Humidity influence on natural drumheads

Extract from a master's thesis:

Nagl, Wolfgang (2020)
Humidity influence on natural drumheads.
Department of Music Acoustics – Wiener Klangstil, University of Music and Performing Arts Vienna.

Abstract:

Changes in humidity cause problems that timpanists who use natural skins must counteract. Moisture, which is often produced in concert halls by the audience or the musicians onstage, changes the elasticity of natural skin, altering the tension and thus the intonation.
The present article deals with the effects of moisture-induced changes on the intonation of drums—mainly timpani—using natural skin and presents an experiment to measure the relationship between air humidity and skin tension respective pitch change. An experimental setup was created in which the skin was mounted on a copper mold with an ultrasonic humidifier underneath. The humidification process was computer controlled. The resulting change of the timpani membrane parameters were measured with humidity sensors and tensile force sensors at all attachment points of the skin. Measurements were made in a relative humidity range of up to 97%. The results should also guide musicians and help them to better counteract humidity fluctuations during their concerts.

Experimental Setup

The setup was built out of a massive crossed-metal foundation, which carried the copper pipe to serve as the kettle substitution. The head got attached to this kettle and was tuned manually with six rods. On the bottom of every rod a load cell was installed to scale the pressure and the traction force of the timpano head.

The humidity was generated with an ultrasonic water nebulizer placed in a water container beneath the foundation. Using pipes, the inside of the copper kettle was moistened using a fan, which was recorded by three humidity sensors inside and one humidity sensor outside as a reference. The process of drying was considered to happen by using a second fan, which should blow the outer air into the inside of the copper shell but was in the end not installed. Due to the fact that standard timpani are almost closed (except for the little hole for air interchange), climate changes almost exclusively affect the grain side of the skin. In the presented setup the climate changes were produced in the inside of the kettle, hence the skin was attached upside down to ensure the most common conditions. The bottom of the timpano was sealed with a plate so the whole volume of the construction functions as a climate chamber.

To record the assumed changes of the skin’s surface an optical observation system was installed. For exciting the timpano head, a shaker was mounted supporting an impact hammer for automated and even strikes on the head, with a microphone above the system to record the gathered response sounds. All changes in humidity, tension, or pitch were recognized by sensors that were fully automatedly driven by software (see Figure 1).

 

Figure 1: Total setup (Mayer & Nagl, 2017)

Figure 2: Humidity vs. fundamental frequency (goat skin)

Measurements

For this work, two different types of drumheads (goat and calf) were under investigation. The first one was made by Werner Edlauer, the other – a “Kalfo Super Timpani” head produced by the Vellum & Parchment Works Limited Company. In the first step, both drumheads were successively attached to the experimental setup in a total wet condition as is common for attaching new drumheads. After putting them under slight tension of approximately 25 Newtons on each load cell, the heads were left to dry on the setup. As expected, the skins tightened while drying.

Drying process

For the goat head, this procedure caused the pitch to rise from 25 Hz to 96 Hz. This means nearly a two-octave step from G0 (+34.99 cents) to G2 (-35.67 cents).5 After drying, which took place in about 17 hours, the timpano head stayed stable on the reached pitch for the remaining 31 hours observed. The measurements of the calf head are similar to those of the goat head experiments but with a faster drying episode of 11 h. The fundamental frequency starts at 31 Hz and culminates at 107 Hz. Expressed in musical notes, this is a step from B0 (+7.40 cents) to A2 (-47.86 cents), after which the calf skin arrives almost exactly a full step higher than the goat skin.

Direct proportionality

Changes in humidity affect the intonation quite immediately. The chart in Figure 2 shows this close connection is unambiguous (example goat skin).

Conclusion
The calf head is in every experiment more strongly affected by environmental changes than its goat counterpart, whether this is the faster drying in the first measurements or the stronger reaction to moisture in the short time measurements (please see the further experiments described in the diploma thesis ). It also needs less load to achieve the same pitch than is the case with the goat head.