How A Resonator Guitar Works

Ever wondered how a resonator guitar produces its unique sound?

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 body of the guitar. The metal cone/s direct the resonance of the guitar and produces a brighter tone than traditional steel-string acoustic guitars.

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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 looks different from your standard steel-string acoustic there’s, even more, going on inside the body that gives 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 work.

What Do Resonators Sound Like?

If you are anything like me, growing up during the ’80s, chances are your first sighting of a resonator guitar was the Dire Straits album, Brothers in Arms, and the first time you heard one was during the intro to ‘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 cone (or cones) found inside the body.

While the materials the guitar is constructed from 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 the tone that the resonators provide as opposed to the soundboard.

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 tonewoods 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 guitar’s 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 tend to be either round, a mesh configuration, or an F-hole design depending on 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, utilizing an open-angle. They tend to have good acoustic properties as no distinct sides run 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 despite being of 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 would usually 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 utilizing a thin, yet stiff configuration.

The depth of the cone is also important, with regard to the sound waves being transmitted most prominently and the ensuing 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 emphasized. Resonator guitars, by being housed within a small body, promote what many describe as a ‘brighter tone because of this.

Are Resonator Guitars louder?
A common misconception about the cone-shaped resonator is that the cone shape contributes to an increase in volume. While it can certainly seem that way it’s more the case that the cone shape allows the sound waves to be focused/directed.

In the case of the ‘spider style‘ resonator, the sound is directed up through the cover plate, providing greater projection of sound in a specific direction but not necessarily increasing the volume overall.

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 a 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 soundhole. 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 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 10-inch 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 favored 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 in response to guitars being drowned out by banjo and horn players found its mark due to the unique tone produced by attempting to increase the guitar’s volume with the addition of resonator cones. In some ways, you could say the resonator was the first ‘mechanically’ amplified guitar.

Nowadays the inclusion of acoustic pickups and amplification mean volume is far less of a concern. It’s a testament to the unique tone resonator guitars produce that they remain popular.