ii Health and Safety - Noise at Work Focus


for the new idea of datalogging, where complete measurements could be sampled and stored to memory.


The trend in SLM development by now had been a slow increase in sampling rate, and dynamic range. The advent of low power digital signal processing suddenly made it realistic to digitise the output of the microphone preamplifier directly, requiring a sample rate of over 48kHz. Our sound level meter still has to cover the complete range of human perception both in level and frequency, but now we can calculate weighting filters, 1/1 & 1/3 octaves, Leq and statistics completely digitally. The idea of digital dynamic range was no different to the old ways.


This simplifies our sound level meter down to Figure 3. Coupled with vastly increased memory, A/D converter and a DSP, almost anything is possible.


Figure 3 Figure 7 Svantek SVAN 971 Sound Level Meter


principle, but the capacitor is machined on to a tiny silicon wafer, which is packaged into a more manageable pot which can be directly soldered onto the circuit board. In some recent cases, the A/D converter can even be built in to the silicon, making what is effectively a digital microphone. MEMS microphones are also incredibly rugged, and of course, the low price of a few dollars is a real advantage.


Completely Digital?


The microphone is the most costly part of a sound level meter and the last analog bastion in the measurement chain. Since precision sound level measurements began, the condenser microphone (Figure 4) has been the gold standard, the ½” capsule providing the best compromise in dynamic range and frequency range.


Figure 6


extension microphones are also available to improve the acoustics and performance. Some even claim to meet sound level meter standards. Ironically, a few of these even have ‘retro’ analogue displays – a real full-circle!


As with PC-based sound level meters 20 years ago, we should still be sure that standards are met so where do these apps fit in? The spectrum analyser apps are very good at finding


the frequency of an audible tone, but measuring the level is often only achieved accurately over a limited dynamic range. Also bear in mind that the electromagnetic environment inside a mobile phone is particularly hostile to low level noise measurements.


Figure 5


Can these be used for measuring sound? The answer to that lies in those standards that govern sound level meter performance, and right now, MEMS microphone performance falls short of those requirements. There is already a place for them as noise dosimeters (Figure 6) now employ MEMS techniques, as well as specialised techniques such as MIRE for in-ear measurements.


Figure 4


For other much larger markets, such as hearing aids and telephones, a digital revolution has been happening in microphone development. The use of MEMS (micro electro mechanical systems) or micro-machined silicon transducers is now well established.


MEMS microphones (Figure 5) are still based on the capacitor About the Author


John Shelton has been in the sound & vibration instrumentation business for over 30 years, and recently celebrated 20 years of AcSoft Ltd, pioneers of PC-based instrumentation. A member of the IOA, he is a founder member and current Chairman of the IOA Measurement & Instrumentation Group and sits on several committees relating to sound & vibration measurement.


Consumer Sound Level Meters?


Now that everything can be done with an A/D converter and a DSP, we are seeing the rise of the App. Using the life support system of the smartphone (which already has MEMS microphones and DSP to burn), software applications are appearing which can turn a phone into a sound level meter (Figure 7). Specialised


It’s unlikely that Apple, Google or RIM will ever move into the sound level meter market – it’s just too small and specialised. Also, producing a new model or operating system every year will obsolete our phone-based instrument too quickly, but the traditional manufacturers can feed off the crumbs left behind – a Class 1 sound level meter with a MEMS microphone is not far off.


Summary


This article has, I hope, given an overview of sound level meter development over the last few decades, highlighting the move from analogue to digital, and consequent increase in value for money.


Where will it end? In my view, sound measurements will become even more integrated to the internet – maybe one day our digital MEMS microphone will connect directly to the Cloud, and our noise report will be written before we even get back to the office, along with weather, photos, GPS, maps. Plug your microphone into your Google glasses?


IET January / February 2016 www.envirotech-online.com


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