Acoustic properties of wood

A subject that has always attracted me, but which I have avoided until now because of its complexity, is the acoustic properties of wood. As I think it is an interesting subject for many of you, I thought it was time to talk about it, without proposing a scientific approach in which I tell you about the various coefficients of radiation or sound loss. It is a simple presentation of acoustic properties and how they can be used.

The best known of the acoustic properties is resonancewhich represents the property of wood to propagate sound. They propagate within the wood at a certain speed, which is different depending on the intensity of the sound, the direction of propagation and the structural elements of the wood:

• the width of the annual rings,
• pore distribution,
• regularity or irregularity of the fibre,
• apparent density of wood,
• existence or absence of growth defects
• humidity.

Sound propagation along the fibre is 1.3 to 2.3 times faster than perpendicular to the fibre. The speed at which sound propagates along the fibre is between 4000 and 6000 m/s and is higher in hardwood.

When the sound propagates very well inside the wood we say that the wood is resonant. A good resonance wood is one with regular annual rings, uniform grain and no growth defects (knots, cracks, twisted fibre, etc). The combination of resonant wood and a vibration-producing element - the strings - is basically the basis of a resonant instrument such as the violin, cello or guitar.

The structure of the wood, as I said, has a great influence on how sound propagates. Wood with narrow annual rings is used to make instruments for high sounds, such as violins, and wood with wide annual rings for low sounds - cello or double bass.

Another acoustic property of wood is that of absorption. Sound absorption is the ability of wood to absorb some of the sound energy. The absorption power depends on both the internal characteristics of the wood and its external appearance. Sound absorption is higher in wood with high density (hard, hard wood), uneven grain and high moisture content. In terms of external appearance, absorption is better the more irregular the appearance, which is also true of wood-based boards, and increases with board thickness. This property of wood is very useful for the acoustic regulation of concert halls, conference halls, recording studios or radio or TV studios. Wall cladding with the right materials makes the acoustics of the room much better and the auditions more enjoyable.

Sound insulationis the ability of wood to reduce the intensity of a sound as it passes through it. Basically, whereas in the case of absorption it was sound inside an enclosure that is, let's say, adjusted, in this case it is sound that, due to the barrier exerted by the wood, remains outside the enclosure. Sound insulation is necessary to protect us from sounds that are too loud, i.e. noise. To give you an idea of what this noise means, I will give you some examples of sounds, measured in decibels (dB), the unit of measurement of noise:

• 10 dB - the noise made by the rustling of a leaf
• 40-45 dB - level of normal conversation
• 90-95 dB - the noise of heavy, intense traffic.

The most suitable wood for insulation is soft, porous, low-density wood. Hardwood is not a good sound insulator.

One property used in choosing wood for musical instruments is sound damping by radiation of sound energy. It represents the ability of wood to dampen sound through internal friction and is dependent on the speed and direction of sound propagation and apparent density. Basically, sound does not propagate equally in all directions, thus preventing excessive resonance that would lead to destruction of the sound box. And so, even though wood is not the material with perfect acoustic conductivity, it is the most suitable material for the sound box of instruments.

We also have wood in the country very suitable for musical instruments. There is a place in the Gurgai Mountains called the Italian Valley where, since over 200 years ago, the Italians have been finding spruce, a very good resonance wood for violins and spruce. Reghin, the town where the first musical instruments were made in the 1950s, has become world famous for the violins and guitars made here. But about those who make them here wood to sing I've told you about it before.