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SPEAKERS - A Player's Guide
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
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