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Old 9th March 2012, 11:48 AM   #1
epilot is offline epilot  United States
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Default Power and SPL

Hi Guys,

Can you guys tell me please how to calculte the SPL of an speaker while having the acoustical/electrical power of the connected amplifier? For instance is it easy to have an approach/relationship of the ouput SPL by knowing the power of an amplifier, for instance we know the power of the LM3886 is 68W at 4ohms when connected to a supply of say -+28V, So can we make a guess or calculte the ouput SPL?

Thanks a lot
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Old 9th March 2012, 12:25 PM   #2
sreten is offline sreten  United Kingdom
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Hi,

Your amplifier outputs around 70W at its clipping point so maximum
music programme into loudspeakers will be around 10 watts average
with music programme that has low dynamic range. (For high quality
uncompressed programme the average power can be much lower.)

With a pair of 4 ohm speakers you look at its "efficiency" dB/W/1m.

Say that is a typical 87dB/W/1m (90dB/2.83V/1m for a 4ohm speaker.)

10W is +10dB, and you have two channels / speakers + 6dB,
but you don't listen at 1m, room size becomes important.

In a typical room about 97dB maximum average SPL is about right.
Double the room size take away 3dB, double amplifier power add
3dB. If the speakers are 84dB/W/1m take away 3dB.

rgds, sreten.
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Last edited by sreten; 9th March 2012 at 12:30 PM.
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Old 9th March 2012, 12:33 PM   #3
AndrewT is offline AndrewT  Scotland
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If you go back and read the HiFi News reviews of amplifiers you will see thay always quoted maximum power in Watts.

But then showed a table of max Power at various frequencies & test loads in dBW

A 100W amp would show as +20dBW. But actual figures in the table could be +21.1dBW @ 100Hz, +22.0dBW @ 1kHz and +21.7dBW @ 10kHz. all into 8r0.
Then they would show the dBW values into 4r0. and 2r0 if appropriate. These latter figures really did show up amplifiers that could not deliver adequate current

Simply add the yydBW figure to the sensitivity value of the speaker which is usually specified as xxdB/2.83V @ 1m
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Last edited by AndrewT; 9th March 2012 at 12:36 PM.
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Old 9th March 2012, 12:33 PM   #4
Boscoe is offline Boscoe  United Kingdom
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A crude way is in a scientific calc do '10log(base10)(power into speaker)' then add that figure to the efficiency of the driver/speaker. This obviously doesnt take excursion room size or anything like that into account.
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Old 10th March 2012, 03:22 PM   #5
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It is worth noting that speaker sensitivity is given at 1 m in anechoic surroundings. Practical listening rooms usually aren't terribly anechoic - as a rule of thumb, 1 m anechoic corresponds to the typical 2..3 m listening distances there, which is quite handy.

When specifying minimum recommended amplifier power, loudspeaker manufacturers tend to shoot for about 100 dB SPL. That's a fairly sensible value.

Sadly, sensitivity specs on loudspeakers as given by manufacturers are not always reliable - sometimes it seems they are just pulling them out of thin air. If you see something with a 7" / 17 cm woofer that claims 91 dB / 1 W / 1 m, better look for some actual measurements. (Even for a 4 ohm affair and at 2.83 Vrms, this seems quite high.)
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