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-30db means 30 db less, but less than what? |

You have already almost nailed it. It is a ratio, or a means of expressing one things compared to another. The decibel scale is a mathematical means to express large ratios. A negative value means that the quantity is less than some reference. A postive value means higher.

The reference used in your receiver is probably just attenuation from maximum output. That is not to say maximum usable output... the amps will probably reach their limits (clip) long before hitting 0 dB.

When used in audio, dB is a ratio of

**power**. It is calculated as such:

*10 * log (Power2/Power1) = Power ratio expressed in units of dB*. Note that the term (Power2/Power1) is just a power ratio expressed in absolute terms. For example, if Power2=100 Watts and Power1=10 Watts, then (Power2/Power1), or the Power ratio, is 10. (Power2 is 10 times Power1). The value in decibels (dB) is 10*log(10)=

**10**. Don't be confused that it is also 10. What if Power1=1 Watt? Then the ratio is

**100**. Calculate it in dB and it is

**20 dB**. Here is where it can be confusing. Why is it 20 dB? Because

*each* 10 dB

*difference* is

*10 times*. So, 20 dB is two 10 dB differences, or 10 X 10 = 100 times. Likewise, a ratio of 1000 times is 30 dB. A ratio of 2 is approximately 3 dB. A ratio of 1/1000 is -30 dB.

Once you know that 10 dB means 10 times the power (or 10 dB

*less* means 1/10 the power), you will realise just how little power it takes to make loud sound. If your amp clips at -10 dB on the volume readout, and you normally listen to music at -30 dB, then you are listening at (-30dB - -10dB =) -20dB, or 1/100th of the power capability of your amp. If your amp is rated at 100 Watts, you are listening to your music with only 1 Watt (peak) power going to your speakers.

In audio, there is a term "SPL", or "Sound Pressure Level", which is measured in dB. SPL is the difference from the absolute quietest sound that can be heard by average, young, healthy human ears. That "quietest sound" is then by definition, said to have a 0 dB SPL. Any other sound will have some positive SPL in dB. A whisper is about 40 dB; a conversation about 60 dB, loud music about 80 to 90 dB. Prolonged exposure above 90 dB can cause hearing damage; above 120 dB causes immediate pain. The avaerage bass-head's booming car stereo puts out well over 120 dB, often over 130 dB (over 10 times the pain threshold).

Speaker sensitivity ratings are in dB SPL, and are usually the SPL measured when driving the speaker with a 2.83 Volt signal, at one meter (100 cm or 39.4 inches) away. (2.83 volts because that voltage gives one Watt into a resistance of 8 Ohms). If you are comparing two speaker systems, the sensitivity is important. If speaker1 has a sensitivity of 89 dB and speaker2 has a sensitivity of 92 dB, then you will need to double the power into speaker1 (+3 dB) to make it output as much sound as speaker2. So if speaker2 needs 80 W to reach a certain loudness, then speaker1 needs 160 W to reach the same loudness.

I hope that I haven't confused you too much. Understand the dB can be a great help when choosing speakers, amplifiers, and when setting up and EQ'ing your system.