Bricolage IS WRITTEN by Jeff Cable and is based in the United Kingdom.

This blog aspires to provide COHERENT and relevant Material for all cast TECHNICIANS. 

Plaster Room Noise

The environment in which cast technicians work can be rather noisy. We may not be able to hear what other people are saying because of the noise from a cast saw being used. In a cast room with several technicians working, it is possible for several cast saws to be operating at the same time.

This type of intermittent noise can be frequent and debilitating. It may result in hearing loss for the cast technician and our patients. Patients may be subject to long cast room procedures (such as wound care or technically complex cast production) and they are being exposed to the same noise conditions as the cast technician.

Relatives accompanying patients may be exposed to the unnecessary risk of hearing damage. Some patients, such as young children, must be permitted to have a relative with them. It is strongly suggested that adults; who are not actually patients or with children, should be kept outside of the cast room where their presence is not strictly necessary.

To adequately assess the degree of noise in the work environment requires several approaches. One may get a sense or an indication of the noise at work by asking colleagues and employees about the way they are perceiving the cast room environment noise.

One can use any of the available mobile telephone applications which attempt to measure noise in the local environment. These applications will not be very accurate but they will provide an approximation as to how much noise is present and quantifiable.

This is an improvement on relying on our own ears and hearing, which may be faulty or damaged. The nature of the noise is usually best determined by a noise dosimeter; which is a calibrated device that is designed to measure and analyse different components of noise simultaneously and over an extended time period.

Noise can be measured as a single instance, which indicates a peak value in dB (C) but it tells you nothing about the exposure of cast technicians to noise during the whole of a typical working day. We can think of the noise as the unwanted segment of our daily work soundscape and every cast technician will be required to work within this difficult sound environment to some degree.

Sound is measured in decibels (dB) and these measurements are weighted. The weightings are used to produce a subjectively valid measurement, which places an emphasis on the portions of the frequency spectrum that are considered to be more important.   

The above weighting image is by permission of Acousticator : Own work, CC BY-SA 4.0,

The above weighting image is by permission of Acousticator : Own work, CC BY-SA 4.0,

The A - weighting represents what humans are able to hear physically. A - weighting is a standard weighting of the audible frequencies which are designed to reflect the response of the human ear to noise. At low and high frequencies, the human ear is not very sensitive, but between 500 Hz and 6 kHz the ear is much more sensitive.

The A - weighting filter covers the full frequency range of 20 Hz to 20 kHz, but the shape approximates to the frequency sensitivity of the human ear. So the A - weighted value of a noise source is an approximation to how the human ear perceives the noise. The A - weighting is internationally mandated as the value which must be used for protecting workers against noise induced hearing loss.

The C - weighting represents the sounds which humans can hear when the volume is increased. The C - weighting is a standard weighting of audible frequencies and it is commonly used for the measurement of Peak Sound Pressure levels.

The Z - weighting represents a graph of equal sound pressure for all frequencies. Z - weighting is a flat frequency response between 10Hz and 20kHz ±1.5dB excluding microphone response.

The actual noise values presented here were measured on thirteen random work days and two non work days. The non work days served as control days to ensure that the measurements could be assessed within their proper context

The values which were measured and collated; using a modern wearable noise dosimeter were: LAeq, LCpk, LZpk, LEX8h, LAFmax plus the timed duration of data collection. All of these measured values are defined and discussed below to enable the reader to become familiar with the sound values which were measured and considered.

A modern noise meter will measure noise levels at around 16 times per second and this results in a one hour data collection period (16 x 60 x 60) requiring the noise meter to calculate and store 57,600 calculation results. The meter which was used to measure the values presented was capable of running for 29 hours on a single battery charge. The measurements collected were gathered in complete work shifts of just over eight hours.

This value represents the equivalent continuous noise level of any noise level which fluctuates with time. The sound and its fluctuations are measured for the period of time which interests the observer.

