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Live backs and dead backs in acoustic guitars

Why some guitar backs vibrate and others do not

If you spend time reading about guitar making you will eventually come across the idea of a live back.

The idea is simple. Some makers believe the back of the guitar should vibrate actively and contribute to the sound of the instrument. Other makers prefer the back to remain relatively still so that most of the sound production comes from the soundboard.

These two approaches are often described as live backs and dead backs.

Both approaches exist in modern guitar making. Understanding the difference helps explain why instruments built by different makers can behave very differently.

There is also a tendency in modern discussion to assume that a live back must sound better simply because the concept sounds appealing. In practice the situation is more complicated than that.

Quick definition

A live back guitar is built so that the back plate vibrates and contributes to the sound of the instrument.

A dead back guitar is built so that the back plate remains relatively stiff and does not become an energy sink for the soundboard.

In brief

• A live back guitar has a flexible back plate that vibrates with the instrument
• A dead back guitar has a stiff back that reflects energy rather than vibrating strongly
• Many guitars end up with a back that behaves like a live back by accident, which can sometimes work against the sound
• A live back requires careful control of modal frequencies
• The Sobell and Forster design approach use a dead back

Where the terminology comes from

The terminology became widely used after Trevor Gore discussed the concept in his book Contemporary Acoustic Guitar Design and Build.

Trevor suggests that a live back adds a fourth mode degree of freedom to the vibrating system of the guitar. In a dead back design this additional degree of freedom does not appear because the back is not participating strongly in the vibration.

Broadly speaking Trevor suggests that a dead back may produce more volume while a live back may contribute more tonal complexity. Trevor himself is careful to emphasise the word broadly. These are tendencies rather than strict rules.

It is also worth noting that Trevor builds both live back and dead back guitars depending on what the client wants. Both approaches can work well when the design is executed properly.

Unfortunately the subtlety of Trevor’s explanation is often lost. The idea is sometimes reduced to the claim that a live back simply sounds better. That conclusion does not necessarily follow.

The modal framework behind the idea

To understand why builders think about live and dead backs it helps to look briefly at the main vibrational modes of the guitar body.

The modes most builders concern themselves with are the T(1,1) modes.

The first of these, T(1,1)₁, is the main air resonance of the guitar body.

The second, T(1,1)₂, is the main top resonance, sometimes referred to as the monopole mobility of the soundboard. This mode is closely related to how responsive and how loud the guitar will be.

The third mode, T(1,1)₃, is the main resonance of the back plate.

These modes are coupled, meaning that movement in one influences the behaviour of the others.

For most builders the critical relationship is between T(1,1)₁ and T(1,1)₂. If the air mode and the top mode are not working properly together the guitar will struggle to produce strong output regardless of what the back is doing.

The back resonance is another matter.

In a live back design, the back plate is deliberately tuned so that T(1,1)₃ sits roughly four semitones above the top resonance. This places the back within the coupled system in a position where it can move without disrupting the behaviour of the soundboard.

Achieving that relationship reliably requires careful control of the back plate’s stiffness and mass.

In a dead back design, the back is built stiff enough that T(1,1)₃ is pushed well away from the operating range of the air and top modes. In practical terms the back is removed as a significant participant in the coupled system.

The behaviour of the instrument is then largely governed by the interaction between the air mode and the top mode, which makes the system simpler and more predictable for the builder to control.

What a live back means

In a live back design the back plate is intentionally built light and flexible. Mass and stiffness are adjusted so that the back plate vibrates at a specific modal frequency in relation to the soundboard.

When the instrument is played the soundboard excites the back plate and the back becomes part of the vibrating system of the instrument.

Supporters of this approach believe the additional movement can increase the complexity of the sound and contribute to a sense of liveliness in the guitar.

In some instruments this can work very well.

Why live backs are difficult to execute

Many makers prefer the idea of a live back because it seems intuitive that more vibration should produce a richer sound.

In practice it is not that simple.

For a live back to work properly the back plate must vibrate at a very specific modal frequency in relation to the soundboard. If that relationship is wrong the back can interfere with the behaviour of the top instead of supporting it.

For that reason live backs are relatively easy to get wrong. Missing the target modal frequency can create a problem where the maker hoped to create a better sound.

Even when the relationship is correct when the instrument is built, it is not unknown for the modal response of a guitar soundboard to change slowly over time as the instrument ages and deforms. When that happens the soundboard can gradually move away from the modal relationship that originally worked with the back.

In other words a back that was well placed when the instrument was new may later find itself working against the soundboard rather than with it.

Some guitars end up with a back that behaves like a live back even though it was not deliberately designed that way. The back is simply too flexible to behave as a dead back but not carefully tuned enough to function as a true live back.

Where many guitars end up

In practice many guitars end up somewhere between these two approaches.

