Loubeeka Podcast #2: Basics – Impedance Response for Newbies

Loubeeka Podcast #2: Basics – Impedance Response for Newbies

Impedance Response – what’s that? Why do we
need it? What’s it good for? That’s what I’m going to explain today. Hi there, this is Marius Loubeeka from Hamburg,
Germany and welcome to the Loubeeka Podcast number 2! Our today’s topic is Impedance Response.
I also call it Impedance Plot. The German version is linked in the comments below. First of all the impedance plot is a Bode
plot: the x axis is scaled logarithmically as in a frequency response plot. All decades
have the same width. That means that 10 to 100 Hz is for example as wide as 100 to 1000
Hz. Basically it depicts the resistance of a speaker at every single frequency because
speakers are a complex system. They contain a system similar to a mass connected to a
spring which causes the peak at the resonant frequency. The pole piece enlarges the inductance
of the voice coil which causes the rising ramp to high frequencies. Other resonances
within the driver can also cause peaks in the plot. Even a normal resistor does not
have a linear plot because firstly if they are made from a wound wire they have a small
inductance and secondly any in wire the proximity effect increases towards higher frequencies.
At high frequencies electrons tend to move only on the surface of a conductor and therefore
the resistance, the real part of impedance, increases. That of course is also the case
on a wound wire. It can be prevented by using multistrand wires, which have a bigger surface
than solid wires of the equivalent cross sectional area. But inside a resistor you usually have
solid wires. Thus the proximity effect can be observed from about 1 kHz on. The impedance response reveals much information
about the driver and even more about a complete speaker. Mainly the region between 0 and 20,
or let’s say 25, ohms is most interesting. If you only look at that range you have a
higher resolution and are able to recognize resonances better, or whatever interesting
there is to see. It is for example easier to determine the nominal impedance of a complete
speaker which is the lowest value in the whole range of 20 Hz to 20 kHz. A German Industrial
Standard defines that the minimum may be 20% less than the nominal value i.e. e.g. 3.2
ohms for a 4 ohms speaker. If the value is less than 3.2 ohms it has to be called 3 or
2 ohms depending on how low the value exactly is. The top of the peak at the resonant frequency
may be located outside of the diagram because it is only necessary to know the exact point
and value for TSP calculation. But that’ll be another podcast. Usually it is more interesting
to have more resolution to be able to see other things more clearly. Smaller peaks in
other parts of the plot may indicate further resonances. To find out exactly you have to
look at the distortion plot and the waterfall diagram and see if there are also resonances
visible at the same frequencies. Especially if they are visible on all three plots they
might be audible. If they’re not visible on all of them you can’t be sure but you know
what to look for in a listening test. The impedance plot of a whole speaker tells
you something about the enclosure type, number of ways and the crossover network. You can
see if the crossover network contains series or parallel RLC circuits. Series RLC circuits
in parallel to the driver usually cause a dip in the plot and parallel RLC circuits
in series to the driver cause a peak. Parallel RLC circuits in series to the driver are mostly
found in fullrange speakers – another topic for a future podcast. In multi-way speakers
peaks are normally near crossover frequencies. – A closed box has one peak in the bass.
– A high pass filtered closed box has a ramp below that towards the low bass because the
capacitor in series has a high impedance at low frequencies.
– A bass reflex box has a double peak with a minimum in between at the tuning frequency
of the port. At the crossover frequencies there is usually also a high peak.
– A horn speaker has many peaks in the bass which sometimes are very close to or overlap
each other. That is due to the fact that horns amplify several frequencies and also cancellations
appear. – A transmission line cannot clearly be determined
by looking at the impedance plot. There might be only one or even two peaks, that varies
depending on the design of the line. I looked at several projects in the German DIY magazine
Hobby HiFi and their impedance responses look really different. Another reason for that
is that Hobby HiFi often uses internal Helmholtz absorbers, Helmholtz absorber chambers, to
suppress particular resonances which therefore might not appear on the impedance plot.
– A leaky closed box can also be recognized. This is basically a bass reflex box where
the port is filled with absorber material. You use this for drivers with a high Qts,
mostly>1, that do not work well in either bass reflex or closed boxes. In a closed box
their frequency response has a high peak in the bass and the well-defined leak in comparison
