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Discussion in 'Electrical Engineering' started by Dave McMahon, Oct 29, 2003.

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  1. Dave McMahon

    Dave McMahon Guest

    I know that James Watt gave his name to this measurement, Do you guys know
    how to measure it? I am trying to measure the max wattage of a sub woofer I
    have a mutimeter and I know the sub has a rating of 4 ohms, I don't want to
    find out by testing it (bang)
  2. John Gilmer

    John Gilmer Guest

    Watt improved the steam engine invented by Newcoming (sp?). One or the
    other invented the term, "horsepower." It was a natural to name the basic
    unit of power after Watt.
  3. daestrom

    daestrom Guest

    Well, more correctly it was named after him to honor his work in early
    thermodynamics. You're right he didn't have anything to do with it

    (last paragraph)

  4. The easiest way to do this is to read the label on the back of the speaker.

    If you *really* *seriously* want to know what your sub is rated at, you
    would have to:
    1. Hire/borrow/etc/etc. an impedance analyser from some place.
    2. Hook it up in an anechoic chamber (or wear suitable hearing protection,
    or both ;-)
    3. Chart the speaker coil impedance over the range 0Hz to 20kHz (for a sub,
    the impedance after about 5kHz won't tell you anything useful, but if you've
    got the gear you might as well measure it).
    4. Take the lowest impedance value recorded and plug it into the standard
    formula to calculate the Max Power (RMS) at this point.
    5. You now know your maximum Wattage. Congratulations!.. Depending on the
    quality of the speaker, the measured value should be near enough to, but
    slightly higher than, the value written on the label on the back of the

    The only use for a multimeter in an audio environment is to check the
    continuity of your speaker leads. ;-)

  5. Jerry G.

    Jerry G. Guest

    Measuring the wattage rating of a speaker driver is fairly complex. You
    cannot do this with a simple multimeter. For true sinewave power handling
    it would require driving the speaker with a low distortion sinewave,
    sampling at a group of frequencies to represent the full design spectrum of
    the speaker, via the proper amplifier. The current draw of the speaker,
    voice coil temperature, and acoustic distortion factor would have to be also
    measured under driven conditions. The speaker unit may over saturate at
    different levels, depending on the frequency. Speakers usually have
    problems to handle the lower frequencies than the higher ones, because of
    the larger mechanical movement demand.

    Some manufactures publish the base power rating of their speakers at 400 Hz,
    while some at 1,000 Hz. Some will publish the true sinewave power, and
    others will publish so-called "music power". The best power rating spec
    should be published for the full spectrum of the speaker design. Music
    power is not an accurate or real way to rate power. I would guess that they
    put some type of music in to the speaker and then judge the point where the
    speaker would be destroyed, or may start to over-extend itself.

    There are low end speakers that I see at flea markets, and shops that have
    published power ratings that are way out by real standards. I have no idea
    of how they get their numbers. I would think that they feel to put any good
    sounding number on them. I have seen small cheap 6 inch shelf speakers have
    numbers such as 200 or 500 Watts on them! I am sure if I would connect up a
    typical Crown or Altec amplifier on these, the voice coils would be shot
    across the room as soon as the first drum roll or click comes along. The
    best one are these little computer speakers that are almost pocket sized,
    and they say 300 Watts on them. I think they should divide this by about
    100! I have no idea where these numbers come from...

    Usually when testing speaker types for their maximum power handling they may
    be damaged during the process. It takes the proper conditions and
    sophisticated test equipment to do it properly. A speaker is also a
    reactive device, thus this is how the term impedance was derived. The
    actual load or impedance of the voice coil is also dependent on the applied
    frequency. The 4 ohm rating of a speaker means that the impedance of the
    speaker should be 4 ohms at the rated reference frequency. At the same
    time, the voice coil may have a DC resistance of 4 ohms, but using DC to
    measure the voice coil is not a proper test. In many cases, the voice coil
    may measure lower than the rated impedance. Impedance is a reactive
    quantity, not a DC quantity. Most manufactures use 1000 Hz as the reference
    frequency for the impedance test. This frequency is also used as the start
    reference base for the testing process. A very good simplified explanation
    of speaker reaction and characteristics can be looked at:

    Many manufactures do not publish true specs, and only the very high end ones
    can be trusted. Many also estimate the theoretical power handling
    capability. The power handling capability of a speaker is also arbitrary.
    If the amplifier is putting out some distortion, or some clipping from being
    slightly over driven, the speaker voice coil will overheat quicker, thus
    causing a break down at lower power than what the speaker was rated at, even
    if it was properly rated.

