# Estimating Speaker Power Rating

Discussion in 'Electronic Basics' started by Blake, Dec 14, 2005.

1. ### BlakeGuest

I've been wanting to build a guitar amp for some time now. When I found an
amp case with 12" speaker at the swap meet going for a give-away price, I
snapped it up.

The first thing I need to do is to know the speaker power rating, so I can
design the amp to make the most of it, without blowing the speaker. The
speaker's marked "Line 6 t5263 80km 50", but since I haven't been able to
turn up any data on the speaker I'm thinking about ways to estimate it.

My idea is to pass a constant DC current through the voice coil and measure
the voltage rise as the copper wire heats up. Based on a tempco of 3.9E-3
for copper, I would expect to see the resistance rise from 16 ohms at 25C to
about 19 ohms at 75C. If I take 75C to be the maximum safe temperature for a
voice coil (pulled that number out of my ear), then whatever power I need to
apply to achieve that resistance increase should be my power rating.

Ok, that sounds ok in theory. But we all know that theory (especially
interpreted by those with a only little bit of knowledge) can be very
Is there a better way?

2. ### Pooh BearGuest

You're going to have to forget it.

DC measurements are useless since the voice coil uses its movement under actual
working conditions to help air cool it.

I've come across 12 inch speakers with as diverse power rating ( continuous ) as
15 Watts at the low end to 400 Watts at the very high end.

The voice coil diameter, materials and adhesives all play alarge part in
determining the power rating. Some speakers can actually work with the voice
coil hundreds of degrees C over ambient.

Graham

3. ### DeefooGuest

Doesn't that read "Line 6 t5263 8Ohm 50" ?

--DF

4. ### tlbsGuest

You can register at http://www.line6.com/support and ask the tech
folks there, for an accurate answer.

My educated guess is the speaker is rated for 50 W RMS, based on the
"50" at the end of the letter/number sequence you listed. That is a
"normal" power rating for a 12" guitar-amp speaker.

5. ### BobGGuest

I've written a couple programs that graph rms and avg of music, and the
rms always seems to track the avg about 3dB hotter. So a nice S L O W
avg would track voice coil heat pretty close.. just assume the rms
voltage is 3dB hotter than what you are measuring with the avgerager.