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Two Technical Questions

Discussion in 'Home Cinema Speakers' started by nheather, Apr 20, 2005.

  1. nheather

    nheather
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    Question 1 - Speaker Impedance
    What are the relative pros and cons of 8 ohm over 4 ohm?

    I understand that for a given power, 4 ohm speakers require
    1.41 x current and 0.71 x voltage compared with 8 ohm,
    and that amplifiers have a tougher time delivering a greater current.

    But apart from that, what are the issues - why do both types exist?


    Question 2 - Frequency Roll Off
    If a speaker is quoted as having a lower frequency of 80Hz at -3dB, at what frequency would the speaker typically start to roll off?

    Cheers,

    Nigel
     
  2. Mason @ B&M

    Mason @ B&M
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    1)

    Some speakers are less efficient so take a bit more power to get them going so 4 ohm makes that easier.

    They both exists as there are benefits to both in terms of matching with amps but to be honest there are no real disadvantages as long as you have enough amp power you'll be fine with whicever you choose.

    2)

    If a speaker is rated at -3db @80hz it's difficult to tell where it starts rolling off.

    However as a general rule if it's in a ported enclosure it's likely to be quite low, probably 100-90hz but after 80hz it'll roll off very quickly.

    If it's a sealed enclosure it's likely to have started rolling off much higher, but it will continue to roll off slowly..

    Hope this helps..
     
  3. Cliff

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    Just to add a few points.

    4 ohms are specified for in car systems because you can get a lot more power for a low supply voltage of 12 volts. In fact to increase the power you can do three things- reduce the impedance to 2 ohms or lower, use a bridge amp, or as they do now, use a switch mode psu so the amplifiers are running on a higher voltage.

    For home use where you are running off the mains 8 ohm is a better option because the currents are lower and the amplifier will be less stressed. The speaker wire resistance is not such an issue as it will be very small compared with the 8 ohm load you are driving so losses are proportionally less and of course better damping.

    The roll off point of -3db is supposed to be where you start to notice- although as was said in the previous post it all depends on the enclosure as to how this happens in practice.
     
  4. eviljohn2

    eviljohn2
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    The 3dB point is a mathematical/engineering tool.

    If you take a speakers frequency range (say 80Hz to 25kHz with these points corresponding with the 3dB point at either end of the scale), for the sake of argument,the speakers should be able to produce a flat signal between these points. In reality the frequency range doesn't stop as a "brick wall" at these frequencies but will roll off at either end. As we use a logarithmic scale to interpret these results the 3dB point is where the response has dropped by 1/SQRT2 (ie. ~0.707 or ~70.7%). Between these points is generally regarded as useful output from the system. As has been mentioned, the actual shape of the roll-off depends on many things with speaker cabinet design being a key point. :)
     
  5. nheather

    nheather
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    Thanks for the replies.

    The reason I asked is that I'm interested in the MS Premiere Plus speaker set.

    The sats and centre have a 4 ohm impedence so was wondering what issues that might present.

    Also, my local dealer stated that MS quote their frequency range as flat response whereas others (such as Mission quote the ends at -3dB).

    The MS low end is 100Hz and if this is flat before roll-off commences (as the dealer stated) I was wondering how that compares with another speaker at 80Hz (-3dB).

    Cheers,

    Nigel
     
  6. eviljohn2

    eviljohn2
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    Unfortunately there's no easy way to tell as it very much depends on the speaker design. Sealed cabinets will have a gradual roll-off whilst a ported cabinet with the same 3dB point will generally have a much steeper decay. Of course drivers, crossover, port dimensions and so on also contribute. The best advice I can really give is not to get too bogged down in these numbers as they don't provide much information about the overall performance of the speaker.

    With respect to impedence, as well as the reply already given it's a subject that comes up quite frequently so there's a wealth of information in this section and the amplifier section which should come up with a suitable search. :)
     
  7. missionman

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    Let's clear this all up for the benefit of everyone.

    When Valve (Tube) amplifiers reigned supreme speakers tended to be of 15 Ohms impedance - valve amps use transformers to match the high voltage, low current valve output to the high current demands of speakers.

    When transistor amps were introduced they had much lower voltage power supplies because of their physical characteristics, and therefore were naturally suited to low voltage, high current output. The only way to increase current flow from a fixed maximum voltage is to reduce impedance (Power = V x A, Voltage x Current; and from Ohms Law Current=Voltage/Resistance), so speakers quickly became 8 Ohms to increase the available output power from transistor amplifiers.

