Extended (Hi) Bass Speakers
Clarification: With Extended Bass (Hi bass) Speakers, we are NOT talking about subwoofers. Were are simply talking about getting some of the lost bass sound that small speakers normally tradeoff for size reduction.
What is the problem?
The common small and compact speakers used in inexpensive sound equipped engines are a full range speaker. Full range speakers work over a wide range of frequencies but are not optimized for sound reproduction at the extremes of bass and treble. Think "Jack of all trades. Master of none". Complicating things further, when full range speakers get smaller to fit inside our engines, they typically give up more bass range. Hence by definition become a "Less than a Full Range" speaker.
How does one get better bass response from a speaker?
To get strong and/or extended bass frequency response means doing any combination of the following things:
1) Moving lots of air. That means:
a) changing the speaker's diameter to a larger diameter speaker.
b) changing the speaker's design to allow more physical cone excursion or movement back and forth. This is the same is increased displacement.
2) Placing the speaker in a good acoustic enclosure. There are two common types of speaker enclosures:
a) Sealed. In other words a simple air tight closed box. Of all the types of speaker enclosures to build, this is the easiest but not the most efficient. All the acoustic energy radiating out the rear of the speaker and inside the enclosure is wasted and turns to heat. "Infinite baffle" is another name for a sealed enclosure.
b) Ported. A box with a "tuned port" or specifically designed opening/hole that extends the base by converting the sound pressure from the rear of the speaker into usable sound out the front of the speaker over a selective or "tuned" frequency range. In other words this special enclosure can use some of the wasted acoustic energy from the rear of the speaker to extend the bass response of the speaker. "Bass Reflex" is another term for a Ported Speaker.
The why's and what's of a speaker Enclosure
Our ears hear by sensing the difference in air pressure. When a speaker cone moves, it creates a pressure difference between the air in front of the cone versus the rear of the cone. If the air behind the speaker is able to meet the air in front of the speaker, the equal but opposite pressures will cancel each other out. We hear almost nothing even if the cone is moving violently! In other words, the speaker will produce very weak and "tinny" sound.
For a more technical discussion on speaker enclosure design for a locomotive, go here:
http://www.qsisolutions.com/pdf/loco_acoustics_design.pdf
SEALED ENCLOSURES:
There are many off the shelf "sealed" enclosure you can buy for a speaker. Typically the store that sells the speakers will also sell you the matching enclosure. Below are some pictures of "off the shelf" sealed enclosures.
If the speaker is the correct size, some people have use old 35mm film canisters as an sealed enclosure. That said, 35mm Film is getting harder to get given the conversion to digital cameras.
PORTED SPEAKERS:
These speaker are available from: http://www.railmasterhobbies.com/Speakers.htm
What does an extended bass speaker look like physically and acoustically?
If you want to go to a technical link that discusses the entire issue, go here:
www.tonystrains.com/technews/qsi_hb_speakers.htm
If you want more background information, read below first.
To learn more about how a extended bass speaker looks different from a normal speaker, we first need to define the parts of a speaker. Below is a "cross section" picture of a typical speaker construction. There are three key speaker parts to look at is:
1) Speaker depth. The depth of the speaker from the front of the cone (Top of the picture) all the way to the back plate (Bottom of the picture). This deeper/larger this dimension gets, it correlates vary well with the cones ability to travel greater distances front and back without increased distortion or clipping. To allow more cone movement, the speaker voice coil itself needs more room to slide up and down while inside the magnet. A deeper/taller magnet structure allows for increased voice coil movement. Stated another way, a flat and thin speaker make for poor bass response.
2) The surround. Look at the top left corner of the picture on the right. Look at the material, shape and size. A "half moon" or "half roll" surround shape versus a flat surround allows a lot more movement for a given size. A taller/deeper/larger surround typically means even MORE cone excursion is possible. If the material of the surround is very pliable like rubber as apposed to something stiff means more cone excursion is possible.
In the next two pictures, we will show these differences.
Speaker depth. If you compare the two speakers, you can instantly see the extended bass speaker to the left is deeper/taller than the flat compact speaker below.
Surround. You can see the thick black soft half-roll shape of the surround around the edge of the silver metal cone. The compact speaker below is all one piece plastic cone. The surround is actually so small it is lost in the little crevice at the edge of the clear cone.
