This is an
article on loudspeakers, the
things which commonly go wrong and
some basic tips on amplifier and
loudspeaker matching. As with
everything else on this website,
this is merely my own take on things
- this is merely based on my 15
years of owning and repairing audio
equipment and playing a lot of loud
music in that time.
How a cone
loudspeaker driver works
A coned loudspeaker
driver is basically a large,
lightweight (and ideally rigid) cone
with a coil of wire attached which
creates a magnetic field when
current is passed through and hence
moves the cone in relation to the
permanent magnet which is fixed to
the main driver frame; this cone is
then attached to the driver frame
via a flexible suspension. If you've
ever seen inside an electric motor
or a moving coil VU meter, the idea
is similar in the loudspeaker. In
the ideal world, the movement of the
cone will be exactly proportional to
the electrical waves passed through
the coil and because the movement of
the cone moves the air around it, in
that ideal world you'll get sound
which is a near exact replica of the
electrical signals you feed in.
Reading from the
above, you can see that for a cone
loudspeaker to work properly, it
needs to possess several elements,
each with distinct qualities:
- A
suspension system which will
silently let the cone move
freely within its excursion
range (and 'silently' is the key
word here, you don't want the
suspension adding any sound or
distortion of its own to the
music). Hence why the vast
majority of loudspeaker drivers
have foam/fabric cone surrounds
and stiff fabric diaphragms,
these can flex virtually
silently and be doped to produce
the required stiffness which the
cone requires in order to have
the correct audio
characteristics.
- A frame
(or spider as they are often
called). This needs to be rigid
enough to support the driver
when it is fixed into the
enclosure and also be strong
enough to rigidly hold the
working parts of the driver in
position. This is the main
chassis for the loudspeaker
driver, with few exceptions
everything will be mechanically
attached to this part. The most
common frame material is mild
steel, plastic is sometimes seen
on cheaper lower powered
drivers, glass reinforced
plastic or resin is better and
sometimes seen on horn tweeters.
More expensive drivers sometimes
use more exotic materials such
as carbon fibre.
- A cone,
ideally strong and rigid (and
ideally slightly funnel shaped
to increase both strength and
surface area) but still
lightweight enough to react to
fast transients in music and
ideally made from a material
with good resonance
characteristics. The vast
majority of bass driver cones
are made from paper, the next
most common are certain types of
plastic such as polypropylene
and mica. Even carbon fibre and
Kevlar (think bullet proof
armour) is used as a material in
some higher end bass drivers
because of its low weight and
high strength.
- A
diaphragm, this forms a major
part of the cone's suspension
and also serves the main purpose
of keeping the voice coil
aligned perfectly central
between the poles of the magnet.
This is usually situated behind
the cone and glued to the
driver's frame, you won't see it
from outside the speaker
cabinet. Usually the diaphragm
is ribbed and made from some
form of stiff doped fabric, most
of the time they're deep orange
or yellow in colour.
- A
tightly wound coil of conductive
wire, usually copper, and
commonly known as a 'voice
coil'. On most drivers this is
attached to a (usually paper)
tube (otherwise called a
'former') which projects from,
and is rigidly coupled to, the
rear of the cone. This is the
part of the loudspeaker driver
which the power from your
amplifier will pass through, and
the part which will actually
make the cone move and hence
produce sound.
- A
permanent magnet, ideally quite
large and sufficiently powerful
so that it can exert control
over the movement of the cone.
To an extent, you can judge the
relative quality of a
loudspeaker driver from the size
of its magnet. Especially on a
bass driver, a large magnet will
exert more control over the
movements of the cone and create
better acoustics. This may also
be covered by a shielding can,
if your speakers are advertised
as 'magnetically shielded' then
they will most likely have these
metal cans covering the magnets
- these cans help counter most
of the magnetism so that they
can be installed near
traditional TVs or computer
monitors, without wreaking havoc
with the picture.
Failures
Armed with the
information above, you can begin to
get an idea of how delicate, complex
and finely tuned even a large bass
driver can be and hence understand
how a loudspeaker may possibly fail.
There are two common failure modes
for loudspeakers, every single blown
or otherwise damaged loudspeaker
driver I've seen over the years has
failed in one of these areas:
Heat damage:
More commonly found on high
frequency drivers (mainly
tweeters), this is caused simply
by putting too much power
through the loudspeaker, this
will often create a break in the
coil in smaller loudspeakers
(such as tweeters, which have a
very fine voice coil). There's a
great deal more to this subject,
such as the types of sound waves
an amplifier will produce when
driven into distortion. Heat
damage does also occur on low
frequency drivers but because
they often have voice coils
which are both larger and also
wound from thicker gauge wire,
other parts of the driver will
often be damaged by the heat
before the voice coil itself
suffers a break. The two
photographs below are taken from
loudspeakers which suffered heat
related failure.
