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The Fundamentals
of Signal Processing - Part 3
The loudness of a sound, specifically the sound pressure level or audio level, is stated in decibels. A decibel
can be defined as the smallest change in an audio level that the human ear can detect. If the audio level is increased by 10dB, the sound appears
to be twice as loud. For a 10dB increase in sound, it takes ten times as much energy.
The decibel is not additive, it is based on a logarithmic scale. To increase the sound level by 3dB, which is only a slight increase,
you need twice as much energy to achieve that. This is why a 50 watt amp sounds only a little louder than a 25 watt amp. Basically, it takes ten times
the wattage to double the perceived loudness of an amp.
| Sound Level Comparison Chart |
| Instruments |
db (at 1 meter) |
Other Sounds |
| . |
0 |
Silence |
| . |
10-15 |
whispering |
| . |
20 |
Leaves in the wind |
| . |
40 |
Waves at the shoreline |
| . |
60 |
Normal conversation |
| . |
70 |
Shouting |
| Acoustic guitar |
80 |
Vacuum cleaner |
| Cello |
83 |
. |
| Upright bass |
86 |
. |
| Flute |
90 |
Loud stereo |
| Trumpet |
95 |
. |
| Drum set |
102 |
. |
| . |
110 |
Electric drill, car horn |
| Rock band |
110-120 |
. |
| . |
120 |
Jet taking off at 230 ft |
| . |
135 |
Jet taking off at 65 ft |
When speakers reproduce a sound, they convert only a fraction of the energy used into sound. Most speakers are terribly inefficient - about 95% of that energy is wasted as heat and the rest is converted to sound. So the more sound you get from a speaker, the more efficient it is. Speaker efficiency is usually measured in decibels. These values typically range between 90-100dB
1 watt/1 meter, using 1 watt and measured at 1 meter. A speaker's maximum sound pressure level, commonly referred to as "SPL", is also measured in decibels. Speakers with SPL ratings over 100dB are more efficient and are actually a little louder as well.
Besides sound pressure levels, the decibel is also used to measure other things, such as signal-to-noise ratio as well as the line input and line output levels of amps and other devices.
Line levels have been standardized into two categories, -10dB and +4dB. The most common one is -10dB, which is what keyboards, synths, CD players and similar equipment are designed around. Professional sound equipment often uses the +4dB standard. Line levels are also referred to as dBm, dBv and dBu. You generally want to match input line level of a piece of equipment to the output line level of the device feeding it. Doing so will prevent distortion due to lack of headroom.
Signal-to-noise ratio. This is a measure of the "noise floor" of a piece of equipment. Any amplifier will produce a certain amount of self-noise with no input. If you turn up the volume and listen, you will be able to hear a certain amount of hiss. An amplifier with a signal-to-noise ratio of -90dB that can produce a 100dB of sound will have 10dB of noise when you cut the signal (100-90=10). An instrument amp with a -90dB signal-to-noise ratio is a very quiet amp.
The next page in this series will be taking a brief look at harmonic distortion.
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