Hiss is common to all electronic devices. Even something as simple as a resistor or a piece of wire has an effect on the signal travelling through it. Any amount of modification to a signal can be considered distortion or noise. The level at which this noise sits is what will determine whether or not it becomes a problem, and if it has to be dealt with.

Let's look at the electronic component noise specification: S/N ratio (Signal to Noise ratio). This is a number, measured in decibels which compares the noise inherent to a component or product to that of the signal that is passed through the unit. Let's look at something like an good quality equalizer versus a much lesser quality unit. Lets compare the Zapco EQ30-SL with the Clarion EQ-74. Both units are graphic equalizers. The Zapco unit has a S/N ratio listed at >110 dB for which we will refer to it as 110 dB, and the Clarion unit is listed at 100 dB.

If you are not familiar with the Decibel scale, it was created and put into use by Alexander Graham Bell to help describe the large ratios of sounds with respect to their intensity. a Decibel, which is one tenth of a Bel is considered to be the smallest perceivable change in sound at the human ears most sensitive frequency. A change sound levels of 3dB or more is audible at almost all frequencies. Lastly, a change of sound pressure by 10dB is considered by humans to be 'twice as loud'

The decibel scale scale can also be used to describe ratios of large numbers. Something that is 3 db more than something else is actually twice that of the original in level. So, a voltage of 2 volts is 3 dB more than that of one volt. A voltage of 2000 volts, is also 3dB more than 1000 volts. This is a logarithmic scale that deals with ratios. Perhaps a small chart will help.

Relative Decibels (dB) | Voltage |

0 dB | 1V |

3 dB | 2V |

10 dB | 10V |

20 dB | 100V |

30 dB | 1000V |

-3 dB | 0.5V |

-10dB | 0.1V |

-20dB | 0.01V |

-30dB | 0.001V |

A unit with a signal to noise ratio of 110dB is one that will invoke a noise component into the system that is 110dB quieter or smaller than the signal itself. Between the two units we have a difference of 10dB, this equates to a difference of 10 times.

To clarify, the Zapco unit has a noise level which is 110dB or 1x10^11 times smaller than that of the signal. If the max signal is 8 volts, then the maximum noise voltage is 1 Quadrillionth of that. Sure this is small, but it adds up quickly. A head unit, equalizer, crossover and amplifier all have noise, and it adds as it goes down the chain. It also gets amplified, so it can be heard.