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Efficiency, can someone define it for me

Discussion in 'Amps and Cabs [BG]' started by cassanova, Mar 3, 2001.


  1. cassanova

    cassanova

    Sep 4, 2000
    Florida
    ive seen alot of stuff here abount efficiency, now at the risk of sounding stupid can you tell me what it means...i buy my amps cos solely cos they sound good
     
  2. It's quite simple, really. The efficiency rating determines how much of the electrical input is converted into sound energy. Amps have all about the same efficiency (except for the really really big amps, for example QSCs): about 60-70% at maximum power. Speakers can have very different efficiencies. A cheap HiFi speaker about 0.05-0.1% and a very high efficient bass cabinet about 5-10%. High frequency horns as used in professional PA applications can reach 75% or so.
    Normal bass cabinets average around 2-3%.

    Hope this helps.
     
  3. Oysterman

    Oysterman

    Mar 30, 2000
    Sweden
    How does one connect these efficiency measurements (%) with the sensitivy ratings (dB)on a cabinet? Or more like, what does the sensitivity rating really mean?
     
  4. Reid

    Reid

    Aug 25, 2000
    Oakville, Ontario
    the sensitivity is the amount of db's the cab puts out when you put a 1 watt/1 khz signal into it, measuring from 1 meter away. Knowing that you can figure out how loud the cab will be with x amount of watts, I dunno how so don't ask me. I don't know how to convert db's to watts, so I can't really measure the efficieny of cabs. I think I'll ask my physics teacher on mon.
     
  5. MikeyD

    MikeyD

    Sep 9, 2000
    As Joris said, efficiency is the measurement of how much acoustic power is generated for a given electrical power input. Speaker sensitivity is normally obtained by measuring the sound pressure level (SPL) in front of the speaker, while determining the electrical power input. This is a very complex subject - much more so than it seems on the surface. Getting an *accurate* measure of sound pressure level is not trivial, nor is obtaining the actual power input. I'll give you some idea of why this is so difficult:

    1. Measuring the power input: power delivered to a loudspeaker is the product of the "real" portions of the complex voltage and current. This means the components of the voltage and current that are "real" and in phase. (You have to study complex mathematics to know about real and imaginary numbers, etc.) A special power meter - or an oscilloscope - must be used, since just measuring the voltage is not sufficient unless the impedance AT the test frequency is known. Typical loudspeakers' impedance varies widely with frequency, so it's not straightforward.

    2. Sound pressure level (SPL) is obtained by a sound level meter or similar test equipment calibrated for the frequencies of interest. The microphone feeding such equipment is typically placed on-axis, 1 meter from the front of the speaker. Sounds simple, however, lots of acoustic anomalies can happen. If one puts the speaker in a "live" room, the reflections will add to the SPL heard by the microphone. This is not desirable, because then it is hard to determine what the speaker will do in a "dry" (anechoic) setting. So... one must get rid of all the reflections/reverberation at the frequencies of interest. This isn't hard at higher frequencies, but for bass frequencies, it gets very difficult and expensive to do the lower one goes. One would need to use a very expensive anechoic chamber or suspend the speaker in a very quiet, large, and unobstructed outdoor space and make corrections for ground reflections.

    3. Even when the SPL is accurately determined, one then has to consider the directionality of the speaker. With a very directional speaker, most of the acoustic energy would be directed in one place, and so the microphone might hear a high SPL level on-axis, but very low SPL off-axis, for example. All of these directionality patterns are ALSO very dependent on frequency and room boundaries! So you might find a very "efficient" speaker whose sensitivity might be 108 dB @ 1w/1m, but the speaker is only loud directly in front, but quiet at the sides. Or - the sensitivity was measured at a frequency that caused "beaming" and resulted in a high sensitivity, but the lower frequencies might be much more omni-directional, so the frontal SPL at the latter frequencies might be poor. And this might be with an anechoic (no echoes/reflections) test. In this example, the speaker would show high sensitivity but rather low "efficiency" (overall acoustic power output, measured in all directions, is low).

    4. Okay, now you take a speaker that doesn't "beam" its lower frequencies and place it in a real room. Maybe that speaker normally generates decent SPL in all directions at low frequencies, so you put it in the corner of a room, and BAM! you get pummeled with bass. Why? The off-axis acoustic power hits the room boundaries, reflects back and reinforces the direct sound in front!

