Speakers - what you need to know


Some of the Ratings

Impedance

Speakers are rated in ohms for the load that they present to the amplifier in the decoder. The majority of decoders want to see an 8 ohm load. Some can tolerate a lower (4 ohm) load. Most will work just fine with a higher (16 ohm) load, but may not put out as much electrical power into that load.

The goal here is to match the speaker impedance to the decoder amplifier's desired load.

Some decoders, such as LokSound, want higher impedances, as much as 100 ohms.

Watts

The power handling capability of a speaker measures its ability to consume power from the amplifier without damage or distorting the sound.

The goal is to get a speaker with the highest watt rating possible.

Since very few sounds push an amplifier to its power limit, you can get away with a speaker rated from a fraction of a watt to 10 watts on a typical 1 watt decoder.

Sound Pressure Level - SPL

This number, expressed in decibels (dB) is a measure of a speakers ability to convert electrical power into acoustic power. The larger the number, the more efficient the speaker is. Understand that this is measured at a single frequency.

The goal is to get a speaker with the highest number that also has good frequency response.

It is possible to have a speaker that is "tuned" for a frequency (also known as a buzzer). It will be VERY efficient at this frequency - have a large number - but may not reproduce a wide range of frequencies or tones. For example the bell might hit the efficient frequency of a "tuned" speaker and you would hear the bell very well, but almost nothing from the rest of the sounds, except for that portion of them that hit the same frequency or close to it.

Frequency Response

For our purposes in "on board" sound, anything above 10,000 Hz (10kHz) is lost. However, the low end is where we need help. Steam chuff and diesel exhaust rumble fall in the low end. It would be nice to have response to 100 Hz. 300 Hz is more likely and many of our speakers die out above 600.

The goal is to get as wide a response as possible.

Resonant Frequency (also called fo) is, for practical purposes the lowest frequency a speaker can reproduce. So, if that is specified and the speaker is listed as "full range" or similar words, you can use it with confidence, knowing that most everything above the fo frequency will be heard.

FAQ about speakers

How do I select a speaker?

Get the best quality and largest speakers that you can fit into your model and still have room for good acoustic ducting. If you can connect multiple speakers, do so!

At Litchfield Station, we audition all the speakers we sell. We work hard to provide the best sound we can for the least money out of your pocket. Rest assured that we constantly look for the best for you.

If you can fit a Mega or High bass speaker, then you MAY want to go that direction.

How do I get the sound out?

This is a topic all by itself. Bruce has done a clinic on the subject at many NMRA national conventions, the NMRA Arizona Division meets, a NMRA PSR convention and several train clubs. He hopes to have a DVD out with his clinic on it. Some of the information can be found here on the DCC University - CLICK HERE.

What are High or Mega bass speakers?
SP-27R-HB

Firstly, the word is pronounced like BASE, referring to low frequencies, not like the bass fish! It is spelled like the fish: BASS!

Coming out of the personal computer world, these speakers have a metal cone for lower distortion and a flexible surround or support for longer excursions (translates to moving more cubic inches).

They typically have a higher wattage rating than similar diameter speakers and are physically deeper - 1/2 to 3/4 inch in sizes that fit HO scale.

What do you get for that extra depth? Usually about 1 to 1-1/2 octaves of additional bass frequencies. This is frequently the difference between good sound and GREAT sound. But they are not the end of the world, either.

For example, Bruce cannot tell the difference between a SP-27RHB-08 high bass speaker and a SP-28x40-08 Intervox speaker with the same sound material coming from a Tsunami (16 bit 44 kHz) decoder. They fit differently, so Bruce recommends between them based on fit, not sound.

The square versions, for the most part, are a plastic frame on a round speaker. They are mounted so that the cone will frequently come in front of the plastic mounting plate so SHOULD NOT BE MOUNTED AGAINST A GRILLE or other restraint. If you do, you will probably hear the cone hitting the grille when you blow the horn!

Can I mount the speaker up-side-down?
HiFiSpeaker

This question comes from a concept routed in the Hi-Fi systems of the 1950s! Speakers were mounted in enclosures with the cones facing out. This has led some folks to believe that sound only comes off the "front" of the speaker. Not true! The "rear" or magnet-side moves air just like the front.

