I was on one job where I needed to re-design a silencer for a cylinder of air with rapid opening valve at 28 bar pressure. 10" diameter and 5 in stroke. opening to atmosphere... Through a multi-chambered expansion box gave 127dBA at (If I remember correctly?) 100m range... Akin to a small cannon? But it was above the Regulation, so I designed a silencer system to change the "note" Frequency range) of the silencer, which changed the attenuation with distance (across fields) and we passed the test.
But I could only test this "in the factory" (it was almost daily practice to test a device anyway!).
At 1 m range I reduced the 135dBA peak with the old system to below 125dBA. Actually, stood at more than 10m range, with ear plugs, ear defenders and pinching nose, closed mouth and eyes, you could not detect any reduction of volume....
But when operated on-site in the middle of the country, it sounded more like a rifle shot at 1/4 mile, (Nearest road/public access) than the deep boom of the old design. (The old design was satisfactory at 21Bar operation pressure, but my equipment needed the 28Bar NWP = a bigger "Boom"!). At 1 mile it was barely detectable, compared to the old equipment sounding like a distant Shotgun report.
That illustrates to me how we perceive sound differently from the sound meter. Most sound meters measure a metered sound pressure level (dB) metering), then have an algorithm to convert to how we perceive sound with our "variable" meters - the ears (dBA conversion). We do not perceive a linear scale across frequencies. So it is hard to compare and estimate sound levels for even the best experts! But you can think of:
* 1 person talking = 70dBA,
* 2 people talking = 73dBA,
* 10 people all talking in a bar = 80dBA,
* 100 people in a works canteen, talking not eating = 90dBA (above permitted sound levels?),
* 1000 people shouting at a demo = 100dBA,
* 10,000 people cheering at a sports match 110dBA and
* 100,000 people shouting at the cup winning goal = 120dBA... or a racing motorcycle exhaust when at full throttle away from a standing start? (One of my noisy experiences, the guy at the back waving the green flag to the starter!)
* Formula 1 cars screaming off the start line = 130dBA (This can permanently damage ears in seconds), (I guess that speakers at pop concerts produce this level of noise to project to the dancers at the back of the crowd?).
* a fighter jet taking off using after burners 135~140dBA? or small cannon, if you get that close? - One is continuous, the other a single peak. 140dBA is double the "number" of 70dBA, but a million times more "sound power " - RMS power rating of the "noise" spectrum - or energy of a peak pulse. It depends on the recording device as to what it is measuring I think?).
The brain is off-the-scale beyond that!
* But NASA & co. make bigger noises at over 150dBA at source... when rockets lift off!
- Or at least that is my guess of the scale?
Terry, can you post your dBA scale to correct my ideas?