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Understanding driver power rating specs
Understanding driver power rating specs
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Old 18th June 2013, 03:45 PM   #11
Charles Darwin is offline Charles Darwin  United Kingdom
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The 100hrs noise test is probably the best we an hope for but for tweeters it still helps to read the small print.

Pretty much all of them use a crossover or band limited noise, the difference is in the low cut off point. These should be stated but can vary dramatically!
The lowest stated off the top of my head is Vifa with 700Hz (at least on some models) butI've seen as high as 4000Hz which in many cases makes it an almost useless number.
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Old 18th June 2013, 03:58 PM   #12
chris661 is offline chris661  United Kingdom
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Quote:
Originally Posted by Kev06 View Post
Sooo... looking at it another way: Taking a worst case of fairly uniformly/continuously loud music sufficient to cause a high percentage of excursions around Xmax (but not significantly beyond), with say a lowish sensitivity driver in a small sealed enclosure... 'in practice' would power be something I ever need to worry about or is it just not an issue?

Cheers
Kev
Depends on the music.

To take my (sub-optimal) subwoofer as an example...

I have a pair of JBL GTO1214 drivers in one of those 35L pre-fab sealed cabinets. They're sharing that volume, so estimate ~16L each (once the driver displacement is accounted for). This is a very small sealed cabinet, so even with a fairly chunky amplifier (half a kilowatt), cone excursion stays reasonably benign.

I once played a 5Hz sine wave through this system. It felt weird in my small room. I couldn't actually hear much, but the cones were definitely moving. The amplifier wasn't clipping, and yet after around 30 seconds, I started to smell the warm driver smell. I immediately stopped the signal generator, took one of the drivers out and let them run essentially free-air to get them to cool a bit.

On the flip side, even the most demanding movie effects I've tried so far haven't managed to get the warm driver smell.
Things like the hospital explosion in the Dark Knight; the massive shootout in the Matrix when they enter the compound to save Morpheus; the helicopter explosion shortly after that.
The amplifier can clip and yet the drivers don't start to smell: the clipping is brief so that, while the delivered power can be rather silly, its in very short bursts.

Its the prolonged application of power that kills drivers, and, seeing as you're unlikely to run any compressors in your system, nor are you likely to run your amplifier into square waves; I'd say that as long as you make sure nothing sounds particularly unhappy, you're likely to be fine: even the 6dB dynamic range found in some modern recordings won't stick enough prolonged power to kill speakers. Within reason.
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Old 18th June 2013, 05:40 PM   #13
Kev06 is offline Kev06  England
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Thanks Chris, maybe I'm worrying too much then. I wasn't intending to electronically extend the response or play anything like a continuous wave through them, probably the worst they'll get is some rock at higher SPLs if I'm in the room next door..

Quote:
Originally Posted by Charles Darwin View Post
The 100hrs noise test is probably the best we an hope for but for tweeters it still helps to read the small print.

Pretty much all of them use a crossover or band limited noise, the difference is in the low cut off point. These should be stated but can vary dramatically!
The lowest stated off the top of my head is Vifa with 700Hz (at least on some models) butI've seen as high as 4000Hz which in many cases makes it an almost useless number.
Very good point; I've been looking at the x-over point in relation to frequency response, but haven't so far checked how that compares to the power band. If this can begin at up to 4khz then I definitely need to give it some attention!

Cheers
Kev
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Old 18th June 2013, 09:03 PM   #14
Charles Darwin is offline Charles Darwin  United Kingdom
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More specifically the old Vifa D27TG-35-06 is measured from 700Hz, the XT25BG60-04 from 500Hz and the XT25TG30-04 from 400Hz!
The Peerless/Scan-Speak D2608 from 4kHz. Which I find odd because it seems to be generally regarded as quite robust even when crossed low. I haven't used it myself though so far.
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Old 21st June 2013, 10:20 AM   #15
Kev06 is offline Kev06  England
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Interesting thanks, I'll have a look at those.

