Decibels and Power
From Zanecorpwiki
Decibels are how loud something is. 0dB is the softest sound most people can hear. The scale is logarithmic, so negative decibels are possible, and just mean ever softer sounds.[1] 0dB is the sound of rustling leaves, and people with very good hearing can perceive sound to -15dB.[2] 1dB is defined as the just noticeable difference under normal circumstances[1], though given a pure tone, the average person can detect a 0.3dB change.[3]
Decibels are the pressure exerted by sound waves as they move through the air[1], measured as the the deformation of a diaphragm. High decibel sounds can generate enough air pressure to tear ear drums; anything above 115dB, for instance can cause instant long term damage.
This air pressure is dependent on how close you are to the source of the sound. As the front of the sound wave expands, it's expanding the same amount of energy across a wider area. Thus, the energy present at any given point is going to be less. There is also some energy lost due to friction of the air molecules. Rated decibels are measured at 1m directly in front of the sound source.
Decibels are also related to frequency. Remember, it's all about human hearing, and human ears are responsive to a certain range. Some ranges more than others.[1] This is why a decent stereo system might have 100W full range speakers to produce all the sound in the human sweet spot, but a 200W sub-woofer to produce just the low base, which will still sound softer than the sound coming out of the main speakers despite the fact that the sound wave has a lot more power. The microphones used to measure decibels are usually fitted with filters that mimic the human ear response.
If you've ever heard the horribly distorted bass coming out of a candy painted low rider, you've had first hand experience with this frequency response. To get that super-loud base sound, they're pumping in 1,000's of watt's of power. 10,000+ is not uncommon. If you were close to a 10,000W speaker putting out midrange sound, it would be enough to cause permanent deafness. The reason that the same power coming out of a sub-woofer merely sounds like shit rather than being dangerous is that you're ear doesn't respond as well to low such frequencies. Physically, that means the membrane doesn't vibrate in response to the sound wave as much.
Sounds beyond the human range of hearing have no sound no matter how much power you put into the sound wave because the membrane in the human ear simply doesn't respond to those frequencies. The sound is invisible to our ears. Having said this, decibels are also used to measure things like the power output of radio towers and what not. While there are some of the same ideas and all decibel scales are logarithmic, that's obviously a different kind of decibel. The audio decibel is a determination of sound wave pressure vibrating at certain frequencies as it interacts with the human ear.
Some basic ranges are[4]:
- 0 db is the threshold of hearing
- Whisper: 15-25 dB
- Background noise: about 35 dB
- Normal home or office background: 40-60 dB
- Normal speaking voice: 65-70 dB
- Orchestral climax: 105 dB
- Live Rock music: 120 dB+ (enough to cause hearing damage, BTW)
- Pain Threshold: 130 dB
- Jet aircraft: 140-180 dB
In general, it takes about 10 times the power to produce twice the decibels. So, to double the volume coming out of your 100W speakers you need 1000W speakers (and amp). This also means that the 1dB change at 100dB-101dB represents a lot more power than the 1dB change from 0dB-1dB.[4] This is why loud sounds become so damaging so fast.