The noise meter converts the noise to sound pressure levels and the levels are added together and averaged in a logarithmic manner before being converted back to decibels. The single value, in decibels, represents the total sound energy collected for the time period of interest  

The noise level is expressed in dB and the scale is logarithmic so that the values cannot be added directly. A doubling of sound results in a measured increase of 3dB (A). In the UK workplace, a doubling of sound values requires us to cut the noise exposure time for the worker by half.

The reference standard for the workplace in the UK is 85dB (A); which equates to a worker's exposure value of 100% of the nominal 8 hour working day. 

Where the sound level exposure is doubled to 88 dB(A) using an exchange rate of 3dB (A), then the acceptable working exposure time should be halved. In this example case, it would mean that the worker could only be exposed to the doubled sound level for a duration of four hours. This would result in halving the available working time from eight to four hours.

This value represents the C - weighted peak instantaneous sound pressure level of noise measured by the dosimeter. This is the true peak measurement of the sound pressure waves detected. The dB (C) value is not adjusted by a time constant nor has the signal passed through any RMS (root mean square) circuitry to correct the value for any purpose.

This value uses the Z to denote zero frequency weighting which implies no frequency weighting was used. The actual range is 10 Hz to 20 kHz ±1.5 dB (Z)

This value is the equivalent continuous sound level corrected for the length of the working shift.  The value is the worker's Daily Exposure Level with the sound level averaged over 8 hours.  

The lower exposure action value is 80 dB (A) 
The upper exposure action value is
85 dB (A)
The exposure limit value (for LEX,8h) = 87dB (A)

LAmax is the maximum A ‐ weighted sound pressure level recorded during the stated time period. It is known that LAmax is often used as a measure of the most obtrusive facet of the noise, even though it may only occur for a very short time and is the level of the maximum Root Mean Square (RMS) reading. 

The time weighting response of the sound level meter (fast (F), slow (S) or impulse (I) should also be specified to make the reading meaningful, which is reported as LAF,max in dB (A), for example. Unless specified otherwise, it is measured using the ‘fast’ time weighting response.

N.B. Peak levels are measured using LCpk and C - weighting and they are significantly greater than LAFmax. e.g. from the data collected LAFmax = 120.9 dB (A)  while the equivalent LCpk value was much higher at 143.5 dB (C)

The data was collected with a dosimeter produced by Casella. This company specialises, inter alia, in the manufacture, calibration and use of noise measuring equipment and this specialist company offered a convenient portable solution for accurate noise assessments.

A noise dosimeter kit was hired and used with company supplied data reading software and an acoustic calibrator. The acoustic calibrator was used before and after each data set was collected. The acoustic calibrator was set to emit a standard tone of 114 dB (A). 

Acoustic calibrator

Acoustic calibrator

The dBadge2 dosimeter was worn for the length of a working day, at shoulder height and it was pinned to my scrub suit. This ensured that the sounds which were reaching my ears would be the same sounds which were being recorded. 

The dosimeter was worn for 13 randomly selected days at a new work location. This ensured that the local work practices and workloads were unknown and could not influence the selection of work days chosen for noise measurement.The selection of random work days at the start of this new contract was used to eliminate observer bias.

The two control days were non working days. The noise measured on these days was used to establish a normal baseline for noise in the everyday environment. The baseline provided a comparator against which the gathered work day data could be assessed for intrusive noise.

dBadge 2 dosimeter

dBadge 2 dosimeter

Data Collected
This link opens to the spreadsheet of all of the data which was collected. These pages show the cumulative total on a single day for every data category of interest. These example pages show a snapshot of the profile points gathered. They were generated by the dosimeter for each minute that the machine was working and totalled 2000 pages for the duration of the assessment. The example day shows that the data collection ran for 8 hours 19 minutes and 36 seconds and the dosimeter had collected 29,976 data points.

