Solid State Amplifiers > Amplifier Discussion

Watts vs Volume (db)

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Everyone seems to think that they need more watts to get more volume.  While it is true that more watts will be louder than less given the same speaker, it is also true that the speaker is more important than the watts in most cases.

For every 10db gain (double perceived volume) you need a 10x multiplication of wattage.  So a 100watt amp is only twice as loud as a 10watt amp.  The reason this doesn't normally seem so is that 10watt amps normally have poor spl rated speakers, making the 100watt amp sound more than double the volume.  Playing through a better speaker will help increase your db rating, and hence your overall volume.

I've attached a chart to give you an idea of how an efficient speaker is louder than more watts on a less efficient speaker.

With this chart you can see that a 90db speaker on 100watts is less volume than a 102db speaker cab with only 10watts.  Most inexpensive amps come with speakers of about 90-93db.

To keep up with an average drummer you will want about 110db.  To reach this level you could use a 100watt amp with a single 90db speaker or you could run a small 10watt amplifier through a cab with rated 100db or better.

The calculation for total db is this:
{log (watts of amplifier) x 10} + speaker spl rating in db at 1w = total db output

Or if you want to figure out what db speaker (or cab) you need to hook into to get a certain desired db output:
Desired db output - {log (watts of amplifier) x 10} = Necessary speaker/cab db rating at 1w

Here is an excellent calculator for adding up speaker db and finding what the total output is:

And thanks to Roly, another good table:

Speaker sens. (1dB/W@1m) Required power (watts)104db1101db298db395db692db1289db2486db4883db9680db19177db382Listener range 8 feet, desired listener SPL 80dB, amp headroom 15dB

But for what I have seen, most likely, there will not be db marked on the speaker. Usually I see Ohm and Watts.
What can I do if I can just see the two information marked? Perhaps the size can help?

J M Fahey:
The datasheet sure helps.  ;)
If the speaker was a "house brand" one pulled from a guitar amp, as a "Jaguar" from a Randall, a "Fender special design" or an unnamed one from a Crate or Peavey, etc., they are almost always an OEM Eminence; most probably available to the general public as one of the "Legends".
Generic "Celestion inside" boxes most often have "Rocket 50" , sometimes V60 or at best "Seventy 80", being the least expensive ones.
Beware, they all sound good, it's just that they were not "used by Hendrix"

You could possibly rig something up to do the measurements yourself. You can put a sine wave through your amp and hook up an oscilloscope across the speaker terminals. Then adjust the gain until you have one watt across the speaker. You might need to put a known resistance in series with the speaker to measure the current to help determine power since the exact impedance of your speaker at that frequency isn't going to be known (I guess you should use a 1 khz frequency for your test signal).

After knowing the power put through the speaker, you can then measure the dB spl, which I unfortunately am not to sure about how to do. Has anyone here made such measurements?

I bet someone could rig up an arduino board to both generate signals at a variety of frequencies and take measurements of the current through the speaker by measuring the voltage across the sampling resistor. And maybe even read the signal level from whatever microphone you use for db spl measuerments. Maybe after measuring all this, you can compare the impedance and frequency response curves with those from known eminance (or other manufacturer's) speakers to figure out which model was rebranded for your amp.

My little Dean Markley which is running at 20watts on a 92db speaker is pushing 105db or so.  If I ran it through a 4×8 cab with four 93db speakers that would get me up to 112db – enough volume for me to do some small gigs with it.  Although more likely I would get a Fender Frontman 65R as it pushes 111db stock on it’s built in speaker, or 120db with a 4×10 cab loaded with 96db speakers.  This would do small gigs on it’s own and most larger ones simply by plugging in a 4×10 cab.  To get the 120db needed for a larger venue with my Dean I would need to plug it into 3 cabinets of the 8×10 cab variety loaded with 96db speakers…you can see at this point more wattage is necessary.  There is definitely a diminishing point and I would say that 4 speakers is it really.  Maybe 8.


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