    As the technology, and price, of amplifiers increased their power supplies and output stages were able to deliver higher current, so it became possible to lower the impedance of speakers further in order to maximise the power output from 'good' amplifiers. Initially led by the Europeans (Germans?), the 4 Ohm loudspeaker has become almost ubiquitous nowadays.

    It should be noted that the impedance quoted by speaker manufacturers is only a guide to the matching capability of speaker to amplifier. In practice a speaker marked as '4 Ohms' is likely to have a minimum resistance of around 3.2 Ohms at DC. This doesn't matter as amplifiers don't drive (or shouldn't) DC output, so we are only interested in the resistance at audible frequencies (impedance is the combination of resistance, inductance and capacitance at a given frequency).

    If you look at magazine technical reviews of loudspeakers you will often see a wavy line on the graph marked impedance. Notice that it generally dips to its lowest point in the upper bass (around 200Hz). This is where the maximum current is drawn from the amplifier. However music covers a wide frequency band and power is taken from the amplifier over the whole impedance of the speaker. You may also note that the impedance rises in the lower bass and through the midband, often as high as 20 Ohms or more through the crossover region. In fact, because the dip to the minimum impedance around 200Hz is only over a narrow range of frequencies, the speaker may 'look' to the amplifier to be much higher impedance overall. Hence B&W and Mission give a specification that is based on the compatibility of the speaker PLUS the minimum impedance where it dips to its lowest (for example "8 Ohms compatible, 3.2 Ohms minimum).

    In practice modern stereo amplifiers, if they are any good, should have no trouble driving such speakers. The 'perfect' amplifier would follow Ohms Law in doubling its power output as the speaker impedance halves. In practice the power supply can never be big enough to do this, so a practical limit of, say, 3 Ohms minimum is sensible. A 4 Ohm loudspeaker will therefore 'maximise' the available power from such an amplifier and will sound louder for a given volume level setting.

    However AV amplifiers are rarely built this way. Because of the customer's hang-up with power output specifications ("What power is it?" is nearly always the first question asked of an amplifier or, strangely, a speaker), AV amps are usually given the '100W' spec whether they are truly capable of it or not. A better idea of an amplifier's capability is to look at the power consumption (usually quoted in VA - but that's Watts to you and me). If an AV amplifier has a power consumption of 130VA then it can't deliver more than that to ALL the speakers. So for a five channel amplifier divide that figure by five (130/5 = 26W) for a guide to the true power capability per channel. (This doesn't allow for losses in the amp, but it's close enough).

    You'll often see, on the back of an AV amplifier, a switch for 8 Ohms or 4 Ohms matching. This is only there to adjust the current limiting of the output stage to help protect the amplifier and speakers, and is probably worth setting to match the minimum impedance of the speaker. Sometimes amplifiers with two sets of speaker outputs may have '8 Ohms minimum' on the back panel. This is only to ensure that, if two sets of speakers are connected, the minimum impedance doesn't drop too low (the speakers would appear in parallel, so the overall impedance seen by the amplifier is half that of each speaker impedance. If you are only connecting one set of speakers, a 4 Ohm spec would be fine.

    None of this means that modern amplifiers have any more of a difficult time driving nominally 4 Ohm speakers instead of 8 Ohm, just that you should always choose the best quality amplifier you can. In other words regard sound quality when making the buying decision, not the specs.

    Frequency Roll Off

    It is difficult to see the 'beginning' of the roll-off of any loudspeaker, so in practice the -3dB point is some sort of pointer to where the bass output starts to fall away. There is no such thing as a 'flat' response specification. All creditable frequency response specs are quoted with dB limits - if they aren't, ignore them!
    The -3dB point doesn't tell you how a speaker is going to perform in your room either, so it doesn't denote how low the speaker actually can reach in your room. For example a closed box loudspeaker will typically have a higher -3dB point than a ported loudspeaker, yet has a slower rate or roll-off and so can audibly extend lower than its ported counterpart.

    Conclusion

    The great thing about buying hi-fi is that you can try before you buy. For goodness sake ignore the specs and go and listen. A good hi-fi dealer is like a bespoke tailor - he or she will match recommended components to suit your tastes. Find a dealer who you are comfortable with and who will look after you. You may not get the best discount now but it will save you money in the long term as you should get equipment you will be happy with for a long time.

    Mr Mission
     
  8. nheather

    nheather
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    Mission Man

    Thanks for the excellent and comprehensive response. The only thing I would question is the conclusion. Whilst in a perfect world we would be able to demo before we buy, in my experience the reality is that this is only possible if you are buying mid to high end equipment.

