The Sound Primer is an attempt to help our customers understand how sound works and therefore, the steps that can be taken to improve any sound installation. This lesson in sound theory can be applied to any onboard sound installation.
Getting the Best Sound from your Sound System
As
model railroaders, we are trying to create a miniature world that is
as realistic as our abilities allow. But no matter how detailed a
model is, something is missing as our trains go silently down the
track - sound.
Real locomotives don't have Pittman motors in
the cab. They are real working machines - a symphony of sounds!
Little kids realize this and try to make up for it by making 'chuff,
chuff' and 'woo, woo' noises when they see a model train roll by. As
adults this is:
A. Not particularly satisfying
B.
Potentially embarrassing if we are overheard!
Good News!
SoundTraxx comes to the rescue and provides you with a wide range of
options for creating realistic sound effects while preserving your
dignity. However, a bad installation can cause problems, so listen up
while we try to provide some insight into SoundTraxx sound system
technology and show you how to properly apply it.
What is sound?
Everything you ever wanted to know about speakers.
Practical Tips for a Successful Sound Installation
The Four Steps to Great Sound
The Science of Sound (from a model railroading perspective)
In order to understand what makes a good sound installation work, it is
first helpful to understand what sound really is...
What
is sound?
In
simple terms, sound is basically a pressure wave traveling through
the air. A sound source such as a speaker vibrates air molecules back
and forth creating small changes in air pressure and causing a sound
'wave' to propagate to the listener's ears. The eardrums pick up this
vibration and turn it into a signal that the brain interprets as
'sound'.
Sound
Volume
The
amplitude of the air molecule vibrations determines sound volume.
When the molecules vibrate a small amount, there is little change in
air pressure and thus, a low level of sound is produced, discernable
only to those near the sound source.
Similarly, when the
vibration level is very high, the change in air pressure is high, and
the sound is very strong. When the sound is loud enough, we can
actually feel
the changes in air pressure!
Sound
Frequency
Sound
is also made up of various frequencies - that is, the rate at which
the molecules vibrate. Low frequencies (slow vibrations) produce deep
bass sounds such as the rumble of the firebox. High frequencies (fast
vibrations) produce bright sounds such as the ringing of the bell or
hissing of steam.
Frequency is measured as Hertz (Hz) or
cycles per second. Thus, if a sound causes an air particle to vibrate
back and forth 100 times in a second, it is said to have a frequency
of 100 Hertz.
Human hearing works in the range of 20 to 20,000
Hz, which is divided into three categories: Bass, Midrange and
Treble.
Bass sounds are characterized by their low frequency, typically from 20 Hz to 500 Hz and are produced by such sounds as firebox blowers, exhaust chuffs and diesel engine exhaust.
Midrange sounds fall in the range of 500 to 2500 Hz and are produced by whistles, air pumps, radiator fans and injectors.
Midrange sounds fall in the range of 500 to 2500 Hz and are produced by whistles, air pumps, radiator fans and injectors.
Treble, or high frequency sounds have a range of 2500 Hz and up and are made by bells, steam blow-offs, dynamos and turbochargers.
Some sounds, such as exhaust chuffs, are complex and are made up of many frequencies that span several sound categories.
How
a Speaker Works
Model
railroad sound systems use what is called a dynamic speaker to
produce sound. The structure of this speaker is fairly simple and
consists of a metal or plastic frame to hold everything together.
Stretched across the frame is the speaker cone or diaphragm, which is
usually made of a stiff paper or Mylar plastic. Attached to the
center of the speaker cone is a voice coil, which is nothing more
than a bobbin, wrapped with many turns of fine wire. Surrounding the
voice coil is a permanent magnet.
When an electrical current is applied to the voice coil, a magnetic field builds up around it just like an electro-magnet. This magnetic field is either attracted to or opposed by the magnetic field of the permanent magnet. The effect is similar to that of bringing two permanent magnets together and turning one toward and then away. One way, the two magnets are attracted to each other; the other way, they are repelled.
When the audio signal is fed to the speaker, the varying magnetic field causes the voice coil to move back and forth. Since the voice coil is attached to the speaker cone, the speaker cone vibrates and produces a sound wave.
Now that you’re armed with a basic understanding of how sound works, let’s get on with it and see how we can use this information to improve our sound systems…
Getting the Best Sound from your Locomotives
SoundTraxx strives to manufacture the highest quality sound system available. A bad sound system will still sound bad even when connected to the best speakers in the world. However, even a good sound system can sound bad when improperly installed. To help you avoid that mistake, we have outlined the most important factors in determining the successful outcome of a sound system installation:
Speaker Installation
Speaker Type
Speaker Size
Speaker Quality
Speaker
Installation
How
the speakers are installed will have a dramatic effect on the overall
sound quality. All things being equal, this makes the critical
difference. A poor installation can make a good speaker sound bad and
a great installation can make even a mediocre speaker sound pretty
good.
