από site γνωστής Εγγλέζικης εταιρίας
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Loudspeaker sensitivity is a measure of how much sound a loudspeaker will give for 1 watt at 1 metre. It is critical to note that the basic measure of sensitivity is at 1 metre and not at a typical listening distance of about 10 feet or 3½ metres.
Sound attenuates (reduces) over distance at the rate of 6dB with each doubling of the distance. At 2 metres distance from the loudspeaker its perceived sensitivity is reduced by 6dB. At a normal listening distance of about 10 or 11 feet from the loudspeaker its perceived sensitivity will be reduced by approximately 10dB.
This is the factual basis for our claims about how much power a system would need for a decent hi-fi dynamic range. We reiterate that this is not made up or marketing hype, it is scientific fact.
Loudspeaker sensitivity – is it real?
This is the source of much misunderstanding. Amplifier power is specified in watts, which are a measure of heating power. They have no apparent relationship to what we hear, as they are a linear measure. Loudspeakers (and our ears) perceive things in dB (decibel) steps. These are based on a logarithmic relationship.
This is the fundamental mismatch between what your ears perceive and how amplifiers are specified. The solution to the problem is to recalibrate watts into dB steps. The results are below, in a chart of watts converted to dB steps. For convenience we have started our chart at 50 watts. These figures are not made up they are fact.
dBW Watts dBW Watts
17 50 24 251
18 63 25 316
19 79 26 400
20 100 27 500
21 126 28 630
22 156 29 795
23 200 30 1000
As you can see, as soon as you calibrate amplifier power in dB watts, you get a dramatically different view of what amplifier power really means.
First off, you can see that what looks like a large increase in amplifier power, for example from 50 watts to 100 watts, only gives an increase of 3dB.
You can see why amplifier manufacturers want to sweep these figures under the nearest carpet; they make most of their claims look ridiculous as they predict that most loudspeaker/amplifier combinations will have only limited dynamic range.
How much dynamic range do I need?
Some years ago John Atkinson (current editor of the Stereophile) made some measurements of live music using accurate equipment. He recorded 109dB peaks (brass and percussion) and the quietest was 63dB (solo violin) a variation of 46dB from the quietest to the loudest moments – a huge dynamic range.
The question is what sort of dynamic range a really good hi-fi system should have.
In our opinion, an ability to produce unclipped peaks of 105dB is the minimum starting point for a really good hi-fi system. You are welcome to debate different figures, but that is our basic position.
1. Take your loudspeaker sensitivity (better yet check back to a technical review to find out what its sensitivity really is). Deduct around 10dB for the SPL (sound pressure level) attenuation over distance. And then add back 3dB because there are two loudspeakers in the room.
Now you have arrived at the practical, real world, in-room sensitivity of your loudspeaker system.
1. Decide what peak level you want to achieve. We think 105dB is about right. Some people think 110dB is more appropriate. It’s up to you.
1. Deduct the result of 1 above from your decision about 2. This is how much amplifier power you require in dB watts.
1. Use the chart above to translate your dB watts result in to ordinary watts.
General overview.
People have objected to our position claiming that their system sounds great. It might. However, you can’t escape the fact that, if they have a low powered amplifier and relatively insensitive loudspeakers, the system must be clipping, distorting and limiting regularly, which must dominate the listening experience. Maybe the listeners like distortion clipping. Well, each to his own and good luck to them. If you want your hi-fi system to produce as close an approximation to the real live performance as you can get, then you must ensure, for a start, that your amplifier is not clipping.
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- Αν πάρουμε τον φίλο Τζιμίσκε(απ' τις χειρότερες των περιπτώσεων λόγω των 84 db των atc) για να πάρει 105 db στα 10 feet δηλ. στα 3 μέτρα έχουμε και λέμε...
84(atc db) - 10(db πτώση λόγω απόστασης) + 3(db διπλασιασμός λόγω 2 ηχείων) = 77 db
105 - 77 = 28 dbw....δηλαδή θέλει 630 watt ενισχυτή...
