Loudspeaker rating - Page 1
Forum section: electronics / beginners

author: najrao
author_rank: Regular Contributor
timestamp: December 14, 2021, 12:07:33 pm
content: Not really a beginner myself, but need to find an answer to what is nagging me for some time.(Audio) loudspeakers, particularly woofers, are givenrmspower ratings typically 50W for a 4 or 6 inch unit with a voice coil diameter of less than an inch or so. Not just some knockoffs, very reputable audio manufacturers too. Given the efficiency of sound conversion is at best a few per cent, the coil has todissipatenearly all these watts from its surface area of a square inch or so? In the annulus where the coil is situated, there is almost no space for air flow. And I have not even mentioned the 500W "music" power! How hot can such a coil go? Seems the currents are in fact enough to be called 'fusing' current.I want to get the physics right, no audioph***ery please.

author: Terry Bites
author_rank: Super Contributor
timestamp: December 14, 2021, 12:35:08 pm
content: Your questions anwsered by speaker engineers.www.klippel.de/know-how/measurements/power-handling-heat-dissipation/thermal-analysis-and-heat-transfer.htmlclick on the SIM links for rmore detail andwww.klippel.de/fileadmin/klippel/Files/Know_How/Application_Notes/AN_19_Nonlinear_Thermal_Parameters_%28Convection%20Cooling%29.pdf

author: CaptDon
author_rank: Super Contributor
timestamp: December 14, 2021, 03:52:28 pm
content: Many things come into play. Ventilated magnet structures, Ventilated dust caps on the cone. And perhaps with 50wrms only 5wrms becomes sound energy, but what about the energy lost to the stiffness of the spider, energy lost to the stiffness of the surrounding material of the cone (rubber, paper, cloth) energy lost in distorting the cone itself from standing waves, energy lost in the very acceleration and deceleration of moving mass, and perhaps energy lost by pumping air into and out of a ventilated magnet structure. The real heat number I worry about is the I2R loss in the voice coil resistance (DCR specification). I have driven 250wrms rated 'quality' Front-of-House P.A. speakers from Peavey, JBL, Eminence and others with 1kw true RMS rated amplifiers (Carver PM-2.0T, QSC, etc.) which by the very nature of their high power ratings can deliver clean power without clipping and has allowed me to run 250wrms speakers with nearly a kilowatt of 'musical power' and not damage the speakers. I have killed some high frequency compression drivers but actually have never killed midrange or woofer speakers!! Strangely enough you can kill a 100wrms speaker with a 50wrms amplifier when you drive the amp into square wave clipping!!! Your mileage may vary as they say. Cheers!!

author: mc172
author_rank: Frequent Contributor
timestamp: December 14, 2021, 04:21:48 pm
content: Quote from: CaptDon on December 14, 2021, 03:52:28 pmHow?Driving an amplifier into full square wave clipping would sound absolutely horrendous, I think you'd have to be fully deaf (but somehow also in control of a sound system) to drive it so hard to cause it to clip that bad.

author: CaptDon
author_rank: Super Contributor
timestamp: December 14, 2021, 08:33:43 pm
content: What do you mean how??? You 'crank it up'. Every little piece of shit 50 watt amplifier can easily be driven to clipping and every idiot that takes their little cracker box sound system outdoors soon finds that 50 watts isn't very loud at the back yard cook out so they crank it. Sure it sounds like shit, but so does a lot of the music they play!!! As soon as you drive an amplifier into clipping you basically have D.C. component on the voice coil and that kills 'em quick!! You never saw a 50 watt guitar amp pushed into square wave clipping??? I had a D.J. buddy with a 100 watt Bogen amplifier constantly killing 250 watt RMS quality speakers. Solid state 'output transformerless' amplifiers are the worst for killing speakers if they don't have D.C. detection or clipping protection!!! Tube amps driven into square wave distortion tend to kill themselves before killing the speaker because the output transformer saturates and plate current skyrockets and the tubes overload and die. We have some 'Rap' D.J.'s that entertain on the boat I operate. Their systems are so overdriven that the sound totally sucks donkey nuts and the music they play sucks also, but 'their' crowd couldn't care less. The louder the better and who cares if it sounds like a chainsaw, just so those low notes knock the Hennesey or Patrone bottles off the table!!!

author: mc172
author_rank: Frequent Contributor
timestamp: December 14, 2021, 08:53:08 pm
content: Calm down.It's not like DC though, the voice coil isn't being held in a steady state, it's being thrown around quite violently. A 50W amplifier driven so hard into clipping it's gone perfectly square wouldn't deliver more power than the hypothetical speaker in question is rated for. It's marginal (100W delivered, 100W speaker) but if it can't handle 100W, is it a 100W speaker?Besides, the power supply in said amplifier probably wouldn't be able to keep up. The rails will sag heavily and therefore the power delivered wouldn't be 100W, it'd be significantly less.You also don't seem to distinguish between "clipping" and "clipping so hard it's gone square" which are very different things.

