I have a speaker that says it produces 91 db at 1w/1m. As far as I know, this means that it will produce 91 db 1 meter away when it's taking 1 watt of signal. But is this relationship linear? So, if I fed it a 10 watt signal, would I get 910 db from 10 meters away? I'm asking because it's rated 110 watts RMS, and 110 watts would make a lot of db if this is true lol.

 

You better hope not.

http://decibelcar.com/articles/40-everything-else/87-dbequivalent.html

 

The output sound versus power is basically linear but dB's are logarithmic.
So increasing the power by 10 times (1w to 10W) increases the sound level by 10dB (at 1 meter, not 10 meters).
The intensity drops off as 1/r so at 10 meters the sound pressure would be 20dB lower than at 1 meter.

 

Also, our hearing's sensitivity to loudness is also logarithmic. Twice the power sounds only a little louder. 10 times the power sounds twice as loud.

 

Ok. Well all I have is an LM386 (which is 1 watt max, anyway, as far as I know). I have a 2.5vpp signal that I want to send to the speaker (which is 4 ohms). I'd like to amplify the signal to 1 watt, nice and loud (91 db apparently). How should I set up the LM386 here (running on 5 volts).

 

I have a 2.5vpp signal that I want to send to the speaker (which is 4 ohms). I'd like to amplify the signal to 1 watt, nice and loud (91 db apparently). How should I set up the LM386 here (running on 5 volts).

2.5Vpp into 4 ohms is only 0.2Wrms.
If you use two LM386's in a bridge circuit (inverting the input audio signal to one of them) you would get 4 times the power or 0.8W.
For more power you need a higher power amp running from a higher supply voltage.

And note that 92dB is at a 1 meter distance.
Is that how close you are going to sit from the speaker?

 

The datasheet for the LM386 has a graph that shows battery voltage, speaker impedance and resulting maximum peak-to-peak output voltage.
With a 5V supply the output into your 4 ohm speaker is only 2.5V p-p which is 0.88V RMS. Then the power in the speaker is only (0.88) squared)/4 ohms= 0.14W.
The graph shows that with a supply that is 9V to 12V the output power does not change, another graph shows that only the heating in the IC changes. With a 9V to 12V supply the maximum output into 4 ohms is 3.5V p-p producing 0.38W. The peak current is (3.5V/2)/4 ohms= 0.44A.

If you connect two power amplifiers in a bridge then the p-p output voltage tries to double and the output current also tries to double. With a 5V supply the maximum peak output voltage into 4 ohms is 1.25V but the bridging doubles it to 2.5V. Then the peak output current is 2.5V/4 ohms= 0.625A which is impossible because it is more than the maximum available output current of 0.44A above.

A 12V supply and an 8 ohm speaker produces a maximum of 6.5V p-p which is 0.66W. The peak current cannot go higher than 0.41A.
Bridged, a peak current of 0.41A is produced when the maximum output is 3.25V peak which can be done with a 5V supply.

Therefore an LM386 can produce a maximum of 0.66 Watt, or 1 fake What.

 

The value is frequency dependent. I think the frequency used is 1 kHz.

 

The value is frequency dependent. I think the frequency used is 1 kHz.

I agree. The amplifier does not work well at 2Hz nor at 2MHz.
Oh, the speaker? Any half-decent speaker produces a fairly flat frequency response from 60Hz to 15kHz.
Oh, the impedance of the speaker? it is the lowest at about 400Hz.

 

2.5Vpp into 4 ohms is only 0.2Wrms.
If you use two LM386's in a bridge circuit (inverting the input audio signal to one of them) you would get 4 times the power or 0.8W.
For more power you need a higher power amp running from a higher supply voltage.

And note that 92dB is at a 1 meter distance.
Is that how close you are going to sit from the speaker?

That is about how close I'd be. Do you have a schematic for this circuit? If I need to, I can use a higher voltage power supply.

 

The LM386 amplifier does not produce enough output current to drive a 4 ohm speaker bridged. A higher supply voltage simply produces more heat in the IC, not more output power.

If I need to, I can use a higher voltage power supply.

No. The LM386 is designed for driving an 8 ohm speaker with a 9V supply to produce 0.45W maximum. With a 4 ohm speaker and/or with a higher supply voltage it simply gets hotter but with the same maximum output power. Bridging it will just make almost 4 times the heat.

Try a more modern Class-D amplifier like the PAM8302 that is already bridged. It produces 2W (the ad says 2.5 Whats) into 4 ohms at low distortion with a 5V supply and it barely gets warm. Fixed the typo.
An assembled amplifier board is available from Adafruit for only $3.95US.

 

Then the power in the speaker is only (0.88) squared)/4 ohms= 0.14W

My calculator says 0.2W.
Does yours need batteries?

 

My calculator says 0.2W.
Does yours need batteries?

Sorry. A crumb from my dinner blocked one segment on my calculator's display making the "9" in 0.194 look like a "4" in 0.144.