Single wall - airborne sound reduction

When looking at a single homogeneous material, like a single steel plate (bulkhead or deck plate), following factors influence the sound reduction.

Mass of the bulkhead/deck plate
Bending stiffness
Elasticity
How it is restraint in the construction
Angle of the sound
Tightness (any penetrations/holes)
In this respect critical frequencies will occur, where the sound reduction is considerably lower than in the rest of the range.

The critical frequency is the frequency at which the wavelength of bending waves in the wall matches those of the incident sound. Bending waves of different frequencies travel at different speeds, the velocity increasing with frequency. This means that for every frequency above a certain critical frequency, there is an angle of incidence for which the wavelength of the bending wave can become equal to the wavelength of the impacting sound. This condition is known as coincidence.

When coincidence occurs it allows a far more efficient transfer of sound energy from one side of the panel to the other, hence the big coincidence-dip at the critical frequency. In many thin materials (such as glass and sheetmetal), the coincidence frequency begins somewhere between 1000 and 4000 Hz, which includes important speech frequencies.

In practice the problem is often seen in simple thin plate bulkhead divisions, and the theoretical way of improving is to enhance the thickness of the plate. This is of cause generally not an acceptable solution as the weight of the vessel will be unacceptable high.

The solution is to make correct stiffener design and to combine the bulkhead/deck plates with damping material as Rockwool (i.e. Rockwool Marine Slab 80) eventually in combination with visco-elastic materials as fire retardant PU.

As several factors have influence, the related calculations becomes a little more complex. Please see next page (without further explanation) to illustrate.