Speaker Question  - Page 1
Forum section: electronics / projects

author: bostonman
author_rank: Super Contributor
timestamp: May 11, 2022, 06:41:27 pm
content: My friend needed stereo help. Long story short, he reminded me of some confusion I’ve had regarding speaker impedance.I’m fully aware of impedance versus resistance, but is an 8 ohm speaker 8 ohms at the DC component regardless of frequency and the AC (obviously) varies with frequency?It seems the loops of coil will measure consistently at DC, but looking online, I read that an 8 ohm speaker will be near 8 ohms when measured with an ohm meter, but will change with frequency.

author: RoGeorge
author_rank: Super Contributor
timestamp: May 11, 2022, 07:11:51 pm
content: Quote from: bostonman on May 11, 2022, 06:41:27 pmA coil's impedance has 2 components- resistance, R- reactance, X (here XLbecause it's a coil)Image source:https://en.wikipedia.org/wiki/Electrical_impedanceThe impedance, Z, is the diagonal line.My guess is, you are asking if the resistive component alone, R, stays constant with frequency (without talking here about the reactive component, X). Else said, if RDC= RAC. In theory, no, they are not the same.The R part is not constant with frequency, because of the skin effect. At high frequencies, the electricity flows only near the surface of the wire, and thus the conductor will appear "thinner" in its cross section, so the R will grow with frequency.Image source:https://en.wikipedia.org/wiki/Skin_effectMeanwhile X and Z also varies, but that's another story. We were talking about the resistive component only, the R part from that drawing.Though, I don't think this R variation with frequency really matters in practice. In practice there are many other things that will grossly influence a speaker before talking about skin effect.

author: TimFox
author_rank: Super Contributor
timestamp: May 11, 2022, 07:43:38 pm
content: Most of the "8 ohms" is the DC resistance (typically 6 ohms).The inductance of the voice coil matters, but remember that the entire speaker system (including box and free air) is an electro-mechanical device, and has at least one resonant mode.Therefore, the complete plot of impedance vs. frequency at the terminals will show the effect of the "motional impedance", including at least one resonant peak.Example:https://www.google.com/imgres?imgurl=https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fen%2Fthumb%2F7%2F72%2FSpeaker_impedance.svg%2F300px-Speaker_impedance.svg.png&imgrefurl=https%3A%2F%2Fen.wikipedia.org%2Fwiki%2FElectrical_characteristics_of_dynamic_loudspeakers&tbnid=kFCv-8fR53oeAM&vet=12ahUKEwj67_yemtj3AhXKZM0KHfqoCqQQMygBegUIARCZAg..i&docid=cxj7cpI7ObMi4M&w=300&h=154&q=impedance%20plot%20loudspeakers&ved=2ahUKEwj67_yemtj3AhXKZM0KHfqoCqQQMygBegUIARCZAg

author: DavidAlfa
author_rank: Super Contributor
timestamp: May 11, 2022, 08:15:12 pm
content: Yes, it varies over frequency, but the important is the lowest impedance, if you measure 8ohms, that's the lower it will go.The problem comes when you connect lower impedance than what it was designed for (ex. 4ohms speaker on 8ohm amplifier), it'll overload and blow the power stage, higher impedance is not much a problem, it might not be optimal and worsen the sound quality (SNR,THD, all that), but won't damage the stage.

author: TimFox
author_rank: Super Contributor
timestamp: May 11, 2022, 08:23:55 pm
content: Also, note that almost all electrodynamic loudspeakers (voice coil inside permanent magnet) are designed to be driven from a constant-voltage source (low output impedance). The only practical problem is if the speaker impedance (measured in the usual two-terminal way) falls well below the rated impedance at some frequency, thus pulling excessive current from the amplifier output.

