Inside hood width?

Dear Blayne,

IIRC the typical inside width of a long-hood on a HO GP model is around 16mm, so that's your most-likely limiting/"max dimension" measurement.

Happy Modelling,
Aim to Improve,
Prof Klyzlr

Speaker Size

Prototype railroad locomotives emit a lot of bass frequency sound.

In general, the larger the physical size of speaker one can fit into the shell of a scale railroad locomotive the better will be the realism of the bass sounds.  The larger the dimension of a speaker, the lower the frequency of the sound it can emit.  This is a matter of physics wherein the wavelength of bass sounds are greater than those of higher frequency sounds and the length of the speaker opening determines the lower frequency limit of emitted sound based upon acoustic wavelength.

Speakers do *not* have to be mounted with their front (output) face horizontal.  They can be mounted on a diagonal if an edge of the speaker housing is trimmed to fit within the shell and a panel of similar material is used to fit and acoustically seal the rectangular opening that was made when fitting the speaker into the scale locomotive shell -- as required.  Reducing the internal volume of a speaker's rear housing *can* affect the frequencies/quality of a speaker's output in certain cases -- a matter of back pressure on a speaker's diaphragm.  Diagonal mounting allows a larger dimension speaker to be mounted.

Additionally, the larger the dimension of the sound opening through which the sound is emitted, the less clipping of the bass notes will be.  A fairly open under chassis would prove helpful/best.

Before purchasing a speaker, one can make a paper mockup of the various speaker dimensions with their housings to aid in determining which is the largest speaker that can be installed in your scale locomotive shell.

But that's not all there is to it.

With a quality pair of ear buds, you can hear your music almost as good as those mega window breaking speakers in your living room.

I have replaced the speakers in four of my factory sound equipped locos with sugar cube speakers along with the recommended housings and have been more than pleased.  The technology has come a long way!

Bart

Thanks for the input!

Many thanks for those thoughts and tips!  I like the idea of making a paper mockup to help figure out a good size and placement of the speaker or speakers.

Make your own enclosure

I recommend using 2 speakers either in series or parallel. Experiment with each way to find out which will be best.  Make your own enclosure to fit all the available space you can find.  This will help in recreating the lower bass notes as well as increasing you volume.  Once you have it all installed, try tweaking the factory settings on the decoder.  ( I assume it's a factory Soundtraxx decoder).  This will make a huge difference from the factory installed speaker.