Do I have this right? A question on watts [Archive] - Audio & Video Forums

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Mark_IDT
05-06-2005, 04:55 PM
Ok I think I have this right so let me know if I go wrong. Lets say I have a pair of 250w speakers and I'm running a 250w amp to them. As I turn up the volume the watts double for every 3 db until I reach 250w which would be the point where the amp clips and another 3db of volume would push it to 500w or more blowing everything up. So then lets say I run a 500w amp to the same speakers once I reach the volume where it's putting out 250w I'm close to or at the point where the speaker clips. Another 3db of volume from that amp would probably blow up the speakers but that last 3db of clean power is reffered to as headspace. Therefore it's better and dare I say safer to have a bigger amp if your going to play your speakers at a volume close to there limits because a small amp quickly doubles watts when it starts to clip.

Geoffcin
05-06-2005, 05:17 PM
Ok I think I have this right so let me know if I go wrong. Lets say I have a pair of 250w speakers and I'm running a 250w amp to them. As I turn up the volume the watts double for every 3 db until I reach 250w which would be the point where the amp clips and another 3db of volume would push it to 500w or more blowing everything up. So then lets say I run a 500w amp to the same speakers once I reach the volume where it's putting out 250w I'm close to or at the point where the speaker clips. Another 3db of volume from that amp would probably blow up the speakers but that last 3db of clean power is reffered to as headspace. Therefore it's better and dare I say safer to have a bigger amp if your going to play your speakers at a volume close to there limits because a small amp quickly doubles watts when it starts to clip.

IF you have a speaker rated for 100 watts, and you hook a 200 watt amp to them they will be just fine unless you loose your mind and intentionally drive them past the limit. The 3db thing that gets bandied around is a bit of a misnomer. A 100 watt amp can make a sinewave that is 3db louder than a 50 watt amp. Music for the most part is NOT sinewaves, but complex waveforms wth lots of transient peaks in it. It's these peaks that can get clipped first, and sometimes even at a moderate overall SPL. A more powerful amp can give you more headroom, and these transient peaks will remain unclipped.

Mark_IDT
05-09-2005, 10:09 PM
A more powerful amp can give you more headroom, and these transient peaks will remain unclipped.


Ok so do these transient peaks stay unclipped only if they remain within the rms watt rating or within the peak power? Is a sinewave just a constant frequency?

kexodusc
05-10-2005, 04:26 AM
Without knowing what your amp is it's hard to answer your question on the clipping of the transient peaks, but these peaks won't be clipped if the amplifier still has headroom (peak power)

Geoffcin was right.

The 3 dB thing does assume sustained signals, but these are can often be more demanding than musical passages which are instantaneous draws of power of varying amounts on an amplifier. Except for probably bass heavy orchestras in classical music which are pretty intense.

The added headroom the transient peaks enjoy in a larger amplifier is directly proportional to the amount of headroom the sine waves enjoy though. There's a function that determines the output based on the input signal, this remains constant in an amplifier. We could state that the transient peaks also only get 3 dB's more of volume as the power doubles, but that's not exactly true. In some cases it will be more, with frequencies below 500 Hz or so it's probably less than 3 dB's.

Most amps have no problem exceeding their (for example) 200 watt ratings for instantaneous transient peaks that aren't as demanding as sustained signals. Some manufacturers will give you a peak power rating which is not really useful, but can give you an idea of how much room beyond the rating there is.
Say you had a 100 watt amp. It likely won't have a problem with 200 watt transient peaks and sometimes more (depending on the manufacturer). Most amps I've seen will have at least 3 dB headroom at max power...3 dB isn't really enough for peaks which can be 9 dB -12 dB louder though, so if you expect to be drawing 100 watts of power to your speakers sustained, you'll want a much beefier amplifier.

Power's hard to gauge as it swings up and down almost instantaneously.

All things equal it's good to have more. But there's a point for many of us where the extra power becomes useless or wasted, and you'd be better off to invest money into another part of your system.
Best to take into consideration your room size, your average listening volumes, your peak listening volumes (as in as loud as you'll ever play it), your speakers maximum input, their senstivity, and whatever flexibility you want in case your speakers change, you move into a bigger room, etc.
Some speakers need more power to play loud than others. People say there's no correlation between speaker efficiency and sound quality, but I've actually found speakers with lower sensitivities to sound pretty darn good. Not sure if it's because cheap speakers are all really efficient or if there is a link there...

