Answered 4 ohm Speakers - Why do they make them?

Discussion in 'Home Cinema Speakers' started by nheather, Jun 21, 2015.

  1. nheather

    nheather
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    The 5.1 speakers that I want based on size, price and colour are the Wharfdale 5.1.

    I'd have bought them already apart from the niggle that they are 4 ohm.

    Everything I have read suggests that this is bad - draw more current than an 8 ohm for the same power and therefore puts a bigger strain on the amplifier.

    Also given that I'm looking at slimline amplifiers such as the Marantz NR1504 based on size and price leaves me a little worried that it could handle the speakers.

    Also I understand that impedance changes with frequency, capacitive reactance decreases with frequency whereas inductive reactance increases with frequency. So when they say that the speaker impedance is 4, 6 or 8 ohms at what frequency is that and what is the range of impedances for the frequency range of the speakers - that information never seems to be advertised.

    But the biggest question is if 4 ohm is so bad and amplifiers are designed for 8 ohm loads why do they make 4 ohm speakers.

    Cheers,

    Nigel
     
  2. Best Answer:
    Post #4 by BlueWizard, Jun 21, 2015 (1 points)
  3. stephenbarnes

    stephenbarnes
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    4 ohm speakers aren't bad as such. Also your question is more electrical / design question.

    Also a 4 ohm speaker means your amp will output more power as it draws more current. My hifi tower, home theatre towers, and standmount speakers are 4ohm.

    If you have a capable amp you don't need to worry if it's 4, 6 or 8 ohm. For HTIB some are 4 ohm, I don't know why perhaps size of drive units? In that the extra power helps the low sensitivity (more power from amp = higher spl given for the inefficiency of the cab size)

    You will also find as you move to higher end speakers they are generally 4 ohm speakers.
     
  4. KelvinS1965

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    FWIW I run M&K K4 and K5 (both 4ohm speakers) in my conservatory set up (so not used at reference levels) using a Marantz NR1605 slim line amp. It works fine and I've cranked it up occasionally just to try it out without any issues. Note that the crossovers are set to 120Hz for the front (set during the amp set up routine), so that helps take the load off the amp as well since the lower frequencies go to the subwoofer.

    4ohm speakers aren't 'bad' at all and if you have a suitable power amp then it helps get extra watts (a good amp will double the watts output compared to 8ohm). In practice most are a little less than double, such as my Arcam P7 I use in my main system (with MK MP150 speakers which are of course 4ohm as well). The P7 gives about 220 watts per channel into 4ohms, but around 140 watts into 8ohms, which is more than enough in my room as that gives up to 110dB at the MLP.
     
  5. BlueWizard

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    Best Answer
    Several things can force a speakers impedance. First the rated impedance is nominal, or, more or less, the average impedance. At some low frequency, all the inductive, capacitive, and resistive components are going to come together to create an impedance peak. This is typically between 25hz and perhaps 80hz. That peak can be 20 to 40 ohms. On either side of that peak, the impedance drops to its lowest, perhaps in the 3 to 6 ohm range. Above the resonance frequency, and above the drop which is itself above the low impedance value, the impedance begins a gradual rise and many reach 15 to 20 ohms at higher frequencies.

    But, they find something approximating the average as the speakers Nominal Impedance.

    Next, the design of the speakers come into play, let's take the example of the Monitor Audio Silver 6 and the Silver 8 -

    http://www.monitoraudio.co.uk/products/silver/silver-6

    http://www.monitoraudio.co.uk/products/silver/silver-8

    The Silver 6 are Low-Bass, Mid-Bass, and Tweeter with crossovers at 700hz and 2.7khz. While it has two 6" bass driver, because one is low-bass and the other is mid-bass, they do not cover the same frequency range, and as such result in an 8 ohm rated speaker.

    The Silver 8, on the other hand, are a Woofer, Mid-range, Tweeter combination with crossovers at 500hz and 2.7khz. In this version, both 6" woofers run in parallel and cover the low frequencies, and a result of those two 8 ohm driver always running in parallel, the resulting speaker has a rating of 4 ohms.

