Problem with Wein AS-500 Air Purifier
How to measure speaker impedance?8 Solutions
Hello I am not an engineer and my skills in electronics are limited, so here goes: Is it possible to measure speaker impedance in an easy way???? Trying to remember what I learned at school I came up with this solution. What if I use two multimeters attached to my speakers. One in series measuring Amperes and one in parallel measuring Volts. Then playing pure Sinus tones at a fixed volume I simply read the two meters and calculate Impedance using Impedance = Volts/Amps. Plotting this in a graph and I'm home free???? Why doses this sound a little to easy even to me???? There has to do be something wrong here. Am I measuring the impedance to the speaker alone or to my entire system? Please help me. MVH Eirik Andreassen Do NOT Reply Remove the obvious for mail to:
65 Found Helpful
Why not be simple? Speakers typically come in specific flavors, 2ohm, 4ohm, 8ohm, 16ohm.
Measure the dc resistance with your cheapo radio-shack ohm-meter, and round up to the nearest reasonable value. if you measure 14ohms, that's a 16 ohm speaker. If you measure 3 ohms, that's a 4 ohm speaker.
An electrical engineer with access to a signal generator and the knowhow to set up a full accurate test bench isn't going to be asking how to measure speaking impedance on a diy website...
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Feb 19, 2012
Feb 19, 2012
Sep 29, 2012
65 Found Helpful
This question is asked SO often, and I try to answer it SO often, that I thought I would sit down and take the time to write a real FAQ about it. Below is the opus I created today. I am also sending it off to the FAQ masters in hopes that it will be included somewhere. ++ THE IMPEDANCE MEASURING FAQ Dick Pierce Copyright 1998 Dick Pierce. Public, non- commercial distrubution allowed with this copyright noticed attached. All other uses prohibited, including use in for-fee media or distribution of any kind. INTRODUCTION An oft-asked question is "What's a simple way I can measure the impedance of my loudspeaker or driver?" The answer to that depends upon a lot of things, like what equipment you might have at your disposal, how much work you want to put into the enterprise, and so on. I'll present one method here that can give reasonably accurate results with the bare minimum of test equipment needed. WHAT IS IMPEDANCE? Simply stated, it's the obstacle to current flow provided by an electrical circuit to the imposition of an AC electrical signal. It is like resistance in that sense, but different in that it is almost always (in these applications) frequency-dependent (it's value is different at different frequencies) and it is "complex" (meaning that, mathematically, it is a vector quantity, consisting of a resistive and a reactive part) The law governing the relationship between DC resistance, voltage and current, known as Ohm's law, is: E = I * R where E is the impressed voltage in volts across the resistance R in ohms, resulting in a current I in amperes flowing through that resistance. Simple high school algebra allows us to rearrance this basic equation: E E R = --- and I = --- I R AC impedance, voltage and current follow the same basic rules: E = I * Z where, now, E is the impressed voltage magnitude in volts impressed across the impedance magnitude Z in ohms, resulting in a current magnitude of I in amperes flowing through that impedance. And, as above, we can rearrange out equations: E E Z = --- and I = --- I Z Now, I use the terms like "impedance magnitude" here. The AC impedance, as mentioned above, is a complex value: it is vector sum of the resistive (or "real") and reactive (or "imaginary') components of the impedance. That vector sum is computed as (for example): 2 2 Zm = sqrt ( R + X ) where R is the resistive portion and X is the reactive portion. (In this context, real and imaginary have very specific mathematical meanings: an imaginary number is not one that exists only in one's imagination, rather it is a number that has the square root of negative one as one of its factors.) Because of the enrgy storage properties of the reactive portion, the instantaneous current flowing through the impedance is not in step or in phase with the instantaneous voltage across it. Rather is precedes or follows the voltage by some amount dependent upon the ratio of reactance to the resistance, specifically: -1 X P = tan ( --- ) R where P (more properly, the Greek letter Phi) is the phase angle, usually expressed in degrees. It should be noted that in the grand scheme of things, both the resistance R and the reactance X can take on any value, positive, negative or zero. However, in the case of loudspeaker impedance, R will never be negative, and almost certainly never 0, while X can either be positive (inductive) or nagative (capacitive) or 0. Looking at the equation for the impedance phase angle, this means that the phase angle of the impedance will always be outside the range of -90 to +90 degrees. (Indeed, it is quite unusual to find the impedance phase to be outside the range of +- 70 degrees). The fact that the real