Am receiver I built, it can run 32 Ohms, 1.5W spaeker with louder sound than 8 ohms, 0.25W. Why??

Question

I can bearly hear 8 ohms, 0.25W spaeker but 32 Ohms, 1.5W is not realy loud but much better than 8 ohms speaker. I thought it should be opposite, because 1.5W will need lot more current than 0.25W.

Answer 1

The rating of 1.5W and 0.25W is the amount power that the speakers can handle, and not the power that it draws.  The power that the speaker will draw is dependent on the voltage across the terminals and is computed by Voltage square over R.  Besides the power rating, there is the specification of SPL level (http://en.wikipedia.org/wiki/Sound_press... that can be generated at a specific power applied.  This is the efficiency.  Generally this spec is rarely supplied, so experimenting on different speakers that can supply the loudest sound is in order.

Thus it would seem that your 32 ohm speaker is much more efficient than the 8 ohm speaker.  Larger cone area helps in the efficiency.

Answer 2

I answered this for you once before.

Low impedance speaker requires low voltage and high current.

high impedance speaker requires higher voltage and lower current.

The wattage ratings on speakers are just for maximum power you can apply to them before they burn up.

Which one delivers more sound depends on your driver circuit. If it can deliver medium levels of current but not high levels of current then the 32 ohm speaker will deliver more sound.

It also depends on the efficiency of each speaker, which may be different.

for example, to get 1 watt from a 8 ohm speaker, you need

P = I²R

1 = I²8

I = √0.125 = 0.350 or 350ma

E = IR = 2.8 volts.

so your driver has to be able to deliver 3 volts at 400ma

32 ohms:

P = I²R

1 = I²32

I =  or 180ma

E = IR = 5.6 volts.

so your driver has to be able to deliver 6 volts at 200ma

6v at 200ma is easier to get from a medium power driver running from a 9 volt battery.

.