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The Basics Of An Acoustic Horn

At first glance, one would assume that the primary purpose of a phonograph’s horn is to act as a structure that “points” the soundwaves in a certain direction. When this horn is disconnected, the playback’s volume and fidelity becomes greatly reduced. This is true whether the listener is standing right in front of the tonearm opening or behind it.

While the horn paves some direction for the sound, its main purpose is to match the sound wave’s characteristics as it passes from the narrow tonearm and then goes into a big spatial area where the sound is heard. The tapering tonearm is also a miniature horn itself. It works by enhancing the match between the sound pressure wave produced by the soundbox diaphragm and the opening of the horn.

The Sound Wave

Only two characteristics are needed when explaining the nature of the wave while it propagates along the inner part of a tube (tonearm). The first is the level of pressure variation (P) created by the wave at any indicated point. The second is the vibration speed (U) of the individual molecules. Remember that U relates to the moving speed of the molecules while they vibrate back and forth as opposed to being the soundwave itself through the tube. Generally speaking, the movement of the sound wave is described by the wave moving relatively slowly which is in contrast to the much faster vibrational speed of the molecules.

Straight And Untapered

In the case wherein the tonearm tube is straight and untapered while the soundbox diaphragm vibrates at a constant level, each molecule would vibrate back and forth about the center point. As each molecule vibrates, it causes them to bump into adjoining molecules. The later is then compressed and decompressed as they bounce back. As time progresses, this “compressing and decompressing” movement of the molecules creates a pressure wave that “moves” down the tonearm. Since pressure refers to the force or area, so long as the tube’s diameter is unchanged, then the cross sectional area and the pressure variation are the same and constant respectively.

When The Tube Ends And Opens Into A Room

Considering that the same situation occurs although this time, the tube ends and opens into a room abruptly, a distinct change happens in the soundwave. Once the soundwave exits the tube, there is a dramatic change seen in both characteristics of the soundwave (P and U). Since the room’s area is much bigger than the tube’s, the pressure drops at a significant level. Moreover, the air molecules found at the end of the tube, which are said to be vibrating and transferring the so-called “acoustic energy,” suddenly meet with a big number of molecules found in the listening room. As a result, the vibrational energy of the soundwave is reflected back into the tonearm as opposed to moving into the room. The sound that is heard in the room is described to have poor fidelity and decreased volume.

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