coupled to a diaphragm which was vibrated by a needle which was placed in the groove of a rotating disc or cylinder even earlier. The disc attached to the needle was about 2 inches in diameter and barely audible if removed from the horn but assembled it could be louder that normal speech so much so that some came with a soft plug that was placed in the horn to act as a volume control. Times have changed.
In the music world, we see what is referred to as the hybrid horn; usually a half horn and half direct radiator system. In it, the loudspeaker may be rear horn loaded as in the JBL (James B. Lansing) systems dubbed "scoops" or the Altec A7, "Voice of the Theatre" which was a bass reflex for rear loading with a 600 Hz to 800 Hz horn for front loading. There were numerous horn loaded systems available on the market until about the end of the sixties. The only home horn loaded system that has survived the onslaught of 'major breakthroughs' is the Klipschorn from Klipsch and Associates. The low frequency section is a twin path folded horn designed to fit into the corner of a room thus utilizing the walls to complete the horn. Patented sometime in the early 40's, it has been in continuous production since 1945. Much more on that illustrious system later.
Horns were also very popular in movie theatres due to their high efficiency and the low power available at the time. A well designed horn loaded system could easily deliver sound pressure levels well in excess of 110 dB down to 35 Hz with as little as 40 watts in a movie theatre and with distortion levels a magnitude lower than the best systems available today. Their physical size prohibited home use as these were straight axis horns placed behind the perforated screen. To give you an appreciation for the physical size of such a system, a straight axis horn, one that is NOT folded, with a low frequency cutoff of a modest 35 Hz would be 15 feet long and terminating with a mouth size, if square, of 14 feet by 14 feet.
There is another horn that is somewhat smaller in length due to a more rapid flare rate than that of the exponential flare and that is the Tractrix, named after the mathematical curve that bears the same name and now a trademark. The equations of the tractrix curve do not lend themselves to the easy transpositions as do those of the exponential which are easily manipulated by using the natural logarithmic functions.
JBL, James B. Lansing; Altec, A7, Voice of the Theatre are registered trademarks/tradenames of James B. Lansing Sound and Altec Lansing, resptively. Klipschorn and Tractrix are registered trademarks/tradenames of Klipsch & Associates.