about continued

About Drums by John Roberts

Now we come to the major source of confusion surrounding this topic. Why can drums be adjusted to make the same note pitch at a given resonance, but exhibit such different sound quality? The attachment method of using multiple lugs spaced around the drumhead circumference to tension the drumhead can cause very audible sound quality differences when the lug tensions are not very closely matched to each other. The fundamental note pitch tracks with the average head tension so half of the lugs could be too tight and the other half too loose while still making the same fundamental note pitch. However as we inspect the behavior at higher vibration modes we find progressively more significance attached to individual lug tension and the matching between these lugs. These slight tension differences along with tiny mechanical imperfections in the drum construction can cause significant audible differences.

If the lugs are not in perfect agreement with each other, instead of just making the lowest possible number of overtone notes, sounding pure when played, we hear instead a much more complex dissonant sound character from multiple closely spaced overtones. This state of perfect tune when all the lugs are in complete agreement with each other is called “clear” or “clearing” the head, because the sound character of the drum appears to open up, or clean up, sounding more pure, or as pure as it can possibly sound from that mix of non-related drum notes.

The Bass/Kick drum: is generally just a larger tom, laying on it’s side. One import distinction about Kick drum sound is related to how it is struck. The fixed foot pedal beater strikes the drumhead dead center. That preferentially excites the fundamental resonance. As we recall from the concert tom discussion, when the resonant head is not mounted, the fundamental note pitch generated is lower frequency, perhaps desirable for a kick drum.

There are also after-market products that form a port with a hole in the kick drum resonant head. Without doing a detailed analysis of the physic involved, this is probably a combination of adding mass to the head with some acoustic effect from the port. I’ll leave it to others to determine what is really controlling resonances and note pitch with those.

The Snare drum: is another variant on the two head tom for dominant head resonances, but in addition there are wire snares strung across the resonant head. While these snares do not vibrate like a musical instrument string, instead they rattle up against resonant head, but in a musically pleasing way. The shell of typical snare drums is shorter than most toms, and far more rigid, often made of dense metal or sometimes very thick wood. This increases the mechanical transmission though the shell from the top batter head down into the resonant end rim and snares.

The conga drum is a variant on the one-headed concert tom with similar resonant series note ratios. In addition the longer shell forms a tuned resonant air column, like a very short organ pipe, or a long loudspeaker port. This extra acoustic structure can enhance a lower note resonance.

The tympani and kettledrum are special cases of one headed drums designed for orchestral use that are perceived as making single note pitches when played. Their sealed back air cavity suppresses resonances like the typical lowest fundamental mode (0,1), that would compress the internal captive volume of air. Their sound character is dominated by vibration modes (1,1), (2,1), (3,1), etc. where half the drum head area is moving up while the other half is moving down so don’t increase or decrease the internal air pressure.

The tympani/kettle drum is designed so that these lower resonances fall on a pitch spacing that while not perfect harmonics of each other, this spacing mimics them being upper harmonics above a missing lower phantom fundamental note. Our brain is trained to interpret that specific overtone spacing, based on our experience with naturally occurring musical sounds. Our brain assumes this missing lower note must be present, and hears the complex note as if the missing fundamental was present. This musical psycho-acoustic trickery allows the tympani/kettle drum to appear to make notes at lower note pitch than their physical dimensions can actually support.

Only one major drum manufacturer, that we are aware of (DW) provides a note target for their drums. They call this “Timbre-Match” (r) and identify this as the shell note pitch. From discussion with DW techs this should be coordinated with the lug resonance so it can also be targeted while using tap tuning, or any of the electronic sniffer variants providing specific pitch information. Timbre-Match (r) is a registered mark or Drum Workshop, Inc.

While discussing all of the head variants is also beyond the scope of this overview I have some general observations. Many of the exotic drumhead designs attempt to squelch the higher overtones to reduce the apparent dissonance of a poorly cleared drumhead. If the upper overtones are damped you can’t hear how un-clear they are. Damping also reduces sustain, if the drumhead doesn’t sound great when you hit it, why let it sound that way even longer? So my suspicion is that many of these trick drumheads are designed to conceal the sound of not being well tuned or “cleared”.

