The Digitrax SP28288 is a 28mm round 8 ohm speaker, the classic full-size cone for HO and larger sound installs. The bigger diaphragm moves more air than the small oval and 13mm speakers, so prime movers and whistles come through with real body - mount it in a sealed baffle and a single 28mm speaker will fill a room.
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Which impedance do I need, 8 ohm or 32 ohm?
Match the speaker to what your decoder manual specifies. Most DCC sound decoders drive 8 ohms. If yours calls for 32 ohms, use the Digitrax SP282832 - the same 28mm speaker in a 32 ohm version.
Will a 28mm speaker fit my locomotive?
It needs roughly 1-1/8 in. of clear width plus depth for a baffle. That is comfortable in most HO diesels, larger steam tenders, and anything O or G. For N scale or a tight HO steam boiler, use the 13mm SP13138 or the oval SP10188 instead.
Do I really need an enclosure?
Yes, if you want it to sound like a locomotive rather than a transistor radio. Sealing the back of the cone stops the rear wave cancelling the front wave, which is where all the bass comes from.
Can I run two speakers off one decoder?
Two 8 ohm speakers in series make 16 ohms and are safe on most decoders. In parallel they make 4 ohms, which can overheat the decoder amplifier - check your decoder manual first.
