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Simple method for determining Speaker Imp?

Discussion in 'Electronic Design' started by Arch-lab, Nov 4, 2003.

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  1. Arch-lab

    Arch-lab Guest

    Hi,
    I was wondering if there is a simple method to determine the impeadance of a
    speaker?
    I could bring a meter and a function generator etc. to do it how I would in
    the lab.... but this is at a store and I'm not sure I want to lug my equipment
    there yet.
    Incase you are wondering it is a store that has a bunch of raw speakers, they
    know the power rating but don't know where they came from, or if they are 4 or
    8 ohm.

    Thanks
     
  2. Arch-lab

    Arch-lab Guest

    .... or I guess I'll just take my ohm meter if nothing simpler... It seemed fine
    when checking an 8 ohm and 4 ohm speakers I had kicking around...
     
  3. Boris Mohar

    Boris Mohar Guest

    You were measuring the DC resistance of the voice coil and not the speaker
    impedance.
     
  4. default

    default Guest

    DCR is probably good enough. Many of the 8 ohm large speakers I
    worked on measured 6 ohms and 4 ohm ones measured 3 ohms DC.

    I think you'd have to know the impedance the manufacturer tests at to
    make a real determination - I don't know for a fact, but it seems a
    little daft to be testing a tweeter that runs from 8 K to 20 K at
    1,000 HZ. It wouldn't do to test at some mechanical resonance of the
    speaker.
     
  5. Arch-lab

    Arch-lab Guest

    Yes, I know that, as my first post mentioned needing a Frequency generator I
    figured that was enough to prevent someone thinking I didn't know what
    impedance was...
    and it also mentioned that all I wanted to do if figure out if these unknown
    raw speakers are 8 or 4 ohm ones.
    By doing a tests I was able to seperate all the raw speakers I had kicking
    around into their correct markings of 4 and 8 ohm just on their resistance
    measurement.
     
  6. Right. But does everyone agree that if a voice coil measures 6 ohms, it
    can't be a 4 ohm speaker?

    This is just a way to eliminate some possible values of speaker
    impedance. It's not an exact measurement.
     
  7. Arch-lab

    Arch-lab Guest

    Exactly, it is not an exact measurement, I'm looking for an easy method that
    fits the goal?
    I'm looking for an easy way to figure out what the raw speakers are in a STORE,
    and would prefer to not bring in my Frequency generator etc...
    The DC Meter seems to be the only thing that is easy and "appears" to work. My
    8's actually read 8, and my 4's actually read 3.99 with just a resistance
    measurement.

    p.s. regarding your 'eliminate some possible values.... exact measurement' ?
    You realize it is an 'exact DC measurement', ;-) just like if we were looking
    at impeadance it would have to be specified at a particular excitation
    frequency which just gives a value at that frequency. Just getting at you would
    need to make a complete chart of frequencies, if you want an 'exact
    measurement' AT a particular frequency, but we're just going for are these 8 or
    4 ohm speakers? Sorry, just having fun - just like bugging people who talk
    about 'exact measurements'.

    message Right. But does everyone agree that if a voice coil
    measures 6 ohms, it
     
  8. N. Thornton

    N. Thornton Guest

    Spot on. Holds true for moving coil units, but not the more uncommon
    things or piezos.

    Regards, NT
     
  9. Ben Bradley

    Ben Bradley Guest

    You could get a BK 878 L-C-R meter like I have, it's
    battery-operated, the size of a handheld DMM and could measure the
    inductance (and 'Q' of the equivalent inductor) of the speaker at 1kHz
    and 120Hz, and from that you could calculate the impedance at those
    two frequencies. It's even possible to program a microcontroller to do
    a frequency response scan over a full audible frequency range and save
    dozens of such scans in flash memory, all in a handheld device, for
    later transfer and display on a PC.
    But that still won't tell you the Theile-Small parameters, which
    are essential to match a cabinet to a low-frequency driver (read most
    any book on speaker building). Also, to answer your original post, if
    this dealer doesn't know the T-S parameters, know the impedance of
    these things, nor where they came from, I wouldn't trust him to know
    the power rating either.
    There are ways to measure impedance and T-S parameters (at home,
    it's not practical in a store), and if you're willing to sacrifice a
    few drivers to destructive testing, you can measure power rating as
    well.
    It may be easier to buy drivers from a seller who makes all these
    parameters available. Here are two:

    http://madisound.com/
    http://partsexpress.com/
     
  10. Robert Baer

    Robert Baer Guest

    An impedance plot like you mention is meaningless, as the
    characteristic changes according to enclosure - which includes the room
    and contents of the room.
    The "definition" for a speaker is at 1KHz, and i think is the real
    term, but some might use the magnitude.
    Any decent DC meter should read a resistance far less, close to a
    short circuit at the voice coil of a speaker; anything higher and most
    espically close to the rated impedance is extremely suspect, to say the
    least.
     
  11. Arch-lab

    Arch-lab Guest

    What? "close to a short circuit" ? you don't think there is resistance in a
    huge coil of wire? Have you tried using a DC meter on a speaker - or coil of
    wire for that matter, doesn't seem very "suspect" so far.
    I mean the model of an inductor includes Rs of the wire for a good reason...
     