author: graybeard
author_rank: Frequent Contributor
timestamp: May 11, 2022, 08:30:04 pm
content: The impedance of the speaker is frequency dependent and depends on resonances of the speaker, it's enclosure, and can be dependent on the area it is being listened to in (especially for car speakers).The nominal impedance of the speaker is the value for the range of frequencies it was designed to reproduce, away from the resonances. Even that can be altered some by interaction with an enclosure.Typically the impedance of a speaker over that range will look domanantly resistive since the applied energy is being dissipated in the sound waves produced and heating of the coil.I have a video on measuring impedance in which I use a speaker as one of the examples and show how the impedance changes when the speaker is moved into different positions.Chris

author: bostonman
author_rank: Super Contributor
timestamp: May 12, 2022, 01:08:31 am
content: To explain more about why I initiated this question, my buddy bought a new receiver (https://www.sony.com/electronics/support/res/manuals/4726/f63b8edb2b154409daa55875111f95e4/47269071M.pdf).He called and said it's two-channels, but he's connecting four speakers, but two speakers aren't loud, and then the amp shuts down. Upon saying it's "two" channels, but he's using four, I began questioning a few things.My background with stereos dates back many years ago when I had a stereo system. I thought 4 ohms was normal for a car stereo, and 8 ohms for house. Apparently his speakers are 6 ohms. In any case, I know from experience you can "bridge" an amp, but it decreases the resistance.What I discovered by looking at page 38 (and assume): the amplifier is seeing the two 6 ohms speakers as 3 ohms and the same on the other channel.I'm not asking to troubleshoot his amplifier issue, and, I do know the inductive reactance will change with frequency.What I've wondered for years, but haven't actually had a reason to research: if the coil of wire is X length, then the DC resistance can't change even with different frequencies (ignoring minor resistance changes caused by things like heat or whatever) because the wire has Y ohms/foot. So if I went over with an ohm meter and measure 6 ohms (or relatively close), then I can safely say they are 6ohm speakers.Thinking about the electronics, the transistors driving the speakers should also always see 6 ohms, but the AC component (inductive reactance) will change with frequency.Am I totally wrong in my thinking?Another reason that contributed to starting this thread: I quickly questioned whether I should say his speakers are 6 ohms resistance, or 6 ohms impedance. If what I stated above is correct, then they are 6 ohms resistive; and the impedance can't be stated (unless you're looking at a frequency versus impedance graph).QuoteI try not deviating from the topic, but doesn't all electricity (electrons) flow on the surface of an object (wire in this case)?

author: bdunham7
author_rank: Super Contributor
timestamp: May 12, 2022, 02:08:31 am
content: Quote from: bostonman on May 12, 2022, 01:08:31 amPretty much! A speaker driver is a complex combination of a resistor, inductor and motor, and a multi-driver speaker throws in a complex network of inductors, resistors and capacitors. The nominal impedance of a speaker is just a number they put on it, the DCR is usually somewhat lower and the effective impedance (Z) can be much, much higher at times. There are also examples where it can be lower than the DCR as well. Well-made and well-specified speakers should give you a minimum impedance as well as the nominal.The amplifier may see an impedance that looks like anything from less than the DCR to many hundreds of ohms. And it isn't resistance, there is a phase angle with the complex impedance of a speaker. At some points in time the amplifier may be putting out a current that is opposite in polarity to the voltage input. So no, it doesn't see '6 ohms'. Fortunately audio amplifiers are well adapted to this task of being ultra-low impedance voltage sources. Except those that aren't.

author: bostonman
author_rank: Super Contributor
timestamp: May 12, 2022, 02:19:30 am
content: This certainly makes sense, and, without getting into the details of a graph over time (which would be a more in depth discussion - and an area I'd need to study before discussing), somewhat connects with my thoughts.Sometimes terminology is tossed out the window with advertisements. So in the case of a speaker, at DC, it's (in this case) 6 ohms, but certainly not the case due to pumping a frequency into it.Does this mean it's incorrect to say the "resistance" of a speaker is 6 ohms even though it's technically 6 ohms at DC while sitting on the table without anything connected to it? I'm feeling that due to advertisements and salesman selling stereos, the term "impedance" has been ignored, so, a person like me is thinking the DC properties of the speaker rather than speaking correctly and I've been saying ohms in the sense that it's the DC we are talking about.

author: TimFox
author_rank: Super Contributor
timestamp: May 12, 2022, 02:54:50 am
content: If you measure 6 ohms with an ohmmeter, then that is theresistance.However, theimpedance(which varies quite a bit with frequency) is what your amplifier has to drive, since it should not have any DC on its output.Remember that the speaker is an electromechanical device, and the reaction of the air to the moving cone reflects back to the electrical connections.