Recalling our earlier physics lesson, sound is basically a pressure wave. The speaker cone makes sound by pushing against the air molecules to create a disturbance in pressure. The speaker however, must be properly installed in housing in order to build up any pressure. Otherwise, as the front of the speaker cone pushes out against the air, a vacuum is created on the backside of the speaker cone that absorbs the pressure from the front so no sound wave is projected. Without an enclosure, an opposite pressure behind the speaker cancels any pressure developed by the front of the speaker. The enclosure isolates the front and back surfaces of the speaker, thereby increasing the sound pressure and hence, the volume.
Here’s a simple demonstration to prove the point: clap your hands together. A nice loud sound is easily produced as the air is trapped and squeezed out between your palms. Now, try making the same sound by waving on one hand through the air. As you wave your hand around, the air in front of hand simply rushes around to the backside producing a refreshing breeze at best but no sound whatsoever.
It
is important to properly size the enclosure to the speaker you are
using. If the enclosure is too small, it will interfere with proper
operation of the speaker (it’s almost as bad a not having an
enclosure in the first place.). It’s hard to make the enclosure
too large although thee is a point of diminishing return. As a rule
of thumb for small speakers under 4” in diameter, the minimum
length, width and height of the enclosure should equal the speaker
diameter. Thus a 1” diameter speaker should be housed in an
enclosure that is at least 1”x1”x1”. Similarly, a
2” speaker would require a 2”x2”x2”
enclosure. As this is a general guideline, exceptions can and must be
made in many circumstances.
However,
the use of a proper speaker enclosure cannot be overemphasized and is
almost always the cause of poor sound quality!
Besides proper enclosure size, the enclosure should also:
Be sturdy with stiff walls to avoid vibrations and buzzing
Be airtight or nearly so. If a sound system is being mounted in a piece of rolling stock, you should use a boxcar and not a stockcar for example.
Have a speaker opening that is at least 50% of the speaker area. It is usually easier to drill a pattern of small holes than one single large hole. Just be sure none of the smaller holes fall outside of the speaker face.
Speaker
Type
A
speaker is a speaker, right? Wrong! Every speaker has what is called
its frequency response – which is a measure of its sound volume
over a range of frequencies. The perfect speaker would produce the
same volume level for all frequencies. Unfortunately, the perfect
speaker is pretty hard to build and most speakers have a limited
range in which they work. Thus, speaker types are divided into four
broad categories each classified by the range of frequencies they can
reproduce:
Woofer – designed to reproduce low frequency sounds
Midrange – designed to reproduce (you guessed it!) the midrange sound frequencies
Tweeter – designed to reproduce the high or treble range of frequencies
Full Range – designed to reproduce most of the entire range of audible frequencies. Full range speakers usually represent some compromise at the extreme ends of the audio frequency spectrum.
High fidelity speaker systems often use a combination of speakers to produce a finished speaker that evenly reproduces frequencies across the entire audio range. For locomotive sound systems, installing multiple speakers is usually impractical, so a full range speaker is the best choice. When selecting a speaker, look for one with as wide a frequency response as possible.
However, as we shall see, a speaker’s frequency response is limited by its size, which is in turn limited by the size of the model, so often a compromise must be reached.
Speaker
Size
When
it comes to selecting a speaker for your sound system, remember: Size
matters and bigger is better! All other things being equal, a bigger
speaker will:
Produce more volume. The larger speaker cone has more surface area and is thus able to vibrate more air particles creating larger pressure changes.
Produce deeper bass sound for realistic chuffs. The voice coil in a larger speaker can move back and forth along a greater distance than that of a small speaker improving the low frequency response of the speaker.
When a large, single speaker is not practical, you can often use two or three smaller speakers together to improve the frequency response and sound level.
Important!
Individual
speakers in a multi-speaker system must be wired properly or the
sound level will actually get worse. Speaker terminals are usually
labeled plus and minus. Be sure to connect plus to plus and minus to
minus. If you are unsure which terminal is which, listen to the
speakers when wired one way, then listen again with the speakers
wired the other way and choose the arrangement that produced the best
sound.
Speaker
Quality
Finally,
the quality of the speaker should be considered. Despite appearances,
speakers are not identical and there are a number of parameters that
should be considered:
Frequency Range – Look for a speaker with as wide and flat a frequency response as possible. Be especially wary of small speakers under 2” in diameter as many of these are designed to work as alarms or beepers and have a very peaked response. They might make a good bell sound, but they’ll be lousy for anything else.
Magnet Size – A larger magnet often allows the speaker designer to us heavier wire in the voice coiling which allows the speaker to handle more input power. It does not necessarily mean the speaker will sound better but it might last longer.
Power Handling – When possible, pick a speaker that has a power rating comparable to the sound system’s amplifier (SoundTraxx Digital Sound Decoders have 1 watt amplifiers). If the power rating is too low, you run the risk of eventually burning out the speaker.
Cone Material – Look for a speaker with a stiff cone and avoid ones that have a flimsy feel. For the outdoor railroader, a polypropylene or Mylar cone has the advantage of being moisture resistant.
Impedance – all SoundTraxx Digital Sound Decoders are designed to work with 8-ohm speakers.