Mechanical
damage: Usually found on bass
drivers, this is caused where
the loudspeaker is 'overdriven'
by being fed too much power and
forcibly overrides its
suspension limits and hence the
central part of the cone which
contains the voice coil becomes
distorted. Symptoms after this
will often be a chafing or
rubbing sound when the cone is
moved (sometimes only on a
certain position of its travel
or at a particular sound
frequency), in severe cases it
will manifest as a low, tinny
sound output as the cone
struggles to move but then heavy
distorted sound peaks (sometimes
a sound best described as
'farting') as the driver cone
overcomes the friction of the
voice coil former against the
central magnet pole and scrapes
against it. As the cones on high
frequency drivers tend to be
rigidly held with virtually no
visible cone movement and
because higher frequency sound
doesn't contain the heavy
transients that low frequency
sound (bass) does, mechanical
damage on tweeters is very
uncommon (although not to say
impossible), depending on the
design of the tweeter.
Another form of
mechanical damage is physical damage
to the loudspeaker driver from
another object, usually it's the
cone that suffers this. Knocking or
dropping the loudspeaker or
accidentally impacting the cone with
another object - the impact might
not be forceful enough to visually
damage the cone but may be enough to
skew it slightly and make it rub on
the suspension, with the same
resulting symptoms as the mechanical
damage above.
A badly heat damaged bass driver,
card voice coil former is completely
burned through.
Remarkably the voice coil is still
intact although the speaker was
unusable in this condition.
Same idea but this time a heat
damaged tweeter. Not much to see
here except a charred voice coil
former.
This voice coil went open circuit
before heat did the amount of damage
which the woofer above had suffered.
Degraded foams
Also mechanical is
failure of the foam surrounds used
on some types of bass/midrange
drivers. Foam is an ideal material
for loudspeaker surrounds but foam
inherently dries out and becomes
brittle with age. How long does foam
take to degrade? It depends on a lot
of factors including
temperature/humidity and the quality
of the foam itself. I've seen some
vintage foam edged drivers still
working fine, I've seen some foam
edged drivers losing large parts of
their surrounds after as little as
8-10 years - in my early audio days
(this would be 1997), I bought a new
pair of small Wharfedale Valdus 100s
to use as test speakers, as of me
writing this article in 2009 the
foams on the edges of the woofers
are beginning to disintegrate even
though I barely used them after the
first year.
If you've got
vintage loudspeakers which you know
for certain have foam surrounds and
which still work fine, it's fair to
say that you're lucky. It is
possible for foam surrounds to
appear okay but then begin to
disintegrate if you drive the
speaker hard, sometimes old, dried
foams won't be able to stand the
stresses if you haul the speakers
out from the attic after 15 years
and suddenly drive them at high
volumes. Dried foams will be easily
visible if you look at the cones,
large parts of the cone surround
will likely be missing and cracked,
pieces of foam will fall off both
inside and outside the cabinet.
Foams which are
intact but feel dry and rough are
probably nearing the end of their
usable lives too, hairline cracks or
visible surface fractures in the
foam are another visible warning
sign. To an uncritical ear, bass
drivers with bad foams will often
sound fine at low volume (to a
critical ear bass will likely sound
much thinner than usual because of
lack of air pressure difference on
both sides of the speaker cone),
turn the volume up so that the cone
begins to move visibly and you'll
probably hear a vibrating or
'farting' sound on music peaks as
the unsupported edge of the cone
distorts and strikes against the
loudspeaker's frame.
I've heard about
people using products to coat the
foam or replace some of the
elasticity that it has lost, I
really don't know much about these
methods and certainly wouldn't
recommend that you try them unless
you were willing to take a risk. I
would imagine that some products
used to protect automotive rubbers
and plastics could, in theory, work,
but again you're on your own if you
decide to try, this is merely an
assumption on my part taken from the
results I've had with some of these
products on the rubber/plastic trim
of vintage cars. In any case, you'd
need to act before the foams started
to visibly crumble, after that time
there would be no point and you
would probably just cause more
damage just by applying the coating.