There is one alternate head technology with some actual physics behind it. The “dot” heads have a small mass added right at the center mid-point of the drumhead. For the vibration modes (like 1,1 and 2,1) where the exact mid-point node is sitting dead still, this added mass has no effect. However for the fundamental mode (0,1), this midpoint is now an anti-node and moving vigorously up and down. As we should recall from the mass/spring rate discussion, mass is in the denominator of the natural frequency equation, so more mass makes a lower note. The goal is to shift the fundamental note of a two head drum down from the nominal 1/1.593x, to 1/2x or one full octave spacing a useful ratio that makes the same note only in a different octave.

We still decline to give specific voicing advice but in general the tension and mass of a head affects stick feel as well as rate of sound decay. Using different weight heads on the top and bottom can make subtle voicing differences similar to de-tuning. Experiment and try different combinations, if you like the way some combination sounds use it. If you don’t like the sound, don’t use it.

Before you lose too much sleep trying to decide which resonant series to target to be precisely on note, since they all can’t all be tuned exactly on full notes, lets look at the popular practice of establishing a musical spread of note pitches across your different toms. If all the batter and resonant heads are the same weight or same ratio of weights, with similar relative tuning, a musical note spread of lug resonances will map out directly to the same note spread of fundamental resonances. While only one or the other set of resonances can be tuned to fall precisely on full notes. Don’t lose any sleep over this. One clue about whether you might favor tuning the fundamental or lug resonance to be on note, when you play a run across your toms do you hit them dead center making the “thud” fundamental sound, or do you hit them off center exciting more of the lug overtone ring? Again do not lose one moment of sleep over this.

A well-tuned drum kit that sounds great is more fun to play and sounds better to everybody else too. If a drum is worth playing it is worth tuning as well as you can. Despite writing far too much about specific notes, the perceived sound quality of a drum kit is most influenced by clear quality, and how pure it sounds, Not making just one of the several different notes fall precisely on some note target. Adjust the lug tension so it feels good when you hit it. Clear the lugs to each other so the head is only making the smallest number of different notes instead of a complex dissonant mess, and enjoy yourself. Even if you don’t have access to the latest technology, do the best job you can using whatever tools you have available (even if just your ears and a well placed finger). We hope this information is helpful.

The math and physics describing these resonances is beyond our basic discussion but for more information on drumhead physics see Acoustics and Vibration Animations – Dan Russell, Kettering University

I also highly recommend the book “Science of Percussion Instruments” by Thomas D. Rossing for a broad technical discussion of several different types of percussion instruments.

A link describing old school “tap-tuning” , by an old friend Steve Volpp who knows a little about drums.

A link to lots of general information about tuning and voicing drums. Prof sound

Batter Head: This is the top head for toms or snare, and the head on the bass/kick drum closest to you that you strike with the beater. The batter head is the one you hit, makes the loudest sound, and has the most influence on your sound quality. Since it is the one you are constantly hitting, it is also the one most likely to drift out of adjustment from playing so needs more re-tuning and re-clearing attention.

Resonant Head: This is the bottom head for toms, and front head on bass/kick drums. The one you don’t hit. As the name suggests this head mostly vibrates sympathetically with the batter head to shape the total sound envelope including sustain and decay after the batter head is struck. These should be cleared when changed, but generally do not require as much touch up as the head being hit. Concert toms do not use a resonant head, and many kick drums have holes (for microphones) in the resonant head.

Clear: This describes the state of agreement between the multiple lug tensions on a given drumhead. When all of the lugs are precisely matched to apply the exact same influence on the drumhead standing waves, the drumhead will make a single set of resonant notes. This single set of resonances will sound more “pure” and “open sounding” compared to the same drumhead that isn’t clear. Clear quality is different from note pitch targeting and a drumhead can be cleared for any arbitrary pitch.

Resonance : In mechanical systems with moving mass and spring compliances, the efficient transfer back and forth of kinetic energy between the movement of the mass into the potential energy of a coiled spring and back again can express as a sustained natural frequency, only diminished by damping that dissipates the energy and causes the sound to decay. Drumhead/ drum systems can express multiple resonances.

Voicing: this describes the subjective personal selection of head weights, absolute and relative lug tension or resonance target pitches between both heads to realize a desired overall sound character. This is mainly a consideration for two-head drums because one head drums like concert toms have fixed resonance ratios, so can only be tuned higher or lower as a group, not relative to each other.