The decibel (dB) is a unit that has various and sundry uses, though almost all of them have to do with the power of a given signal.  To be short and ambiguous, the decibel is 10*log(ratio of two powers), and is 1/10 of the bel, a unit that is used about as often as grandma’s roller skates.  But though there are many different types of decibels (seriously, see for yourself), I just want to talk about one: the absolute sound pressure level, or how loud something is to our ears. This decibel is referred to dB SPL.

Fortunately, for this dB, there are absolutes. 0 dB SPL is the threshold of human hearing. If you were in a space that was completely silent, a sound that was 0dB SPL would be perceived by you half the time (in theory anyways, I have no idea how they actually test this since most of us have less than perfect hearing).  To give a reference you can really sink your teeth into, normal ambient noise is usually between 30-50 dB SPL. That’s stuff like air conditioners, lights buzzing, wind blowing, that kind of thing.  If there’s more ambient noise, it will probably be a little higher, but not much.

Since decibels are logarithmic, and our ears perceive loudness in a logarithmic way also, an increase of 10dB is perceived by our ears to be about twice as loud, but really is an increase 10-fold in actual “sound,” or power.  70dB is twice as loud as 60dB, which is twice as loud as 50dB, you get the idea, and if you wanted some speakers to be twice as loud, you’d need 10 times the watts, or power.  A change of 3dB is noticeable, like a click or two of the volume on the tv, but represents a doubling or halving of the “sound” or power required.  So the volume difference between a 600W amp and a 1200W amp? noticeable, but nothing that huge. It’s the same for a 300W amp and a 600W amp. A change of 1dB is difficult to detect, but trained ears can definitely detect it.

So, now for something interesting and applicable (these are from wikipedia’s table here):
0dB: threshold of hearing
10dB: calm breathing
20dB: really calm room
30dB: calm room
40dB: normal conversation
50dB: washing machine, dishwasher
60dB: TV (because we all watch tv at exactly 60dB)
70dB: maximum to prevent hearing loss or other adverse reactions like stress, sleep disturbance. Also, a normal car from 10 meters.
80dB: traffic on busy road
85dB: hearing loss possible over long time. funny enough, this is the level at which our ears have the flattest frequency response, and ideally the level you should mix audio at. could be a problem…
90dB: even louder traffic, or some really loud music at home
100dB: jack hammer at 1 meter
110dB: rock concert if you’re not standing next to the PA, also, vuvuzela at 1 meter, because we all know exactly how loud that is
120dB: hearing loss possible, especially if polonged. rock concert if you stand right in front of the PA
130dB: THRESHOLD OF PAIN. the most metal of the dB’s
170dB: Rifle from 1 meter. if you’ve ever shot guns w/o ear protection, you’ve experienced the “threshold of pain.” If you’ve done it so much that it doesn’t hurt anymore, save for some hearing aids when you get older.