Cheers
kev
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Old 21st June 2013, 11:31 PM   #16
BlueWizard is offline BlueWizard  United States
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I'm surprised that no one else has pointed it out, but technically RMS can't be applied to power, it is a reference to voltage. But, the term "RMS" has come to be associated with Continuous Power, and so is accept as a reference term for this type of power.

Let me use amps to explain the difference between 'RMS' and Music Power. RMS power rating are based on a sustained tone. It is not pulse or peak or transient. It can still be measured relatively quickly, but the fact is that a continuous sustained tone especially at low frequencies can drain the power supply and result in low Power ratings.

Onkyo is a good example, in the USA, their power rating are very straight forward, but in the EU and UK they are a bit sketchy. I suspect they are either using a different power supply or the power supply is not able to deliver the same SUSTAINED power with 50hz.

But music power, that is, the amp functioning under normal dynamic music conditions, performs very nicely because they don't have the drain of the sustained tones dragging on the power supply reserves.

For the record, I have two Onkyo stereo amps that I'm very happy with.

If we look at Pink Noise or other similar noises in general, you tend to get all frequencies at all levels playing randomly. It sounds like the static hiss between FM radio stations. In a sense, it is a form of random noise. However NAMED noise like Pink Noise, Brown Noise, White Noise have consistent and predictable characteristics to them. Because of that they can be used to simulate music in a consistent and predictable way.

While Pink Noise may sound like a constant HISS to us, it is actually very dynamic. Because it is dynamic, it gives the power supply time to rest and re-charge, which means it is able to recover more quickly.

Now let's apply this to speakers. First, and I'm sure people are tired of hearing this but -

It is never overpowered or underpowered amps that destroy speakers; it is always the guy running the volume control.

The best advice I can give is - don't be that guy.

RMS power ratings on speakers involve sustained tone sine waves. Music power or other power ratings might use Pink Noise or by some other means try to simulate actual music conditions.

Sustained tones cause heat to build up much faster in the voice coil and as such will lead to pretty early failure of the driver.

A simulated Music tone like Pink Noise, again is dynamic, always changing. This gives the voice coil time to rest and time for heat to dissipate resulting in a much longer time to failure.

However, as has been implied, an underpowered amp can still damage the speakers. It is about sustained volume (or voltage) over time.

The next aspect is Excursion. One very large speaker can sound significant and can reach high SPL very easily. To get the same SPL (sound pressure level; loudness) from a smaller speakers, the only solution is to push that smaller speaker harder.

So, besides electrical meltdown, the other way to damage speakers is to over drive them. This is based on pure VOLUME or Loudness. To push them beyond their mechanical limits. Typically what happens is you crank the bass to the max, then crank the volume near the max, and you push the speaker so hard, you push it forward out of the magnet.

Under these conditions, the cone is so physically distorted that the voice coil can't and doesn't neatly fall back in to the Gap. More likely it slams into the frame of the magnet and is physically damaged. The alternate is that rather than moving forward, the coil and cone move so far back that the smash into the structure of the frame and magnet and again are physically damaged.

Many people use 100w speakers on 150w and 200w amps with no problems. How and Why? Well they simply understand the limits of their equipment and stay within those limits. Good sound is not necessarily loud sound. Even at a house party, it is possible for the sound to get too loud. You can usually tell when it is too loud because people start moving out of the room.

And many people use 100w speakers on 50w amps an go through them like confetti on New Years Eve.

Continuous or 'RMS' imply a sustained tone over a sustained period of time, but that time period is not indefinite. Even at very low levels is a tone is sustained without relief it can damage a driver.

Music and other such higher rated powers are an attempt to imply how the speaker will function under real work conditions. Though they can't allow for every circumstance, such as the circumstance in which the local village idiot has a beer in one hand and the volume control in the other.

Peak Power is likely the near instantaneous failure point.

Time is a factor, a low level sustained for a long time can build up enough heat to cause damage.

Heat is a factor, how quickly it builds up and how quickly it is allow to or is able to dissipate determines how long your speaker will survive.