    But the Silver 8 speaker has the advantage of moving twice as much air in the low frequencies under 500hz range. So the designers have made a trade off, in exchange for greater bass output, they have settled for a 4 ohm rating.

    Then cabinet design can effect the final impedance of the speaker through electro-mechanical interaction. The mechanics of the cabinet design can have some effect on the final electrical impedance of the speaker system. On either side of the Resonance Frequency of the bass driver, the cabinet resonance can force the electrical impedance a bit lower than it would be for the stand-alone bass driver, and this could result in the speaker receiving a 4 ohm rating.

    Many time you see speakers that have a official impedance rating like this - 4-8 ohms - that means that overall the speakers is 8 ohms, but at one particular frequency, the impedance drops lower than normal. Because the low impedance range is so small and isolated, it is not enough to give the speaker a 4 ohm rating, but not quite insignificant enough to give the speakers a flat out 8 ohm rating.

    I think some of the better B&W speakers, while they may have an 8 ohm rating, also have a note indicating that the impedance can drop as low as 3.2 ohms. Keep in mind that with a normal speaker, at any given frequency, the impedance can be above or below 8 ohms. In fact, at only 3 or 4 point in the frequency spectrum is it precisely 8 ohm, the rest of the time it is above or below.

    Here is the example of the 6 ohm rated Wharfedale Diamond 10.7 -

    http://www.stereophile.com/content/wharfedale-diamond-107-loudspeaker-measurements

    In the first Chart, we are interested in the Solid Line -

    "...the (impedance) magnitude reaches a minimum value of 3.15 ohms at 120Hz, it stays above the specified 6 ohms for most of the midrange and treble ..."

    This is somewhat unusual in that it has a high impedance peak around 1200hz, but it still illustrates the point that for every speaker, the impedance is all over the place.

    However, if you move down to the 4th chart, you will see that the frequency response is pretty normal.

    In the first Graph at the top of this next page, you can see the impedance across frequency of the KEF LS50. Again, the Impedance is the Solid Line. -

    http://www.stereophile.com/content/kef-ls50-anniversary-model-loudspeaker-measurements

    The Kef LS50 is rated at 8 ohms.

    Generally anyone who can afford a 4 ohm speaker can also afford an amp capable of driving it. Many consumer grade amps are 4 ohm rated, meaning they will have no problems with 4 ohms speaker - Yamaha, Marantz, and more .....

    AV Receivers can be another matter, as they tend to have under-rated power supplies, that is, if the per channel amp power adds up to say 700w, very likely that amp will only have a 500w power supply. That is typically the limiting factors. You see many 7 channel amps that are rated at 100w/ch when one or two channels are driven, but only 50w/ch when All-Channels are driver. That is reflecting the limits of the power supply.

    Fortunately, a AV Receiver under normal operation, does not use all channels equally, the bulk if the demand is on the front three channels (Left/Center/Right) and that power demand is softened further by diverting all the heavy low bass to the Subwoofer.

    So, if you have 4 ohms speakers on a AV Receiver, as long as you are not exceeding the absolute limit of the Power Supply, you are fine.

    One last aspect of 4 ohms speakers, because the Amp is pushing out more current, it is going to get hotter. That is the other limiting factor, if the amp gets too hot, it will shut down in an effort to protect itself from damage. But, that can be offset by improved cooling.

    In my case, I had only one place on my equipment stand large enough to house my Amp, and that was an opening intended for a VCR/DVD/BluRay. That compartment only had about 1.5" of space of above the amp, and was closed backed. So, I added a couple of fans over the heat-sinks to make sure it stayed sufficiently cool. I used 12v fan run at 9v to keep them quieter, and I never had a problem.

    [​IMG]

    So, there are a number of reasons why a speaker design might result in a 4 ohm speakers. In the old days, you used to find a lot of 16 ohm drivers, you could add two 16 ohm drivers in parallel to get an 8 ohm speaker system. Today, though, on the DIY market, you find virtually no 16 ohm drivers.

    However, manufacturers are having their drivers custom made, and they have a bit more flexibility. I've seen surplus 12 ohm speaker on the DIY maker, and I suspect companies like Wharfedale us two 12 ohm drivers to achieve a 6 ohm speaker with two bass driver running in parallel.