    To find out the rating of any speaker from what the manufacture says it is,
    it is best to contact the manufacture for such details.



    Jerry Greenberg GLG Technologies GLG

    I know that James Watt gave his name to this measurement, Do you guys know
    how to measure it? I am trying to measure the max wattage of a sub woofer I
    have a mutimeter and I know the sub has a rating of 4 ohms, I don't want to
    find out by testing it (bang)
  6. Kind of.. I must admit that the ratings they put on amps and speakers these
    days can be a bit erroneous depending upon where you are and where they were

    Most (all good) amp/speaker manufacturers will give a rating in "True RMS" -
    probably somewhere in fine print on the back page of the manual and as far
    from any advertising material as they can get - and you can reliably use
    these ratings to configure your system. Rule-of-thumb is that the Peak RMS
    power of the speaker should always be higher than the per-channel Peak RMS
    power of the amp.

    Totally ignore ratings like "PMPO" (Peak Music Power Output) - usually the
    absolute peak (not RMS) power of all channels of the amplifier into 2(!)
    ohms x the maximum possible number of speakers (note: 1 mid/high/sub = 3
    speakers) you could physically connect with destroying the entire unit - but
    sometimes they include a large fudge factor as well to improve sales.

    To put all this in perpective (FWIW), a comfortable listening level for
    music at 1 meter from a speaker is 1 Watt RMS. And since volume is
    logarithmically related to power, 10Watts is twice the volume of 1Watt...

  7. Don Kelly

    Don Kelly Guest

    You are assuming that at the lowest impedance value the speaker is
    resistive. However, the power with this impedance depends on the voltage-
    that is a measurement of impedance alone will tell you nothing about the
    power unless you have this other information.
    However, given knowledge of the impedance and the voltage or current- you
    then only have a figure for the power input under the conditions of the
    test. This is not a measure of the maximum allowable power for that
    speaker. Crank up the volume and the power into the speaker goes up but the
    impedance at a given frequency will not change or if it does- it will be at
    or near the point of no return. That is a sure way to test so that you know
    what not to do with the replacement speaker.
  8. No...ish. Given the impedance at it's lowest value and knowing the max
    current rating of the coil (Amps) should be enough to calculate the max
    power, assuming coil burnout is the most likely cause of failure (not
    over-extension of the core, etc. etc.). P=I^2*R last time I checked (but I
    haven't got a textbook nearby ;-).
    But you don't have any control of the voltage - that's completely up to the
    amplifier manufacturer. At some point in the impedance curve, the
    *amplifier* will current limit (at Max Output Power of the amp), but the
    speaker may be stuffed by then...
    Not the power per se - the *voltage*.
    Certainly one way to calculate max power would be to feed the speaker under
    test from a humungous amplifier and wind up the volume on white noise until
    the coil burns out. Instantaneous volts and amps at the time would give you
    a good indication... but I assumed the OP wanted to use the speaker

    Regardless of voltage, if the current that the amp delivers exceeds the coil
    rating you get burnout - so the Max Power of the speaker must be related to
    impedance and current only (excluding coil dynamics and acoustic effects).

  9. No you bloody don't.

    I'm merely trying to be helpful and learn something on the way.

