Estimating Sound Machinery Sound Levels

Estimating Sound Machinery Sound Levels

Description

The purpose of to this bulletin is to provide a simplified tool for estimating sound levels for silenced engine enclosures such as enclosed generator sets. 

Background

The science of acoustics, and “noise reduction” is complex. It is therefore important to simplify some aspects pertaining to engine designs and ensuring that the user is also aware of the risks in utilising these ‘simplified’ tools.

The universal standard for testing generator sets is ISO8528-10. Testing basically consists of analysing, via sound meter, various point around the unit, then adding results and establishing a Sound Pressure Level (SPL), Lp (dBA), typically at 1m at 75% rated power.

Genset manufacturers use this standard as a basis for rating their gensets but will test the genset at 7m, rather than 1m, and at 75% rated power.  This is a safer alternative in acoustics because the closer to the source sound recordings are taken, the less accurate the result can be due to effects such as background noise, direct noise resulting from measuring at inlets/outlets, etc. Another major factor for measuring at 7m, is that the noise source can be treated as a “point” source, rather than a more complicated “plane” source. (This is another source of discussion!)

The terms “Reverberant Field”, “Free Field” and “Near Field” and “Far Field” also come into play.  Essentially the “Near Field” is an immediate area around a noise source (Approximately < 1 metre) where the sound waves are being generated. Sound measurement in this area can be very inaccurate. Far field is the region immediately following the “Near Field” condition. A “Free Field” condition is where there are essentially no external influences, such as walls, etc. affecting acoustics, and “Reverberant Field” condition is an area whereby ‘complicated’ external influences, such as walls, etc., affecting acoustics. Refer to Figure 1.0 below. 



Calculating Sound Pressure Levels at A Distance Away from Source

 

As a quick rule of thumb when calculating sound pressures at distances further than the source and the application is assessed as being in “Free Field” condition, we use the rule “for every doubling of the distance away from the sound source, we subtract a maximum value of 6dBA (3dBA is a more conservative value I like to use).” So, for example, when a genset is rated for 65dBA at 7m, and we require a “free field” sound pressure level (SPL) at 14m, we subtract a maximum of 6dBA from the rated value, resulting in 59dBA at 14m. If we use the conservative value 3dBA, we obtain a final value of 62dBA at 14m.

 If the less conservative 6dBA reduction value is used, the following formula, called the Inverse Square Law can be used:

 

Lp2        =          Lp1   –   20log(d2/d1)

 

where:

Lp2 (dBA) is the required SPL at required distance d2 (m), and Lp1 is the known SPL at known distance d1 (m).

 

Note that the resulting answer is a general estimation which excludes many factors in sound measurement. A true value would have to be obtained by utilising sound measure techniques in the field where effects such as physical conditions such as barriers, background noise, etc., can be taken into effect.

 

National Engineering has created a simple calculator called “Inverse Square Law.xlsx.” 

Calculating Sound Pressure Levels at Distances Closer to Source

 As a quick rule of thumb when calculating sound pressures at distances closer to the source value, where we are approaching “Near Field” but the application is still deemed as a “Free Field” condition, we use the conservative rule “for every halving of the distance to the sound source, we add 6dBA to the value” and the formula above can be used.

Please note that if the SPL is already given at 1m, any distance less than this would be deemed as being in “Near Field” condition and therefore the Inverse Square Law is not used.

 

Calculating Sound Pressure Values in the “Reverberant Field”

 This is a complicated condition and beyond the scope of Penske Technical Support Engineers. This would typically require the services of Acoustic Consultants.