    5. What the above says is that "efficiency" and "sensitivity" are actually two different things that may or may not relate to each other. If one accounts for the directionality pattern (at a given frequency, remember), then one can develop the relationship between the measured sensitivity and overall efficiency FOR THAT FREQUENCY. Efficiency accounts for the sum of ALL the acoustic energy generated by the loudspeaker, in all directions - not just on-axis!

    6. Sensitivity can give you some idea of how loud a speaker will be, but you must consider at what frequency that sensitivity was measured, whether directionality is important, etc. For "dry" applications - such as playing outdoors away from reflective surfaces, sensitivity might be more important, because it tells you something about how the sound will get developed in front of the speaker, with little or no reinforcement from reflections. For other applications, such as playing in a typical club, actual percent efficiency might be more important at the lower frequencies, because even if most of the acoustic power is propagated off-axis, the reflections will yield a strong overall output.

    As you can see, this is a very messy subject! So - a very oversimplified guideline might be: pay attention to sensitivity ratings for the higher frequencies (say 500 Hz. and up) and to "efficiency" percentage for lower frequencies - especially if you need good output in reverberant spaces. If you only play in nearly-anechoic environments (e.g., outdoors in the middle of a big field), then sensitivity gives a better indication of loudness vs. power.

    So, to answer your question, sensitivity is ONE number that represents a very small piece of a loudspeaker's performance. It's like rating a person's personality with one number! I hope the above helps everyone see what the meaning of this number is.

    I welcome anyone's comments on this.

    - Mike
     
  6. cassanova

    cassanova

    Sep 4, 2000
    Florida
    thanks for explaining that to me, Mikey D you kinda left me a bit more confused than I already was...lol

    Ok 2nd question then, are Carvin Cabs rl 4x10's to be specific, efficient. Dont get me wrong I love the sound of mine, but just dont know how efficient they are.
     
  7. MikeyD

    MikeyD

    Sep 9, 2000
    Yeah, this subject confuses me, too, and I've studied it a lot. Bottom line is, try to listen to other players' equipment as much as you can in *real* playing situations to see how it sounds. Even that is not always clear, because you may not know about the amp and/or equalization used, etc.

    I have a Carvin 2x10, and it uses the same drivers. They are not that efficient, IMO. Raw PS10 drivers in free air are rated at around 95 dB; I've seen other 10" drivers at 99 dB. In practice, I have had to dump huge amounts of power into my 10's to get them to be loud enough - especially on lower notes. Fortunately, they can take a lot of power - and Carvin amps give good output per dollar. The 4x10 cabinet is 8 ohms, so getting enough juice to it to make it loud may be a challenge. If you got the R1000 head and ran the 4x10 bridged, you'd get maybe 550-600 clean watts into it. If the cabinet's sensitivity were 96 dB @ 1w/1m (a guess on my part - Carvin doesn't show this figure for their bass speakers), then you might get up to 124 dB maximum at 1 meter. That's fairly loud - at least at the frequency associated with the sensitivity measurement (I'd guess 1 kHz). But the speaker rolls off the lows quite a bit, so your bottom notes won't be as loud. Compare my guess of 96 dB (sorry, Carvin, if I'm way off, but it's a guess) to SWR's 4x10 claimed at 105 dB and Eden's XLT at 106 dB. So - the Carvin just isn't a "loud" cabinet as far as I've been able to tell. They have nice tone, however.

    - Mike
     
  8. cassanova

    cassanova

    Sep 4, 2000
    Florida
    RE: ^
    My 4x10's get pretty loud, I think I know what you mean though, its not as loud as some of the others out there, but i usually use it as a referance moniter when i gig and mic it. It does suprise me that they arent as loud as some of the other cabs out there that are 400 watts as they have most cabs Ive seen beat by 200 watts. But yes they do have a very nice tone. I need a more powerful head also i think that will help, Im only running 120 watts into a 600 watt cab.
     
  9. jvtwin

    jvtwin What it needs is a little more cowbell

    Jan 26, 2001
    LA Calif.
    Thats easy - one word, Aguilar! Now THATS efficient!