    However, when you adjust one parameter of a speaker's design, if effects all the other parameters, so you have to strike a compromise between changes you do need, and effects you don't need.

    So, there are a variety of reasons why a speaker might end up at 4 ohms, but they are all design driven. In some case, it is a compromise in the design of the driver to achieve a specific goal, other times the cabinet can compromise the impedance of a design. Sometime the need for multiple bass drivers compromise the impedance.

    Generally, assuming you have a good amp, and the amp is operating in its normal range, you aren't going to have a problem.

    Steve/bluewizadr
     
    Last edited: Jun 21, 2015
  6. PH001

    PH001
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    Superbly detailed response from Steve as usual. I think virtually all 8 ohm speakers these days can and do dip below 4 ohms at certain frequencies and the lowest impedance is often almost the same irrespective of whether the manufacturer classes it as a 4ohm or 8ohm nominal speaker. In practice you will not have any problems.
     
  7. hifix

    hifix
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    It's just a balancing act by the manufacturer - compromise here and there to improve other areas - it just depends on which aspect of a speaker's performance that a manufacturer wants to bring to the fore.
     
  8. BlueWizard

    BlueWizard
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    Here are the Specs Sheets for two identical speakers, Dayton RS100 ...except one is 8 ohms and the other is 4 ohms. These are 4" full range speakers.

    Dayton RS100-4 (4 ohms) -

    Note the Re specification, this is the pure DC resistance of Voice Coil. Typically about 0.75x of the rated impedance, meaning an 8 ohm speaker typically has a Re of about 6 ohms.

    Look for the graph on the right marked "IMPEDANCE/PHASE". The Impedance is the Blue Line.

    The Re of the 4 ohm speakers is 3 ohms. It is generally assumed that is as low as the driver will go, though not necessarily true within a given speaker design.

    The Peak Impedance at the Resonance Frequency (Fs) is about 14 ohms.

    http://www.parts-express.com/pedocs/specs/295-378-dayton-audio-rs100-4-spec-sheet-revised.pdf

    Dayton RS100-8 (8 ohms) -


    The Re of the 8 ohms speakers is 6.6 ohms.

    The Peak Impedance at the Resonance Frequency (Fs) is about 26 ohms.

    http://www.parts-express.com/pedocs/specs/295-359s.pdf

    Notice that in both cases, the Impedance dips on either side of the Resonance Peak. Above the Dip on the high side, the impedance gradually rises to 9 ohms at 20khz for the 4 ohm rated driver, and to about 16 ohms at 20khz for the 8 ohm driver.

    While the numbers vary from driver to driver, these two examples illustrate the general nature of the impedance of a given driver.

    As an example of how these 4 ohm drivers might be used. I've been thinking of using the Dayton RS100-4 (4 ohm) as midrange drivers for a 3-way speaker design. (on the Right)

    [​IMG]

    I have two choice, one is to wire two 8 ohm drivers in parallel for higher output, but a resulting 4 ohm impedance for the midrange.

    The other possibility is to wire two 4 ohm drivers in Series. When this is done, you only get the equivalent output of a single driver, but each driver has half the excursion and twice the overall power handling capacity. The low excursion, but moving more air, results in a lower distortion midrange section.

    Ideally, though simply not found today, you would use two 16 ohm drivers in Parallel for a resulting impedance of 8 ohms. But, as I mentioned, you find very few 12 ohm or 16 ohm drivers available on the Build Your Own market today. Though they were more readily available in the past.

    These are the design trade offs when you are building speakers. The power drain in the midrange is not that high, so two 8 ohm drivers in Parallel would work in the illustration above, and would result in a 4 ohm rated speaker, even though the woofer and the tweeter are 8 ohms. Myself, at this stage, I think I'm of using two 8 ohm drivers in Series for the Midrange. It is just a personal design choice.

    Speaker Manufacturers have more choices as their drivers are custom made to their specifications.

    Steve/bluewizard
     
    Last edited: Jun 25, 2015

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