Instrument sound in controlled humidity conditions

To treasure hunters, the most precious artifact of Christianity has always been the Holy Grail, but musicians have a Grail of their own: the perfect sound, with just the right balance of each of its components – highs, mids, and lows.

The secret to the sound of a chordophone, or an instrument that produces sound through a vibrating string, are the acoustic qualities of the wood it’s made out of. The most important of these are elasticity, sound velocity, and damping. The value of these qualities depends on the species of wood and its physical properties, the most important of which are specific gravity and moisture content.

The industrial manufacturing of guitars and basses to meet market demands for an affordable product has resulted in the decrease of quality criteria in available instruments. This applies mainly to the parameters and resonating qualities of the wood. Custom instruments (produced by small shops or hand-made by luthiers) are usually constructed from selected wood that has been air-seasoned, a process that allows for a gradual reduction of moisture content in stable conditions. Seasoning also results in an even distribution of moisture and equalizes the internal tensions in the wood. Cheaper, mass-produced instruments are generally made from lower-quality material that has been dried or briefly seasoned and then artificially dried. This can produce an uneven distribution of moisture in the dried wood, and thus affect its acoustic qualities. The characteristic behavior of artificially-dried wood can be observed during the winter, when the humidity in closed, heated rooms does not exceed 25%. Instruments made of wood with uneven moisture content have a tendency to warp.

While nothing can be done to change the way an instrument’s wood was treated in pre-production, it is possible to influence the distribution of moisture in an instrument, and thus improve its sound by achieving an optimal balance between its damping ratio and sound (acoustic wave) velocity, which is affected by the specific gravity of the wood.

Damping ratio

 

Average Sound Velocity in Woods (F. Kollmann, 1968)

Wood species	Average specific gravity	Average modulus of elasticity		Average sound velocity
		׀׀	┴	׀׀	┴
	g/cm3	MPa		m/s
Fir	0,45	11000	490	4890	1033
Pine	0,52	12000	460	4760	932
Spruce	0,47	11000	550	4790	1072
Beech	0,73	16000	1500	4638	1420
Oak	0,69	13000	1000	4304	1193
Maple	0,63	9400	915	3826	1194
Lime	0,53	7400	250	3700	680

׀׀ – parallel to grain

┴ - perpendicular to grain

Even moisture distribution in the wood of the instrument can be achieved through what is known as “forced vibration,” produced by regularly playing the instrument. This is an extensive process that requires consistency. An alternative method involves sounding the instrument in controlled humidity conditions using a constant source of vibrations.

The results of procedures conducted in Guitar Help’s Hygro Chamber suggest that in conditions of controlled, stable humidity, forced vibrations increase the stiffness of wood while reducing it damping ratio, in effect improving the sound of the instrument.

 

The Hygro Sound Procedure

To illustrate the role of forced vibration in sound improvement, we conducted test procedures on instruments built with non-lacquered resonating components, which are more susceptible to changes in ambient humidity.

Vibrations were applied to soundboards of solid spruce (The Loar LH 600, 2011), laminated wild cherry (Godin 5th Avenue Kingpin, 2011), and laminated maple/poplar (Gibson Custom ES 175, 2011). 1/f noise was used as source of vibrations. Also known as pink noise, this sound has even levels of acoustic pressure in each octave across its spectrum, from 20 Hz to 20 khz. This means that a pink noise signal will produce the same amount of energy in each octave, measured in dB on a logarithmic scale.

Humidity was set to 45%, in keeping with production standards employed by Gibson, which, according to the company, help retain the wood’s optimal acoustic qualities and resistance to changes in ambient temperature and humidity, even after leaving the factory.

The average temperature in the Hygro Chamber was 23°C. The duration of the procedure was 120 hours.

1. Sound sampling methodology

In order to compare the effects of the Hygro Sound procedure on the parameters of the instrument, a sound sample was taken using a specially-constructed arm that produces a sound of equal volume from each string. The device is triggered mechanically, guaranteeing that the quality of the produced sample will be sufficient for the purpose of comparison.

 

2. Instrument placement

The instrument was placed horizontally on three specially-designed aluminium cones, reducing the points of contact with the test surface as much as possible in order to avoid the dispersal of vibrations onto objects other than the instrument itself.

 

3.Comparison of results

After completing the procedure, another sound sample was taken and compared to the control sample.

The Loar LH 600

 

Godin 5th Avenue Kingpin

 

Gibson Custom ES 175

 

The waveform in particular bands of the spectrum was also compared.

The Loar LH 600

 

Godin 5th Avenue Kingpin

 

Gibson Custom ES 175

 

Results of the Hygro Sound procedure

In the case of two instruments subjected to the Hygro Sound procedure (The Loar, Godin), a change in sustain was observed, extending by an average of 15% in both guitars. The sustain of the sound produced by the Gibson changed to a much lesser degree. This may be a result of the fact that a carefully crafted, custom class instrument such as the Gibson has significantly better wood moisture parameters and, subsequently, better moisture distribution.

The marked improvement in sustain may suggest a decrease in the damping ratio of the wood and an increase in its stiffness. This is the result of the gradual shifting of water molecules from levels of high tension – and thus high energy – to to levels of lower energy (multi-layered water molecule relocation).

The above results raise the following question: given the possibility of improving an instrument’s sound, is this procedure worth trying? The answer to this question will be left to musical instrument owners.