If you have rubber
surrounds you should be fine, at
least I've yet to see degraded
rubber surrounds as they all seem to
be synthetic (this doesn't mean that
rubber surrounds can't degrade, just
that I haven't seen any that have).
A similar situation with fabric
edged cones, with the exception of
the glue which bonds the surround to
the cone or the frame failing with
age, I'm yet to see any loudspeakers
with degraded fabric driver
surrounds.
Repairs
So, if you have a
damaged loudspeaker, what do you do?
Well, there's a couple of commonly
known repair routes:
Re-foaming: If you
have a problem with dried foam
surrounds, it's not the end of the
world. If you have some patience, a
steady hand and a few small tools,
you can find re-foaming kits if you
look around on the internet. for
some more common models of driver
(some JBL bass drivers etc), you can
buy foam kits which are claimed to
match the originals in terms of
stiffness and profile, so in theory
a driver repaired with a re-foaming
kit should sound exactly the same as
it would originally.
If you have a lesser
known driver (for instance a bog
standard manufacturer's driver from
a vintage loudspeaker), then you may
still be able to find a new set of
foams the correct size/profile to
repair them but they may not match
too well compared to the originals.
A word of warning, the stiffness of
a loudspeaker cone is very important
to its acoustic characteristics and
part of this stiffness comes from
the foam surround. Hence changing
the surrounds might alter the
acoustics of the driver, but
hopefully not by a great amount.
Re-coning: This is
the next step up from re-foaming,
and not anywhere near as easy to
perform unless you can find parts
which are tailor made for your model
of loudspeaker driver. Basically,
this keeps the original driver frame
(the spider) and magnet, but
replaces the cone, voice coil and
suspension. For this to work
properly you need to find
replacement parts made especially
for your driver.
If you don't, then
even if you're lucky enough to find
a kit which will physically fit,
you'll likely change the
characteristics of the driver (and
hence the resulting sound) by a
large margin. If you need to go this
far, you might be better to just
start searching for some replacement
loudspeaker components, unless
you're repairing a pair of
loudspeakers which were something
really special or you are keen on
experimenting and don't mind the
risk of ending up with something
unusable.
You can sometimes
replace voice coil 'diaphragm' units
on some better quality horn tweeters
and even on some professional grade
bass drivers, but on most
non-professional drivers things are
a great deal more difficult as most
often they weren't even designed to
be repaired in the first place.
I remember the days
when, long before I began repairing
higher end audio equipment, I tried
'repairing' some foam edged bass
drivers in cheap, low end
loudspeakers (we're talking
mini/midi systems here) with tissue
paper and PVA glue. Of course this
is a bodge, and I wouldn't consider
trying it on a good loudspeaker or
on an item that I would sell, but it
got old systems working again and
the restored, stiffer bass drivers
actually sounded reasonably good and
worked well despite the obvious drop
in bass response. If you value sound
quality at all however, you need a
higher quality repair which matches
the original driver specifications
far more closely. There are cheap
repairs which will physically get a
speaker working again but not
something you'd want to do if you're
looking for high quality sound.
Can music damage
loudspeakers?
Yes, it most
certainly can. Many people will
rigidly obey loudspeaker and
amplifier power ratings, hence would
never consider buying a pair of
speakers rated at 50 watts to run on
a 100 watts amplifier. However,
there's a great deal more to this
than some people realise.
Many years ago when
I began getting into audio, I bought
myself an old junk shop amplifier
and got it working. This amplifier
was rated at 45 watts per channel,
it sounded really nice. Soon
afterwards I came across a pair of
huge speakers with horn tweeters and
15" bass woofers, rated 350 watts
(yes, three hundred and fifty). I
assumed these speakers would be all
but indestructible on my small amp
but within weeks I had terminally
damaged both 15" bass drivers. Now
admitted, these weren't high grade
speakers, but they should have
managed far more than I put through
them. This taught me an important
lesson about amplifier power and
loudness.
Amplifier power
vs. loudspeaker power
If you look at the
specifications of a loudspeaker,
you'll likely find a maximum power
rating somewhere. This rating is
basically the amount of power which
the speaker can physically take
without suffering damage, this
damage usually being heat related.
So, you could be forgiven for
thinking that this means that
amplifier 'A' with 100 watts/ch
amplifier will kill your 50 watt
speakers but that they would work
fine on smaller amplifier 'B' which
was rated at just 20 watts per
channel. This is where you may run
into trouble.
