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| Thread ID: 30846 | 2003-03-04 18:18:00 | OT - measuring the power coming out of a car stereo... | tango (2697) | Press F1 |
| Post ID | Timestamp | Content | User | ||
| 125573 | 2003-03-04 18:18:00 | Ok. If you have speakers rated at 230w, and you're running them out of an amplifier, you have to be careful not to throw an overly powerful signal at them. Well I need to know a way of measuring how much power is getting to them. I know you can probably use a multimeter, and then do the appropriate maths (which I have since forgotten:P) to find out how much power in watts is getting to the speakers. Is there another way of doing it? and if not does anyone know the right way of working out power, using the voltmeter/ammeter/pen and paper method? (also bear in mind there is a pretty big current involved, 8 gauge wire is used for most of the connections:)) |
tango (2697) | ||
| 125574 | 2003-03-04 19:41:00 | Sheesh tango! You call 230W a lot of power? Just drive your car to CHCH and we'll compare it to my concert PA system! (homepages.paradise.net.nz) OK first thing measuring the power with a multimeter has to be a huge waiste of time, as this is audio and while a multi meter would measure say 12vDC accuratly there is no way it will measure the constant change of audio in practice with any accuracy. This is one reason why mechanical VU meters fell out of favour. Secondly 300w rms rated speakers will gladly take 600w in small bursts provided they are in efficiant cabanets that allow them the get the air moving without over extending the cones. Thirdly the big killer of speakers, particualy high frequency stuff is not over powering but underpowering. This leads to distortion, distortion of this type tends to turn the nice easy flowing sine waves that natural sound is largly made of into nasty squarewaves that want to jerk the cone a bit too harshly. Often amplifiers will create nasty harmonics under overload conditions. Assuming you are using a passive crossover, this distortion will find it's way directly to your tweeters and tear them appart, or melt them. Fourthly, there are alot of ways of measuring the power rating of speakers. Computer speakers and cheap home stereos are measure the power rating using systems that are designed to multiply what they can really do with the last 2 or 3 digits of the manufacturers phone number. It sells speakers, it means nothing. RMS and "peak" are the only ratings that mean anything. RMS is effectivly the avourage amount of power. In the case of a speaker, so long as you dont let the cones overextend, or the coils melt, you can pump short excessive bursts just fine. In the case of an amplifier, you can't get even short bursts that are well over its RMS rating, thus causing distortion. (see above). OK, theres more to it than that, but this is a working explanation. And the direct answer to your question? Get one of those little LED VU meter kits for when you cant use your ears to hear the speaker distorting. If you are going to use your ears open the car doors and listen from outside the car. If it sounds unclear, you are overloading it. Simple as that. .Clueless |
Clueless (181) | ||
| 125575 | 2003-03-04 19:48:00 | You cant realistically do it with a multimeter. Music is a highly complex waveform, where a multimeter accuracy is generally aimed at sine wave. A pure sinewave input to the amp would help I guess. Then its Watts=Volts x Amps, or Watts = (Amps x Amps) x Impedance of speaker. Trouble is the speaker impedance (resistance) will vary with the frequency of the sound. You need an Audio Analyser to do it properly, as many factors come into it. If the speakers are 2 way or 3 way with crossover filters, then the power going to each portion (woofer, mid-range and tweeter) will be totally dependent on the music being played at any time. Mr car system is rated at 45 watts per channel, 4 channels. My speakers are each rated at 200 watts, and are 2 way, so no measuring is required as the amplifier would run out of puff first (and my hearing would be damaged as well. considering that some speakers are mounted inside the headrests!) |
godfather (25) | ||
| 125576 | 2003-03-04 20:04:00 | ... footnote Why do people get so obbsessed with power ratings? The big one to look for is efficiancy. If the speaker is efficiant, then it will deliver with a LOT less put into it. I personally only use speakers are such that they can deliver nearly 100Db from 1 watt measured at 1 metre from the speaker an open inviroment. 100watts into a speaker of that efficaincy will cause pain and hearing damage in a car. The best car sound i have ever heard was a 25w amp driving a 15" JBL and horn in the back of my wagon. It sounded awful till we lost the windscreen in the Lewis Pass. The rain was like ice picks, but the sound became delicous! .Clueless |
Clueless (181) | ||
