SPEAKERS - A Player's Guide

The purpose of this article is to give some kind of grounding in the area
of (loud)speakers. It's not about product recomendations but about
some basic knowledge to help in your choices.

Background
The man who really wrote the bible as far as speakers are concerned
was Harry Olsen of Bell Labs. His research from the 1930's is the
foundation of most of our current knowledge. Progress in recent times
has been more linked to materials technology rather than the discovery of
any new principles. If you want more theoretical underpinnings to
your understanding of how loudspeakers work then Harry (H.F.) Olsen's
book 'Accoustical Engineering' is a very good place to start. 
You could follow that up with AES (Audio Engineering Society) Journals.

Some Aspects of Speakers:
number of drivers 
size of boxes
power handling
shape
Auditioning
Will they still be good when I get them home??


Number of drivers...

One of the problems with speaker design is that one driver isn't capable
of carrying the full frequency range necessary for the reproduction of
music..
For example a driver that is capable of producing deep bass has to be 
large and needs to be placed either in a large box or on an extremely 
large flat panel. Because of various factors including the inertia of the 
large mass of this speaker it is not able  to accurately reproduce
higher frequencies.

Electrostatic speakers (Quad makes some) cover the whole
frequency spectrum but they are expensive and most reviewers say
they are a bit light in the bass department. Very nice for classical
music though.


A Frequency Aside

Sound travels in waves. The frequency of 
the wave (number of waves that pass per 
second) describes the pitch of the sound. 
Bass is usually described as being some-
where between 20Hz and 250Hz (Hz = 
Herz =cycles per second), mid-range from
there to say 3kHz, and high from there on
up to the generally recognised threshold 
of hearing for reasonably acute human ears, 
20kHz. (What we might  take in above that
and how, is the subject of some argument.)


The basic principle here then is that drivers are optimised for a portion
of the overall frequency responce - bass speakers do the lows and
tweeters do the tops. The idea is that responce should be flat - that
the loudness of a sound at 40Hz will be the same as one at 400Hz,
or 4kHz.

A very basic setup in something like a cheap boom-box might have a 
single small speaker for stereo left and right with a metalic center to
help radiate the tops. Such a speaker will have no bass and no real
tops, merely an accentuated upper-midrange. This set up will be
quite OK for rendering the human voice in an intelligable way (but not
a faithful way) and will be completely hopeless at playing music of
any kind. 

The next step up is a two way system where there is a bass speaker and
a tweeter. Quite a big range here;  some of them very good and some
very bad.  What makes a good one good? 


What makes for good?

well designed speaker components which are carefully matched
crossovers which are matched left to right and "fit" the speakers
solid enclosures that alleviate edge-diffraction and phase distortion
God is in the details!



Speakers built for studio use (not near-field) are usually no more than
four way. These tend to be split along the lines of bass, lower-mid,
midrange, and top. This leads to the subject of crossovers ...

For every speaker with more than one driver there
should be a way of sending bass to the bass driver and tops to the tweeter
(and whatever else). Crossovers are electrical networks which
divide the frequency spectrum and send appropriate signals
to the drivers.  

There are two main types of crossover - active and passive.
Passive crossovers operate on the signal just before the individual
drivers in the speakers (and after the signal has been amplified) 
while active crossovers operate on the low-level signals before 
they are amplified. Active is  superior because low-level signals 
are managed more easily (in a nutshell!). 



Size Matters!

To get bass responce you need bigger enclosures than will fit on the
average bookshelf. There are no exceptions to this rule!  Why it is so
is because where a speaker is mounted in a box the springiness or
compliance of the air combines with that of the driver to make a system
resonance that is higher than the speaker resonance. The smaller the
box the higher the resonance figure - and responce drops swiftly
from that point. 
As an aside to this, if you're mounting a speaker on a flat panel, to 
have a cut-off point of 50Hz the panel will have to be 12 feet square.
You can gather from this that small, open backed speakers won't
have much bass reponse.
Some speakers have tuned holes in the cabinets which are said to
boost bass output at certain frequencies. These are called bass reflex
speakers and aren't recommended because of the difficulty in 
aligning them and because the reason for having them in the first
place (getting more sound pressure level out of small amps) is made
redundant by readily available, comparitively cheap powerful amps.