Amplifiers have a
rated output power, just like
loudspeakers have a rated input
power. As with speakers, when
thinking about power output on an
amplifier a lot of factors need to
be taken into account. Power output
capability depends on many factors
including speaker impedance (speaker
impedance dictates how much current
that amplifier need to produce as
opposed to voltage), how much bass
control you use, even what type of
music you play. On paper, your
average 20 watt amplifier will give
those 50 watt speakers plenty of
breathing room, and if you always
run the amplifier at modest volume
and never push it past its limits,
you'll live happily ever after.
However, in reality,
most people like their sounds and
many people tend to like them loud -
thus your 20 watt amp will soon run
out of steam, especially if you like
your bass. It will then reach the
limits of the amount of clean power
it is able to produce and will begin
to 'clip' the signal, which
literally means that the music peaks
are cut off past a certain point
because they reach the limits of the
voltage swing which the amplifier
can output (clipping is loosely
known as distortion, you likely have
an idea what severe distortion
sounds like). Once you get clipping,
the voltage waveform which your
speaker is fed changes drastically
and creates a great deal more stress
in the loudspeaker than a clean
signal would, hence you'll now be
pushing your loudspeakers far harder
than the audible power output of
your amplifier would have you
believe.
This can cause both
thermal and mechanical damage to the
loudspeaker, if you put masses of
low frequencies through your
loudspeakers you could easily drive
the woofers hard enough to exceed
their 'X-max' and damage the
cone/voice coil alignment.
I'm really sticking
my neck out here but I don't ever
look at the power ratings on my
speakers, I just drive them with
whatever I think will work well.
Subsonics and
noise
Turntable 'wow' or
'rumble' is also another factor with
the possibility to damage speakers
if you're playing vinyl, this is
made up of low frequency signals
caused by combinations of mechanical
turntable noise, room acoustics, or
even record warp. Even something as
simple as being in a room with
suspended wooden floors (i.e. joists
and floorboards) can cause low
frequency sounds from footsteps etc
to be picked up by the turntable
stylus and reproduced by the
amplifier, with the resulting low
frequency instability it can cause.
This low frequency
sound isn't part of the original
music and because it's often very
low in the audible frequency range
it will very quickly consume huge
amounts of amplifier power, creating
extra distortions and also
potentially causing serious damage
to your speakers if the drivers are
over driven by those low
frequencies. Just because this sound
is barely audible, doesn't mean that
there's not a lot of amplifier power
going in to reproducing it.
Subsonic filters are
one method of countering this,
they're effectively a high pass
(usually -6 or -12dB/octave) filter
which gradually cuts any sound below
the low bass region and hence
removes a large portion of the
subsonic signal before it gets
amplified. Subsonic filters aren't
the ideal solution and there's an
argument that to some people the
subsonic portion of sound is
important even if the ears can't
hear it, subsonic signals are
filtered from most digital music and
tape is highly unlikely to be able
to reach anywhere near that far, but
vinyl in theory can reproduce
frequencies well outside the audible
spectrum.
Much higher end
equipment won't have any kind of
subsonic filtering because it's
deemed to be something which should
be corrected in the turntable
mechanics - this is fine if you have
the perfect turntable standing on a
perfect acoustically dead unit, in
an acoustically perfect room - many
people don't have this luxury.
If you can see your
loudspeaker's woofers jumping
visibly in and out during quiet
passages or between tracks you
really need some kind of subsonic
filter to cut out those low sounds
that shouldn't be there and that you
can't hear anyway. If you're playing
vinyl and have this problem and your
amplifier has a switch marked '15hz'
or 'subsonic' or 'rumble', you
really should use it to safeguard
both your amplifier and your
speakers. This should never be much
of a problem at low volumes but at
anything over mid listening volume
it has the potential to cause
problems.
Conclusion
In a nutshell, a low
grade, underpowered powered
amplifier driven hard will begin to
distort at relatively low volume
levels and will push your
loudspeakers far harder than an
amplifier four times the power
output would at that same volume
level. That's not to say a good 200
watts/ch amplifier won't cook a pair
of 50 watt loudspeakers if you push
it hard enough, of course it will.
But by the time it's
creating anywhere near enough heat
to cook them, your ears will already
be going numb from the volume
levels, your loudspeakers would
already be producing far higher
levels of sound than they were
designed for.
Be wary of using too
much bass, nothing will suck the
output power from your amplifier and
give you little audible loudness
more than low end bass boost will.
Also, even pops and clicks can
damage loudspeaker drivers if they
are loud enough, the high signal
level and high frequency for that
split second could be enough to
fracture the voice coil in a small
tweeter or even midrange driver.
