Pleading the Fifth
The listener needs to learn to listen for, and then harvest mode 1,1 and ignore mode 2,1. Mode 2,1 should be present in the spectrum, but subdued, while mode 1,1 should have the most presence. This technique can take years of practice to master when tempering by ear alone.
Since mode 2,1 tends to have more energy than mode 1,1, it will add sustain to the sound which is desirable. However, don’t be deceived by this sustain; mode 2,1 can become a wolf in sheep’s clothing if you follow it too closely.
When the frequency of the principal tone (mode 1,1) is not strong and consistent from lug to lug, the overall strength of perceived pitch is severely diminished and permutations of the more audible mode 2,1 (the fifth) can tend to dominate the spectrum. An overbearing fifth generates a pitch shift once the principal tone (mode 1,1) begins to decay.
The graphic below shows the prominence of mode 2,1 (the fifth) in a normal timpano spectrum. Once the overbearing fifth becomes pervasive, it can quickly mask the other preferred modes because it has the most energy, which makes it more audible with a longer sustain.
Waterfall chart (frequency, time and amplitude) of a timpano sound spectrum
(single struck note) highlighting six preferred modes (1,1), (2,1), (3,1), (4,1), (5,1) and (6,1)
(Fleischer & Fastl)
This overbearing fifth is especially noticeable once the head becomes worn or becomes out of clear. It can also happen if a head has not been tempered properly during the initial mounting and is then continuously played; this causes it to become unevenly stretched the more it is played. Unlike animal skin heads that have more natural elasticity, Mylar heads tend to develop a stretch memory. Once the head is consistently stretched to where this overbearing fifth is prominent in the spectrum, it is often best to just replace the head and temper the new head properly.
Ironically, what the manufacturers of synthetic timpani heads head don’t publish about their products is that excessive stretching and flexing are the primary cause of deterioration with Mylar. Mylar’s ability to stretch and return to its normal state makes it a great replacement for natural skin heads. However, the constant fluctuation in tension and beating of the head contributes to an elongation of the material causing it to become brittle and shortening its service life. This is the primary reason why you can’t remount Mylar heads with any great success; the collar crease is too brittle and has lost all malleability. Other culprits contributing to deterioration are heat and humidity, especially direct sunlight and water. Applying a polymeric coating to the head surface occasionally can extend the chemical integrity of the Mylar. Heat guns, hair dryers, irons etc. will all suck the life right out of a head and the dollar$$ out of your wallet.
If a head replacement with correct tempering is not an option, the overbearing fifth can be mitigated to some degree by following Step No. 5 and Step No. 6 found in Chapter Five.
When the head is properly mounted and tempered, the virtual pitch of the preferred modes convinces the human auditory system that it is hearing harmonic pitch even though it is not.
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