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What is efficiency ?

Discussion in 'Amps and Cabs [BG]' started by 6L6, Apr 10, 2005.


  1. 6L6

    6L6

    Apr 7, 2005
    I think having an efficient loudspeaker is important.

    Usually efficiency i measured in db at 1 meter distance
    from the speaker, receiving 1 watts.
    But since a speaker has a variabel impedance vs frequency,
    this has later changed to 2,83 volts input.

    at the frequncy the speaker is 8 ohm, this vil give 1 watt.
    If you have 4 ohm, 2.83 volts vil give you 2 watt.
    So this means that a 99 db/W 8 ohm speaker is
    as loud as a 4 ohm 102 db/w speaker.


    Statement 1:
    After having tried some different speakers
    the 15" always seems louder, even when they
    are rated at the same SPL and have the
    samme lower frequency range than a 2 x 10".

    This is even more apearent, when i compare an older
    Peavey 15" with newer 2 x 10" systems.

    I got the samme impression about 15" vs 18"
    in PA. the 18" always seems so loud.
    Even when the 15" had the samme -3 db point.

    This makes med wonder, when the manufacturer
    measures his speaker: at what frequency is this done ?
    Could it be that let's say a 4 x 10" system is measured at
    let's say 500 hz ?
    And a 15" at let's say 100 hz ?
    (A smaller speaker like a 10" , are usualy a lot
    more efficent at higher frequency's than a 15")

    Statement 2 :
    The lagest cabinet always seems to win.
    And this makes me also wonder if the SPL
    of the driver is measured in a very efficent cabinet,
    and then
    later, this data is used on the smaller cabinet. ?
    Or is it there something i have missed about the larger cone "
    Of 15" gives a higer SPL in smaller roms (at lower frequencys) ?
     
  2. I had some interesting discussions with the guys at EA and Acme through the years on effiency. There is a great article on EA's website regarding efficiency. Part of the reasons that Acme and EA describe their cabs as somewhat inefficient is based on impedence differences. While cabinets are rated at 4ohm and 8ohm, what I didn't realize was that this is just average impedence. For a 'typical' cabinet design, the impedence greatly increases at very low frequencies. Therefore, the speaker is drawing much less power at those very low frequencies, and hence, the low frequency response is not as good, all other things being equal. However, since those low frequencies are not 'sucking up watts', you get a higher general db level per watt. For the 'inefficient' designs, like the EA transmission line and the Acme cabs, the cabinet design results in lower impedence at the lowest frequency level... resulting in 'better' bass response, but requiring more wattage in general due to the fact that the amp is working very hard to reproduce those lowest frequencied (i.e., the lower impedence below 50hz draws a massive amount of power from the amp.

    I realize the above is from a layman's perspective and is probably not exactly accurate, but I think it's generally correct. It was a real eye-opener to me that the impedence distributions of cabinets are very different, resulting in both different sound and differences in efficiency. Check out the info section on the EA website... very interesting and explains why the Acme and EA cabs are such different beasts compared to other cabs.
     
  3. IvanMike

    IvanMike Player Characters fear me... Staff Member Supporting Member

    Nov 10, 2002
    Middletown CT, USA
    well, 1st off, (and btw, the comments form ea and acme were very enlightening), you're not going to get anyone to change the common practice of measiring efficiency/sensitivity @1 watt @ 1 meter @ 1K. the whole world does this.

    what you'd need to really compare cabinets (on specs alone) would be the whole frequency response chart of both cabs (which you're not going to get unless an independent tech does it for you).

    even then, it's not always easy to translate what that graph looks like into how the cab will sound, or how loud it will be compared to cab #2.

    part of what you describe can also be due to the differences in surface area of the drivers. two 10" have slightly less surface area than a single 15", and an 18" driver has almost as much surface area as four 10" drivers. however, i think there may have been other factors that made the 15 sound louder than the tens (the difference in surface area really is slight).

