Sound Attenuation – Diesel Powered Equipment

Diesel-powered equipment, with its many advantages, does have the drawback of being somewhat noisy. When placed in a room / enclosure with poor sound-absorption characteristics, this noise can be deafening and quite disturbing to those who find themselves in its environs.

As a manufacturer of removable insulation blankets, we are often called upon to provide sound attenuation as well. While standard removable insulation blankets do offer some sound attenuation properties, the correct solution is often a combination of removable blankets, composite foam barrier materials, insulation boards, and barrier curtains. The right choice depends on a number of factors particular to each situation.

Background: What is Sound?

Sound is a change in atmospheric pressure that is interpreted by the ear. The sound pressure level heard by a person in a room is determined both by the sound power radiated by the source of the sound and the acoustic characteristics of the room.

The most common way of measuring sound is in Decibels [dB] which measure both sound power and sound pressure levels and are expressed in logarithmic terms. Tolerable noise is accepted as 85dBs. (OSHA standard 29 CFR 1910.95).

To identify the root cause of a noise problem, sound has to be further analyzed into frequency levels [Hz Hertz], which for convenience are separated into 8 or 9 octaves on a frequency scale.

In order to engineer sound attenuation solutions, ideally, both the dB ratings and Octave band Frequency Analysis report are needed. This enables an Acoustics Engineer to address the complete situation within the practical limitations of the available materials and constraints of the individual case.

Sound Attenuation Solutions – Materials

Insulation solutions are based on using combinations of materials to absorb sound, to offer barriers to sound, and to close up places where there is sound leakage.

1. Some materials, such as Fiberglass and Mineral wool, are particularly good in absorbing higher frequency sound in the 1000 to 4000 Hz ranges.
2. Low frequency sounds are difficult to absorb because of their long wavelengths. In general, the absorption of low frequency sound increases with the thickness of the absorber or barrier.
3. Sound will leak through openings where no sound insulation is in place.

In order to give some perspective to the effect of Sound barrier materials and their effectiveness over a full full Octave band we show data for some typical materials used.

Comparitive Charts of three typical absorber materials showing Sound Absorption and NRC [Noise Reduction Coefficients]

Frequency [Hz]
	125	250	500	1000	2000	4000	NRC
Material
Fibrous Glass 4lb/cu.ft Sound Absorption Coefficients
1” thick	.07	.23	.48	.83	.88	.80	.60
2” thick	.2	.55	.89	.97	.83	.79	.81
Fibrous Glass 11lb/cu.ft
1” thick		.29	.86	.95	.92	.95	.75
Polyurethane Foam [Open Cell]
1” thick	.14	.30	.63	.91	.98	.91	.70
2” thick	.35	.51	.82	.98	.97	.95	.82

Chart showing Transmission Loss and STC [Standard Transmission Loss Coefficient] for a Composite Absorber/Barrier used for lining Engine Rooms and Containers

Frequency [Hz]
	80	125	250	500	1000	2000	4000	STC
Material Transmission Loss [dB]
BTMM14C	22	22	22	32	44	50	56	34

Typical Solutions – Panels, Absorbers, Blankets

Typical solutions for Engine Generator sets in engine rooms or containers are to insulate the walls with absorption materials made from fibrous glass/mineral wools or open cell foam panels butted tightly against each other. However this concentrates mainly on the upper Hz range and not the lower Hz range, so often composites such as Firwin BTMM14C are used which offer better attenuation over the full octave spectrum. Composites include absorption materials and barrier materials together with a decoupler layer to separate the barrier from the room/container wall.

Where space considerations limit the use of wall/ enclosure insulation, Removable Insulation Blankets can be used for the manifold, turbo, exhaust piping, silencers, etc. contained inside the room/enclosure. These blankets contain composites to bridge the full frequency band. In some situations, both wall insulated and engine and exhaust blanketed approaches together achieve the best solution. These blankets have both heat and sound insulation properties with a side benefit of energy conservation.

Factors to Consider for Proper Sound Attenuation

1. Thickness of the materials used will affect the attenuation.

2. All panels must be butted up to each other to assure that there is no sound leakage.

3. Engines and other equipment must be mounted on vibration isolation mounts or pads to stop vibration and sound transmission.

