Secrets of Amplifier and Speaker Power Requirements Revealed
Although it's not the easiest thing to comprehend, doubling the amplifier power does not double the loudness. In the above example, the sound from the speakers would not be "twice as loud"; it would only be "a little louder," an increase of 3 decibels. How loud is that? Hearing tests with large groups of people have revealed that a one-decibel (1 dB) change in loudness is approximately the smallest audible step that the average listener can detect, so an increase of 3 dB most listeners term "slightly louder." So why doesn't that 100-watt amplifier always sound twice as loud? Because the acoustic decibel--the decibel (dB) being the unit of measurement used worldwide to quantify the acoustic loudness of sound--has a peculiar relationship to amplifier power output measured in electrical watts. That relationship is called "logarithmic." If that word gives you an instant headache (nightmares of high-school math), then here's a simpler explanation: If a sound gets louder by 3 decibels or "slightly louder," it takes twice as much electrical power from your receiver or amp to produce that modest increase. Therefore, a 100-watt amplifier will produce sound only slightly louder than a 50-watt amplifier. Incidentally, if you'd like a kind of immortality, be terribly clever and work out a system of measurement. It may be named after you. The "decibel," one tenth of a bel and named for Alexander Graham Bell, recognizes his contributions to the understanding of sound. Likewise, we have to thank James Watt, Georg Simon Ohm, and Heinrich Hertz for their contributions to the industry. And then there's the Lofft, a measurement of neighbors' tolerance to testing new speaker systems . . .So far, so good. But what if it's party time, and you're listening to music "very loud," a level defined as about 90 dB Sound Pressure Level (SPL), and your speakers are gobbling up swings of 15 to 20 watts per channel on those musical peaks. Drink in hand, you advance to the volume control on your receiver thinking, "I'll just crank this up to make the music twice as loud," and you turn up the volume control until there's a 10 dB increase in the sound level. Now your party-time goal of "twice as loud" will make huge electrical demands on your nice little multi-channel receiver or power amp. The receiver must deliver ten times as much power to double the subjective loudness. Between 6 dB and 10 dB is double the volume level, where 6 dB is four times the power and 10 dB is 10 times the power. In the aforementioned example, the amp must produce 150 to 200 watts per channel for those peaks in loudness. Therefore, every 10-dB increase in acoustic loudness--from 80 dB to 90 dB, or 90 dB to 100 dB--requires ten times as much electrical power in watts. That's all very well if you have a monster amplifier or multi-channel A/V receiver with huge reserves of power output (most of us don't). If not, watch out. Your receiver or amp may "clip" or distort (or both), which will put a clamp on the output of the amp. When you push your amplifier into overload or "clipping," several things may happen. First, the top and bottom of the waveforms (representing the audio signals) are clipped off, generating distortion. Next, the amplifier's protection circuits are activated, removing those portions of the signal that are causing the overload, generating distortion. And finally, the amplifier's power supply may fluctuate according to the demands of the music signals. Not everyone is affected by this scenario, of course. Some people (increasingly few, it seems) don't listen to loud music. They like background levels, and with average speakers, background levels demand 1 watt or less of amplifier power. Or they may have very efficient speakers (Klipsch, Cerwin-Vega, Tannoy, and the like) that will play extremely loud using modest amplifiers, the trade-off being a very large degradation in tonal accuracy, a definite harshness, and a complete loss of off-axis performance that accompanies horn-loaded designs. But in many situations, speakers will be damaged and distorted sound will offend many ears. No discussion of decibels, acoustic loudness, and electrical watts is complete without an explanation of loudspeaker "sensitivity." (Another way to define a speaker's sensitivity is to look at how efficiently the speaker converts electrical power, in watts, to acoustic sound output in decibels.) Let it be said in a general way that speakers are not very efficient or sensitive devices. They need a lot of electrical power input to produce relatively little acoustic output. Nevertheless, speakers do vary quite a bit in sensitivity. To determine a speaker's sensitivity, we feed the speaker with 1 watt of amplifier power, using a test signal of pink noise, and measure in decibels how loud the sound is at a distance of 1 meter (about 3 feet). A lot of domestic hi-fi speakers measure in at about 89 or 90 dB SPL at 1 meter. Larger speakers, with bigger woofers and more drivers, typically produce greater acoustic output; smaller bookshelf models have to work harder, and their output is typically less, often between 86 and 88 dB SPL at 1 meter. Placing the speaker