My tower speakers in my studio are about 88 dB, not the most efficient. At my listening position in that room (about 10 feet back), 1 watt of power from my amp to each channel will produce 85 dB at the listening position. I've actually used a multi-meter and spl meter to test this. My room is adding about 3 dB of volume since the speakers are only a few feet from the walls. Most of the time, I don't actually listen to music that loud, I'm usually in 10 dB lower or so. I'm not even drawing 1 watt of power.
I can listen to music at about 95 dB or so in there, this is where the cheap sound proofing really starts to fail though and you can hear stuff in the rest of the house. Kind of sucks when the boys are over practicing, but it was the cheapest solution when I built the room and lady of the house is patient with me. Anyway, at 95 dB I'm drawing about 10 watts per channel. My Rotel Integrated is rated conservatively at 100 watts per channel. I have probably 15-18 dB of headroom at 10 watts for any musical peaks...maybe more. This is ear bleeding loud, and I could turn it up even more, but it becomes unenjoyable at that point.
In that 20 X 18 room with those speakers, I don't need anymore power. I could probably get by with some older 40 watt/channel NAD amps I have actually, but I like the Rotel.

N. Abstentia
05-10-2005, 10:21 AM
I don't think you have to worry about getting 250 clean watts into a home theater speaker...unless of course you're trying to make those speakers fill up the Superdome or something.

You normally won't use more than 20 watts to get to a level where you can't stand it for more than 30 seconds. It's nice to have 50 watts or so for headroom/transients.

Now keep in mind I'm talking CLEAN watts here..not the "100 watts per channel!" you see in the receivers at Best Buy. Even on their best day you'll never get 100 watts per channel from one of those.

Mark_IDT
05-10-2005, 08:48 PM
I thought learning about 2strokes buy reading on the net was sometimes confusing but audio is just mind bleeding painful physics. I'm going to go buy a spl at ratshack tomorrow to see how loud I listen to music.

kexodusc
05-11-2005, 04:06 AM
Good idea on the SPL meter...you might be shocked at how loud you actually listen to music at versus what you think you listen to musica at, I was...I've heard so many people say they crank their stereos to 120 dB. Yeah right.
Report back and let us know.

This Guy
05-11-2005, 11:28 AM
Yeah i thought I listened to it really loud, but when I listen and it hits 110 dB PEAKS it just hurts my ears and isn't enjoyable. It's a great tool for setting up your stereo.

kexodusc
05-11-2005, 01:03 PM
Atta boy...:D
Actually, 110 dB peaks is even too loud for me quite often. that puts you into the 90dB's.. Unless I really want to rock the house, for some classical music I'll even go a bit higher. Until I get yelled at (and don't hear it which makes the woman have to come downstairs all ticked-off...). I use ear plugs when the band plays, I like music too much to risk not being able to hear it, repeated exposure to music/movies at these levels for extended periods adds and can cause permanent damage.
Just be careful.

Geoffcin
05-11-2005, 03:54 PM
I have a little anecdote about something I witnessed recently.

My buddy in Tucson has some vintage gear that he got from a defunct NY recording studio. The speakers are a killer set of EV 2-way studio monitors with a dome mid/tweeter, and an 8" woofer. All of the EV pro gear that I've heard is very sensitive, and although there wasn't any rating on these, my guess is that they were at least 90db-watt. He was driving them with a vintage Nikko amp that had a logarithmic power display, and darn if we weren't pinning the meters on the 100WPC amp during some semi-loud passages. The 100 watt light might not have been exactly acccurate, but the tempo of the music registered a rise to the upper ranges more than once, and it wasn't ear splitting loud either.

Also;

From a review of a Home Theater DIY speaker using Adire woofers;

"Power was our QSC PLX3402 on the L&R speakers (~850WPC), and our Plinius amp bridged into the center (~600W). I think I heard the Plinius clip once, and the main speakers bottom just once or twice, but that was at volumes where we had to shout at each other."

Clipping a 300 watt Plinius amp would be hard enough, but bridged?! HT demands are a lot greater than most musical demands, but MOST people are using severely current limited receivers to drive their HT.

kexodusc
05-11-2005, 05:15 PM
Good point, in home theater applications (DVD's, mostly) you can really stress a receiver quickly in a scene with lots of noise.
I have an old stereophile magazine that talks about how just about every Dolby Digital track has at least a few scenes, musical score or otherwise, where 5.1 channels are used at relatively even volumes...this was from 1999. I'm sure today's sound effects are even more stressful on receivers.