  10. As stated below. Please read the *entire* post (and the thread too)
    Please read the entire post. My point was that amplifier manufacturers can
    use whatever output voltage they like. If they want to drive a speaker
    using 2 volts @ 100 amps or 100 volts @ 2 amps there is nothing to stop
    them - the output power is unchanged.
    Crap. That *is* the power rating of the amplifier - regardless of what they
    might say in the manual.
    Like I said - please read the entire thread.
    ...And exactly where did I talk about "instantaneous power"??
    Oh, go away. I was having a good discussion with Don until you turned up.

  11. Don Kelly

    Don Kelly Guest

    No problem here -provided that you know the maximum current rating.
    That was not addressed or stated in either your post above or the
    original thread.
    I will stand by the statement that impedance <alone> won't give the power
    rating. That was the point I wanted to make.
    True -I should have said current rather than voltage (recognising that
    losses and cone excursions are current related) but, for the minimum
    impedance situation under discussion , the speaker will appear purely
    resistive and the voltage and current are intimately related and power can
    be expressed in terms of voltage. (P=VI =I^2R=V^2/R)
    At other times where the impedance is not resistive,, it definitely makes
    sense to consider current rather than voltage as you indicate.
    True enough -However, at any point in the impedance curve, knowing the
    voltage is sufficient to determine the current. You can't have one without
    the other. If you know the impedance in both phase and magnitude (hence the
    reference to the minimum being the resistive case)then knowledge of the
    voltage can be used to determine power. Use whichever approach is convenient
    and fits your comfort zone best - although, admittedly measuring current is

    The speaker may or may not crap out at the amplifier current limit so the
    current and the power at this point is not a measure of what the speaker can
    handle. What the amplifier can do is not a measure of what the speaker can
    handle (without going to destructive testing).

    It boils down, as you indicate to having to know the maximum allowable
    current. The manufacturer may be the only one with this (or the "power"
    rating) information. The power rating would also have to assume that the
    nominal impedance is resistive and applicable -otherwise it is a can of
    As the voltage goes up with a fixed impedance (as stated above) so will the
    current and the power. (I^2R or V^2/R are not basic but are derived
    In the sense that the thermal limit of the speaker and the maximum force
    produced (affecting coil excursions are current related -Yes.
    In the sense that voltage and current are directly related at any impedance
    and, in the minimum impedance case, as discussed, ignoring the L/C and
    mass/spring factors , the speaker appears as a resistance -then using I^R or
    V^R are equivalent. At other impedances, it is best to use curren tas the
    criterion. That is - don't exceed a given current is a better rule than
    don't exceed a given voltage as the latter requires more information than is
    generally available.
    Is it more sensible to use current as a limit rather than voltage ?
    Definitely- because the impedance may not be resistive and the terminal
    voltage higher but the current limit and the approximate real power will be
    the same. In fact it makes more sense to have a current rating on a speaker
    than to have a "power" rating.

    However, we have been discussing the resistive case -mid frequency
    situation. ignoring L,C,Mass ,spring etc and being stuck with coil
    resistance assuming coil losses far exceed the mechanical power output- in
    that case -use of V or I are a matter of choice in determing power.
  12. Don Kelly

    Don Kelly Guest

    2V @100A implies a speaker load of 0.02 ohms.
    100V @2A implies a speaker load of 50 ohms
    Since the speaker impedance is independent of the amplifier, one must
    consider what that amplifier can do with a given speaker load.
    2V across a 4 ohm speaker results in 0.5A and a power input of 1W (assuming
    4 ohms resistive)
    100A in a 4 ohm speaker requires 400V and a power of 40KW
    Quite different kettles of fish.
    The voltage/current/power characteristic of a speaker is determined by the
    speaker (i.e. what is downstream of the speaker terminals), not the source.
    You mentioned instantaneous voltage and current as a measure - these
    determine instantaneous power, not average power. There is no relationship
    between instantaneous current and voltage and average power. If you are
    referring to the rms voltage and current and average power -that is
    different but still it is not a good measure unless failure occurs after a
    sustained period of time at this level.
    .. -----
  13. Guest

    C'mon Keith, there *must* be. After all, if the manufacturer
    rates the equipment with that term, people will pay more for
    it. Of course, if you want to maximize the "true RMS power",
    you must connect the amp to the speakers using gold plated,
    argon filled, 4000psi pressurized, pre-stretched monster cables
    that you wash weekly with "Signal Kleen". Someone will be willing
    to sell you a bottle of that for 10 dollars an ounce. :)
  14. Given the resultant confusion, I should probably have originally stated that
    the only thing you need to *measure* to do the calculation is impedance.