in a room helps (the walls, ceiling, and floor reflect and reinforce the speaker's sound), adding about 4 dB to its output. For example, a speaker like Axiom's M80ti has a measured sensitivity in an anechoic chamber of 91 dB SPL at 1 watt at 1 meter. But putting the M80ti in a room raises its sensitivity rating to 95 dB SPL at 1 watt, 1 meter. A 95-dB sound level happens to be "very loud," as most of us would subjectively describe it. And it is--from 3 feet (1 meter) in front of the speaker. But let's move our listening seat back twice as far, to 6 feet. Guess what happens? We instinctively know that sound gets weaker as the distance from the source is increased, but by how much? A formula called the "inverse square law" tells us that when the distance from the source is doubled, the sound pressure weakens by 6 dB. Among sound engineers, there's a common saying: "6 dB per distance double." So at a 6-ft. distance, the M80ti is now producing 89 dB. Now let's double that distance again to 12 feet, a fairly common listening distance. The speaker now produces 83 dB, which isn't all that loud at all. And if you sat 24 feet away, a not uncommon distance in big rooms, the speaker would produce 77 dB SPL. But what about stereo, I hear you shout. Here's another oddity of loudness and the decibel. When one speaker is producing a level of 90 dB, adding a second speaker playing at the same level only increases the overall loudness by 3 dB! (The loudness does not double!). So the two speakers in stereo produce a loudness level of 93 dB. So adding a second M80ti will raise the loudness at 12 feet from 83 dB to 86 dB. And don't forget we're still using 1 watt of amplifier power output into Axiom's most sensitive speaker. But how loud are real-life instruments, orchestras and rock bands? Now, while 86 dB SPL is "fairly loud," it's not nearly as loud as what you might hear from a good seat at an actual rock concert or from an orchestra or pianist in a concert hall. A solo grand piano can reach peak levels of 109 dB SPL, a full orchestra and chorus in a concert hall will measure 106 dB, and a rock group, 120 dB SPL. Now let's try and get our peak speaker sound levels to 96 dB, "twice as loud" as our 86-dB listening level. That isn't that difficult because right now we're only using 1 watt per channel to drive the M80ti's to 86 dB. So we'll need ten times as much power, or 10 watts, to reach 96 dB. Big deal. We've got lots more. But things begin to change, and rather dramatically. Let's push the M80ti's to what we might experience from a solo grand piano, 109 dB. We're at 96 dB with 10 watts per channel. Let's go to 106 dB. So that requires 10 x 10, or 100 watts. Close, but not quite there yet. Just 3 dB more. Remember, we have to double the power for a 3-dB increase in sound level. So 100 watts becomes 200 watts. Yikes! Our receiver has only 110 watts maximum output! We've run out of amplifier power! And what about the rock concert? Let's lower our expectations and aim for 119 dB. Going from 109 dB SPL, which needs 200 watts per channel, to 119 dB SPL (get out your ear plugs) is another 10-dB jump and--you do the math--that requires 10 x 200, or 2,000 watts per channel! From all this you can see the huge power requirements inherent in reproducing real-life acoustic sound levels in average or big rooms. The M80ti's are tested to levels of 1,200 watts of input power so they come very close. But the truth is that if we are seeking real-life acoustic sound levels in our listening rooms, there's a very persuasive argument for very large, powerful amplifiers. And if your speakers are less sensitive (and many are), then the power demands rise even more dramatically. Sizeable rooms and greater listening distances will also increase power demands tremendously. And what many of us don't realize until we hear it, is that clean undistorted loud sound often does not sound that "loud" The key here is that in most or our home listening, there are small amounts of distortion caused by a lack of dynamic headroom (but more on that next month). It's the distortion that makes it sound "loud" in a domestic setting. To remove those distortions and increase dynamic headroom relates to even more power. We've become accustomed to accepting some distortion with our reproduced music, because all amplifier's distortion ratings gradually increase as they approach their output limits or slightly clip the audio signals. When that happens, we turn down the volume, because distortion starts to intrude on our listening pleasure, and it sounds "too loud." The lesson in all this is that you can never have too much power, and that big amplifiers rarely damage speakers. Little amplifiers driven into clipping burn out speakers. In the scheme of high fidelity, that last barrier to realism is having enough power and being able to approximate real-life loudness levels. by Alan Lofft, Axiom Audio (reprinted with permission) |
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     Anonymous |