If the back is too flexible to behave as a dead back but has not been deliberately tuned as a live back, the T(1,1)₃ mode can end up in an unpredictable position within the system.

This is often the case in what most people would recognise as a traditional American steel string guitar.

In that design the back plates are usually sanded to a fairly uniform thickness. The upper two braces tend to be relatively stiff while the lower two braces are wider and lower, making them more flexible. The back radius is also relatively shallow.

From a modal point of view this construction does not strongly favour either a true live back or a true dead back.

The back resonance simply ends up wherever the construction happens to place it.

In other words the back can become something of a wild card within the coupled system.

Sometimes that works well. Sometimes it does not.

What a dead back means

The dead back approach begins with a different assumption.

Instead of encouraging the back to vibrate, the back is built relatively stiff or with enough mass so that the soundboard carries most of the acoustic work. Rather than joining the vibration the back reflects energy back into the system.

The goal is to prevent the back from becoming an energy sink for the soundboard.

This allows the top to behave more predictably and avoids the risk of the back interfering with the behaviour of the soundboard.

Many classical guitars and many steel string guitars have been built this way. Makers such as Simon Marty, Greg Smallman and Stefan Sobell make guitars with dead backs.

When I worked for Stefan we had no idea that the backs were dead. It was only in conversation with Trevor Gore years later that I realised that was what we had been building. This terminology simply was not around in the 1980s that we knew. At the time we were just building guitars that worked.

With a dead back guitar the sides become more important. The sides effectively act as a lever that allows the maker to modally influence the soundboard. Once the back is removed from the vibrating system the process of shaping the sound of the instrument becomes simpler and more predictable.

The sound of a dead back guitar

A well designed dead back guitar can have a very distinctive voice.

Martin Simpson, for example, plays many fine modern guitars but only really sounds like Martin Simpson when he is playing a Sobell. The sound associated with those instruments comes largely from the way the soundboard is allowed to dominate the acoustic behaviour of the guitar.

A similar observation can be made about Django Reinhardt. Django only really sounds like Django when he is playing a Selmer guitar. Selmers also used a stiff dead back design rather than an actively vibrating back.

Another way to think about the difference is that a live back tends to impose more of its own character on the sound. Because the back is actively vibrating it introduces its own resonant signature into every note.

Some players enjoy that sense of richness.

With a dead back the instrument itself contributes less of that additional character. The soundboard produces the sound and the back stays largely out of the way.

In practice this leaves more room for the player’s right hand to shape the tone. For an experienced player this can result in a wider expressive range.

You can often recognise this style of construction by the arching of the back. A laminated back with considerable mass is usually “dead”. A strongly arched back tends to be structurally stiff, which makes it less likely to participate actively in the vibration of the instrument.

The greater the arch, the more likely the back is functioning as a dead back, regardless of whatever bracing pattern may be used underneath.

So deadening a back can be achieved through arching, bracing, adding mass or thickness. Or a combination of these factors.

My own approach

When I worked with Stefan Sobell his instruments used a dead back design.

The back was built relatively stiff and did not move very much. The soundboard carried the main acoustic load.

That approach has influenced my own work.

In my instruments the shape and structure of the soundboard determine the fundamental voice of the guitar. Once that structure is working correctly the rest of the instrument supports it rather than competing with it.

The method I use to create a dead back depends on the materials a client wants. With more stable woods I work with curvature and bracing. With less stable woods deadening is achieved through lamination.

These days I try to use the method that achieves the result with the least added mass while still providing the support required for woods that are prone to movement.

A final observation

One thing worth remembering is that the popularity of the live back idea owes as much to theory as to experience. Guitars are more beautifully made than ever. The language used to describe them has become more complex, and the theories used to build them more scientific. But it would be difficult to argue that they sound dramatically better from the guitars built twenty or thirty years ago.

The idea that the whole instrument should vibrate feels intuitively appealing. It sounds as though it ought to produce a richer instrument.

In practice many of the most recognisable guitar sounds have come from instruments where the back stayed largely out of the way and allowed the soundboard and the player to do the work.

Frequently asked questions

What is a live back guitar

A live back guitar is designed so that the back plate vibrates as part of the sound producing system of the instrument.

What is a dead back guitar

A dead back guitar has a relatively stiff back that prevents it from acting as an energy sink for the soundboard.

Which approach sounds better

Both approaches can produce excellent guitars. The result depends on the experience of the maker and the details of the design.

Do Sobell or Forster guitars use live backs

No. Sobell and Forster guitars are built with a stiff back design where the soundboard carries most of the acoustic work.

From the workshop of Nigel Forster

Nigel Forster has been building guitars, mandolins and Irish bouzoukis since 1988.

Live backs and dead backs in acoustic guitars explained