Data Interpretation
The collected information may be interpreted in several ways. Initially, before reading any data, a subjective impression can be formed by the cast technician. This assessment would be helpful while attempting to understand unwanted sounds which were contributory to the local soundscape noise.

Subjective Data Interpretation
The noise levels in the cast room used for this collection of data was one where noise levels were often quite high. It was difficult to hear any of the other technicians when they were speaking from one to two metres distant, especially where the cast saws were in use. It was sometimes hard to hold a normal conversation with patients.

One phenomenon I have noticed after a noisy environment becomes quieter is that it takes some time for my hearing to readjust itself. I experience a sensation where the noise level appears to persist, even though it has become quieter. I also feel that the noise is far too loud for comfort and wish for a control to adjust the volume down.

I sometimes think my appreciation of the noise (in a residual sense) remains for several minutes after it becomes quiet. This is my own subjective sensation and it indicates to me that the noise experienced was probably too loud for my comfort. Empirically speaking, where the sounds heard are mainly comprised of unwanted noise, then the level is already too high.

Technical Value Interpretation
Another way to interpret the data is to look at the purely technical value data for each category. The values can be regarded as absolutes which are either on one side of the line or the other. That is to say that either the noise level is legally permissible or it is not. The Control of Noise at Work Regulations (2005) is a statutory instrument which deals with the noise levels which are permitted.

This is a sterile approach to noise at work and cast technicians do not work under identical and reproducible laboratory conditions. For example; A child may be inconsolable and screaming, even when a Softcast splint is being removed with scissors or unravelled. This may well bring the overall noise level above the acceptable legal level. Damage to a cast technician's hearing may ensue because of the child's proximity and the duration of screams.

Working Day Relationship Interpretation
Another approach to the data interpretation is to examine the values obtained from the noise dosimeter assessment. Overlaying the data interpretation with its relationship to a working day is likely to yield more useful results than rigidly sticking to a purely technical line.

It will be clear from the spreadsheet of collected data that the average equivalent continuous noise level LAeq (A) was well below the prescribed 80 dB (A) lower exposure action level at 75.5 dB (A). The averaged measured continuous noise level was less than half of the permitted value. 

The averaged exposure limit value for an eight hour working day (LEX,8h) was also well below the prescribed limit of 87 dB (A) at 76.5 dB (A). The averaged 8 hour values reflected noise exposure levels which were less than an eighth of the permitted volume.

The average values for the two non working control days produced values which were only one sixteenth as loud as the work day values for both the LAeq (A) and LEX, 8h (A). On its face, these noise values show that all is well within the cast room environment. The general perception of cast room noise is probably quite different.

Cast room saws may easily generate noise values above the 85 dB (A) upper exposure action value. Many cast saws, which I have assessed with a sound pressure level application, have displayed instantaneous values ranging from 86 to 104 dB (C).

The noise levels can increase where there is more than one cast saw in use (there are four in my current working environment). The increase in noise is very likely when the incorrect blade is fitted for the casting  material which is in use.

Contributory Factors
It is common to see the wrong blades being used with the de facto cast room standard saw in the UK. The cast saw is made by deSoutter Medical and one can see it around 90% of all cast rooms in the UK. The issue concerning the fitting of wrong blades appears to be based upon the unit price and the longevity of the blades.

The normally used blade is the 64mm circular blade number 6250. (page 9 of the linked deSoutter Medical catalogue) It is a hard chrome PTFE coated blade. This is optimal for plaster of Paris and listed as suitable for fibre glass casting tapes such as DeltaLite and Scotchcast. A diamond blade is listed as optimal for fibre glass casting tapes. 

The most commonly used light weight, casting tapes are polyester and these are produced by Benecast, 3M and BSN. The hard chrome PTFE blade is not recommend by deSoutter for cutting polyester materials. It is recommended that cast technicians use an iron or titanium nitride blade. Iron or titanium nitride is listed as optimal for all casting materials.