For the quality of sound we are dealing with here in models, you can mount the speaker with the majority of the sound coming off the magnet side or the cone side. What ever works best in your model!

Check out an examples of each below! Both sound GREAT! The Southern Consolidation installation was done by Bruce in June 2010, based on a design that he has been doing for five years. The coal load fitted over the speaker has about 200 holes drilled in it to let the sound out.

The photo below shows the first Tsunami installation Bruce did for a customer. This is a Life-Like 2-8-8-2 HO loco. Note that the speaker is pointing UP throught the coal load, not down, as in the Bachmann, above. The sound is the same! In both cases, the sound off one side of the speaker moves through the perforated coal load to the ear of the listener, while the sound off the other side of the speaker is contained within the tender shell. The shell acts as a VERY LARGE box enclosure.

Do I need to worry about polarity?

Decoder: No, for the most part. If your decoder manufacturer cares they should provide you EXPLICIT INSTRUCTIONS - follow them!

Coupling Capacitor: Most decoders that use a bipolar (un-polarized) external coupling capacitor. If not, the decoder manufacturer should provide you EXPLICIT INSTRUCTIONS - follow them!

Speakers: In a single speaker install, NO! See the multiple speaker information, below!

How do I connect multiple speakers to my decoder?

I covered this topic in my January 2014 column (CLICK HERE to view).

First, find out the impedance that your decoder wants to see and what it will tolerate. Most of the decoders on the market, the significant exception is the LokSound series, want to see 8 ohms and will tolerate higher, frequently with little or no reduction in audio power generated. Thus, this discussion will revolve around presenting an 8 to 16 ohm load to the decoder!

Bruce recommends IDENTICAL SPEAKERS. In advanced installations a mix of speakers may be used with special connections. See that topic below.

One speaker is easy - select an 8 ohm speaker and connect it to the decoder, with a coupling capacitor, if the decoder manufacturer recommends it. Don't worry about polarity.

Speaker1

One speaker installation

Two speakers - only slightly harder. select IDENTICAL 4 or 8 ohm speakers. If they are mounted side by side or across from each other in an "A" frame, wire them SERIES-AIDING. That means that you connect the positive lead of one speaker to the negative lead of the other and then connect the remaining two terminals to the decoder, through a coupling capacitor, if necessary. The capacitor can be connected BETWEEN the speakers, if that is more convenient! If your speakers are not marked with a polarity mark and are IDENTICAL, you can physically line them up together and mark the terminal on the same side of each of them. Notice that neither terminal is marked in the example below.

Speaker2

Two speakers in series-aiding (wired in phase)

Two four ohm speakers will give an 8 ohm load when wired this way. Two 8 ohm speakers will provide a 16 ohm load.

Speaker4

Four speakers in series-aiding / parallel (wired in phase)

Four speakers are easy, too! If you have the room this is a FANTASTIC installation. The customer whose loco is shown HERE is still raving about the sound! You simply wire two sets of 8 ohm speakers in series, as shown above and then connect them in parallel as shown below. This provides the desired 8 ohm load.

SP-27RHB-08-Series-Parallel-2

Here is a photo of a 4 speaker series parallel array for an O scale F3A.

Three speakers are only practical when you have four ohm speakers available. When Bruce ran Litchfield Station, he stocked them whenever he could find appropriate ones. With the release of the LokSound Select and its tolerance of 4 ohm loads, LokSound has released more sizes. Wire the three 4 ohm speakers in series, just as shown for two speakers above. The resulting impedance is 12 ohms, fitting the design criteria.

Can I mix a large and a small speaker somehow?

If you wire non-identical speakers as shown above, your results will be limited by the weakest speaker in each series link. Think about water flowing through a ditch. It will be limited by the narrowest point of the ditch!

If you want to do a "woofer" and "tweeter" installation, you will need specific information for the speakers you are dealing with. We suggest you NOT attempt one of these unless you are studied in acoustic design and electronics, or have SPECIFIC INSTRUCTIONS for your type of acoustic design and speakers. Preferably, hear an installation you like and get the EXACT information to precisely duplicate it.

Copyright © Bruce F. Petrarca 2007 - 2017; All Rights Reserved