wrecho

Noise & Vibration

Minimising Noise And Vibration
“ Overall the noise and vibration performance of this vessel is impressive. When one compares this vessel to a ‘similar’ powered European quality mono-hull, one can only conclude that ‘White Rabbit’ can compete with the top of the range. Considering the build material (aluminum, from noise point of view worse than steel), the speed and length, this is a very good achievement for NWBS”
(61M Trimaran Motor Yacht “White Rabbit” Sea Trial Measurement Report, Van Cappellen Consultancy, April 2005)

Good noise and vibration performance is a combination of:
. Layout of the vessel such that accommodation/living spaces, and machinery spaces are separated as far as is practicable.

. Minimising the noise and vibrations generated (inside the vessel) at source, through quality equipment selection and mounting design, and

. Containment and absorption of the residual noise and vibrations through the use of insulation and damping treatments.

LAYOUT
The trimaran platform with its large accommodation areas above main deck lends itself to good noise and vibration performance from a layout perspective. All guest accommodation is on the upper deck level, and the owner’s suite on main deck is at the opposite end of the vessel is buffered from the engine room by the electrical room and from the bow thruster void by the laundry. The only areas abutting a machinery space are the aft saloon and dining area, which are located directly above the engine room.

MACHINERY
In addition to being located within the acoustic enclosure of the engine room itself, both main generators, are enclosed in separate custom sound shields to ensure that harbour noise is minimal. This is particularly important when the generators are running while the vessel is at rest.


The main engine exhaust system comprises of a large primary muffler with a secondary muffler/rain maker to ‘silence’ and cool the exhaust prior to overboard discharge. The primary mufflers are isolation mounted to reduce structural transmission of noise,

The size of the primary muffler on the main engines important:- “Bigger is better”.

A comman noise issue aboard boats is gearbox “whine”. This is the gear meshing frequency, a medium pitch noise 400-800Hz, which is almost tonal, and very annoying to the human ear. Aluminium is particularly good at transmitting this structure borne noise and insulation is not efficient at containing it.

The solutions on White Rabbit was to mount the gearboxes on semi-rigid trunnion mounts. These absorb the drive shaft thrust and minimize the transmission of gear meshing vibrations.

Gear Meshing Frequency
INSULATION AND DAMPING TREATMENTS
Extensive containment and absorption treatments were untilised including:
. Decoupling of floor soles, internal stairs, wall and ceiling linings from the vessel’s structure.
. Insulation treatments comprising multiple layers of sound absorption material are used throughout the vessel.

(The deck partition between the engine room and the main lounge comprises 150mm of acoustic material, excluding the structure and the carpet).

Absorption Treatment

AFT BULKHEAD IN LAUNDRY ON LOWER DECK
Note: Multiple layers of insulation material visible on this bulkhead. The lining grounds are isolated from the bulkhead with rubber mounts,

UPPER DECK INTERNAL BULKHEAD
Stud partitions constructed of acoustic composite sandwich ply wood and packed with multiple layers if insulation materials, to provide acoustic privacy between adjacent cabins.
NOISE ATTENUATION
A quiet luxury yacht makes boating much more pleasant. However, spending a fortune on a yacht does not automatically imply the yacht is acoustically comfortable. Van Cappellen Consultancy believes that spending millions of dollars on a yacht should mean buying a silent one, even through a little bit more of this money needs to be spent on acoustic consultants.

The company’s consultancy work, which includes state-of-the-art 3D simulation software, also used in the automotive industry, can make a yacht’s stateroom as quiet as the average office.

The main advantage of this simulation software is that consultant activities do not have to stop with a sound prognosis. Prediction of noise and vibration levels is often not enough, as we need to understand how levels are generated.

When a deck segment, engine room bulkhead or other major construction element   is excited in its natural frequency at for instance propeller blade rate, vibrations will be amplified. This can be compared to pushing someone on a swing. If one keeps pushing at the right moment (at the right frequency), the person swings higher and higher. This is called resonance.

If the bottom hull structure would resonate at the propeller blade rate, the structure is very likely to move and thus receive energy. Just like with an engine bed, a thorough structural analysis can result in a tremendous reduction of energy transfer.

Using finite element modeling, the input mobility of a foundation can already be anticipated in an early design stage. Local parts of for instance hull and engine girders are modeled.

It is evident that mobility is strongly frequency dependant due to modal behaviour of the structure. An engine bed is built out of plate-like structure parts which inevitably results in many natural frequencies. However, the magnitude of vibration can be reduced by thoroughly considering geometric properties if the foundation and isolator mount positioning.

Performance of the foundation is judged by comparing characteristics of the vibration mount.

Most people tolerate noise coming from the propulsion system to acceptable limits. It is part of the boating fun and is associated with power. However, at anchor with people sleeping, a quiet surrounding is essential. The design of mounting systems and sound shields for generators is a company’s specialty.

To get a boat that is both quiet and performs well, the philosophy is to strategically add relatively heavy damping and insulation materials. For each project, our objective is to obtain an insulation package. Which has a good balance between the noise targets, insulation weight and total insulation cost.

The effort to find innovation, lightweight and improved insulation materials has enabled us to achieve a respectable 25% saving in both insulation weight and costs during the past years, while on average the yachts have become quieter and quieter.

Noise in yachts is most often measured in decibels A- weighted, dB(A). The dB(A) scale is intended to give an accurate single number loudness scale for noises, by accounting for the combined various pitches of noises, by accounting for the combined various pitches of noise in a given environment and relating each pitch to the sensitivity of hearing of a typical human subject.

The dB(A) scale is universally known and it does a reasonable job of rating noises.

Some brief decibel factoids are appropriate before discussing actual decibel limits, through there is growing understanding of the scale amongst members of the marine community.

An increase of 10 dB(A) is heard by the human ear as a doubling of loudness, through it actually represents a factor of ten increase in sound power (i.e. increasing stereo system power from 25 Watts to 250 watts gives a 10 dB(A)  increase, which is heard twice as loud.).

A one decibel change of noise is audible, when it happens, but is it difficult to discern from day to day. A three decibel change of noise is distinct and clearly discernible, from day to day.

Improving insulation of a single panel wall by 10 dB(A) generally requires an increase of the panel mass by a factor greater than three.

Sound Shields
The below shown chart with typical noise levels aboard luxury yachts related to common sound-sources are average levels given for 70’ to 200’ displacement, disregarding relevant characteristics such as properties of hull material and the ships speed.

In addition to this, the overall noise level in an engine room at cruising speed would be in range of 105 to 115 dB(A) depending on engine revolutions and engine room dimensions.

The overall noise level in an engine or generator room with one generator set running would be in range of 75 to 105 dB(A) largely depending on whether an acoustic enclosure is provided for the generator set, and on engine revolutions and engine room dimensions.

(Comment from Van Cappellen Concultancy)

Click here to view table
A noise survey was recorded by Van Cappellen consultancy during sea trials on 29 March 2005.
Click here to view Sea Trials Measurements table
This performance can be put in perspective by comparison with the Del Norske Veritas comfort class criteria: Comfort rating number (c r n) 1 represented the highest comfort level, and  (c r n) 3 represents an acceptable level of comfort.

Comfort class criteria
Click here to view table