dB vs dBA: what the "A" means

What a decibel measures

Sound is a rapid rise and fall in air pressure. A sound level meter turns those changes into a number of decibels. The scale is logarithmic, which matches the way our ears respond: 3 dB more means twice the sound energy, and 10 dB more sounds about twice as loud. What 65 dB sounds like puts the scale next to everyday sounds.

A plain decibel reading treats every pitch alike, from a deep rumble to a high whine. Our ears do not.

What the "A" means

The human ear is most sensitive to middle pitches, roughly between 400 and 4,000 hertz, and much less sensitive to very low and very high ones. A meter picks up a far wider range. So meters apply a frequency weighting: a filter that turns some pitches down before the level is worked out.

The A-weighting is the one used to judge the effect of noise on people. On the CAA's chart of the standard weightings, it turns a sound at 100 Hz, a low hum, down by about 20 dB, and one at 50 Hz by about 30 dB, while leaving the middle range almost untouched. A reading with no weighting at all is called Z-weighted.

For a plane, this matters most in the low rumble. The CAA's guide shows the noise of a Boeing 787 taking off, measured 6 km from the start of its take-off: at low pitches the A-weighted curve sits well below the unweighted one, and from about 500 Hz upwards the two almost meet.

Why aircraft noise is measured in dBA

Because the question is how the noise affects people, and A-weighting is the closest simple match to human hearing. The FAA has adopted the A-weighted level as its accepted measure of aircraft noise, and the CAA applies A-weighting to every measurement used to assess the impact on people. Europe's Lden and Lnight maps are A-weighted too.

That is also why official figures often just say "dB". The CAA's guidance treats it as best practice to put the A in the name of the measure and give the unit as plain decibels: "54 dB LAeq,16h" is already A-weighted. "dBA", "dB(A)" and "dB" next to a measure starting LA all mean the same thing.

C-weighting, in brief

C-weighting is much flatter. It trims only the very lowest and highest pitches, so it keeps most of the rumble that A-weighting turns down. One common use is for sudden, sharp peaks: UK workplace rules set daily noise exposure in A-weighted decibels (action from 80 dB) but peaks in C-weighted ones (action from 135 dB).

Low-frequency sound is what you hear from a distant jet as a rumble, and what can make loose windows vibrate when a take-off passes close by. The EU's noise directive lists strong low-frequency content as one case where extra measures may be worth using alongside its A-weighted maps. Aircraft noise maps and monitor reports still use A-weighted levels.

LAmax, LAeq and SEL in plain words

Most aircraft noise terms are built the same way: L for level, A for the weighting, then what is being measured.

LAmax is what most people mean by "how loud was that plane". For aircraft it is measured with the meter's slow setting, the standard in aviation, so it is sometimes written LASmax.

SEL, the sound exposure level, takes account of how long the noise lasts as well as how loud it gets. If two planes peak at the same level but one lasts twice as long, its SEL is 3 dB higher. For aircraft, SEL is typically about 10 dB above the LAmax of the same flight.

LAeq adds up the SELs of every flight and spreads them over a period, such as 07:00 to 23:00 for the UK's LAeq,16h. Because the average is taken on the energy, loud flights count for more than quiet ones, and doubling the number of flights adds only 3 dB. Lden and DNL are LAeq over a whole year with extra weight for evening or night flights, and Lnight is the night on its own; Lden, DNL and Lnight explained covers them.

You may also see EPNdB, the effective perceived noise level. It is used to certify aircraft, adjusts for whining tones and duration, and cannot be measured directly with a meter; the CAA puts it about 2 to 6 dB above the SEL.

Reading a figure

A peak and an average cannot be compared directly. The Australian government's noise assessment for Western Sydney's new airport gives an example: a sequence of flights whose loudest peaks at 75 dB LAmax works out at 55 dB LAeq over the period. Both numbers are correct; one describes the loudest moment, the other the energy spread over time.

So when you see a noise figure, check which measure it is before comparing it with anything. A plane "at 70 dB" is a peak; a house "at 57 dB" on an official map is an average, and behind it can be many planes a day far louder than 57 dB. That is why we count the loud planes rather than quote one average.