attenuates that peak a bit. The impedance plot looks similar to a normal closed box
with the difference that the impedance peak at the resonant frequency is asymmetrical.
The lower slope of that peak is not as steep as the higher slope. It looks more like a
sawtooth. I’m planning on making podcasts about enclosure types and then I’ll also tell
something about leaky closed boxes. You can also determine how big the output
impedance resp. the damping factor of the amplifier needs to be from looking at the
impedance response of a speaker. If the speaker has a big range in impedance the output impedance
of the amp needs to be small i.e. the damping factor has to be big. Valve amps for example
have the disadvantage of high output impedances which may be even 3, 4 or 5 ohms. That forms
a voltage divider with the speaker which causes a deformation of the frequency response of
the whole system. The more the impedance varies the more the frequency response gets deformed.
The impedance response is also probably important in combination with class-d amplifiers, aka
digital amps. Tests of those in Hobby HiFi showed that some have the same effect at high
frequencies. This is caused by the low pass filter which is installed on their speaker
terminals to filter out the carrier frequency. The cut-off frequency of this low pass is
determined by the impedance of the speakers and can cause am amplification at a high impedance
or an attenuation at a low impedance. How much depends on the inductance of the coil.
It might be up to 2 to 3 dB. Another thing you can see in impedance responses
of bass reflex boxes is the Q factor of the enclosure. In Hobby HiFi issue 4/2004 for
example you find an article about how to calculate this from measuring the height of the minimum
between the two peaks in the bass in comparison to the DC resistance. Basically this shows
how leaky the box is and especially in a bass reflex box you want the acoustical energy
to leave only through the port and not lose it through gaps or deformation of walls. Furthermore with the help of an impedance
response you can measure capacitance and inductance values of crossover parts. By measuring the
impedance curve of capacitors, coils and resistors in the software LIMP, which I use, you can
calculate resistance and capacitance or inductance by one click at any frequency where you place
the cursor on the plot. That works, because LIMP also measures the phase of the impedance.
Impedance, in equations represented by the letter Z, not only consists of the argument
of Z which is shown in the impedance response but also of the phase difference between voltage
and current. Okay, buckle up, now we take a short detour through the kingdom of complex
numbers and the complex calculation of alternating current. You can depict impedance as polar
coordinates for one frequency. The argument of Z from the impedance plot is the length
of the pointer at one particular frequency in polar coordinates and the angle between
the pointer and the positive x axis is the phase. You could also calculate Cartesian
coordinates from that. The x value is the real part of the impedance, the y value the
imaginary part. For an ideal resistor the real part is exactly the resistance and the
imaginary part should be zero. For ideal capacitors and inductors the real part is zero and the
angle is plus or minus 90 degrees. In German we have two nice mnemonics: “im Kondensator
eilt Strom vor” something like “in a capacitor the current hurries to be the first” and “in
Induktivitäten die Ströme sich verspäten” which means “in inductors the currents are
late”. I also found an English mnemonic for that: “ELI the ICE man” E stands for voltage,
I for current, L is an inductor and C a capacitor. Now ELI, E-L-I, means: E leads I in an L,
i.e. voltage leads current in an inductor, and ICE means: I leads E in a C, i.e. current
leads voltage in a capacitor. This shows the shift between voltage and current. Basically
this a bit too complicated to describe it only with words. It might be a topic for a
video. On the other hand there’s the possibility to look it up at Wikipedia. There might be
a descriptive sketch to understand it better. That’s all for today. Next planned topics
are crossover networks, distortion plot, waterfall spectrum, Thiele-Small parameters and various
enclosure types. If you have other suggestions please write a comment below. If you found
a mistake please have a look at the comments to see if I already corrected it. If I didn’t
then leave a message! Thank you for listening and I’m happy if you listen to my other podcasts
or watch some of my videos. If you like what I do please subscribe. Until next time! Bye!

1 thought on “Loubeeka Podcast #2: Basics – Impedance Response for Newbies

  1. zum messen der Lautsprcher kann man sehr schön SATlive nehmen von Thomas Neumann www.satlive.audio – es gibt div PDFs mit Erläuterungen und Background als auch einen youtbue kanal der div Anwendungen zeigt

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