The muting of organ pipes

Many musical instruments (such as the stringed or brass sections) are capable of being muted in order to change their harmonic structure or acoustic output. The present investigation is aimed at determining how well wind instruments can be muted by studying ways of muting an organ pipe. Three methods of muting were investigated: 1. The use of thin sheets of perforated plastic applied to the open end of the organ pipe. 2. Application of porous absorbers to the open end of the organ pipe. 3. The construction of a “double organ pipe”. The perforated sheets proved to be the most useful form of mute with reductions in the total SPL of up to 10dB and any required value from 0-10dB obtainable. The open area of the perforated sheets was correlated, to a high degree, with the SPL reduction of each mute. Drops in the fundamental frequency of A f/f < 0.02 were observed as the perforated sheets were applied. The absorptive materials appeared to be less reliable in correlating the muting performance with relevant factors (such as the acoustic impedance or airflow resistance) but useful correlations were obtained with the airflow resistance of the absorptive materials. The double organ pipe demonstrated larger reductions of over 20 dB in the SPL but this value could not be altered as easily as changing a perforated sheet applied to the open end of the organ pipe. The method by which the double organ pipe mutes the single organ pipe was found to be active cancellation of the sound emanating from the open end. Factors that were thought to be of importance in determining a “good” mute were a large reduction in the total SPL, a small change in the fundamental frequency. The change in the relative intensities of the harmonics was briefly investigated as this may be important in determining the timbre of the muted organ pipe