    My budget for a 5.1 speaker set for home cinema is £500 and there are a number of good contenders in this range. But finding a hi-fi store who is

    (a) Interested in demoing this level of equipment
    (b) Can demo a selection of contenders to enable comparison

    is virtually impossible

    I'm am usually told by the stores (and I'm sure their points are valid) that the mark ups on these packages are not sufficient to justify their time and effort particularly when their are many internet box-shifters waiting to take the business off them once they have done the demo.

    I understand the store's position entirely, but at the end of the day it generally means that if your budget is less than £800 then you must rely on specifications, reviews and opinions.

    Cheers,

    Nigel
     
  9. eviljohn2

    eviljohn2
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    Excellent, thorough and informative post there Missionman.

    Mods, any chance that we could have that up as a sticky in the speaker and amp sections like Nimby's subwoofer sticky? :)
     
  10. missionman

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    Understood, but I do personally question why you are seriously considering only spending £500 on a 5.1 system? Think about it. You want to buy 5 loudspeakers with good cabinet build quality, high performance drive units and crossover PLUS a subwoofer with a high power, long throw drive unit and powerful amplifier. PLEASE, it's difficult enough making a subwoofer that can sell for £500 on its own!

    In 1977, when Mission started, a quality pair of bookshelf speakers sold for £109. In 1977 this was a typical wage for a production worker for one MONTH. If you use the GDP index this equates to £600 today, which I think is rather less than the minimum wage for one month.

    On that basis you should be looking at spending around £2500+ on a quality 5.1 speaker system. The reason that speakers are cheaper today than they have ever been is because all the component parts are made in China. The reason that speakers HAVE to be cheaper today than they have ever been is because we all want so many things. In 1977 there wasn't so much to buy, hi-fi was number 3 on the desirability list of things to own, and it was appreciated that quality performance came at a cost.

    Whilst I agree that quality retailers who do not have oodles of customers coming in and buying boxes (a la Currys/Dixons) are not interested in selling cheap speakers because there is no money in it for them, note that they are in business to advise you to buy quality, high performance products so that they can count you as a satisfied customer. Their service is worthwhile to avoid getting into the vicious circle of upgradeitis which, as I pointed out, can be costly in the long run.

    Don't get me wrong, at Mission we know some people have limited budgets and produce ranges of speakers to suit those like yourself. We put the same emphasis on the design of these speakers as we do for more expensive items, but with the best will in the world we can't deliver the performance that, say, our Elegante speakers can provide because, with the latter, we can use some quite esoteric components and a build quality that allows us to offer exceptional sound quality.

    I sympathise that you can't get decent demonstrations of cheaper speaker systems, but please raise your budget to a sensible level if you can as it really will pay dividends in your long term listening enjoyment.

    Mr Mission
     
  11. eviljohn2

    eviljohn2
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    Worthwhile 5.1 is certainly tricky for £500 but isn't impossible. I think the second hand market is really the place to be looking for much of your equipment though.

    Myself being the case in point as most of my speakers have been purchased second hand:
    * Mission 771e's as stereo pair cost me £80.
    * Mission 77C1 cost me £90 (plus a little extra for a replacement driver but it's still the best centre I've heard!)
    Those combined with a BK XLS200 sub (for example) would come in at around the £450 mark leaving a little for some cheap rear speakers and cable (Richer Sounds here we come) and hopefully some stands (Argos here we come).

    Clearly those speakers are a bit dated now but there are plenty of bargains out there if you look into it.

    I suppose it really depends where your priorities lie but it's certainly worth investing much more heavily in the front 3 than the back speakers which will be fine and could be upgraded if you wanted. Personally, I wish I hadn't gone 5.2 at all and spent the equivalent money on a stunning stereo system though... :)
     
  12. missionman

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    That's good advice - quality speakers for less money.

    We still advise, if you are a two channel fan who wants 5.1, to get the best front left and right speakers you can and add matching centre and surrounds. In your example a pair of 773e floorstanders would have been nice at the front.

    It also explains the popularity of the e82 system.

    Mr Mission
     
  13. nheather

    nheather
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    missionman

    It's an interesting point of view but not everyone has the same priorities.

    If were to apply this principle to everything in life we would all be in serious debt - where do you stop with the highest quality - car, house, food, cutains, lawn mower, china tea service.

    Some people may be happy with a £500 speaker set but spend £3500 on a new fireplace - it's all down to personal priorities

    There are some people that are not audiophiles and just want a reasonable 5.1 AV system for home cinema use. Mission sell plenty of them

    m30 Cinema
    m2s
    m3s
    m7s
    fs1
    fs2

    All under the £500 mark. I was interested in two of these but couldn't find a dealer who was prepared to stock them not alone demo them.

    Cheers,

    Nigel
     

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