Noise assessment, dB, dB(A), NR

Humans are champions in the assessment of things. We assess the quality of things, other people’s work output, car damage after an accident, and we don't shy away from assessing noise levels either. This subject is somewhat complicated, but I will try to describe it in simple words without falsifying the facts.

A noise consists of low, medium and high frequencies. Many of us are familiar with low-frequency sounds from good concerts where you not only hear the bass, but feel it in the pit of your stomach. I like this! And high-frequency noise is what you experience when you’re woken from your Saturday nap by your neighbor urgently sawing wood with a buzz saw. I’m not so keen on this.

Sound therefore consists of tones with different frequencies. Altogether, these make up the frequency spectrum. To present this in figures, it is commonly divided up into so-called octave bands. Those of us who can read music or play an instrument know our way round octaves. Sound levels are mostly stated at the frequencies 63Hz, 125Hz, 250Hz, 500Hz, 1kHz, 2kHz, 4kHz, and 8kHz. These individual sound levels are added together using a logarithmic scale to determine the overall sound level. We won’t go into this logarithmic addition process here - that would be too much for this blog.

Here I’ve just provided you with an introduction. Now we’re going to get down to business:

Human beings perceive tones of different frequencies as being of different loudness. A-weighting adapts measured sound levels accordingly to reflect the response of the human ear. Both sound power and sound pressure levels can be A-weighted, that is, stated in dB(A). The A-weighting is the most common rating used in industry, but we occasionally also find the Noise Rating, or NR.

It’s important to realize that all these methods of defining a sound level are describing the same thing, but the figures themselves differ dramatically. That’s why it’s so important to state the unit clearly and correctly.

To take an example, let’s look at a commercially available 100kW air conditioning unit. This unit has a sound power level of 73.8 dB(A), or 92.4 dB, or NR 72, all of which are describing the same noise, or same sound power level. These figures demonstrate just how important it is to use the correct unit.

Let’s examine this example in more detail:

The linear or unweighted frequency spectrum looks like this:

Frequency [Hz]	63	125	250	500	1k	2k	4k	8k	Total
Sound power level in dB	91.5	84.4	77.6	68.9	63.8	58.6	55.8	43.9	92.4

The A-weighting is calculated by adding or subtracting values defined for each frequency, as you can see in the table below:

Frequency [Hz]	63	125	250	500	1k	2k	4k	8k
A-weighting	-62.2	-16.1	-8.6	-3.2	0	+1.2	+1.0	-1.1

If we now take the above linear spectrum and add or subtract the A-weighted values for each frequency band, the result is the spectrum in dB(A). The total is calculated by adding all the values of the frequency spectrum together using a logarithmic scale.

Frequency [Hz]	63	125	250	500	1k	2k	4k	8k	Total
Sound power level in dB(A)	65.3	68.3	69.0	65.7	63.8	59.8	56.8	42.8	74.1

The Noise Rating is somewhat different. For each NR value, a curve is defined over the frequency spectrum with values in dB (not dB(A)!). This family of curves is shown in diagrams. The values in dB are now plotted on this diagram for each octave band frequency, and the value that reaches the highest NR curve determines the NR value for this sound.

In this case, with the above spectrum in dB, the resulting value is NR 72.

So as you see, a sound can be represented in figures in different ways. The values differ from one another greatly, and so it is vital to clearly state which unit you are using.

About the author

Benjamin Petschke was born in 1969 in Germany. After studying physics he joined STULZ in 1996 and worked since then in the R&D, Export and Marketing department on different positions. With 19+ years' experience in the Data Center cooling industry, Mr Petschke is specialised in Data Center cooling design, energy saving and acoustic issues.

He works closely with the Joint Research Centre of the European Commission for the Code of Conduct on Data Centres on the Best Practice section and recently with the German DKE in development of the DIN EN 50600, Information technology – Data Centre facilities and infrastructures.

Mr Petschke authored White Papers on subjects like Best Practice for Data Centre Cooling and Indirect Free Cooling with Dynamic Control Logic.