We hear you!

In response to many enquiries from the mining, quarrying, construction, and infrastructure industries, here is session 101 on Sound Level Meters and noise measurement.

The general theme is, while we respect the need to comply with noise and vibration management plans, we also must deliver on outcomes.

As we all appreciate, demonstrating on-going compliance with the DA or consent conditions is no small task. It requires a constant balance between technical and human factors for all stakeholders.

This blog will cover what is probably the most common of the environmental monitors required on any site – the sound level meter (also known as a noise monitor).

Texcel now offers sound level meters as part of our full-service packages or you can also purchase or rent them.

A few basic issues to get started:

The basic unit of measure for sound is the decibel (dB). The dB scale is a logarithmic scale – i.e. a 60 dB level is not double a 30 dB level – it is 31.6 times higher.

A question often asked of us is:

• The Texcel blast monitors come with a microphone and measure dBL.
• A sound level meter comes with a microphone and measures dBA.
• What is the difference?
• Can I get dBA from my dBL reading and vice versa?

Measurements in dBL:

• This measurement treats all frequency levels in the sample equally. The reported level is the highest level recorded across the measured frequencies.
• The typical frequency range sampled is from 2 Hz to about 350 Hz.

Measurements in dBA:

• This measurement is adjusted (weighted) to reflect, as accurately as possible, the functioning of the human ear.
• The ideal human ear can detect frequencies from 20 Hz to about 20,000 Hz, but the sensitivity to frequencies varies. Basically, this enables the ear to detect everything from a faint scratching of skin to a jet engine.

Can I transition my dBA reading to dBL?

• Given the above, the obvious answer is – NO.
• Basically, the frequency levels measured by each microphone type are very different.

Types of Sound Level Meters available:

• As per international standards, there are two types:
  • Class 1
  • Class 2

What are the differences and when do I use each type?

• The full explanation of the differences is quite complex, but fundamentally:
  • A Class 1 sound level meter is more accurate across the full frequency range. It is generally referred to as a “precision” grade meter.
  • A Class 1 sound level meter is able to measure over a wider frequency range.
• There are usually no hard and fast rules, but generally:
  • Compliance measurements require a Class 1 meter and this will usually be defined in the operating conditions
  • In most other situations a Class 2 meter will give adequate results.
  • Both, of course, need to be kept in calibration.

What sort of levels could I expect from my operation?

• It is obviously difficult to be prescriptive here, so what follows are approximations.
• If you are measuring dBL (usually from a blasting event where it is often referred to as air overpressure), a pretty low level would be 100 dBL and permitted upper levels are usually between 115 dBL and 120 dBL. The dBL microphone will typically not even be able to measure below about 75 dBL.
• If you are measuring dBA, the range is much greater. Some indicative levels are:
  • Leaves rustling, whispers 30 dBA6
  • Normal conversation 60 dBA
  • Heavy traffic 80-90 dBA
  • Shouted conversation 90-95 dBA
  • Rock concert 120-130 dBA
  • Siren at 30m 140 dBA
  • In general, levels above 85 dBA are considered harmful.
  • In theory, 0 dBA is the lowest level of hearing.

Derived sound levels:

• Levels in dBL are straightforward – they get reported as a simple number (there is a frequency associated with the number which may or may not be reported automatically).
• Levels in dBA usually come with other measures such as:
  • LAmax – this is the maximum level recorded over the period stated.
  • LAeq – this is the average level recorded over the stated period.
  • LA10 – this is the level exceeded for 10% of the stated time – usually referred to as the “average maximum level”.
  • LA90 – this is the level exceeded for 90% of the stated time – usually referred to as the “average minimum level”.

Like many other things associated with compliance monitoring, accurate measurement of sound can get very complicated. The Texcel full-service packages (Guardian by Texcel and 360 by Texcel) provide a managed service that is completely turn-key. This absolves the operator/contractor of the responsibility of selecting, installing, maintaining, and reporting. Importantly, Texcel’s managed services provide an independent, expert, third party facility – i.e. environmental monitoring made easy.

As always if you have any other questions or would like more information, pleased don’t hesitate to reach out to us on +61 7 3237 8111 or www.texcel.com.au