For the mathematically inclined, you get 10 * log(p1/p2) more dB when you go from power level P2 to P1. So if you go from 30 to 300 watts, you get 10 dB more of SPL. From 30 to 60 (doubling), you only get 3.01 or 3 dB. It takes +10 dB to double the apparent volume (-10 dB to halve the apparent listening volume). "Sensitivity" is measured with 1 watt of power *only* if the speaker is rated at 8 ohms (2.83 V). If the speaker is not 8 ohms, 2.83V would not yield 1 watt; and 1 watt would not require 2.83V. Most mfg measure sensitivity with 2.83V across the speaker terminals regardless of the impedance (and hence the power delivered). "Efficiency" on the other hand is measured with 1 watt and ignores the voltage needed to deliver that 1 watt of power. So with sensitivity you hold the voltage at some level (2.83V) and with efficiency you hold power at some level (1 watt). At 8 ohms, they come out the same but many speakers today are lower than 8 ohms nominal. The difference between efficiency and sensitivity is important because power is related to the square of voltage (P=v2/R); it is not a linear relationship. |
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toysub Unregistered guest |
Thanks for the clarification. I have been looking for that answer to the 2.83V measurement and the 1W/1M differnece in ratings for a while now. When it comes to speaker volume, doesn't it have something to do with Voice Coil diameter times the Xmax? or the amount of air that the driver can move? Is the max power of a speaker the amount of power it takes to drive it to Xmax? I'm trying to build a home theater sub and I see all these ratings of massive Xmax and power handling but when it comes to spl they're only 85 or so. But cheaper subs that don't have as high of Xmax and power handling have a higher spl of 95 or so. Which sub would be better as a home sub. Is there a trade off of loudness with sound quality somewhere in that process? Thanks for any response. |
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Unregistered guest |
What effect does the ohm rating [say 6 ohms rather than 8 ohms] have on speaker sensitivity and the wattage required to produce a given loudness? |
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Unregistered guest |
So if you gain 3db by adding a second speaker to the same 100w amplifier, what do you get if you add a second 100 watt amplifier AND a second matching speaker |
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toysub Unregistered guest |
I'm no sound expert but here's my input. If you put a 6 ohm speaker in place of an 8 ohm speaker, you will have more current flowing through the voice coil. Obviously if your input voltage stays the same while you decrease the resistance of your load, more current will flow (ohm's law) therefore making a stronger magnetic field from the voice coil, creating more excursion (which equates to loudness). You could put a 2 ohm speaker on any given amp and make it a lot 'louder'. However, amps are rated for a certain ohm output. Puting a 2 ohm load on an amp that is designed for an 8 ohm load, will cause the amp to overdrive and probably cause damage by overheating. to the second question: To gain 3db with a second speaker, it has to be powered by the same power as the first speaker. For example you have a single 8" driver with 100 watts driving it giving 100 db. To get 103 db output, you'd have to put another 8" driver with a 100 watt amp on it too. If you were to put both drivers on the same amp, you'd decrease the power to each sub to 50 watts each. You'd lower the initial 100db on the first driver to 97db and then by adding the second driver at the remaining 50 watts bring back the output level to 100db. It does no good to add a second driver on the same amp unless you're not using the full potential of the amp's output. |
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Unregistered guest |
I ve been a speaker builder since 1987, as a hobbyist. I am now given the opportunity to mix at church. In my opinion, the present sound sucks. We have (one) Tannoy V12 (200 watt-97db) running in mono, supplying the general seating area(40' X 50'). There is a sub box in tandem with the tannoy V12. We also have four Community monitors (400 watts-97 db each) supplying the stage (25' X 40') I am looking for a formula('s) to configure the amount of SPL I need from my speakers output, to fill the church with a warm, soft, easy sound. I am looking for a formula that I can use the following parameters. Room surface area- 3,000 Square Ft. (40' X 75') Room cubic volume- 36,000 Cubic Ft. (Vaulted Ceiling) Watts- ? SPL- ? I was thinking about running multiple (spaced, positioned, and directed accordingly), lower SPL two-way cabinets to supply sound instead. This way it would fill the room with sound, but not cranky... I am interested in professional advise. My main concern is having enough volume to fill the room (not one really loud speaker in the middle... lol). Your advice will be greatly appreciated. God Bless You Ben M. |
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Unregistered guest |
Remember the inverse square law...every time you double your distance from a point of energy, the apparent intensity falls to 1/4. The "trick" is to position enough loudspeakers with wide dispersion to provide proper fill to the area of interest. In doing so, as you move away from one source of sound, with it's decreasing intensity, you'll be moving closer to another, with it's increasing intensity. Done right no "hot spot-cold spot", and you won't have to crank anything up to really high spls. Quality counts! |
