Course:PHYS341/Archive/2016wTerm2/snaredrumvibrations

Snare Drum Vibrations

Introduction

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. 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 it’s spherical 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. 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 it’s 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 releases 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.

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 f1 as displayed in diagram 1 is a circular mode. (This lowest pattern, the entire membrane moves up and down as one piece, as the whole thing moves in unison.) 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 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 f1 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.