or resistive portion of the impedance is always positive ensures that the impedance phase angle never exceeds these 90 degree limits. (for thosewith a mor etechnical inclination, that means that the entire impedance is confined to the right of the imaginary axis in the complex s-plane). Basically, all we need to do is then to put a voltage across the unknown impedance, measure the current going through it, plug the numbers into the following equation (from above): E Z = --- I And out pops the impedance, Z. In principle, this is abosolutely correct, but in practice, it is more difficult. The main reason for this is the range of typical values for the impedance of most loudspeakers and drivers (from a few ohms to a few dozen ohms) combined with the sensitivity of most common measurement instruments. Imagine putting a voltage of 10 volts across an 8 ohm loudspeaker. Ohms law says that the current going through that speaker will be: E 10 I = --- = = 1.25 amps R 8 While 1.25 amps is a convenient current to measure (it's large enough to ensure reasonable accuracy with many comment meters) it is a LOT of current to put through the voice coil, and that poor speaker and the people near it will be subjected to a rather deafening level of sound. Additionally it does pose some risk of damage to some drivers. MEASUREMENT SCHEMES A common assumption is that one needs two meters: one to measure voltage placed across the impedance and one to measure current placed in series with the impedance. Then, by Ohms law: E Z = --- I However, this poses some problems. As mentioned above, it requires a hefty amount of current to get enough of a reading to be dependable. Most commonly available meters that measure AC current at all well aren't very sensitive. There is also the issue of having to go through the calculation for each and every frequency being measured. Another method that seems to have escaped many peoples' attention is the "impedometer." This is nothing more that a calibrated constant current source. When properly set up, there is no calculation required and it is reasonably accurate over a wide range of impedances. Another advantage is that it requires less equipment than other methods. It is the impedometer method that we will discuss here. TEST INSTRUMENTS Very little is required for a properly working impedometer. We'll enumerate the requirements here. 1. AC sine wave generator This can either a function generator (usually meaning an instrument that has the capability of sine, square, and triangle waves, and often has pulse output as well) or a Wein-bridge or twin-T audio oscillator. The major requirements are stable AC output, stable frequency, reasonably low distortion (less than 1%), flat frequency response over the audio bandwidth, and reasonable voltage output (10 volts or more into 1 kOhm is good). There are a lot of new instruments that are acceptable, funtions generators by B&K, Tenma, Leader and others can be had, but often cost several hundred dollars new. Their performance is generally more than good enough, and they are versatile instruments for other purposes as well. Often they have frequency ranges far in excess of what's needed, like 0.02 Hz to 2 MHz, but that's okay, too. On the other hand, you can often find used equipment that is very serviceably as well as inexpensive. I have seen excellent units from the likes of Wavetek an Krohn-Hite for under $100. In working order, they have superb specificationa and are ideal for this sort of use. Their distortion is not the lowest (because, like other function generators, they synthesize the square wave frm the triangle output), but, for impedance and frequency response meaasurements, they are superbly accurate for audio use. One of the all-time best sine generators is the venerable HP 200 audio ooscillator. I have seen them at swap meets and even at yard sales for as low as 5 dollars. The have good frequency response, good stability and high output voltage (5 vots into 600 ohms). There are several variants, the 200 AB and 200 CD are the most common and both are equally good. Look for examples from General Radio or GR as well. The GR1309 can often be had for $50 and can be tuned to have very low distortion, under 0.05%, while 1304 will do 20-20 kHz without range sweeping and has high ouput voltage as well. Be preapred for a little tune-up work, like cleaning and lubing dial shafts, amybe replacing a tube and an electrolytic capacitor or two. Otherwise, these units last absolutely forever. I cannot recommend them too highly. 2. AC voltmeter This can either be an analog or digital unit. Ideally, it must be capable of reading down to about about 10 mV full scale with reasonable accuracy. It must also have flat frequency response over the audio range. Unfortunately, the sensitivity requirement eliminates most "passive" VOMs (volt-ohm-milliammeters), including the ubiquitus and venerable Simpson 260 (which is truly unfortunate, because the 3 I have here of different vintages all have ... read more »
18 Found Helpful
Really don't understand why people like to make simple things complex, I see so many examples.