Not sure about the SPL meter not being fast enought to detect it though. Perhaps for some digital ones? If the current to the driver and drivers subsequent displacement of air happens so fast it is actually generating more force and should create even more SPL and lingering echo effects.
If it it happened too fast, this could be bad for sound too...wonder if a driver ever broke the sound barrier...talk about timing, phase, and delay issues...

I have an inexpensive AudioSource amp I bought broken and repaired that has reasonably accurate VU meters too...at loud volumes it's easy to hit the maximum rating on peaks, and I've often seen them exceed double the amps ratings as I creep above 105 dB's...I'm usually afraid I'll fry this little workhorse gem and that's when I turn things back down.

Remeber the days when Kenwoods and Pioneers all use to have those things?

Geoffcin
05-11-2005, 05:36 PM
Remeber the days when Kenwoods and Pioneers all use to have those things?

But I get all misty when I see those old VU meters bopping away.

How'd you like to see this one tapping those upper tics!?

http://gallery.audioreview.com/data/audio/504/medium/75191IMG_4323.jpg

Florian
05-12-2005, 02:17 AM
Mmmmh.... sweet. Well we measured that with the VMPS RM30 and the vacuumstate electronics that at aprx. 100db average we were only using 12wpc. !!! Power is not created equal :p

kexodusc
05-12-2005, 03:38 AM
Mmmmh.... sweet. Well we measured that with the VMPS RM30 and the vacuumstate electronics that at aprx. 100db average we were only using 12wpc. !!! Power is not created equal :p
Power isn't created equally, but 1 watt is 1 watt. The problems only start when the marketing departments decide to measure how much power a 5 channel receiver will output at 900 Hz on a cloudy day on the 3rd moon of Saturn. Suddenly you've got 1 watt under one set of conditions, and only 0.3 watts under others.

It's not helpful that companies like NAD, Bryston, etc have long been purposely under-rating their amps too. But this is the lesser of two evils...I'd rather have more than less.

It's pretty easy to see why people get confused - we get some people with Sony receivers believing they're using 500 watts, and some with Arcam amplifiers that think they're only using 20 watts. Both playing at the same SPL.

kexodusc
05-12-2005, 03:44 AM
But I get all misty when I see those old VU meters bopping away.

How'd you like to see this one tapping those upper tics!?

http://gallery.audioreview.com/data/audio/504/medium/75191IMG_4323.jpg

I always get misty when I see those lovely glass faceplates on the McIntosh gear...

One would think in todays digital age it wouldn't be anything to stick those into more modern equipment in a subtle, non-intrusive LED format or something. Then again, it wouldn't be doing a lot of receivers any favors when the meter has a hard time reaching 40 watts...

Mark_IDT
05-12-2005, 11:50 AM
I took some readings today with my new toy. I set it on a tripod 1 meter away from one of my speakers and about half as tall as the speaker. My bass plays around 100 db with peaks up to 106. This is at the highest volume before I start to hear poping in the woofers actually it's about 2clicks below when they would start. Using weighting A 500hz up it comes in around 90 db which makes sence cause I have the woofers turned up 10db. I also measured my friends band playing at 110db with peaks to 116db easy. It was just the drums and bass playing.

Dave Lindhorst
05-26-2005, 01:47 PM
There is a very good reason why a lot of manufacturers as you say "under rate the power of their amps". If you saw the distortion figures at the ratings the amps can really produce I doubt if you would buy thier product. So to keep the THD figures respectable they rate the amps at a lower power level than they can produce. Simple demographics is what it is. Like I care if the amp produces a little too much distortion just when my ears are ready to implode. I don't normally play the stereo that loud at any rate.

bjornb17
05-26-2005, 08:07 PM
My HK AVR130 receiver is 45 WPC, and my speakers are rated at 500 WPC peak (i doubt that they'd actually handle that much). the speakers are 94 db sensitive. i used a SPL calculator online and with my configuration, it said my max SPL was about 130 db (when i set power at 45 watts per channel). i think 130db is highly exaggerated and that the real volume level would be much lower than that.

how loud would you expect a receiver like that with 5 of thsoe speakers connected play?