4. Baffle chambers and good Silencers are also important.

5. Where space is at premium and enclosures cannot be used, Removable Insulation blankets with built in barriers are an answer. Because the Sound Insulation package is installed directly on the equipment the attenuation is usually limited to no more than 8 to 10dB [provided the installation is done with care and diligence]

6. Very often space and economic limitations will mean that the ultimate practical possible solution is not attainable.

Each project should be regarded as a separate case due to the multiplicity of variants from application to application.

Sound Attenuation – Case Studies

Firwin Case Study #1

Large Hydro Corporation

Problem:	High-pitched Sound of 110 dB on Screw pump used for pumping Glycol Coolant in Large Diesel Power Plant.
Objective:	Sound level had to be reduced to 85dB level to meet National Standard for Industrial plants. Easily removable blankets required for regular maintenance and inspection.
Solution:	Velcro fastening removable sound blankets. Special attention to all pipe openings using tie-downs onto adjacent permanent insulation to ensure no sound leaks. A muffler shroud was fitted on the end of the electric drive motor.
Result:	Using a full Octave band Sound analysis, the customer confirmed that sound levels had been reduced to an average of 85dB at 3 meters. Customer was satisfied with the result meeting the Industrial Standard.

Firwin Case Study #2

Quiet Diesel Generator Set Builder for Movie Industry

Problem:	Although the Container enclosing the generator-set had been lined with mineral wool boards the customer was not satisfied with the sound level outside the enclosure.
Objective:	Due to the size constraints required for the container size an innovative solution had to be found to attenuate the sound from the engine exhaust parts to between 65 dB to 75 dB. Any cover on the Engine had to be able to be removed for maintenance and inspection.
Solution:	Removable Sound blankets using a composite of 12 lb/ft^3fiberglass and high temperature heavy density membrane. SS Capstan rivets and SS lacing wire and drawstrings were used to tie down onto adjacent piping [see photo].
Result:	End Customer was satisfied with the results - this was a very demanding customer.

Firwin Case Study #3

Large Underground Mining Corporation

Problem:	High-powered underground vacuum unit mounted on a truck. The sound level in the driver’s cab was 95 dB and outside the cab 120dB. There were major complaints from both the drivers and the Mine Health and Safety Committee. Due to the constraints of narrow and limited height of mining tunnels/hauls regular enclosures were not feasible.
Solution:	A combination of sound curtains, and removable sound insulation blankets [on the power pack, muffler, compressor and pipes].
Result:	Sound levels in the cabin were reduced to 75 dB, outside the cabin to 105 dB on one side and 110 dB on the other. Although not entirely happy everyone agreed that within the restrictions of the situation there had been a marked workable improvement.

FIRWIN FAQ

Fire Safety and Insulation Blankets - What are the Risks?

As a manufacturer of removable insulation blankets for high temperature applications, Firwin's blankets are designed to withstand extremely high temperatures. All the components that go into our blankets, be it the insulation fiber matt or the outer blanket cover, are rated as non-combustible and conform to various fire standards (MIL and UL).

The risk: Engine oil or hydraulic fluid lodging in the insulation matt.

In most cases, Firwin's Removable Insulation Blankets acutally increase fire safety. By containing the extreme heat generated by diesel engines and their exhaust systems, engine rooms are safeguarded from overheating, and heat sensitive components are protected.

Our insulation solutions are often found on diesel powered equipment, and one of the most used applications is underground mining. Given its rugged enviroment, underground mining equipment has been known to spring an engine oil leak or hydraulic hose burst. If this fluid seeps underneath the insulation blanket and lodges into the matt, there is the possibility of spontaneous combustion.

The solution: Firwin MineWrap™ Mark II and Mark III insulation blankets.

Firwin produces insulation blankets designed specifically to meet the stringent safety requirements of the underground mining community. Our MineWrap™ Mark II and Mark III insulation blankets are lined with a stainless steel foil inner liner, which effectively seals off any leaked oil or hydraulic fluid from coming into contact with the insulation matt material.

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