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Frequency, Resistance, and Power

Discussion in 'Beginners, General Questions' started by Belgarion115, Jan 21, 2012.

  1. Belgarion115

    Belgarion115 Member

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    I bought some 6 ohm speakers a little while ago and I've been shopping for receivers for a 5.1 surround system. I keep seeing this kind of thing: Power Output Per Channel (20Hz-20kHz,.08%THD@8ohm): 80W Power Output Per Channel (1kHz@8ohm): 110W Power Output Per Channel (1kHz@6ohm): 125W I'm wondering why the frequency is different for 8 ohms vs 6 ohms and how is that going to affect the audio quality? I didn't see any sticky threads for beginners. :huh:
     
  2. gene c

    gene c Well-Known Member

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    There are different ways to measure a receivers power output. Just like there are different ways to measure a car engines horsepower. At the flywheel without the accessories connected (maybe 300 hp), with acc connected (275 hp),at the back of the transmission (maybe 235 hp), at the rear wheels (maybe 210 hp). Same with receivers. It's easier to drive one speaker of smaller impendence, at a higher point of distortion and at a single frequency. So, two channels at 8 ohms, from 20-20,000 hz and .08 thd = 80 watts per channel. One channel driven at 8 ohms, 1khz, with 1% thd = 110 wpc. At 6 ohms, 125 wpc. 4 ohms, maybe 140 wpc. Same amp, different ways to rate it. Some htib's are rated as "1200 watts of total power!" but it's 200 wpc, one channel driven, at 3 ohms, 1khz, and 10% thd X the 5 speakers + the subwoofer. In reality, it's more like 15 wpc with all channels driven at a volume level that doesn't provide so much distortion that it's un-listenable. Non of this will really affect sound quality, just volume.
     
  3. Belgarion115

    Belgarion115 Member

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    Thanks for the explanation. The power/resistance thing I understood, but I was really after the importance of frequency response and why the range is so vast at 8 ohms, but somehow restricted to 1khz on the 6 ohm speakers, and what that really *is*. Sounds like sound wave frequency to me, but that can't be, otherwise the speaker would only produce one frequency which would be stupid. I have to be missing something important.
     
  4. gene c

    gene c Well-Known Member

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    "but I was really after the importance of frequency response and why the range is so vast at 8 ohms, but somehow restricted to 1khz on the 6 ohm speakers" The frequency response isn't restricted to 1khz on a 6 ohm speaker. They chose to measure the power output on a 6 ohm speaker at a single frequency, which is easier to drive and therefore gives a higher output, rather than measure it at a full range response which is harder to drive and would therefore give a lower output. They're just fudging the 6 ohm numbers to make things look better. They could have measured the 8 ohm speaker at a single frequency response as well. And they could have measured the 6 ohm speaker at full range. It's much more difficult for an amplifier to drive a low bass frequency, like 50 hz, than it is to drive a higher response, like 1khz. This is why a powered subwoofer is much better than a passive one. The receiver doesn't have to drive the low frequencies. The subs internal amplifier will. Remember, they're measuring power output there, not frequency response.
     
  5. Belgarion115

    Belgarion115 Member

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    Ahhhh I understand now. Thanks for the explanation. So basically, for 6 ohm speakers (or any resistance rating not advertised across the frequency spectrum), get a receiver with a higher maximum wattage per channel (compared to the speakers' advertised maximum rating) to ensure the receiver is able to produce as much current as the speaker will request. Have I got that right?
     
  6. gene c

    gene c Well-Known Member

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    Sort of. But there are other things to concider. When buying a receiver to match the speakers you already have (or the other way around) the first thing you want to do is compare the ohm ratings. Virtually any modern receiver will power a 6 or 8 ohm speaker. It's 4 ohm (or less) that cause problems. Running 4 ohm speakers on receivers that aren't certified for 4 ohm might cause the receiver to go into "self-protect mode" and shut down. However, many people, myself included, have used 4 ohm speakers without any problems. The more expensive the receiver, which hopefully has a stronger amp section, the more likely you can use 4 ohm speakers with it. Some receivers like the Onkyo 709 and higher are certified for 4 ohms and even have a switch on the back panel or a 4 ohm setting in the setup menu. But all that does is limit it's power output to prevent damage to the receiver. Also look at a receiver power consumption. This isn't a tell-all spec but it is something else to look at. That 1200 watt htib might only consume 175 watts. How is that possible??? Getting a receiver certified for use with 4 ohm speakers by the UL lab is apparently VERY difficult and expensive so most brands simply don't bother to try and get their receivers certified for 4 ohms, even though they may actually power them quite well. The other thing to look at is the speakers senitivity or PSL (Sound Pressure Level). This tells you how loud the speaker will play compared to some others. An average SPL is 88-90db. I've seen some as low as 80-82db and some as high as 104db. They say a 3db increase is effectively twice as loud but that seems a bit of a stretch to me. But you can't question science. It also yakes twice as much power from an amplifier to get a 3db increase. So SPL is kind of important. The sze of the room and how loud you want your system to get is also important. But the actual power output of the receiver and the listed power input of the speaker are the least important specs to look at. What you want is enough good, clean power to give you the volume you require. Distortion is what usually damages a speaker, not over-powering it. A 88 db speaker and any good quality 75wpc receiver will play ear-bleeding levels, in most cases. Really want to play it loud? Get a seperate outboard amplifier.
     
  7. Belgarion115

    Belgarion115 Member

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    I just looked up the sensitivity of the speakers I bought. Sadly they are 82dB. On the upside the area I'm filling with sound is pretty small, only about 20x20, and no one is trying to listen to music or movies deafeningly loud. Thanks again for the information, it's helped quite a bit and I know far more about audio equipment than I did before.
     

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