Dee-bee, eh?

Measuring noise, part 3

Congratulations if you have slogged through and understood my two blogs of 2 Jun and 6 Jun; you are now in a position to comprehend the units used to measure sound levels. There is an assortment of such units, but I will concentrate on the ones that you will find in the documents that have been published for the public consultation on HS2.

The first unit that I will explain is the dBA (pronounced as in the title of this blog), which is a basic unit for expressing instantaneous sound power. To makes things easier for me I will split the term into two parts “dB” and “A” and explain each part separately.

The “dB” part is our old friend 10log10(Pm/Pref) with Pref defined so that sound power can be expressed directly, rather than as a ratio. Pref is specified as the power of the quietest sound a human can hear, which produces a sound pressure of 20 micropascals or apparently roughly the sound of a mosquito flying three meters away. Of course this level was specified before the iPod was invented; today the quietest sound that can be heard by the iPod generation is probably equivalent to a hydrogen bomb going off three metres away.

You will remember from my blog of 6 Jun that Pref is the zero point of the dBA scale, since 10log10(Pref/Pref) is zero.

The “A” part of dBA is there because the human ear does not respond equally to all frequencies. The frequency of a sound is the rate at which the molecules in the air are being vibrated by that sound and is measured in hertz (symbol Hz), where one hertz is equal to one vibration cycle per second. When we are dealing with sound and the human ear the most commonly used unit is the kilohertz (symbol kHz), which is one thousand hertz. Generally, the human ear is much more sensitive to sounds in the frequency range from about 1 kHz to 4 kHz than it is to very low or high frequency sounds. To illustrate this range in musical terms, middle C on a piano has a frequency of about 260 Hz. Since an octave jump in music involves doubling the frequency of the note, 1 kHz is about two octaves above middle C and 4 kHz is two octaves above that.

The quirk in sensitivity of human hearing means that low frequency or high frequency sounds can contribute less to the nuisance value of noise than the sounds in the 1 kHz to 4 kHz band. This effect is reflected in the measurement by the application of weighting, which reduces the impact of some frequencies relative to others. “A-weighting”, which is the “A” in dBA, is the most commonly used weighting standard, but there are others such as B-weighting, C-weighting and D-weighting which specify different weighting factors for each frequency.

The variety of weighting standards available reflects the annoying fact that there is no “one size fits all” solution. The way that the human ear responds to various frequencies can be affected by a number of factors, such as the loudness of the sounds and the complexity of the noise waveforms. However, the A weighting standard tends to be the one that acoustical engineers use by habit, even if it is not always the best fit to the actual behaviour of the human ear in any given circumstances. Well we all have our faults!

So a measurement of sound expressed in dBA means that it is an absolute measurement on a scale where the zero point corresponds to a sound pressure of 20 micropascals and A-weighting has been applied.

In the next blog I will examine how we actually measure sounds using the dBA unit.

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2 responses to this post.

  1. Posted by Bob Day on April 3, 2012 at 3:16 pm

    You seem to be confusing dBa and dBA! Firstly the dB (decibel) is a dimensionless unit and cannot be used to measure or represent any absolute electrical power or sound level. As it is a (logarithmic) unit of ratio between one level to another (reference level) it can only be used properly if the reference level is clearly stated or implied.

    The dBa (lower case a) is generally accepted the unit used for sound level measurements where 0dBa is 20uP (20 micro pascal rms) which is considered to be approximately the onset of human hearing (human hearing varies a lot from person to person).

    The dBA (upper case A) is normally used to indicate that a weighting network has been used to take and/or modify measurements.

    I hope this helps you to sort out your rather confusing and incorrect explanation. Any incorrect use of the dB used in a legal challenge to HS2 could prove to be very expensive.

    Regards Bob Day

    Reply

    • Hi Bob and thanks for your comments. I have read the offending blog again very carefully and, although I hesitate to pick a fight with a representative of academia, I don’t think that I have got it wrong. There appears to be some grounds for your accusation that my blog was confusing, since I seem to have confused you, at least.

      Firstly, after a long working life using the decibel every day as an engineer in the telecommunications industry, I can assure you that I realise that the dB is a ratio and, therefore, dimensionless. Please look again at the third paragraph of my blog, where I have used Pref so that “sound power can be expressed directly”. I have not referred to dBa at this point in my blog, since this (seldom encountered in the real world) unit is not employed in any of the HS2 literature and would merely serve to confuse.

      As regarding your point about dBA and A-weighting, please look again at the fifth and sixth paragraphs, where I cover this.

      I think that where I have confused is to try and get directly from the previous blog, where I treated dB as a ratio (and said so), to using dBA to measure absolute sound levels (including weighting) in the space of one blog.

      Regarding your point about a legal challenge to HS2. I am not party to any of the applications that have been, or may be, made for a Judicial Review and my blogs are not included in any evidence that has been, or may be, presented in support of any such application.

      Reply

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