author: Benta
author_rank: Super Contributor
timestamp: December 14, 2021, 09:04:38 pm
content: Quote from: mc172 on December 14, 2021, 04:21:48 pmI think CaptDon is talking about 2, 3, or 4 way speakers.You have to look at the power spectrum in a "normal" signal.There's not a lot in the tweeter range (they are normally rated at much lower power), mid-range is significant and are normally rated at RMS or a bit lower, and bass is rated at full power, because they have to move the largest volume of air.Turning up an amplifier to clipping, a lot more power power is transmitted to the tweeters, and they burn out quickly. The mids may or may not survive, the bass units will.

author: TimFox
author_rank: Super Contributor
timestamp: December 14, 2021, 10:01:37 pm
content: Repeating one of my pet peeves: "rms power" is a defined mathematical quantity, but it is useless. What people mean is "mean power". One can calculate that true "rms power" is (3/2)1/2= 1.225 times the "mean power", for a pure sine wave. Other waveforms will give different results.Unfortunately, the industry was led astray more than 60 years ago and perpetuates this mistake. However, that is when the marketing departments started to develop further lies to exaggerate what their amplifiers could actually produce.

author: Benta
author_rank: Super Contributor
timestamp: December 14, 2021, 10:29:25 pm
content: Quote from: TimFox on December 14, 2021, 10:01:37 pmYou forgot to add: "it is uselessin audio"I'm totally with you hereAnd considering that all audio specifications (except power amps) are voltage specified, perhaps a better specification for power output would be voltage these days. Modern power amps have almost perfect voltage outputs. Limiting factor is just current capability.The "power" specifications go back to the tube days with output transformers and continued into the DIN41000 "HiFi" times. All outdated... but still used for marketing.

author: Zero999
author_rank: Super Contributor
timestamp: December 14, 2021, 10:42:34 pm
content: There really are speakers which can take up to 2kW RMS, so 50W should be no problem. Here's a video of photonicinduction testing one to destruction.

author: TimFox
author_rank: Super Contributor
timestamp: December 14, 2021, 10:56:11 pm
content: Quote from: Benta on December 14, 2021, 10:29:25 pmActually, mean power is a generally useful concept, related to “variance” in statistical questions.The confusion results from a proper measurement of rms voltage across a resistive load, then squaring it to calculateP=V2/R. IfVis an rms value (for a sinusoid, square wave, DC-biased waveform, or noise voltage), then that equation gives a mean power, which can be compared with the power dissipated by a DC voltage in the same load. I have never seen a legitimate use of rms power for an electrical power, but perhaps it might be useful in optical power.

author: Benta
author_rank: Super Contributor
timestamp: December 14, 2021, 11:22:05 pm
content: You've lost me completely. You bring three or four themes together in one post, which just results in... I don't know what.Sorry.

author: CaptDon
author_rank: Super Contributor
timestamp: December 15, 2021, 12:14:43 am
content: Indeed my systems are 2, 3 or 4 way depending on venue size. As for the squarewave clipping. Assuming no rail sag a 50wrms amplifier with ZERO headroom must push 20vrms into 8 ohms to produce 50wrms. That very same amplifier must then be capable of 28.2 volts peak. A squarewave has the same heating value AS D.C.!! Therefore 28.2vdc at 8 ohms is 99.405 watts. So much for the life of that 50 watt rated speaker!! In addition an 8 ohm speaker tends to have a DCR of 5.5 ohms. Let's see, 28.2vdc at 5.5 ohms = 144.589 watts!!! Get the picture!!! Just for giggles look up the typical 'true' impedance of an 8 ohm speaker, typically varies anywhere from maybe 6 ohms to over 20 ohms as it hits resonances and is measured using a pure sinewave. All that impedance stuff goes right out the window when you hit clipping, even a little clipping!! I suspect the high frequency compression drivers that were destroyed were taken out by the transient but powerful clicks and pops that occur in a front-of-house multi-kilowatt system when EMI/EMP events occur from big refrigeration equipment, big trash compactors and so forth switch on or off and are picked up by hundreds if not thousands of feet of audio cable even if it is shielded to some degree. It is even crazier when our power comes from generators at county fairs where the same generator runs rides with 50hp motors!!! Ferroresonant transformer regulators ARE A NO-NO!!! Frequency shift on the generators makes the regulator voltage vary wildly, even more than the generator voltage itself!!!

