The difference between a 2-ohm speaker and a 4-ohm speaker is the amount of sound each device generates. The speaker itself in a car serves to amplify sound. The number of ohms reduces the sound as it climbs.
The relationship between ohms and power is determined by the amount of wattage in the amplifier. A 4-ohm speaker generates twice as much resistance to this power as a 2-ohm speaker, resulting in half as much energy passing through the speaker in the form of sound. The higher the wattage and associated ohm count, the more energy moves between the amp and speaker.
A voice coil generates sound for the amplifier to process and differ. This sound becomes pressure that acts on the speaker and requires dispersion into sound waves. Depending on the kind of speaker, the amount of sound released when pressure is applied will vary. For instance, the same power source creates 100 watts of power through a 4-ohm speaker, while 200 watts pass through a 2-ohm speaker.
This increment holds true with higher rated speakers as well. The mathematical constant here helps car owners determine what kinds of speakers they need in their vehicles depending on whether they want more or fewer amps.