Power Handling
The power you need is governed by what your application is. Big
crowded parties in accoustically dead rooms will need PA rig
power outputs whereas a small monitoring system in a one person
studio will not need much at all. There are points to remember though.
Big speakers built to take a pounding are usually less sensitive and
because of their increased mass are slower to respond than an
equivalent lighter built model - that means distortion.
Materials technology has helped a lot but the results are quite
expensive. 
As a general rule powerful amps driven lightly sound better than
amps working at their limit. There's room for transients as well.
You need to work out your usual power levels taking into account
the size and brightness of the room (with the usual number of
people in it) and work backwards from there to get the suitable 
power handling of your speakers. 

Shape 
All ninety degree angles with sharp corners and the speakers mounted
straight on the front without being recessed are bad things!
These bad things lead to diffraction of the sound waves which mean
dips and peaks in the frequency responce. Some speakers with shaped
fronts are quite effective in this way - and some are completely not.

Auditioning
Listening to speakers to decide their quality is a reasonable approach.
But remember that when you do so you're listening to the whole
audio chain - CD player/turntable/computer -> preamp -> amp -> speakers
-> room.
If you're shopping for expensive-ish speakers from a good dealer,
you should be able to take them home for a try-out. Failing that you
need to be careful. Listen to an instrument you know well, human voices.
Don't make it too complicated for yourself but make sure you have the
frequency spectrum about covered (bass, mid-range, tops) with any
materials you take with you.
Get a first impression and then listen to particulars. Remember also that
volume increases can often sound "better" so if you're your listening in
a shop make sure the person doing the demo-ing doesn't muck with
the controls.


John Watkinson's Do's and Don'ts for speaker buying

Look For:
Speakers that include a plot of the time-response-to-a-step
 in the spec
Speakers that show a minimum migration of accoustic centre 
 at low frequencies
Speakers that can reproduce square waves
Speakers which are minimum phase
Speakers with a small number of wide-band drive units
Active crossovers
Tweeters with rare earth magnets
Speakers with flowing shapes or rounded corners
Speakers that reveal how many parts were used in a 
 mutitracked vocal


Avoid:

Speakers with reflex ports, ABRs or transmission lines
Speakers with sharp corners and flat panels
Speakers with lots of narrow band drivers and steep crossovers
Speakers that make a noise when rapped with the knuckles
Speakers that won't reveal the difference between a MiniDisc 
 and the CD of  the same recording

Audition Using:
Natural, non-musical sounds such as traffic, building sites, 
 conversations in a pub
Percussive sounds such as Tympanum, Bodran, Marimba
Complex sounds such as choirs
Distinctive sounds such as piano

John Watkinson, Studio Sound, Sep. 99, p112


Will they still be good when I get them home??

Not as silly as it sounds ... some speakers do sound better with some amps.
The reasons have to do with the speaker's impedance under load and
the resultant effects on amplifier damping (cables come into this too but
we'll take them as a constant). Some speakers and amps are a better "fit"
than others and the best you can do is listen to what people say or write
and test the conclusions yourself.
Quite a few speakers with active crossovers actually come with built-in
amplifiers so going that route can remove the problem through having no
other choice.
And then there are the rooms - different furnishings and wall coverings
together with room shape and size as well as speaker placement will give
vastly different sounds. Most speakers have some sort of average room in
mind when they're built so generally, getting the placement right will
be all you need to do (unless you're after an optimum, in which case you'll
need a good accoustics book and some measuring equipment - or you can pay
an expert). 
As far as placement goes, bass is accentuated most when the speaker is
in a corner on the floor (or cieling). The accentuation lessens as the
speaker is moved to just being backed by a wall, and then lessens again
as it's moved away from a wall.
Speaker placement at the shop might be interesting to notice.





updated 26 May, 2000

John Littler