author: mag_therm
author_rank: Frequent Contributor
timestamp: December 15, 2021, 12:20:20 am
content: Motional Impedance and back EMF.I don't know how to type math functions in here, but recommend to refresh about motional impedance, equivalent circuit and multi-resonance of loud-speakers.I recall doing that microphone and speaker theory and lab in year one, prior to much about electrical machines and magnetics etc..Also the driving voltage AC of an ordinary ( classical) linear amplifier into the impedance is limited by the rails.

author: IanB
author_rank: Super Contributor
timestamp: December 15, 2021, 12:21:33 am
content: Surely what nobody has clearly mentioned here is the RMS power rating of a loudspeaker is how much RMS power you can safely feed into it (how much it can handle).How much sound comes out of a loudspeaker depends on the sensitivity. I could build a speaker out of a coil of nichrome wire and it could safely handle 50 W RMS. It might not produce much sound, but it would still be rated at 50 W.

author: mc172
author_rank: Frequent Contributor
timestamp: December 15, 2021, 12:54:00 am
content: Quote from: CaptDon on December 15, 2021, 12:14:43 amWhy on Earth would a 50W "RMS" amplifier be expected to be able to deliver double or beyond its rated output power with no rail sag, especially if it's aQuote from: CaptDon on December 14, 2021, 08:33:43 pm?Quote from: CaptDon on December 15, 2021, 12:14:43 amYou've moved the goalposts, it was a 100W speaker originally. That was my point of asking "how?" in the first place. I originally wrote a post about knocking out tweeters due to the spectral content upon the onset of clipping as Benta suggested but instead thought I'd leave it as just "how?" to see what happened.Quote from: CaptDon on December 15, 2021, 12:14:43 amNo, because the fundamental of a square wave is not DC, it's a sine wave at the same frequency as the square, which is probably between 40 and 100 Hz for the bassline and/or kick of commercial music, at which point the impedance is easily double its "nominal" 8R. As you even said yourself:Quote from: CaptDon on December 15, 2021, 12:14:43 am

author: mc172
author_rank: Frequent Contributor
timestamp: December 15, 2021, 01:04:08 am
content: Quote from: IanB on December 15, 2021, 12:21:33 amA twist on a classic - how strong is a piece of string? I buy PD and JBL string so mine is pretty strong...

author: bdunham7
author_rank: Super Contributor
timestamp: December 15, 2021, 01:29:21 am
content: Quote from: najrao on December 14, 2021, 12:07:33 pmRating an amplifier and a speaker are done differently, although you might expect them to match somehow. An amplifier is generally rated by its ability to produce RMS power into a specific resistance or resistances, so all in-phase and constant impedance. An amplifier that can maintain output voltage into decreasing resistance is generally deemed to be better. Amplifiers generally are supposed to behave as voltage sources and have very, very low output impedance, good ones will often be less than 100 milliohms. Amplifiers are also tested quite rigorously to actually meet their specifications.Speakers have complex and varying impedances, so determining the actual RMS power input would require measuring voltage X current simultaneously under all sorts of varying input frequencies and combinations. The ratings are a good bit more subjective and AFAIK, nobody takes a speaker and sets it up to dissipate exactly 50W (or whatever it is specified for) for an hour to see if it burns up--that really isn't the point. Typically what they actually mean is you can use a 50W amp up to clipping with a 'normal' music program without setting it on fire. Of course it matters a lot what the spectral content of the program is, etc. So speaker power ratings are a good deal less well defined. Loosey-goosey if you like.As for how they dissipate the power, in a woofer or lower midrange if there is a lot of power, the voice coils are moving. This will circulate air internally to the magnet structure, which is big and can absorb, transmit and dissipate quite a bit of heat. If you run the speakers hard, the magnets should warm up a bit. Tweeters are another issue because they don't move as much and have much less thermal mass, so they will burn out much quicker if you overload them. They are generally rated for more power than would typically be allotted to them in normal music material just as a safety margin. Some of them have a ferrofluid that bridges the gap between the VC and the magnet structure.

author: CaptDon
author_rank: Super Contributor
timestamp: December 15, 2021, 03:38:25 am
content: Years ago I had some ferrofluid cooled midranges around 6" if I recall correctly. Used them in 3 way cabinets for D.J. work and a small band in small venues. I noticed over time they seemed less and less efficient but thought maybe I was imagining it. Put the cabinets away for a couple years. When I got them back out NO MIDRANGE!!!! Ripped the speakers apart destructively. Found the ferrofluid had basically turned to tar!!!! No more ferrofluid mids or highs for me!!!