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  • Samantha Hathway

"It's Complicated" - 10 Things That Make Acoustics Confusing


Confusing Acoustics

Acoustics is one of those subjects that people just find it hard to "get".

Barely a week goes by without someone telling me that acoustics is a "black art", or that acoustics is like law - you need your own acoustic consultant to decipher another acoustic consultant's report; like you need opposing lawyers to write and interpret a legal contract.

As an acoustic consultant, I can obviously "get" acoustics (phew!) but I can also see why non-acousticians find it hard to understand.

The maths of it is quite complex, but it should be a subject that its easy to explain - after all, as a minimum you can use descriptive words and even your own verbal sound effects to describe what something sounds like!

Over the years though, the discipline has taken on more and more obscure elements that muddy the water. Lets look at the evidence:

1. It's Logarithmic - Imagine the familiar scene:

Human being: OK, so 60dB + 60dB = 120dB, yes?

Acoustic consultant: Nope, the answer is 63dB.

HB: Er, excuse me?

AC: Yes, its logarithmic.

HB: What about 60dB + 70dB?

AC: The answer is 70dB.

HB: Eh?

Is it any wonder that people struggle when it seems that even the rules of normal maths don't appear to apply?!

2. Various Uses for Decibels - dB's are obviously a descriptor of how loud something is, but they're also used to describe sound reduction i.e. how good a wall/floor/partition is at reducing sound - and that's also a positive value.

So, we have the noise level in a room, which is 45 dB. The partition between that room and next door is rated as 45 dB. The two things are different (usually differentiated by some seemingly meaningless subscripts - see item 5 below) but are often assumed to be the same thing!

3. NR levels and dBA's - Both are ways of describing how loud something is, but the same noise usually has an NR level and dBA level that are numerically different. Thanks again, acoustics!

4. Sound Level Differs with Frequency - OK, so you've got your head round the difference between sound level and sound reduction, then you've differentiated the NR level from the dBA, then someone mentions that sound actually has different levels at different frequencies!

For example, a sound that is dominant at the low frequencies will sound low, rumbly and 'bassy', whereas a high frequency sound will sound more like a whistle or a hiss.

HB: So you can't just look at a sound as a simple single number?

AC: Er, not really.

HB: <sigh>

5. Long Meaningless Parameters - In acoustics, we do love a subscript! Unfortunately, sound can be difficult to describe with numbers as it varies from person to person, and room to room, so we need to add corrections, which are normally shown as subscripts.

Check out these little gems, bearing in mind that everything after the 'D' is normally a subscript:

Drt,2m,nT

DnT(Tmf,max),w

Even the UK's most prominent acoustic document (Approved Document E) contains the notoriously catchy 'DnT,w + Ctr' - I can guarantee your attention span is shorter than the time it takes me to explain all those letters!

6. Sound Absorption vs Sound Insulation - Another familiar conversation:

HB: They said it was a high performance acoustic ceiling...

AC: High absorption or high sound reduction?

HB: Is there a difference?

AC: Well, if its high absorption, it'll reduce reverberation but may not be that good at sound reduction. Conversely, if its good at sound reduction, it'll probably be dense and acoustically reflective.

HB: It just said 'high performance' on the box.....

7. Diminishing Returns - Because sound is logarithmic, you get a steep initial improvement, but as you try and improve more, you get less of an effect. For example, if you double the amount of sound absorption in a room, you theoretically halve the level of reverberation. To halve the reverberation again, you need to double the sound absorption again. Infuriating for the quantity surveyor!

8. Its Subjective - What might sound fine to one person (e.g. the sound of children playing), might get on another's nerves. This can make the subject difficult to explain and also difficult to determine the potential impact of a new noise source.

9. It Varies from Site to Site - "But we did this on a virtually identical site just down the road" is a familiar quote. Sorry, whatever you did down there doesn't apply. This site has different features, different surroundings, different requirements - so we need to start from scratch!

10. Myths - Egg boxes, non-existent duck quack echoes, trees absorbing sound, "silence", "soundproof" - all still used in the press and by many manufacturers! As if the water need any more muddying!

Of course, here at auricl acoustic consulting our mission is to prevent acoustics being a black art, or even a grey area, and bring it into the open as a clear, comprehensible subject.

We do this using a combination of the latest monitoring and modelling technology, combined with committed, helpful staff.

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