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mixneffect Unregistered guest |
Yes, the location, spacing and direction at which they will be pointed to, will be taken into careful consideration. However, I don’t know what speaker size to use, how many, and what power handling, and SPL. In my opinion, I designed a custom box consisting of two 8" woofers (80W ea., 8 ohm @91 db ea.) with a 1" tweeter (90 W, 6 ohm, 92 db) in a closed box. The x-over will be designed to accommodate balance between the woofers and the tweeter. This design will produce down to 60 Hz. It will be used for everything except the bass guitar, and keyboard on the low frequencies. There will be a custom subwoofer box for those frequencies. I am thinking about using 8 of these cabinets. If anyone has a professional opinion, please pass it along. Should I use less or more speakers, watts, SPL...? God Bless You, Ben M. |
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Unregistered guest |
im not very knowledgeable with this stuff, but i hav a 70 watt guitar amp and another 100 watt speaker, if i connect the 2 will it end up being much louder |
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Unregistered guest |
first of all, does your amp have a speaker integrated with it (both in one box)? if so, then does it allow you to plug another speaker into it? if so, then the total amount of power that your amp will push will still be 70 watts, just split among both speakers. remember watts is what your amp pushes out. SPL or decibels is what the speaker produces. on the other hand, if you have a 70 watt amp and a separate speaker, that can handle upto 100 watts, you will get only enough volume that your speaker will produce with 70 watts. when two or more equipment pieces have a different wattage power handling, the lower one will always get satisfied first. then any more volume that you may give (by turning up the volume button) will distort the sound. once a speaker has reached its peak performance, it will distort. you will notice that it seems overloaded or forced beyond its capacity. a 70 watt power amp and a 100 watt speaker should be ok. even: a 100 watt amp and a 70 watt speaker is not a terrible matchup. |
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JAKE123456789 Unregistered guest |
ok i have no idea what that means... but could some one tell me what sort of dB output will a marshall avt150 head with 4x12" 300watt 8ohm,will have? |
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Unregistered guest |
I'm looking to get a low volume guitar amp for recording in my apartment. Will the difference between 20 watt (specifically, the Mesa Reverb Rocket) and 30 watt (Mesa F-30) tube amps make much of a difference in volume? I want to be able to push the tubes as much as I can without getting evicted. |
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Unregistered guest |
I want some suggestions for designing outdoor sound system solution. I have a open air lawn of 13500 sq. ft. and I want to cover it for sound coverage. We will use that lawn for Live speech, debate and music functions. Kindly, suggest that how many speakers of how much power required? and moreover how much power or watts required in amplifiers and mixer for mentioned speakers? Thanking in anticipation. |
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Unregistered guest |
I have a 70 watt, 4 or 8ohm amplifier with two output channels i.e. I can have two sets of speakers or Bi-wire one. I'm wanting to build some 3-way, Bi-wireable speakers but have no experience in speaker design. I don't want to build crossovers, could someone help either by A: telling me what to do or B: suggesting a site to look at for driver selection and box design. Cheers |
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Unregistered guest |
i have a '65 fender 20 watt deluxe reverb and 85 watt twin reverb guitar amp. im building a extension speaker cabinet so what size speaker(s)?could i use to be able to use with both amps? thanks ucimthedude@hotmail.com |
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Scarenius Unregistered guest |
Just wanted to add that the efficiency of a driver is directly related to the enclosure it is in when playing. For a very direct comparison, check out the description of the Pyramid sub in the Imperial Folded Horn at http://www.decware.com. IIRC, generally, for an enclosure, sealed < slot/vent ported < single reflex bandpass < dual reflex bandpass < folded horn. Efficiency increases with each of the aforementioned enclosure designs respectively. Aperiodic bi-chamber enclosures and transmission lines are somewhere between single and dual reflex bandpass for the curious. Of course, this is all relative to the build quality of the box. |
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Pramod Unregistered guest |
I have 8 speakers each of 6Watts output (Total 8*6 = 48W). How much should be my amplifier output wattage? |
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