Page 14 - Summer 2021

MUTES FOR MUSICAL HORNS
Figure 2. A: British cartoon, c. 1939. Reproduced from the National Archives Image Library, with permission. B: His Masters Voice horn gramophone, early twentieth century. C: long-term average spectra of radiated sound with horn open (blue line) and muted by sock (red line).
Putting a Sock in It
The phonographs marketed by the Edison Company and other manufacturers at the beginning of the twentieth century relied on a large flaring horn to radiate the sound energy derived from the vibrations of the stylus in the record groove (available at bit.ly/3atI1gI). The timbre of music played through the horn was inevitably colored
by its internal resonances, and the poor radiation effi- ciency of the horn at low frequencies resulted in a sound that was often strident. A large consignment of cheap Edison phonographs was sent out to entertain Ameri- can troops fighting in the First World War after 1917. About this time, the colloquial injunction to “put a sock in it” became current among groups of soldiers, and the cartoon in Figure 2A illustrates a common (though con- tested) view that this phrase originally referred to the use of a sock to mute a phonograph.
What are the acoustic consequences of stuffing a soldier’s sock into a phonograph horn? To answer this question, we carried out an experiment with an early twentieth- century horn gramophone (Figure 2B), playing a 1926 recording of Sousa’s “Stars and Stripes Forever” by the band of H. M. Coldstream Guards (Multimedia 1 at acousticstoday.org/campbellmultimedia). We recorded the sound radiated by the gramophone with a micro- phone about 1 meter in front of the horn in a domestic room. The A-weighted equivalent continuous sound level, measured over the final 64 seconds of the recording, was reduced by 12 dB when a woolen sock was pressed into the horn. The long-term average spectra in Figure 2C show that the attenuation was significant over a broad frequency range, rising to over 20 dB around 3.5 kHz.
In 1919, the Edison Company introduced the model H19 Hepplewhite disc phonograph, which included a volume control. This control was, in effect, a more sophisticated version of the sock (available at bit.ly/3u90sPF). The horn was mounted inside a cabinet, and an externally operated mechanism allowed a soft “muting ball” to be inserted into the mouth of the horn. By changing the degree of insertion, the operator was able to adjust the loudness of the radiated sound. This device was apparently effective since the model continued in production until 1927.
Muting Brass Instrument Horns
A sock does not make a successful brass instrument mute because inserting it into the bell changes the playing pitches as well as the timbre and loudness. On the phono- graph, the vibration frequency of the needle is determined by the undulating profile of the groove and the rotation speed of the record (see bit.ly/3b8KBIi starting at 12 min- utes 45 seconds). Altering the resonant properties of the horn has a negligible effect on the needle vibration rate, and the pitch is therefore unaffected by muting. On a
14 Acoustics Today • Summer 2021