Noise Measurements and Human Noise Perception.

Today we introduce a very important aspect related to noise level measurements. That is “decibel –A weighting filter”

Sound or Noise level in an occupational environment is a very important safety related measurement dictating the relative safety and the time duration that a person can safely work within a certain noisy environment. Therefore sound pressure level (in dB), often referred as the noise level,  have to be measured whenever workers are suspected to be exposed to unsafe noise conditions in their working environment, or when we need to determine the maximum duration that workers are allowed to work within a certain environment.

Audible noise occurs at various frequencies throughout the 20 Hz – 20 kHz  frequency range (i.e. approximate audible frequency range for human hearing). But, (similar to many manmade microphones, and speaker sets), the human ear is not equally sensible to all frequencies within this audible frequency range. It has been discovered that human ear is highly sensitive to the sounds within the frequency range of 1 kHz to 4 kHz, while human ear is less sensitive to sounds of frequencies below this range and also above this range. That means, humans can tolerate more loud noises outside this sensitive frequency range.

Due to the above mentioned reason, if we measure the total noise level in an environment using a measuring instrument without any “human like” adjustment, that measurement will not represent the actual noise condition perceived by a human.

So, acoustic engineers have developed various types of “noise weighting curves” (or “noise filters”)  to adjust the noise measurements in accordance with human perception of noise under different conditions.

There are several noise filters denoted as, decibel-A filter, decibel-B filter, and decibel- C filter (a rarely used decibel – D also existed). The graph in the image indicates the characteristics of above mentioned filters.

Out of these, decibel –A filter is the most common one and it gives a satisfactory representation of human sound perception at “not too loud” noise conditions. Many modern safety engineering standards also use this decibel – A weighted noise filter for their reference values. Hence many noise measuring instruments have incorporated this db-A filter into their instruments. What it really does is that the instrument automatically subtracts a certain dB level from the actually measured decibel level at each frequency (based on the A weighting curve  -see the graph). For example, at 100 Hz, the instrument shall reduce 20 dB from the measured noise level. Eventually the instrument will collect such adjusted dB levels for all measured frequencies and give you the total noise level as a single value with the unit denoted as db-A. (Remember, this is not a simple arithmetic addition process. You cannot just add 2 decibel levels together to take a summation. This is due to the fact that dB range is a logarithmic range, not a normal linear measurement unit. We will discuss this phenomenon further in future posts).

The image shows the characteristics of db-A, db-B, and db-C weighting filters.

HART protocol for process safety instruments

HART protocol is a vendor-neutral communication protocol used by many modern process plant equipment including Combustible Gas detectors, Flame detectors, Temperature, Pressure, Level and Flow Transmitters, etc. 

HART stands for Highway Addressable Remote Transducer. 
This communication foundation enables safety instruments to communicate their diagnostic information and other information such as remote calibration, clock setting, selection of calibration gas, operational history (such as installed date, current detection gas type, alarm history, fault data, temperature extremes faced, etc).

HART is a master slave arrangement where slave devices (field instruments such as detectors, process transmitters, actuators, controllers, etc) respond to the commands send by master devices such as a PLC controller or a PC.

A major aspect of HART is that HART devices can transmit digital signals on the same two wires used for typical analog communication (typically 4 -20 mA signal). This allows easy retrofitting of existing systems with HART enabled devices.

Alternative field digital communication protocols such as  Modbus, and Fieldbus, etc. exists.

One major advantage related to safety critical devices using HART is that HART communication allows continuous monitoring of diagnostic state of a field device. If a device become faulty, it can be immediately recognized by the monitoring system.

Continuous Fault Monitoring can enhance the Safety Integrity Level (SIL) of safety critical systems such as Emergency Shut Down SYSTEMS, Fire and Gas Systems, etc.

Picture shows a HART enabled Flame Detector

Valve Car Seals on Safety Critical Valves

Car seals in different colors

Car seals are simple locking devices (such as a steel cable strand or a plastic tie) used to “lock” (or seal) safety critical valves in a predetermined “safe position” (either “open”, “close” or an identified middle position).
Once a car seal is used, the valve position can only be changed by cutting and opening the car seal. (then this will indicate a possible tampering, or an authorized change).

Color coding of the car seal devices are sometimes used to easily identify the valve position (e.g. red for closed, green for open).

Car seal requirements on identified safety critical valves should be shown on respective P&IDs. (for examples: valves on a fire water mains line). Following notations are generally used with respective valves.

CSO – car seal is required at open position

CSC – car seal is required at closed position

Car seal put on a Valve's hand wheel

Bathtub curve of Engineering System Failures

Understanding the risk of failure of an engineering system is very important to avoid catastrophic accidents from happening due to the failure of safety critical items of an engineering system.

The following bathtub curve gives a general idea on expected failure potential through the life cycle of a system. The expected risk level will also behave accordingly, if necessary risk management practices are not followed ( including increased testing, observations, risk assessments, etc.).

As indicated in the “Bathtub curve”, the early phase of high failure risk can be due to many reasons such as  improper designs, untested technologies, use of unsuitable materials, lack of operator experience, etc. ).

Bath tub curve

Then the system enters a long period of reduced and steady risk level region where the system is quite safe as long as it is kept undisturbed (by not making significant changes).

Eventually the system will enter a rapidly increasing failure rate region. This is due to the aging of components and subsystems which are failed by wear, fatigue, corrosion, end of design life, or inability to properly maintain due to the old technologies or out of production parts. Manufacturers often end their warranty period before their product reaches this region.

While doing risk assessments, it is important to understand this general bathtub curve behavior of the engineering systems.