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Topic: Understanding ohm/watts in terms of speakers (Read 15071 times) previous topic - next topic


So I'm almost finished working on a project of making a bluetooth speaker/amp.  Audio needs some tweeking to filter out the noise.  I have that under control.  Just waiting on a picoferrad capacitor.

It was recommended that I use either a 4 or 8 ohm speaker.  I see online a 8ohm 1 watt and 4ohm 3 watt speaker.

To understand this correctly...  8ohms has more impedance thus lower wattage, less decibel range than the 4ohm 3 watts?  The amp is supposedly able to push 8 watts, I have yet to test this.  Just for clarification.  all running off 9v battery if that matters.


Is it a small class-D amplifier ? Then the filter is either for 8 ohm or 4 ohm.
8 ohm requires more voltage for the same Watt, but 4 ohm requires thicker wires and good connections between the amplifier output and the speaker.

If good sound doesn't matter, and it is a common amplifier, then 4 ohm is louder than 8 ohm.
But If the 4 ohm is build with less quality, then the 8 ohm is better ;)


To understand this correctly...  8ohms has more impedance thus lower wattage...
That's generally correct.   If you cut the impedance in half, you get twice the power.  

In order to understand that you first need  to know that when operating normally, most amplifiers are "voltage limited", and like most electronics, they are "constant voltage"* devices.

Related - If you put two 8-Ohm speakers in parallel, that's 4 Ohms total...   If you have one speaker running at 3W, and you stick a 2nd speaker in parallel, they both see the same voltage, they are both running at 3W and you have twice the total power (as long as the amplifier holds-up).

Wattage ratings on a speaker are a little complicated...    A 3W speaker is designed to handle a 3W amplifier operating at 3W peak with undistorted program material.    Regular program material (voice or music) usually has an average power of about 1/10th the peak power...   So, a 3W amp (at full undistorted volume) is putting-out about 0.3W average and the speaker can handle that.    You could probably burn-out the 3W speaker with constant full-volume test tones, or with a distorted guitar.   

Or, if you use an 8W amp with a 3W speaker and play it a full volume, you could burn-out  the speaker.    But as long as you don't "blast" the volume, you should be OK.

You won't get 8W from a 9V battery.  An AC waveform swings positive & negative, which means [theoretically[/b]  you could get +4.5V and -4.5V peak.    But, an AC waveform only hits the peaks for an instant, and the RMS (a kind of average) is about   .707 times the peak.    If you could get the full 9V peak-to-peak voltage swing, that's about 3.18V RMS and you could get about 2.5 Watts into 4 Ohms.   ...But, you'll get some "drop" through the amplifier so you won't get the full  voltage, and the 9V battery can't supply that much power (voltage and current at the same time) and you'd be lucky to get 1W.   

...less decibel range than the 4ohm 3 watts?
Only if the speakers have equal efficiency.    If the spec is published, speakers are usually rated for a dBSPL rating with 1W at a distance of 1 Meter.    (Technically, that's sensitivity, not efficiency, but they are directly related.)


Ohm's Law and Power formulas:

Ohm's Law
defines the relationship between voltage, resistance, and current.    Basically it says more voltage = more current, and more resistance = less current.   (Resistance is the resistance to current flow).

(Impedance and Resistance are both measured in Ohms in some cases (like with speakers) they can be considered the same.)

Current (Amps) = Voltage/Resistance

With a little algebra, you can get:
  Voltage = Current x Resistance
  Resistance = Voltage/Current

The basic power formula is:
Power (Watts)  = Voltage x Current

And, if you know Ohm's Law, you can get:
  P = V2/Impedance   (This one is handy for amplifiers & speakers.)
  P = I2 x Resistance


If good sound doesn't matter, and it is a common amplifier, then 4 ohm is louder than 8 ohm.
I'm not sure what you are trying to say, but if an amplifier is designed to work at 4 Ohms, there is no quality loss at 4 Ohms.


Most hobbyists still use the ancient LM386.
That one might perform better (on a 9volt battery) with an 8ohm speaker.

Class-D is the way to go on battery power (higher efficiency).
4ohms is better for a low voltage bridged class-D amps, like this bridged class-D stereo amp.
Low voltage (1C LiPo or 3xAA) might also be a better match for your bluetooth receiver.

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