  12. Arch-lab

    Arch-lab Guest

    Also, to answer your original post, if
    Power rating is the only thing stamped on these raw ?sonys they might have
    been?... so I can hope the MFG did it right. Pretty silly they didn't put
    4,8...
    They don't just sell speakers, they move batches of 'anything', so when
    something like this comes along that they know nothing about, they sell for
    next to nothing because they could care less.
    I've bought some very good deals because they are just trying to move
    everything and they might not even know what it is.
     
  13. N. Thornton

    N. Thornton Guest

    We could take it further and point out that there is no exact
    measurement of a speakers impedance when used in real world apps. The
    reason is that impedance varies a lot with frequency, and real music
    is a varying mix of different frequencies. Moving coil speakers have
    no genuinely fixed impedance. If you measure v/i as the music content
    changes, you'll find the total impedance keeps changing. This is why
    impedance is no more than a nominal figure.

    If you move away from moving coil speakers into the less common types,
    generally the picture is much worse. Look at the impedance/frequency
    plots for piezos, moving iron, or electrostatics. Old books from the
    20s and 30s often point out that a single moving iron speaker's
    impedance can range as wide as 100 ohms to 10k ohms. With speakers
    like that amp design becomes challenging - or one gives up and resorts
    to 'yeah, that transformer tap sounds good'.

    depends what you're doing.
    maybe true, tho power rating is often stamped onto the speaker magnet.
    Just beware of the usual mix of rms, peak power, music power, and
    pmpo. Or they might have been stripped from speaker systems with known
    ratings.

    It does depend what you're doing. If you buy full spec units, they
    cost. If you buy stuff with almost no specs, as I gather the OP is (or
    perhaps looking to sell some) they're a fraction of the cost. If you
    take a look at them and stick them in the nearest guess of box size,
    you then have very very cheap speakers, which are usually fine for a
    number of apps, if not all. So it does depend. Sometimes the 'right'
    advice isn't always the right advice.


    Regards, NT
     
  14. Yzordderex

    Yzordderex Guest

    Spot on. Holds true for moving coil units, but not the more uncommon
    yea, just use dc resistance to measure. After all it's just a little motor.

    73
    Bob
     
  15. Jim Meyer

    Jim Meyer Guest

    Build a small battery powered 1 Watt audio power amp and a
    variable frequency audio sine wave generator. Each section could be
    built with a single suitable chip. Make a two position switched
    attenuator to go between the generator and amplifier. One position
    with no attenuation and one that delivers half the voltage to the
    amplifier. A second section of the attenuator switch should either
    insert or remove a 0 to 20 ohm rheostat (pot) in series with the
    amplifier output. The pot should be in the circuit when the
    attenuator is set for no attenuation.

    Connect the system to a speaker and set the generator to a
    frequency that is appropriate for the speaker you're testing. Switch
    the attenuator switch back and forth while you vary the pot setting.
    At some point you will be able to hear that the volume level at either
    switch setting will be the same. If you've calibrated the pot and
    added a scale, you will be able to read the speaker's impedance on the
    scale.

    Jim
     
  16. I like this. It's only the "calibrate the pot and adding a scale" part that
    might be a bit of a pain. I suppose that starting it out with some speakers
    that are marked might work, unless they end up giving me some radically
    different results for similar (8 ohm, say) markings...
     
  17. The DC resistance will generally be somewhat lower than the nominal
    impedance. As for measuring the actual impedance, that can be done
    with a frequency generator, meter and variable resistor but cannot be
    reduced to a single number as the impedance varies widely with
    frequency.

    So, unless you are measuring carefully with the intention of
    characterizing the impedance vs. frequency for design purposes, the
    single DC measurement gives you the basic info.

    Kal
     
  18. default

    default Guest

    Seems like a lot of effort. Why not use the amp/generator to feed a
    balanced bridge and calibrate the pot directly in ohms? You can do
    that with your ohm meter.

    You intend to use an instrument as subjective as human hearing to
    calibrate with? Monsieurs Fletcher and Munson are twisting in their
    graves.

    You'd be doing all this to establish the impedance at one frequency
    when a speaker is an electromotive device and has mechanical
    resonances. That is to say, the impedance will vary with frequency up
    and down as you go through the mechanical resonances. You'd be
    testing a bare speaker without the enclosure which is part of the
    system, at a frequency that may entirely arbitrary, to arrive at an
    impedance that is nominal at best.
     
  19. James Meyer

    James Meyer Guest

    The pot calibration should be straightforward. Measure its resistance
    and mark the scale accordingly. It should be a 1:1 correspondence. The pot
    resistance should be equal to the speaker impedance when there is no volume
    change between the gain switch positions.

    Jim
     
  20. James Meyer

    James Meyer Guest

    A bridge circuit is an unnecessary complication.
    No. You would only be listening for a volume *difference*. That's the
    beauty of the scheme. No matter how nonlinear your ear might be to volume
    changes, you can pick out volume differences very accurately. As soon as your
    ear cannot detect the difference between the attenuator settings, then the
    "bridge" is balanced and the pot setting is equal to the speaker's impedance.
    Exactly! Using my suggestion, you could determine the speaker's
    impedance at *any* frequency that interests you. In an enclosure or bare.
    Isn't that a lot better than trusting the manufacturer's nominal impedance
    rating?

    Jim
     
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