    IME, many fifteens have some midrange peaks that make them sound "barky" and not as "smooth" as tens. often we end up perciving this as "louder" since we hear midrange so well.

    finally, although companies that just make drivers give efficiency ratings for them, the efficiency rating of a cabinet is done with all of the drivers in place in the cabinet in question.
     
  4. Halftooth

    Halftooth Supporting Member

    Nov 24, 2002
    Tri-Valley, NorCal
    The whole sensitivity issue has to be looked at with a grain of salt, especially when comparing woofers where SPL is as big as an importance as DB. It's easier to produce a higher DB at a higher frequency than a lower because it takes less work to produce a higher DB at a higher frequency. A larger woofer, "less efficient" cab in terms of DB will often times put out more SPL than a smaller woofer cab that is considerred to be more efficient in terms of DB, and the result is that the player will feel that the "less efficient" cab is louder because the amount of air moved is greater. It's really confusing, but in my experiences with home and car audio, the woofer that claims to be the most efficient end up being much less efficient because when pushed at high levels, they require much more power than some other drivers which can go much louder or produce greater SPL at lower power ratings. This is in a round about way what EA is saying. The bottom line is that you can't rely on the manufactures DB results to impact your decision. You have to look at the whole enchilada which is FR, DB, and SPL.
     
  5. Mike Dimin

    Mike Dimin

    Dec 11, 1999
    Clinician: EA, Zon, Boomerang, TI. Author "The Art of Solo Bass"

    Actually, the EA cabs are pretty efficient and the Transmission Line increases efficiency at lower frequencies. For more info check these two articles:

    Transmission Line

    and

    Sensitivity

    Mike
     
  6. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    A few points. Efficiency is acoustic watts output as a percentage of electrical watts input and is expressed as a precentage figure. The average bass cab runs at perhaps 5%. Sensitivity is expressed in decibels, generally dB per watt at 1 meter. However, this figure is not constant with frequency, so without referencing sensitivity to frequency you cannot make any valid comparisons between different speakers.

    Transmission lines are very seldom used in pro-sound as transmission lines work best with drivers that have low fs and high Qts figures, while high sensitivity pro-sound drivers get their high sensitivity via high fs and low Qts figures. That doesn't mean that one can't use a TL for pro-sound, but does explain why you see very few of them.

    Impedance higher than the speaker average impedance at low frequencies, and high impedances in general, do not necessarily translate into lower output. An amplifier is a constant voltage output device, and cone movement is dependant on voltage input, so a high impedance doesn't necessarily equate with less SPL. High impedance does lower the amperage draw of the speaker, which lowers the wattage draw, but if the cause of the high impedance is resonance in a reflex alignment or TL or the increased acoustical impedance of a horn then far higher sensitivity can be a result which more than offsets the lower wattage draw. Conversely, lower impedances will allow more power to be expended by the system, but that does not result in more volume being produced if the system is operating at lower efficiency, which more often than not is the case when the system impedance is low.
     
  7. Mike Dimin

    Mike Dimin

    Dec 11, 1999
    Clinician: EA, Zon, Boomerang, TI. Author "The Art of Solo Bass"
    Not really true. The reason you don't see Tl in MI applications is that the design is extremely difficult and time consuming. Most copmpanies do not have either the expertise or the desire to sacrifice the bottom line. There are no computer models for TL, no easy way to design it for mass production.

    I understand that TL is not the way you choose to go in your designs. John Dong, EA's cabinet designer, is a true genius in the industry. John has had a great deal of success doing what others said could not be done, which is to create a TL in a small, lightweight and terrific sounding cabinets. He has demonstrated this time and time again.

    In this post and others, you have derided the use of TL. Obviously you have your own business - selling your designs. But instead of promoting the benefits of your ideas, your criticize others. Personally, I feel that you only serve to hurt your own ideas that way. If you are posting to create business demand, then be upfront, talk about the benefits of your product, it's design, etc (of course you must stay within the rules of the Commercial Users Policy).

