My speakers are 4 ohms but my amp is 8 ohms,
can I use this combination? And what is ohms for heaven’s sake? Can I use two speakers per channel and why
do they put ohms in it anyway? Well, time to shine some light on those illusive
ohms. In this video I will try to explain the concept
of loudspeaker impedance to people that do not have a degree in electronics. To do that I will cut corners and simplify
things. So to those that do have a degree in electronics,
this is not for you. OK, now that we are amongst normal people,
let’s look at the topic at hand. Most loudspeakers have motors that use electro-magnetic
force to move the cones: there is a coil that is attached to the cone and mounted inside
a magnet structure. If you apply a voltage by connecting for instance
a 9 volt battery, a current starts to flow through the coil which generates a second
magnetic field. Depending on the polarity, this makes the
cone move inwards or outwards. When the voltage is disconnected, the cone
returns to the middle position. Now it is time for a very important law of
physics, and I quote Albert Einstein: “Energy cannot be created or destroyed, it can only
be changed from one form to another.” Now, the coil in the speaker motor works in
such a way that it resists the current flow to a certain degree. This causes the magnetic field and heath. The magnetic field is what we want, the heath
is something we don’t want but we have to live with. The battery we connected produces a direct
current, the currently flows only in one direction and the degree of resistance is called …. electrical
resistance. It is expressed in ohm. Engineers use the omega sign for ohm. But sound is caused by variations in air pressure
and if a loudspeaker want to produce sound, it needs to cause variations in air pressure
by moving back and forth. So the current though the voice coil needs
to go back and forth too. Technicians call this alternating current
since the current alternates. It changes from one direction to the other
so to make the air move and cause differences in air pressure. Just as instruments do. It’s as simple as that. Unfortunately the resistance of the coil is
not the same at every speed of change. The resistance might be lower at fast movements
and higher at slow speeds or vice versa. High speeds produce tones we hear as high
pitch and slow speeds produce low pitched notes. OK, let me tell you another engineers secret:
the speed of change is called frequency and is expressed in vibrations per second or hertz. Low tones give low hertz figures and high
tones high figures. The lowest tone we can hear is 20 hertz, the
highest 20,000 hertz when we are young, say around 15,000 hertz half way our life and
perhaps as low as 10.000 hertz when rather old. Unless we turn deaf, but that’s another
story. Now, why does the resistance change when the
frequency changes? Simply put it has to do with the slowness
of things, including electrons. That is perhaps best explained using the behaviour
of a car. If your car is stationary and you put your
foot on the accelerator, the engine starts applying force to the wheels and the car will
start moving. But it will take some time before it reaches
the speed you want. At the crossing you will apply the brakes
but again it will take some time before the car stops. The mass of the car impedes acceleration and
deceleration. All kinds of properties of loudspeaker parts
impede the current in a similar manner, meaning that when the amp applies a voltage to the
loudspeaker, it will take some time before a current starts flowing that in turn starts
the secondary magnetic field that causes the cone to move. The reverse applies too, if the voltage becomes
zero, the cone doesn’t stop right away. The kinetic energy in the cone makes it move
somewhat longer. Electronically, similar behaviour is present
that impedes the flow of electrons. The degree in which the loudspeaker impedes
the current is aptly called impedance and again is expressed in ohm. Why do they bother us with this impedance. Can’t they take care of it? Well, no. Loudspeaker technique might be one of the
most complex technique of your stereo since it involves electronics, mechanics and acoustics. This means that all kinds of compromises have
to be made, some of them involving the impedance. The key for you is that the lower the impedance
the higher the current. And the higher the current, the more heath
is generated in the amp. That is why amplifier manufacturers specify
‘the minimum load’, the lowest impedance. If the amp is able to work with 8 ohm speakers
without getting too hot, the brochure, the manual and the rear of the amp will state:
for use with 8 ohm speakers. Will the amp break down as soon as you connect
4 ohm speakers? No, and if you don’t play too loud, the
amp will keep working. Under normal domestic conditions the amp only
has to provide little power, often around a single watt. But be aware: if you play at higher level,
the amp might over-heath. Many modern amps will simply shut down and
work again when cooled down. But some amps that lack such protection circuit,
might simply fry their power transistors or equal. If your amp is specified to work with 4 ohm
