TECHNOLOGY OF MATERIALS
Being a mechanical device, each horn has its own resonance due to material behavior. Horns are normally built of metal (steel-aluminum), wood (natural –MDF) and plastic. The above materials have a resonance signature of their own. We could all familiarize with their sound behavior by knocking them with our finger. Some are characterized by the “easy listening“ sound (wood, plastics) and others by the harsh sound (metals). Selecting one with a rich colorful sound, to another with a brilliant one, and visa versa, depends on one’s personal preference. The truth is that all of them distort badly, quite far beyond the neutral.
WOOD
Wood is a live, natural material, influenced by humidity, temperature, constantly changing its shape within the time domain. Moreover wood is not an acoustically inert material. It vibrates, and some type of resonance is extremely colorful, but smooth and easy to listen (musical). That is the reason why it is extensively used for musical instruments. By selecting the appropriate wood, each instrument creates its specific and unique tonal character.
MDF does not exhibit as strong a resonance peak as solid wood does. MDF and chipboard also resonate at 150-400 Hz, a sensitive region in the human ears. Being cheaper and more inert than natural wood, it is preferable for horn solutions, unless you like the more distorted but musical sound of natural wood horns.
METALS
Metals such as aluminum or steel are geometrically stable and non-ageing materials. However, high energy resonance is the worst enemy of metal, unless it is used for bells and cymbals. A damping material would control some resonance energy without eliminating it.
GRP – ABS – PVC (PLASTIC)
Despite the easy manufacturing process of casting, these materials resonate badly, producing a dull, cheap and not so musical sound. Thin and unbraced horn walls, the most common process, make the structure much more resonance sensitive. This is the cheapest solution of all the available materials.
COMPOSITE TECHNOLOGY (GLASS-CARBON-KEVLAR FIBERS)
Modern technology has developed acoustically inert materials such as carbon and Kevlar composites. Composite technology offers the freedom to design stiff, resonance-free products, with virtues of mechanical and acoustical properties. These materials comprise one of the best stiffness to mass ratios (better than titanium) , with the additional benefit of self damping (Kevlar). With the further application of vibration physics, an almost inert result could be produced:
1. By combining materials of different resonance character, the overall frequency resonance could be reduced.
2. Longitudinal wall bracing (constructed from special foam sandwiches), improve the stiffness even more, minimizing the resonances.
3. Variable wall thickness improves and controls the resonance peak.
4. Double sided walls improve the structural rigidity.
5. Despite the inert neutral behavior of such a design, additional damping could be used. Structural damping dissipates vibrational energy in the structure before it can build up and radiate as sound. Damping materials reduce the vibrating energy of the walls, transforming it to heat. There is no ideal damping material, whatsoever. There are materials compromising excellent properties at high frequencies, poor at low and visa versa. The appropriate damping material matches the horn structural resonance behavior at the operating frequency range. This material could be placed as a sandwich between the inner and outer walls of the horn. Special mixtures of marble, sand, lead, mixed with elastomeric compounds could be used.
The final cost of such a structure must be under no consideration at all, using the best vacuum-bagged epoxy, carbon-Kevlar-glass fibers. This is the most inert structure of all.