Mechanics are the 3rd factor. If you stretch a rubber band too far or blow up a balloon too much, they break. If you mechanically push a speaker too far, it breaks.

For what it is worth.

Steve/bluewizard
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Old 21st June 2013, 11:38 PM   #17
BlueWizard is offline BlueWizard  United States
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One additional point, for purposes of another post in this forum, I tested the output of my system with the volume control at 60% which is about 2 o'clock on the volume dial. 1 o'clock is about 100db, so I'm guessing 2 o'clock is about 105dB. That is LOUD, especially in my small room.

What do you suppose the average long term highly smoothed highly averaged power was at that volume level?

It was in the range of 2w to 3watts. That's all. Though again that we very averaged long term sustained power.

Just though you might like to know.

Steve/bluewizard
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Old 22nd June 2013, 10:09 AM   #18
Kev06 is offline Kev06  England
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Thanks for the extensive post! You seem to be supporting some things I've been discovering whilst researching this, which is reassuring.

Now that I understand what the noise in the 100hr noise test is, I can see why thats a better measure than an unrepresentative RMS value. I like how the crest factor is a part of the 'noise' used in the test too, which probably relates to your point about the difference between peak power/volume and the much lower average power.

I also now understand how the use of compression in recordings and/or clipping at playback both start to reduce the crest factor, effectively closing the gap between average power and peak power, increasing the chances of damaging drivers through excess power even with a modestly powered amp.

So I'm now going to just use the 100hr noise test figures as a very rough check on how stressed the drivers are likely to get playing actual music at my intended SPLs. I'm also going to start making much more distinction between this and the peak power of the amps, which will be significantly higher to avoid clipping.

Thanks
Kev
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Old 22nd March 2018, 12:32 AM   #19
suzyj is online now suzyj  Australia
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I'm trying to get a handle on some of this with my own speakers. The driver I'm working with is a SB Acoustics SB12PFC25-4, which is specced at "30W". It's well and truly run in - I've been listening to lots of music on this speaker for months. It's a nice little inexpensive 4" mid-bass, that I use in a small TL enclosure with a bit of EQ to do ~40Hz, albeit at reasonably low level (noiseUnit - transmission line computer speakers.).

I've just built a new amp to drive this speaker, and at the moment it's capable of putting around 12W into a 4Ω load before clipping. I ran the speaker with REW this morning to test the whole system (active crossover, power amps, speaker) and drove it pretty hard, to the point where it was obviously distorting below ~100Hz. Wondering if it was the speaker or 12W amplifier that was the limiting factor, I put a CRO across the speaker and had a look.

I was surprised at just how low the level needed to be before the voice coil was literally hitting it's stops - with just 5.4V RMS (7W) at 60 Hz. I guess the impedance will vary at these low frequencies so it's not necessarily 7W, but still.

It's making me re-evaluate the power needs for them, at any rate. If I really did put 30W into this poor thing it'd just melt.
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Old 22nd March 2018, 10:01 AM   #20
Kev06 is offline Kev06  England
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It has been several years since I worked on this and so I'm I bit rusty, but yes the peak travel of the coil can easily limit things in some cases; initially by becoming non-linear and then by reaching mechanical limits.

There is a huge difference between instantaneous power (of the peak) and more sustained/average power. The former doesn't cause much heat but may hit excursion problems, the latter may be 'much' lower in magnitude and not cause much excursion at all but its (considerably) more relentless nature can cause over-heating.

By way of example, both were factors in my build. Using/misusing a full-range driver just as a mid driver meant excursion capabilities were far more than would be needed, so sustained power handling was the limitation to consider there (especially as a lot of power is present in my chosen mid-range part of the spectrum). The bass drivers could handle lots more power and so wouldn't be troubled by that when matched to the mids, but the low frequency peak excursion could be reached relatively easily (for high power use, I elected to use a rumble filter to constrain low-end excursion).

Last edited by Kev06; 22nd March 2018 at 10:09 AM.
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