| 125577 | 2003-03-04 21:34:00 | Well Clueless, no offence intended, but your thirdly is the biggest load of old rope I have ever read . And not like you at all I might add . ?:| Underpowering does not harm speakers at all . Following your logic, listening quietly will destroy them completely . In the context of audio, sinewaves are ultimately converted into squarewaves by overdriving the amplifier well past the point of simple peak-clipping and deep into gross overload . You can also create a rough approximation of a squarewave effect by grossly overdriving a speaker with a pure sinewave simply by pushing the cone travel beyond the limits of its suspension mechanism, but cone break-up will make the sound pretty awful long before that stage and the smoke from the voice coil will soon tell you to stop if your ears or wallet don't . We shouldn't overlook the fact that music does not consist of just sine waves either . It would be very bland and boring if it was, and very little content is true sinewave at all . It is the complex harmonics created by musical instruments that give music its character and warmth . I think what you were really trying to say was that using under-rated speakers and overdriving them would create distortion . I'm not so sure about destroying tweeters either, they normally have adequate power rating for most conditions and you would be unlikely to over-drive them long enough to do damage because the woofers would have smoked themselves long ago . Overload protection is also cheap so there is no reason to destroy any speaker by intentional or accidental overload . Leaving aside any questions about why an audiophile would want to listen to that kind of rubbish when all they had to do was lower the volume to stop overdriving of course . :| Now, getting back to Tango's question, it is difficult to measure music power output accurately because the dynamic impedance of the voice coil changes with frequency, but if I wanted to get a rough idea of peak power input to a speaker system, I would put a digital ac voltmeter with max & min recording capability across the speaker I wanted to check, play my favourite music at my preferred volume (without distortion effects) and note the maximum recorded voltmeter reading . Squaring the meter reading and divided the result by the nominal speaker impedance value in Ohms will then give an approximation of the power input to the speakers . Note: Unless you use a quality digital meter with selectable peak and RMS capability, the max hold reading will be something closer to average than peak, and in this context max-hold simply means "maximum recorded value" . However, if using a true RMS meter, the max-hold reading will be a reasonable approximation of the maximum real power applied . If able to choose true peak-hold then that will show the peak watts applied rather than RMS . Of course 230 watts per channel (continuous) will require around 50 amps from your battery (allowing for the ineffficiencies of the amplifier) so there may be some source impedance limitations unless you go all the way with cabling and connectors, and a second battery right beside the power amps . Cheers Billy 8-{) |
Billy T (70) | ||
| 125578 | 2003-03-04 22:16:00 | Billy T . . I should never post here before noon, it always comes out wrong >Following your logic, listening quietly will destroy them completely . Well um no . What i meant to say is that SEVERE demands on a smaller amplifier can do more damage than over powering speakers . Of course caning a larger amplifier will produce more damage, however caning a 200W amplifier is more likely to damage tweeters than running a 400W amplifier with little or no distortion . Many systems are built on the concept that less than 10% of the systems power will end up at the tweeters . While there is debatable truth in this under "normal" clean conditions, there is not under overload . If the amplifier powering a system using a passive crossover cannot deliver the bass because it is getting caned, it may well still be able to deliver the tops, with distortion only too well . This tends to give a bit more than the 10% of clean power that the systems tweeters were built for . Thus ends the life of many tweeters . Underpowering had caused hundreds of dollars of damamge to the diaphrams behind the horns of my own tops over the years . They very rarely fail now, as i upgraded the amplifier that is used for the above 125 Hz section of my system . It also sound ALOT better under "normal" conditions . And the best protection is always the ears of someone who cares . People who just want volume but can't hear distortion should not be allowed near good equipment . This is why i do not hire to DJs! . Clueless |
Clueless (181) | ||
| 125579 | 2003-03-04 22:20:00 | > You call 230W a lot of power? Just drive your car to > CHCH and we'll compare it to my > concert PA system! (homepages.paradise.net.nz > 3.jpg) Are you the man with the acoustic?!? :D ;-) |
Chilling_Silence (9) | ||
| 125580 | 2003-03-04 22:33:00 | >Are you the man with the acoustic?!? No, at the time that picture was taken, i was the grump behind the unseen mixing desk. .Clueless |
Clueless (181) | ||
| 125581 | 2003-03-05 00:07:00 | You could try a calorimeter. Three 2300W electric jugs paralleled works out to 7.66 ohms (and would handle considerably more than the rated 6.9KW). And after calibration (to account for heat absobed by the metal bits and radiated) it would be quite accurate. Without hightech electronics. :D | Graham L (2) | ||
| 125582 | 2003-03-05 01:31:00 | Graham... I think i'm having a somewhat dim day today, but allthough i can see that 3 Jug elements would make an excellant dummy load that is as near to 8 ohms as makes no odds, I fail to see where one would get accurate info on and amplifiers power output using a dummy load alone! Also why use a dummy load when one wishes to measure the power levels going into the speakers on site? ?:| .Clueless |
Clueless (181) | ||
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