1. of course you need sine wave because the impedance is a function of frequency. But the speaker shall have min power so that it is all most in sleep.
2. add a know resistor in series with the speaker, thus the resistor and impedance makes a voltage divider, Vo=Vi(R/(R+Z)), meaasure Vi,Vo and calculate Z.
8 Found Helpful
In <6se5ku$2i.net at 02:43 PM, "Eirik Andreassen" < Using your multimeter to measure the current and voltage may be a problem because very few multimeters have a frequency response that is good enough for audio work. Check the meter's specs. You may find that it is rated for 60 to 400 Hz. Always know the accuracy of your meter. Digital meter specs are interesting. There is a percentage and a digit spec. While you may have a 0.5% or better basic accuracy, they'll also throw in a "digit" spec. which says that the least significant digit can't be trusted within "n" digits. I've seen the digit spec. as high as seven -- which means that the last digit can't be trusted! In any case don't use a mutimeter to directly measure the current because you will be using a sweep of frequencies and you don't know the impedance of the meter. A standard trick for this measurement is to use a constant current source -- a resistor in series with the generator and the speaker. If you use a resistor value that is much larger than the expected speaker impedance, the current won't change enough over a range of frequencies to bother the measurement. Calculate a few typical values and satisfy yourself that this trick works. Pick a resistor value (don't use a wirewound resistor) and generator output that yields a convenient current -- 1 or 10 mA. Measure the voltage drop across the speaker, shift the decimal point, and you're done! Keep in mind that you don't need three or four digits of precision for your results. Also, since you will be dealing exclusively with a single frequency sine wave at each data point, you can use an RMS or average responding (but RMS calibrated) voltmeter. Conceptually it is possible to use your sound card and some programming to make the measurement. If you attempt the feat, be sure that your sound card has a decent frequency response at the frequencies of interest. (Barry Mann)
2 Found Helpful
Insert the probes of a digital multimeter in the correct sockets of the multimeter. There may be several different sockets for the probes depending on what parameters you're measuring. Look for the sockets marked "impedance" or displaying the ohm symbol. Different models and brands of multimeters will vary in their labeling, so check your multimeter's manual if you're not certain about which one to use.
Select the range for the multimeter. Since you'll be measuring fairly low resistance levels with a speaker, choose a range that will encompass readings in the 2-16 ohm range. It's extremely rare to find speakers that will have an impedance outside of that range.
Touch one probe to each of the speaker terminals and check the reading. This will be the impedance of the speaker. Record the measurement, and when you're done, turn off the multimeter to preserve the batteries for the next time you need the multimeter.
1 Found Helpful
I like innovation when you are designing quality assurance tests.
If you want a professional test you have the option of looking up ASTM tests which are developed by the engineering community or else using software - some is free like ARTA if you have a computer. I assume you want something handheld and accurate. The previous posts pointed out that it is harder to come by than you may think. I have found that low currents like speaker coil movement output (the actual output impedance, not the input impedance being assumed by other posts) is probably best approximated by vibrating the cone at 1000 herz (a standard) and using a handheld RF meter - I perfer UHF as digital has gone UHF instead of VHF. The new digital ones run about $50 but the used coil needle older types run about $20. The point is that that type of instrument is sensitive enough to pick up the speaker output.
Then there are other concers about quality. Thicker coil wire makes current stick to the inside of the wire because of shorter path length around the inside of the wire. The center of the coils also is worst because of coils next to it. But when doing engineering DIY - its ALWAYS best to start low cost and work up to better measurement equipment and quality methods.
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