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DB and sensitivity questions

Discussion in 'Home Cinema Speakers' started by Brad_Porter, Aug 12, 2002.

  1. Brad_Porter

    Brad_Porter
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    Dear all,

    I was reading my amp manual at the weekend and I noticed the specs at the back say that my amp is 105W at 8 Ohms and 210W at 4Ohms.

    Now, my speakers are 4Ohms. Does this mean that my amp, with 4Ohm speakers will be giving out the equivilent of 210W?? This doesnt feel right!!

    And to throw another spanner into the works, how does 'sensitivity' come into all this? Mines around 90Db.

    Confused.

    Regards - Brad
     
  2. GJC

    GJC
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    Brad_Porter

    Now, my speakers are 4Ohms. Does this mean that my amp, with 4Ohm speakers will be giving out the equivilent of 210W?? This doesnt feel right!! -

    E.G. - AMP is rated @ 50w for 8ohms.

    This means that any speaker which is 8 ohms will be fed 50 watts from the AMP. If it says 90db on the speaker then that is the Sound level it will produce for 1watt of amp power at 1 metre distance from the speaker (correct me if I am wrong please).

    E.G. - AMP is rated @100w for 4ohms

    But the speakers are 4 ohms - 90db, then the AMP will need to output 100watts of power to supply the speakers with the same level as the 50w AMP - 8ohm Speakers .equivalents.

    For every 3db of increase in volume you need to double the power of your amp. Or get speakers that are more sensitive. E.G.

    Simply put - 90db Speakers with a 50w amp will sound the same as a 87db speakers with a 100w AMP
    ----

    I am sure someone could explain this in more detail or clarify any of the above for you.

    ----

    GJC
     
  3. Brad_Porter

    Brad_Porter
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    So does that mean that my 105W amp will only output about 50W to my 4ohm speakers?

    Damn - that half my power gone if it is!
     
  4. snelly

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    Well,

    From the sensitivity and db point of view...

    The specs normally quote db/W/m therefore if the figure is, say, 90 db/w/m then at a 1m distance from the speaker a nominal signal amplified with 1W of power would create this db (SPL?) level.

    Bear in mind that db are logarithmic and that a 10 db increase equates to a doubling in apparent volume i.e. 100 db is twice as 'loud' as 90 db to our ears (at the same distance of course). To yield 10 db increase (double the volume) will require ten times (not double) the power to be delivered to the speakers. Thus (all other things being equal) a 100W amp will not play 'twice as loud' as a 50W amp rather you would need 500W.

    Additionally (**think I am correct on this but not too sure**) as the distance between the listener and the sound source (speaker) doubles the apparent volume halves. However this fact will also be seriously subject to the environment around the speakers too.

    Hope this is some help and not too far off the mark - if it is I am sure that the more learned members will soon let us know.
     
  5. GJC

    GJC
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    If your amp is 105 watts for 8ohm speakers then it will double to 210 for the 4ohm speaker. But be carefull as some amps will switch off if pushed to hard at these levels.

    ----

    GJC
     
  6. Brad_Porter

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    Cheers everyone - interesting stuff, especially the fact that to double the audiable output sound for a 50W amp you need a 500W amp! Thats unreal! So really, there isnt much difference between a 105W amp and a 150W amp. Well, its not twice as loud anyway?

    Cheers
     
  7. MikeK

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    Yet another area where the hifi industry just confuses what should be a relatively simple issue. :rolleyes:

    You get some quoting dB/W/m, some quoting dB/2.83V/m, and some quoting one when they really mean the other.

    At 8ohms, it's really not an issue - as they are both the same.
    However, few speakers are actually 8ohms, even if that's their nominal value.
    For 4ohm speakers, it "might" be an issue, as the two figures are then not the same, but then you never really know whether they have quoted one, but actually mean the other :).


    For the most part what they mean is that you will get XYdB at 1metre distance, when the signal level from the amp is 2.83V.
    It's an average anyway, because it also depends on the frequency of the signal - no speakers have a truly flat response.
    If they do actually mean 1W though, then the sensitivity figure for 4 and 8 ohm speakers won't really mean the same thing, and in reality, it wouldn't mean the same thing for many speakers with the same nominal impedance rating either.


    Personally I think the hifi industry should be banned from using the word "watt" :)
     
  8. snelly

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    Say what?

    How about kgm[sup]2[/sup]s[sup]-3[/sup]

    Grrrr :mad: cant do superscript. Is there no way we cam slip in HTML tags here? Or can we only use the vB Code ones (fair enough I suppose) is there a tag reference for vB Codes
     
  9. Reiner

    Reiner
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    I was reading my amp manual at the weekend and I noticed the specs at the back say that my amp is 105W at 8 Ohms and 210W at 4Ohms.

    In theory a an amp which delivers 100 Watt @ 8 Ohm should give e.g. 200 Watt @ 4 Ohm or 400 Watt at 2 Ohm.
    However most amps can't reach this due to limitations of the power supply and/or components in the power amp section.
    Thus it is common to find an amp with 100 Watt @ 8 Ohm but "only" e.g. 150 Watt @ 4 Ohm.
    Just check some specs from (budget) AV amps to see what I mean. ;)

    If your amp can deliver 108 Watt @ 4 Ohm and 210 Watt @ 8 Ohm then that's very good, given that it refers to a measurement over the entire frequency range and with an acceptable THD (<0.1%).
    Mind you, any amp can be quoted with 1000 Watt if you want but the THD would be >10% or whatever. Just look at some specs of 'Ghetto-Blasters' which offer this - driven by 6 AAA batteries. ROTFL. :D

    The lower the impedance the higher the power demand. Note that impedance is not a constant and changes with frequency, i.e. bass requires more power than highs as impedance drops lower in the first case.

    Now, my speakers are 4Ohms. Does this mean that my amp, with 4Ohm speakers will be giving out the equivilent of 210W?? This doesnt feel right!!

    Nope, this is the maximum the amp can deliver in connection with 4 Ohm speakers. It is not the permanent load (under normal listening conditions) which may be a few Watt only.


    And to throw another spanner into the works, how does 'sensitivity' come into all this? Mines around 90Db.

    Simply put Sensitivity means how easy the speaker is to drive or how loud it will go when fed a reference signal (as it has been explained already).
    90dB is a very good value and those speakers should not stress the amp too much even they have only 4 Ohm.
    If the sensitivity is low the amp has to work harder, increasing the load and thus the power output required.

    Now the next question: what has the "damping factor" to do with all this!? :devil:
     
  10. Brad_Porter

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    Thanks to all for the advice. Really apprecaited.
     

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