    The maximum current rating is directly proportional to the wire size (in
    mm2) used to make the coil (basically, from manufacturer's charts). I'm
    sure there is a formula to calculate this, but I have only ever seen the
    charts - and then only in distant memory.

    Note here: I didn't really expect the OP to try my method when it is easier
    to read the label, so I didn't bother spelling it out in fine detail -
    perhaps I should have.

    I don't disagree. I was trying to clarify your point that by changing the
    volume of an amplifier you are directly controlling the voltage only - not
    the "power" (voltage x current) as such. The load determines the current
    draw. It's a minor nit - assuming the impedance is fixed, which being
    frequency dependent to some extent, it is not.

    This comment was relating to empirically calculating the maximum current
    rating of the coil. The amplifier voltage & current at the time of failure
    will give you an upper limit on the max power handling capability of the
  15. Guest

    Actually, the impedance of the speaker has nothing to do with
    how much power it can handle. The op said:
    "I am trying to measure the max wattage of a sub woofer"

    Did you have that in mind when you posted the above? Or
    are you thinking of doing the calculation based on wire
    size in the coil of the speaker? If the latter - you would
    have to measure the wire size in addition to the impedance.
    I guess I'm totally missing your point.
  16. Guest

    Yup! And I didn't even think of it. Where's the dunce cap
    when I need it?
  17. Don Kelly

    Don Kelly Guest

    Wire size is not necessarily a good measure of current rating. Note that
    nominal ratings apply to wire in air- not wire wound in a coil and enclosed.
    There may be charts for this situation which fit particular devices (they
    cannot be general) - in fact, I am sure that there are. However this does
    beg the question- if one knows the manufacturer and the speaker model, then
    the manufacturer will have the information needed, without the need to run
    If not, then the impedance test won't tell you anything about the power
    capability of the speaker and looking at the wires won't help. .

    No problem, It is like using a fuse tester which runs the current up until
    I thought that I said or at least implied or assumed as "a priori" knowledge
    that the load determines the current for a given voltage. It is true that
    one doesn't control the power directly- in fact I don't know of a device
    where this is true- one controls something else to control the power and
    often the power change is a side result of trying to control some other
    variable. In this case: Double the voltage- double the current and
    quadruple the power- This is true at any frequency independent of the
    impedance at that frequency. I stated fixed frequency to avoid comparing
    thepower/voltage relationship at one frequency to that at another frequency.
  18. Cite? Now you're making yourself out to be the Audio Fool.

    The output impedance of most audio circuits is *never* nil - it is always
    some intrinsic value. Some examples: Speakers = 2/4/8ohms, Balanced Line
    = 600ohms, AES/EBU Digital audio = 110ohm..

    Have fun.

  19. Guest

    You've missed the point. It's an ellipsis, based on the content
    of the post he was replying to, where the impedance changes
    with changes in the frequency.

    Ellipsis: "the omission of one or more words that are obviously
    understood but that must be supplied to make a construction
    grammatically complete." (From Webster's)

    Had Keith not used that ellipsis, he would have written something
    "Sure, in most audio circuits the output impedance DELTA is
    nil, so you're varying the voltage."
  20. Yes, I guess I *did* miss the point. I let his groundless insults get to
    me - and I shouldn't have done that (this is Usenet after all).

    In my (brief) time doing concert audio, all we ever needed to do to measure
    speaker characteristics was to hook them up to a Tektronix impedance
    analyser, run the tests, print the graphs and hand them over to the cabinet

    I have learnt quite a bit though this discussion - thanks to you and Don.

    Keep up the good work :)

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