speakers, as most modern amps are nowadays, you can of course use 4 ohm speakers. But also 8 ohm speakers for the higher impedance
will lead to a lower current and thus lower power. That is why amps have a lower power rating
in 8 ohm speakers than in 4 ohm speakers. Well that’s clear then: use only 8 ohm speakers
when your amp is specified for 8 ohm speakers. If your amp is specified for 4 ohm, you can
use 4 and 8 ohm speakers. Or more correctly: any speaker with an impedance
of 4 ohm or higher. Nowadays manufacturers often specify a range:
4 to 8 ohms, for instance. It is good practice to stick to that. Is it all clear now? OK, let’s make it a bit more complicated:
there are no 4 or 8 ohm speakers. Yes, the spec’s are absolutely legal, at
least in most cases, for the standard is to measure the impedance at 1kHz and if a lower
impedance occurs at another frequency it should not deviate more than 8% or the spec should
be adjusted. All fair and square. But let’s have a look at real world measurements
and let’s start by measuring an 8 ohm power resistor I use for measuring amps and that
does nothing more than convert power into heath. Hence the cooling profiles. I use my automated measuring beast, the Audio
Precision System 2 Cascade and it is set up to apply a range of frequencies at a given
voltage and measure the voltage drop over the loudspeaker terminals and the voltage
drop over a sensing resistor. From this two measurements the computer calculates
the impedance. You can easily forget this if you don’t
get it or google loudspeaker impedance measurement if you want to know more. When you listen to this video over headphones
or a large stereo, you can hear the measurement tones start at 20 Hz. From there it goes up to 30 kHz in 200 steps
and after each measurement the next frequency is generated, measured and plotted. Whatever you do, don’t increase the volume. Measurement tones can cause damage when played
too loud. Here we go. As you can see, the green line is nicely at
8 ohm, as indicated on the left scale, ignore the m’s behind the values. Now it is time for a normal loudspeaker. This speaker is specified at 8 ohms. It takes the Audio Precision relatively long
to do the measurements below 100 Hz. This is because the impedance isn’t too
stable at low frequencies. The Audio Precision does several measurements
and is only satisfied when the outcome seems reliable. Again the green line is the impedance, the
other line I’ll explain later. Here we go. As you can see, the measurement at 20 Hz starts
at 5.8 ohms to go to 15.5 ohms at 48 Hz. Then goes down again to 6 ohms at 70Hz and
even 22 ohms at approximately 120 Hz. It even goes to 27 ohms at 2.5 kHz. As you see, this 8 ohm speaker is anything
but an 8 ohm speaker. And most speakers are like this, some are
a lot worse even. Then there is the light blue line that indicates
how much the current will lead or lag the voltage. Remember the car where you apply the horsepower
before the car reaches the desired speed? Here the horsepower is ahead of the speed. And when braking the speed is ahead of the
power. The light blue line indicates the comparable
phenomenon for speakers. The scale is on the richt and in degrees. At 20 Hz the phase angle is - 20 degrees,
meaning the current is slightly ahead of the voltage. At 55 Hz the phase angle is +20 degrees so
here the voltage is ahead of the current while at about 150 Hz it is about +35 degrees. And so on. It might be clear that the best efficiency
is achieved at zero phase angle. If the phase is negative at a frequency where
the impedance is very low, it causes big currents provided the amp can deliver. If not, the amplifier is no longer in control
and that leads to poor performance or worse. There is much more to say about loudspeaker
impedances, but I won’t The interaction between the amp and the loudspeaker of course
is very important. If the loudspeakers behave nasty, you better
find an amp that can deliver shiploads of current. But how do you know what current an amp can
deliver? Even professionals only know for sure if a
combination of amplifier and speakers will perform well when they have tried. The information in this video is good to know
for it gives more insight making you les prone to false claims made by sellers or resellers. But unless you know for sure a combination
of amp and speakers works fine, like from a friend who owns that combination, always
try to listen to the set, using your own music. For now I have decided not to review speakers,
with the exception of those that offer wireless streaming at minimally my setup 1 quality. And I am told some are in the pipeline. So subscribe to this channel or follow me
on Twitter, Facebook or Google+. See the show notes for the links. If you liked this video, please consider supporting
the channel through Patreon or Paypal. Just one dollar or more a month is appreciated. The links are in the show notes, just as the
link to a description of my three setups. Help me to help even more people with their
stereos by telling your friends on the web about this channel. I am Hans Beekhuyzen, thank you for watching
and see you in the next show or on theHBproject.com. And whatever you do, enjoy the music.
