Tuning a Birdcage Action using Double String Unison, DSU. 

A Birdcage Action, sometimes called an Over Damper Action, is found on some older European pianos of the early 1900's and late 1800's.     They got their name because the wires that actuate the dampers are in front of the hammers, and they resemble a birdcage.  While some manufacturers may have produced quality work, in general the design of the birdcage action was inefficient and the sound of the strings would continue on after the key was released.  The biggest problem with these pianos is how to tune them.  Some people remove the action, insert mute strips, replace the action, tune, remove the action, remove one strip that frees up one string per note, replace the action, tune the bichords and double string unisons that were created by removing the one strip, remove the action, remove the final mute strip, replace the action, and tune the remaining trichords. Not an easy or quick task.  Some use Papp's mutes. They look like long thin plastic pliers. There is one inserted into the action in the photo above and below. With these you can insert the mute between two strings of a trichord, creating a single string, tune it, remove the mute, insert between the tuning note and the next note, thereby muting only the third string of the trichord and freeing up the second string to be tuned, tune it, remove the mute and tune the third string of the trichord. The hard part with this method is trying to stick the Papp's mute into the tiny space between the two strings of the same note.  I have found that the fastest method to tune birdcage actions is to use Double String Unison, DSU.  DSU is a technique some advanced tuners use and that I have systemized using my Bisecting Beat Speed Windows temperament sequence and NSL analysis for superior stability.  The tuner inserts the mute between two notes so only one string of a note is muted. The space between the strings of adjacent notes is usually much larger than the space between strings of the same note, and therefore easier and faster to insert.     This creates a DSU where two strings of a trichord are free to vibrate. (No mute is needed to tune bichords using DSU) Then the tuner cleans up the DSU, makes a judgement on the pitch using rapid beating intervals like M3 and M6, changes one string of the DSU in the direction they want to move the whole pitch of the DSU, if it needs to be changed, moves the other string to clean up the DSU, and continues to recheck and move until the pitch of the DSU is acceptable.  Once perfected, DSU is a very fast technique for producing  accurate equal temperament with pure and beatless unisons that have superior stability, and it has the bonus benefit of being an easy way to tune birdcage actions.  I will be producing a book and video course on the DSU technique soon. Contact me if you wish to receive the 50% prepurchase discount.  

About the author 

Mark Cerisano, RPT, B.Sc.(Mech.Eng.)

Instructor and Founder, howtotunepianos.com

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