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Wood, Temperature, and
Humidity



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by B.J. Fine

Wood always contains some moisture, except under very extreme conditions of heat and dryness. The amount of moisture contained helps determine both its density and strength, among other physical properties. Density and strength each affect the acoustical and structural properties of any stringed musical instrument.

Wood swells and shrinks in proportion to the magnitude of change in its moisture content. Even with properly seasoned wood, the shrinking and swelling can affect the acoustics, structure and function of a harpsichord. It is the functioning that is most likely to be adversely affected by the swelling and shrinking, even when the instruments are kept in typical indoor spaces in temperate climates. Small, short-term changes in moisture content primarily affect tuning. Large changes in moisture content can greatly impair function. Large, cyclic changes in moisture content, if repeated often enough, can weaken the structure of the instrument.

The air surrounding the instrument is the primary source of moisture. Humidity is the capacity of air to hold moisture. Absolute humidity (AH) is the amount of moisture in a given amount of space e.g., a cubic foot, at any given time. Relative humidity (RH) is the ratio of the AH of a space at a given time to the total amount of moisture the space can hold at that time, referred to as its saturation point. If the amount of moisture in the air exceeds the capacity of the air to hold it, the air is "super saturated" and the excess moisture will either condense on cold surfaces or be suspended as mist or fog.

The capacity of the air to hold moisture is proportional to ambient temperature; warm air can hold more moisture than cold air. Given a proscribed space e.g., a room, as warm air in it is cooled and the capacity of the air to hold moisture thus decreases, relative humidity in the space will rise. As cold air in a space is warmed and its capacity to hold moisture increases, relative humidity in the space drops. Such changes in temperature and moisture characterize the temperate climates in which most harpsichords live. In general, in winter, the greater the difference between inside and outside temperatures, the lower the interior RH will be (assuming no inside humidification).

For any given set of air temperatures and RHs, there is a corresponding moisture content of wood at which no further gain or loss of moisture will occur. This is referred to as the Equilibrium Moisture Content (EMC). EMC varies directly with RH ( assuming temperature is held constant). That is, at a given temperature, the equilibrium moisture content of wood will increase as relative humidity increases and it will decrease as relative humidity decreases. On the other hand, EMC varies inversely with temperature (assuming relative humidity is held constant); that is, EMC will increase as temperature decreases and it will decrease as temperature increases.

The moisture content of wood is slower to reach a state of equilibrium than that of air because of the relatively slow speed at which water diffuses through the cells of the wood. Surfaces of wood will exchange moisture with air quickly, but core sections will lag significantly, acting as a buffer.

EMC is more sensitive to small changes in humidity than to small changes in temperature, and, as Hendrik Broekman has pointed out, our musical instruments generally are in rooms in which we are more likely to restrict the range of temperature than of humidity.

Small changes in EMC occur hour-to-hour, day-by-day during stable weather. Occasional dramatic weather shifts will make for larger changes. However,the largest changes occur from season to season.

Since manipulation of temperature is unlikely to play a meaningful part in attempts to control relative humidity, one must manage the moisture content of air directly by artificial means. Humidifiers increase the moisture content of air by evaporating water either by heating or by increasing the surface area of the water. People and plants are two further modest sources of evaporation. Moisture may be removed from the air by condensation. Dehumidifiers and air conditioners both do this by passing the air over cooling coils. Modern air conditions are designed to both cool and dry the air, dumping their waste heat outside. Dehumidifiers, because they circulate the waste heat directly into the area in which they are working, do not cool the air, but elevate the temperature somewhat, further lowering the relative humidity.

Well-constructed harpsichords have had the moisture content of their soundboards reduced just before installation and the gluing on of cross members. If moisture content at time of construction is below the lowest EMC that will occur in their lifetime, no cracks should occur due to low humidity.

Most builders of finished instruments and, I assume, most kit builders, have observed proper building techniques. Nevertheless, common sense dictates that you try to keep your instrument within reasonable bounds with respect to temperature and humidity rather than risk damage. In particular, if you have no knowledge of the "pedigree" of your instrument, that is, if you acquired it used and don't know the climate for which it was made or, if a kit, who built it and whether or not they observed proper techniques, the environmental limits should be even more strict.

Common sense also suggests that when dealing with the opposite problem of too much moisture in the air, care be taken not to subject the instrument to conditions where the moisture content of the soundboard becomes sufficient to cause it to swell so extremely as to affect performance. Dehumidification is obviously recommended.

A major advantage of controlling humidity is with regard to tuning. An instrument kept in a controlled environment should be quite stable insofar as tuning is concerned. We keep ours at 68-70degrees/40percent RH which is a reasonable comfort level for humans. Small differences in either direction will make little difference as long as the conditions are kept constant and you will be rewarded by more playing and less tuning time.

Finally, from Hendrik Broekman,

"I have found that the optimum moisture content of the soundboard is essentially that at which the soundboard was installed, if it was installed correctly in the first place. Thus, most instruments sound best in winter, since most boards are installed relatively dry,for obvious reasons. The actual mechanism at the level of the wood, I would wager, probably involves a complex relation between the weight of the moisture content of the board and the effects of the variable weight and the moisture itself thereof on the stiffness and acceleration functions of the spruce. Certainly, to my ear, sodden boards are dull, dry boards sound much better. I have the sense that boards sound harsh and edgy in the summer, but even while still short of sodden. But of probably the greatest importance is the effect that the moisture content has on the downbearing of the strings. We want to persuade the soundboard to go up when it expands, but try to achieve this result with the minimum crown. Since we ship all over, we try to approximately match the EMC at installation with the likely winter EMC to be encountered in the instrument's ultimate environment."

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