Course:PHYS341/Archive/2016wTerm2/SnareDrum
Snare Drum Vibrations
The snare drum is a percussion instrument that produces a distinctive sharp fast sound when the head is struck. It’s sound is often recognized as a loud crack. It is a central piece in a drum kit and is used in many different genres of music. The snare drum is primarily played with drum sticks, and can also be played with brushes for a variety of different sounds.
Structure of the snare drum
Snare drums can be made from different types of materials from wood, metal, composite or fiberglass materials. In general, snare drums made of wood tend to radiate more warmth in their sound whereas metal snares tend to give out a brighter and louder sound that is popular in contemporary music. Snare drums can come in different shapes and sizes depending on the use and playing style. The snare drum shell is constructed and pressed into a cylindrical shape with reinforcement rings on the inside to maintain its shape. Like the bass drum and other orchestral and band drums, the snare drum is a two-headed drum. Both open ends of the snare drum have a plastic membrane also known as a drumhead stretched over each side. the top head is the playing surface in which the player strikes while the bottom head is acting as the resonance head, the top head is almost always much thicker than the bottom head because it must be robust enough to endure the battering by the player. Most snare drums have a snare strainer, when the strainer is engaged, the snare wires vibrate against the bottom membrane head as it is hit giving it’s instantly recognizable cracking sound. When the strainer is not engaged, it will sound hollow, imitating a tom drum.
How drums produce sound
The sounds that come out of a drum is largely affected by its shape. In broad terms, the larger the drum the lower the pitch the drum will emit. In the case of a snare drum, due to the fact that it is a smaller sized drum it radiates a sound resembling a tom tom but with a higher pitch when the snare wires are disengaged. When the top head of the snare drum is hit, it vibrates causing the air around it to move, ultimately creating sound waves. The resulting sound waves move around the shell of the drum then ring through the resonance head on the bottom of the drum. Deciding on whether the batter head is tuned either the same of differently from the resonance head is a controversial discussion amongst drummers, but it's merely a matter of preference. The batter head can be either tighter or looser than the resonance head or both of equal tension. If the batter head is tuned looser than the resonance head, it will have a hollower sound, with a longer decay in the lower frequencies and will resonate at a lower pitch. However if the resonance head is tuned tighter than the batter head, the snare will produce a faster attack and a faster decay, especially if the resonance head is tuned a perfect octave above the batter head.
Drum head behaviour and frequency modes and nodes
A drum head is a circular plastic membrane. The sound is highly dependent on where the membrane is struck and how hard it is struck. As mentioned, the size of the drum determines it’s pitch, but also the diameter and the tension of the membrane of the drum. Usually, the tighter the membrane and the smaller the diameter results in a higher pitch. When the plastic membrane is struck while it is put under tension, the moving surface can support transverse waves and will resonate at certain vibrational frequencies. If seen with slow motion equipment, the vibration at the surface of the drum head will move in a pattern that is similar to standing waves. These waves are known as modes. Any drum head can have an infinite number of modes as there are an infinite amount of ways in which a plastic membrane can vibrate. In order to describe frequency modes, the concept of nodes is extremely important. A node is a point along a standing wave where there is little to no motion.
Starting at the lowest frequency mode is called the fundamental mode. The fundamental mode as displayed in diagram 1 is a circular mode. The fundamental mode is the only mode with nodes that surround the perimeter of the drum head, which is why this is called the circular mode. The pitch of this mode is primarily determined by the diameter and tension of the drum head. The second mode pattern shown in diagram 2 occurs at a higher frequency. The drum head is split down the diameter with a linear node, allowing the membrane to move separately, with 1 side of the membrane moving up while the other side is moving down while still retaining it’s perimeter nodes. The third frequency mode occurs at an even higher frequency with the vibration pattern separated into quarters. Two opposite sides of this frequency pattern move up together and the adjacent sides move down together as seen in diagram 3.
Striking the drum head at different locations
When a circular membrane is struck at different locations, the membrane will react and vibrate differently each time, all uniquely different from the previous hit. If you are able to strike the membrane at precisely the centre, then the membrane will produce a circular mode just like in diagram 1, and this will excite less frequencies then if you were to hit it slightly off centre thus producing a non-circular mode. Striking the membrane off centre will result in many more modes getting excited, the more off centre you strike the membrane more non circular modes will appear which will produce a more complex sound with more frequencies being emitted.
Conclusion
The snare drum is a percussive instrument commonly seen and used in all types of music genres. The drum produces sound by having the player strike the top batter head with a wooden drum stick, the sound then resonates through the resonant head on the bottom. Both batter heads and resonant heads can be tuned looser or tighter to the preference of the player. When the circular membrane is struck, the vibration of the surface will move similar to standing waves. Hitting the membrane dead centre will cause the membrane to vibrate at it's fundamental mode which is also known as the circular mode. Striking the membrane off centre will create more complex vibrations thus creating more complex modes with different vibrational patterns.
References
Thomann Online Guides Snare Drums Shell Materials - https://www.thomann.de/gb/onlineexpert_page_snare_drums_shell_materials.html
Errede, Steven. "Vibrations of Circular Membranes (e.g. Drums) and Circular Plates." Web. University of Illinois at Urbana-Champaign. 2002 - 2014. https://courses.physics.illinois.edu/phys193/lecture_notes/p193_lect4_ch4_part2.pdf
Russell, Daniel. "Vibrational Modes of a Circular Membrane." Web. The Pennsylvania State University Jan 21, 1998. http://www.acs.psu.edu/drussell/Demos/MembraneCircle/Circle.html
Diagram Screenshot Captures done with Circular Membrane Applet. Hanson, Bob. http://www.falstad.com/circosc/
http://mydrumlessons.co.uk/2016/04/12208/
http://hyperphysics.phy-astr.gsu.edu/hbase/Music/cirmem.html