How Does A Resonator Guitar Work?

🔑   Key Points
Resonator guitars work by transferring the vibrations from the strings of the guitar, through the bridge, to a ‘resonator’ e.g. a metal cone (or cones) located within the guitars body. Resonator guitars can feature single cones (biscuit or spider style) or three cones (tricones) which are connected by a T shaped aluminium bar. The metal cone/s direct the guitars resonance and produce a brighter tone than traditional steel string acoustic guitars which project volume from the soundboard of the guitar body and are more influenced by the materials the guitar is built from.

What is a resonator guitar?

A distinctive instrument in its own right, the resonator guitar (also commonly referred to as the ‘dobro’ or ‘steel guitar’) is synonymous with delta blues, traditional blues, bluegrass guitar and even jazz and country. And, while the resonator clearly looks different to your standard steel string acoustic there’s even more going on ‘under the hood’ that give the guitar its unique sound.

There’s plenty of information online about resonator guitars, including their history (the Dopyera Brothers) and the brands that make them like ‘National’ and ‘Regal’. Rather than rehash that information, in the following article I’m going to focus on the mechanics of how they actually work to produce their unique tone.

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• Antique Steel Finish brass body
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• Antique Steel resonator cover  
•  1 7/8" bone nut

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If you are anything like me, growing up during the 80’s, chances are your first sighting of a resonator guitar was on the Dire Straits album, ‘Brothers in Arms’ and your first listen was the intro for ‘Romeo and Juliet’ from the same album. Considering the album sold over 30 million copies and was no. 1 worldwide, for many weeks it’s fair to say Mark Knopfler’s brass bodied 1937 National Style Resonator exposed this particular type of guitar to a whole new audience.

Described by some as ‘shrill’ or ‘nasally’, and by others as ‘mellow/twangy’ the resonator is not easily defined as the tonality of the instrument is greatly influenced by the resonator cones (or cones) found inside the body.

While the materials the guitar is constructed from do play a role in combination with the resonator cones e.g. timber bodied single resonator cone guitars tend to sound warmer than steel bodied resonators, all things being equal. The materials themselves play far less of a role than they might in a traditional steel string guitar, due to the influence on tone that the resonators provide as opposed to the sound board.

Generally speaking however, resonator guitars tend to provide more sustain, a brighter mid range and a more pronounced and immediate attack which in some cases provides a ‘banjo’ like ‘piercing’ tone, especially when played with a slide.

How the Resonator Guitar gets its sound

As you can see in the image below, the internal workings of the resonator guitar (this particular configuration is a simplified version of the spider resonator cone model) compared to a standard acoustic guitar are quite different.

While the standard acoustic guitar’s tone relies heavily on the tone woods the guitar is made from the resonator guitar transfers the vibration from the strings to the bridge which is connected to the resonator or resonators (metal cones that reside directly beneath the guitars sound plate).

Depending on the configuration of the resonator cones the sound waves are transferred directly up through the sound plate or into the body before exiting via the sound plate and sound holes found on either side of the neck. The sound holes tend to be either round, a mesh configuration or F-hole design depending on the particular characteristics of the guitar.

Why use a cone shape?

Speaker cones, much like resonators are designed in such a way as to direct sound waves through the air, utilising an open angle. They tend to have good acoustic properties as there are no distinct sides running parallel to each other.

The cone shape is used not just because of the ‘funnel shape’ shape it provides to direct sound waves however, but also due to its capacity to provide greater stiffness using a thinner material. If you think about it, if trying to move air in a fanning motion with a flat sheet of composite paper (e.g. the material speaker diaphragms most commonly use) the paper will fold. However when shaped like a cone it is much stiffer and far less likely to fold, allowing for greater resonance by providing greater strength while utilising a thin, yet stiff configuration.

The depth of the cone is also important, with regard to the sound waves being transmitted most prominently and air displacement. In simple terms the greater the depth the lower the frequency, the less depth the higher the frequency, resulting in the mid to high ranges being emphasised. Resonator guitars, by virtue of being housed within a small body promote what many describe as a ‘brighter’ tone because of this.

The three types of resonator

While there are many differences between models of resonator guitar, for example steel, brass and timber body variations, along with round or square neck models. The major difference between resonator guitars along with being the largest influence on the guitar’s tone is the configuration of the resonators residing in the guitar body. While resonator guitars in general are credited with a brighter mid-range and warm bass, there is quite a bit of variability within the three resonator models.

Biscuit Style Resonator	The biscuit style resonator is the least complicated resonator of the main three. It is essentially an inverted, spun metal cone with a small hardwood center, known as the ‘biscuit’. The ‘biscuit’ houses the bridge saddle resulting in vibrations being transferred directly to the bridge from the strings and in turn directed from a single point through the ‘biscuit’ style cone, into the guitar body. The sound waves react to the body of the instrument, and for lack of a better term, are more filtered than the spider style resonator cone which is inverted and directs sounds directly to the sound plate. As a result there is greater bass response.
Spider Style Resonator	The spider resonator cone, unlike the ‘biscuit’ model is inverted, with the wider opening of the resonator cone directed toward the sound hole. The main difference between the ‘spider’ and ‘biscuit’ resonator design is the spider web bridge pattern. The bridge transfers resonance directly to the center of the ‘spider web’ where it is then distributed more evenly to the outer radius of the resonator cone. The resonator cone will often be housed within a sound well, also contributing to the tonality of the instrument. In the majority of cases the spider resonator will more often be seen in a timber bodied guitar, this combination provides a warm tone, with greater sustain and note definition.
Tri cone: 3 x 10inch cones	Tricone resonators feature 3 x 10inch cones joined by a metal T bar. This design features two resonators on the bass side and one on the treble side of the guitar body. They fall somewhere between the biscuit and spider style with a good balance of volume and sustain, and are favoured by slide players. Tricone resonators tend to be less common as the construction of the three cones is a more expensive process.

Summary

All things considered the resonator guitar, originally developed as a reaction to guitars being drowned out by banjo and horn players when playing accompanied really found its mark due to the unique tone produced by attempting to increase the guitar’s volume with the additional of resonator cones. In some ways, you could say the resonator was the first ‘mechanically’ amplified guitar.

While nowadays with the inclusion of acoustic pickups and amplification volume is far less of a concern, its testament to the unique tone resonator guitars produce that they remain popular.