author: Someone
author_rank: Super Contributor
timestamp: May 12, 2022, 03:35:33 am
content: Quote from: bostonman on May 12, 2022, 01:08:31 amThanks for the extra detail. If people read the manual (novelty):QuoteQuotePretty normal disclaimer, the purchaser should take it up with the retailer.Connecting random/arbitrary speakers in parallel is not always going to work, sensitivity varies wildly (and is usually measured against a referencepower).

author: bostonman
author_rank: Super Contributor
timestamp: May 12, 2022, 03:50:09 am
content: I'm uncertain if I'm being too over particular about terminology, but it was funny because he called and expressed jokingly that I'm his go-to guy when I failed to solve his problem in a timely manner.He was trying to figure out why his four speakers "worked for years" with another amplifier and didn't quite grasp the concept of (what I assumed and tried explaining) the amplifier is seeing the two speakers on one channel as a parallel combination.In any case, I began asking myself, should I be continuing to call speakers "x ohms" (6 ohms in this case) because they measure x ohms with an ohm meter when sitting on the table without being powered, or be "correct" and say "oh, those speakers have an impedance of x ohms" knowing it's x ohms at their lowest frequency even though they measure x ohms with an ohm meter.So the long short is, I think nomenclature came into play and wanting to use correct terminology to avoid being wrong; or sounding very smart.

author: CaptDon
author_rank: Super Contributor
timestamp: May 13, 2022, 05:06:55 pm
content: Just stop over thinking it and over analyzing it. The whole world understands the term '8 ohm speaker' will typically measure from 5.4 to 6 ohms!! This is the standard definition of an 8 ohm speaker. A 4 ohm subwoofer and other woofers will typically measure 3.2 to 3.6 ohms and that is the standard expected resistance of a 4 ohm speaker. The '8 ohm' or '4 ohm' is an indication of the speakers somewhat typical load on the amplifier. 8 ohm speakers will often hit around 40 ohms of reactive impedance near mechanical resonance. Bottom line, your friends receiver was probably designed for an 8 ohm load and hates the fact that it is seeing a 4 ohm load. 6 ohms doesn't even enter the verbal conversation. It is the D.C. resistance and we don't listen to D.C.! Your buddy needs to buy a 'real' system that can slam dunk even a 2 ohm load. No highs, No lows, must be Bose? Buy Other Sound Equipment? Or the other kid on the block JBL, Just Bring Loads (of cash)?

author: themadhippy
author_rank: Super Contributor
timestamp: May 13, 2022, 05:10:20 pm
content: QuoteNearly, jbl= Just Bloody Loud

author: edavid
author_rank: Super Contributor
timestamp: May 13, 2022, 06:20:36 pm
content: 1. Did he try setting the speaker impedance to the 6 ohm setting per the manual?2. If he always wants to have both sets of speakers on, he can put them in series. That's probably what his previous amplifier/receiver did.

author: dietert1
author_rank: Super Contributor
timestamp: May 13, 2022, 06:33:06 pm
content: Matching an amplifier and a speaker isn't something simple, but subject to surprise. If you want something well designed, better buy an active speaker (speaker with built in amplfier). Often those employ multiple amplfiers for different frequency ranges, in order to avoid the pitfalls of crossover design. Sometimes they even include acoustical or motional feedback to avoid/reduce resonances at low frequencies. Only then a speaker gets close to "correct" and "predictable".Lucky enough human ears are well forgiving, that's why many people are satisfied using simple and cheap speakers. Of course, if the amplifier shuts off, you need to find a different solution.Regards, Dieter

author: TimFox
author_rank: Super Contributor
timestamp: May 13, 2022, 06:38:44 pm
content: Modern solid-state linear amplifiers (as opposed to older vacuum-tube amplifiers with output transformers) are essentially constant-voltage (low source-impedance) devices, but they have a maximum current capability and therefore will give less output power with a higher impedance load, and possibly limit the output current into a lower-impedance load, resulting in distortion or (possibly) damage.