Sub-optimal blade use serves to generate heat and increase the risk of skin burns to the patient. This is through increased cast cutting times when used with unsuitable casting tapes. There is an increase in noise duration because it takes longer to cut the cast and the volume of the noise generated is also increased by using the wrong blade to cut unsuitable materials. The blade has to work harder to cut a material for which it was not designed.

The cast room may be relatively bare of furnishings and these help with breaking up sound, which will tend to echo in an empty room. There may be a specific case to make for requesting that cast rooms are constructed as an anechoic chamber. This would guarantee that generated noise is lost, once it reaches the boundaries of the room. Making cast saws quieter from the outset would be a good aim for the manufacturers of such devices. 

My own cast saw is made in Germany and has 6 speed settings which can be adjusted via a rheostat. At its quietest no load speed it generates a noise level of 68 dB (A) rising to 82 dB (A). It can produce oscillation speeds of between 12,000 and 21,000 oscillations per minute and it is fitted with titanium blades which cost about £24 each. They stay sharp and cut every casting material for around 6 months before needing changing.

Casts are cut according to the teachings of cast technicians; which suggest that the blade must be withdrawn entirely from the cast material and moved laterally then reinserted by pushing the blade downwards vertically. The method clearly cuts through the whole cast thickness with every cut made.

My own method is to cut the cast more efficiently. I do this by withdrawing the saw blade (after the initial downstroke) just a couple of millimetres. As I move the blade laterally, it cuts through the cast using the lower forward quadrant of the curve of the blade. This results in less heat generation and removes the need to make a brand new cut with every lateral movement of the saw blade.

The height of the blade is adjusted so that the blade cannot be dragged along the skin. The technique cuts far faster as more of the saw blade is being used. It is cooler because it is more efficient. Finally, it should not need saying that casts which are unnecessarily thick are another major cause of noise increases and expansion of the duration of noise effect.

Significant Data  
The peak pressure levels for initiating action are: a lower exposure action value of 135 dB (C) and an upper exposure action value which equates to 137 dB (C). It is notable that on one day, the LCpk and the LZpk values were 143.5 dB (C)  and 143.5 dB (Z), respectively.

The significance of the recorded numbers is this: 143.5 dB (C) is more than 7 times louder than the lower exposure action level. It is more than 4 times louder than the upper exposure action level. In the Control of Noise at Work Regulations (2005) at section 4 (3) it lays down the exposure limit value peak sound pressure as 140 dB (C).

Using the standard exchange rate of 3 dB we can see that the exposure limit value of 140 dB (C) was broken by 3.5 dB (C) which equals a doubling of the sound intensity. It is against the law on the one hand and on the other these noise levels are considered to be damaging. The measured peak levels were frequently outside the LEX,8h exposure limits.

Recall that 80 dB (A) is the lower exposure action value and 85 dB (A) is the upper exposure action value for LAeq and LEX,8h. The absolute exposure level permitted for LEX,8h is 87 dB (A). The lower exposure action level for peak sound pressure is 135 dB (C) and the upper exposure action level for peak sound pressure is 137 dB (C).

The LCpk sound pressure levels were always over 120 dB (C) and occasionally exceeded 130 dB (C). This is a measure of how intolerable the noise of a plaster room can be. The noise may only be present during a cast removal but it can take some time when the cast is complex. With several cast saws working, the noise may be very intrusive. 

In their Guide To Noise At Work, The Health and Safety Executive identify the use of noisy power tools for more than half an hour per day as potentially a noise problem. Cast technicians know about the noise they endure throughout a normal working day.

We should be asking for hearing protection for ourselves and our patients. Employers should be carrying out risk assessments and annual hearing tests and provide the relevant training. My current work location has conducted comprehensive hearing tests for me.

We should try to influence buying departments in purchasing much quieter cast saws. Companies with supply the majority of casts saws into the NHS (e.g. deSoutter Medical) should be encouraged to produce quiet cast saws. 

Reasonable Workloads

Removable Casts