    Dimin
     
  8. fdeck

    fdeck Supporting Member Commercial User

    Mar 20, 2004
    Madison WI
    HPF Technology LLC
    AFAIK the sensitivity rating of a speaker is based on a "Watt" defined as the equivalent input voltage required to drive a resistive load of the speaker's rated impedance to a Watt. In other words, this is 2.83 V RMS for an 8-Ohm speaker, and 2 V RMS for a 4-Ohm speaker. To measure a speaker's sensitivity curve, one would sweep a generator at constant output voltage across the frequency spectrum.

    This strange convention is actually useful, becuase an amplifier controls its output voltage in proportion to the input signal, not the output power. Likewise, the familiar equations for speaker response assume a constant voltage input. A power amp driven far below clipping (i.e., a bass amp feeding one Watt into typical speakers) will produce a constant output voltage regardless of the load.

    It is unlikely that there will ever be believable sensitivity ratings until manufacturers publish actual graphs. True, these graphs are not necessarily easy to interpret in terms of predicting sonic performance, but a graph is less likely to represent the speaker operating under the most favorable conditions. I don't know if they still do, but Carvin used to have graphs in their catalog.
     
  9. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Mike, Bill wasn't criticising EA or Transmission lines. If anything he pointed out how unique they are.
     
  10. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    Apparantly you don't understand. I do use TLs where I have found them to be most appropriate, in home theatre applications. I don't use them in pro-sound applications as I believe that horns work better there.

    Not true. Here's a link to one:
    http://www.audioxpress.com/bksprods/sotware/sofTLwrx.htm

    I don't think that what I've said qualifies as derision. I have pointed out what I feel are both positive and negative aspects of particular box alignments. I believe I am somewhat quailfied to do so, both from a professional standpoint and from my interpretation of the First Amendment.

    That's what I thought too. Whatever.
     
  11. Mike Dimin

    Mike Dimin

    Dec 11, 1999
    Clinician: EA, Zon, Boomerang, TI. Author "The Art of Solo Bass"
    John used TL as well in pro audio applications when he was designing for John Curl and Saul Marantz. However, he was also able to utilize TL for MI applications, quite successfully, I might add.



     
  12. Actually, those are the articles I was quoting in saying that the EA's are inefficient. An inefficient design is not a bad thing. If I understand the articles correctly, the lower impedence a cabinet has below 100hz, the more watts it will take to drive the cabinet to a given volume in general, since those low frequencies result in a lot of power being used up. Acme is similar... from my many discussions with Andy over the years, he wears the badge of 'inefficient' speaker with honor, since the 'inefficeincy' results in a much wider frequency response. However, maybe there are a lot of definitions for inefficiency, so maybe I'm using too loose of a definition.
     
  13. billfitzmaurice

    billfitzmaurice Commercial User

    Sep 15, 2004
    New Hampshire
    Owner, Bill Fitzmaurice Loudspeaker Design
    You are using the term correctly in this case. It was first succinctly postulated in Hoffman's (the H in KLH) Iron Law, which states that for a given combination of driver and alignment there is a finite relationship between efficiency, cabinet size and low frequency bandwidth, and that the alteration of one factor will alter at least one other. To simplify, you can go small or low or loud, but you can't do all three at the same time. Two out of three is all you can get without changing either the driver or the alignment or both.
     
  14. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    And in striving to achieve all three, we come up with some interesting design theories, some of which have been spoke about here. And as you can see, the debate over which is "best" can get pretty hot.

    The reason andy from Acme wears his "inefficient" badge with honour is because his solution to the problem is "more watts". Overall SPL is a function of both sensitivity and wattage. With a high enough wattage rating, inefficient speakers can achieve the SPL required for most situations. And watts aren't as expensive as they used to be.