One of the most important features of a speaker is its impedance - the load it presents to the amplifier. As a rule of thumb, it is okay to have a lighter load (higher impedance) than the amplifier desires, but not lower. Higher impedance loads will frequently lead to lower volume, but the two speakers working together may be enough more efficient to more than overcome the loss.

Many decoders (SoundTraxx, QSI, MRC) like an 8 ohm load. This can be achieved with a single 8 ohm speaker or two 4 ohm speakers in series or two 16 ohm speakers in parallel. For more information see the speaker FAQ section in the DCC University!

However, if you have the room NOTHING WILL TOUCH the HIGH BASS speakers.

Digitrax SoundFX decoders prefer a 32 ohm load, but can be used (at reduced volume settings) with impedances as low as 8 ohms, although they may need a programming track booster at this lower impedance.

LokSound standard and micro decoders want a 100 ohm load. Thus they have their own line of speakers.

The LokSound XL garden sized decoder looks for 16 to 32 ohm speakers - selected on the decoder.

Speakers

4 Ohm 8 Ohm 16 Ohm 32 Ohm 100 Ohm