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Old 21st March 2007, 09:48 PM   #1
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Smile How much amplifier power do I need

Cheers,

I'm embarking on a new DIY project: DIY Tri/Bi-amp class A solid state amplifier and O/B or horn speakers. It is safe to assume my speakers will have sensitivity of 95dB at 1watt/1meter (maybe +6dB higher if I'll choose front horns but this is less likely to happen at first stage due to complexity).

I'm trying to figure out how much power my amp need to have.

Here is my calculation, is it correct?

Listening position is 5 meters from speakers.
From what i read proper (not ear damaging) music volume should be 80dB with +25dB headroom for transients (Crown site).
so:
at the listening position 80+25=105dB
add 14dB drop at 5 meters from speakers.
so the SPL at the speaker should be about 120dB (thats an enormous figure and undoubtedly will cause hearing damage!)

each +3db at SPL means double electric power.
so 120dB-95dB=25dB which is ~ 8 times 3db.
therefore amp power is 2^8 = 256watt. this figure looks very wrong. Could you point where is my mistake?

Thanks
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Old 21st March 2007, 11:17 PM   #2
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Quote:
add 14dB drop at 5 meters from speakers.
This would be true for omni-directional source and open space.
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Old 22nd March 2007, 12:20 AM   #3
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I rather doubt that 25 dB headroom (sorry Crown). The 85 dB is already loud. Much depends on where you sit 5m away from the loudpeakers: In a room not much bigger or on a sports field? (over-the-top example but you will get the point). For me 3 dB is not much of a difference in loudness, but it is a factor 2 in power. I would then say that practicality/cost also comes into the equation. 256W or 128W? There could be a design convenience trade-off here.
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Old 22nd March 2007, 08:53 AM   #4
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Quote:
Originally posted by darkfenriz

This would be true for omni-directional source and open space.
Thank you for your reply. What is your estimate for SPL drop over distance in a closed 6X6 room? Do you have a link to an interesting reference?


Quote:
Originally posted by Johan Potgieter
I rather doubt that 25 dB headroom (sorry Crown). The 85 dB is already loud. Much depends on where you sit 5m away from the loudpeakers: In a room not much bigger or on a sports field? (over-the-top example but you will get the point). For me 3 dB is not much of a difference in loudness, but it is a factor 2 in power. I would then say that practicality/cost also comes into the equation. 256W or 128W? There could be a design convenience trade-off here.
Thanks for your answer. You are correct (+3dB in SPL does not mean double hearing volume, I _think_ that +10dB is) , it seems that with few hand waves I could drastically reduce this figure:
use 15dB headroom (which is recommended by crown as more down to earth figure - but I suspect them because there are more into PA than HiFi), assume that SPL loss over distance will be 10dB and not 14dB, so the figure is down from 120dB to 105dB at the listening position, which calls for about 2^3=8watt amplifier. Finally, a Class A domain figure.

I'm just not sure How much I'm forcing the numbers to my desired result. Thats why I'm looking for a reference calculation. Do you a Link, by chance?

Again, Thank you.

Cheers.
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