Noise Level and Relation to Distance and Formula to Calculate

OSHA (Occupational Safety and Health Administration) regulate the noise level, in relation to duration of exposure to human mental health and hearing problem, based on OSHA 85dBA is the limit for exposure 8hours. The dBA might increase or decrease if duration is change and vice versa. I believe that noise able to control your mind, it changes mood and able to flip a switch on our head (normal or crazy mode haha).

During my beginning of working in oil and gas 2010-ish, I vaguely remember that the limit was 90dBA, then later getting lower till nowadays is 85dBA at 1meter distance. This number become the acceptance criteria of all rotating equipment in design.

Sound simple talk about noise, if there is noise just go away from it, yes, it is. However, in plant, especially platform, you will not have the luxury to make distance to the noise source. Consequently, no other way, rotating equipment shall be design in such a way, to limit the noise to 85dBA.

Let try to scale what is 85dBA

SCALING

Below graphic, just give a general definition, but still lack of distance in the data. In fact, distance is very important when talking about noise. In my understanding below graphic shown with logical distance following the sample, such as whisper, is less than 30cm from your ears. City traffic and truck we may assume it is 1-2m from our ear to the noise source. While jet engine is let say 1km from where you are, you are never hear full power jet engine 1m from you, your ear will bleed.

Noise Scale 

I just found below table that much more explaining on sound pressure level

Basically, the noise that we talk about is the sound pressure level, which is conversion from Sound Pressure Pa into dB, which easier to understand.

Sound Pressure Level 

Then there is sound power level term, which does not affect by distance. Sound power definition is sound energy transferred per second from the noise source to the air

Sound level vs sound power

DISTANCE

Distance gives lot of effect to noise, to demonstrate please refer to below graph

Relation between noise and distance

As per above graph, by distancing our self 1 meter away from 90dBA(1 meter from sound source) noise, we able to reduce to 83dBA (-7dBA) then when we distance 3 meter away from 90dBA, we reduce further by 12dBA.

There should be mathematical form to estimate sound reduction in relation to distance to one sound source.

The formula is;

Formula relation sound vs distance

Let's give it a try if r1=1m, r2=2m, the log(0.5) = 0.3, times 20 = 6, so in line with above graphic stating that every 1 meter we lost 6-7dBA, ok, this formula we able to use. try once more, if the new distance 4, means log (0.25) = 0.6, times 20 = 12. It is exactly reflecting the graphic.

above lines in dBA, below line is the ratio between previous distance to new distance (r1/r2)

DIESEL ENGINE CASE

The ultimatum for diesel engine is to limit it sound to 85dBA, oh nooo, impossible, the power is reflected by the noise that it makes. Even, in stationer speed of diesel engine it may reach >85dBA, and during peak power run it able to reach 120-150dBA. Just get away bro.

Then it comes to sound proofing, the diesel engine must be located inside enclosure that able to absorb the noise the diesel engine producing. There is detail about each material that able to absorb noise.

The same also happened in GAS TURBINE, or any high-speed machinery. That is why it end-up to be put inside enclosure. However, it is not always the single reason, diesel engine also considered as source of safety risk in hazardous area, that is why sometimes the enclosure also act as buffer zone to expelled flammable gas move around the hot surface of the engine, using pressurize fan on the enclosure ventilation system.

TESTING

This is very interesting, how to test the noise, it is very challenging to have 0dBA environment condition, there are place that made that able to reach 0dBA, but it is troublesome for equipment manufacturer to move the complete equipment to the place, so the rational thing that we able to do, is to measure the ambient environment noise in dBA before the engine/equipment start in the location that we will measure the noise.

Noise measurement location 1 meter away from equipment skid limit

Using below formula (reference to ISO 3746we able to estimate what is the real noise produced by equipment.

Combined noise formula

So let take to real case, in Factory acceptance test, we measure ambient noise without engine running is 89dBA, then when engine running, we measure in the same place 90dBA. What will be the engine sound level?

using the above formula we already knew the Lt = 90, the L1= 89, L2 = engine sound

the L2 = 83.5dBA, the formula is quite hard to simplify, so the easiest way is to keep trying L2 until the Lt ~ 90, with help of excel, it will be fast, as shown below

89dBA ambient

We able to verify that our engine sound is 83.5dBA

What if the ambient noise is 88, easy, if during run you got 90dBA, means your engine producing 85.7dBA

88dBA ambient

It is interesting the 1 point different of ambient dBA resulting far different engine noise level. Thanks to logarithmic scale.

Another case, if ambient exactly 85dBA, and during running measured noise value is 88dBA, means your engine producing 85dBA so still acceptable.

However, if your ambient is lower than <85dBA, while the measurement resulting above 85dBA

85dBA ambient

Finally, above formula able to justify the thousand-dollar question, whether the equipment noise level is within acceptance criteria? or additional scope of sound proofing shall be added. The question is never multiple-choice answer, multi department will be involved do find solution if the noise value measured beyond the acceptance criteria.

END SPEECH

However, please remember by default engineer shall always be right (engineer will be scolded and punished if made mistake but not vice versa), then let the management take the credit on how much potential saving that been done, with sometimes even a simple thanks are forgotten to be said to engineer.

Do not be an engineer if you have soft heart, you will be heartbroken haha. Thanks to my alma mater, the "ospek" that made me as I am now. We love the job, we love to solve problem, but being recognized and being credited for our work, just let say it is a bonus.

There are times in project end celebration, management, construction, procurement is being praised, while engineering team never be spoken about, but everyone shall remember project title start with EPCIC "project names"; EPCIC stands for: Engineering, Procurement, Construction, Installation, Commissioning. Engineering comes first.