What is db sensitivity how and why is it measured with speaker volume from WPC?

[Britchie]

I understand db is decible a unit of sound volume. By why do people use db sensitivity to measure the WPC a speaker can handle before ....POW

Example why would someone run 500wpc amp on speaker that are only rated 50-200 w capacity.

BR

I realize this may be in the wrong thread but the Rotel guys are the smartest on this forum and I am a fellow rotel owner

[Paul H]

I was going to answer this question, but don't unfortunately have any Rotel gear

Paul

[SpOoNmAn]

I might own a Rotel but am by no means "one of the smartest"

A speakers wattage handling capability, the one written on paper, is pretty much meaningless nowadays. Assuming its of decent quality that is.

It is safer to run a speakers that are rated to handle 200 watts max, with a power amp that delivers 300 per channel @ 8 ohms, then it is to run that same speaker with an amp thats rated to deliver say, 70 watts per channel.

I'll tell you something I was told, and leave it a that.

A friend at another forum was invited to his dealers store for a workshop of sorts, Reps from certain companies were going to be there.

They had DefTech 7001's hooked up to a pair of Mcintosh monoblocks, each rated to put out 1,200 watts I believe.

The test began and the Mcintoshs, according to their dials were close to outputting full power, into a speaker that was rated to handle 600 watts. 600 watts handling yet taking almost double?

Ive talked to many people about this and a lot of them say power handling ratings are rubbish. Tons of good clean power is your best bet for speaker longevity.

I know I didnt answer your question appropriately, but Im sure some of the "smart" ones will come in and do that :T

[Andrew Pratt]

By why do people use db sensitivity to measure the WPC a speaker can handle before ....POW

I'm not sure what you mean by measuring WPC in dB...but really from what I understand a speaker's ability to handle power is at least in part determined by how well it can cool itself. As was pointed out above most speakers will do far better with too much power then too little as people tend to push lower powered amps past their comfort zones at which time the amps will no longer be able to generate smooth sine waves. When this happens the tops of the sine wave get cut off (flattened) and this means that for a short period of time the voice coil in the speaker isn't moving...but its still getting power which builds up heat and it melts the coil (worst case). If you'd used a mega power amp it would just loaf along and likely never approach clipping....at least until your ears were bleeding

Where sensitivity comes into play is that all speakers are rated at how loud they'll play at 1 meter with 1 watt or power. This is typically somewhere between 85 and 95 dB/1watt.

[purplepeople]

Here's a simplified way of looking at it...

Start with loudness standards... and don't worry about what the numbers mean... just think relative. Your average jet plane is around 140 dB loud just at the moment you're getting sucked into the engine. At the moment of ingestion, you've probably lost your hearing and your life. Normal speech is around 60-65 dB. Blenders are around 90 dB and chainsaw is around 120 dB. So as you explain how to mix margaritas for refreshment while taking a break cutting firewood, you can imagine how loud things are getting, especially when you look up and notice the Lockheed coming in for an emergency landing on your driveway.

Anyway...if your speakers are 87dB efficient at 1 watt at 1 meter. Then your amp needs to give them 1 watt RMS for you hear 87dB loudness at about 3 feet.

Here's the calculation part. Every 3dB increase in sound requires a doubling of the wattage. So 90dB needs 2W, 93dB -> 4W, 96->8, 99->16, 102->32, 105->64, 108->128, 111-256, etc. Notice how quickly the required power goes up. Think about the power in your voice vs. blender vs chainsaw vs jet engine.

So many of the speakers that are 100W RMS will only give around 107dB of loudness before they begin to break down (in one way or another). There is a difference between RMS and peak power.... look it up and discover why I find myself laughing openly at many electronics salespeople.

One more thing.... AP is correct about clipping, as it is often worse than "over-powering" speakers. But that doesn't mean you can take your Bryston 7Bs and deliver maximum sine waves to your little Paradigms all day long. That's just asking the coils to move the cones so far that they either break down or wreck the surround material in the process.

ensen.

[purplepeople]

I should add that every time you double the distance from the speaker, you need 4 times the power and equivalent of 6dB loss in loudness. So if you sit 6 feet away instead of 1 meter, you will need 4 watts to hear your 87dB from the speaker.

ensen.

[Britchie]

Great Answer! You really got me thinking now. I appreciate your detail and annecdotes.

Brad