Good article on SPL

Anything not covered elsewhere.
Post Reply
Message
Author
Bruce Weldy
Posts: 7743
Joined: Fri Nov 27, 2009 11:37 am
Location: New Braunfels, TX

Good article on SPL

#1 Post by Bruce Weldy » Thu Aug 03, 2017 11:15 am

Just came across this......helps to bolster the argument of whether or not manufacturers lie about their output.

http://www.prosoundweb.com/channels/liv ... fications/

6 - T39 4-25" 2-22" 3012LF
4 - OT12 2512 Melded/NSD2005
1 - T24
2 - XF210


"A system with a few knobs set up by someone who knows what they are doing is always better than one with a lot of knobs set up by someone who doesn't."

User avatar
Bill Fitzmaurice
Site Admin
Posts: 27838
Joined: Tue May 02, 2006 5:59 pm

Re: Good article on SPL

#2 Post by Bill Fitzmaurice » Thu Aug 03, 2017 12:06 pm

There's no argument to be made, they all lie. None of them post measured results, only calculated, which conveniently ignores both maximum excursion and power compression.

Think
Posts: 117
Joined: Tue May 09, 2017 5:37 pm
Location: The Neterlands / Holland
Contact:

Re: Good article on SPL

#3 Post by Think » Wed Dec 20, 2017 2:11 pm

This is a great one http://sound.whsites.net/articles/pa.htm

"Let's look at a simple example. Two systems are set up side-by-side. One has an overall sensitivity of 100dB/W/m, meaning that an input of 1W will give an SPL (sound pressure level) of 100dB at 1 metre distance. Power rating is 100W maximum continuous average for this first example.

The second system is rated at 90dB/W/m sensitivity, but has a power rating of 1,000W - also continuous average. The question is ... which will be louder?

In theory, both will be exactly the same - they will be capable of 120dB SPL at 1 metre with full rated power. In reality, the 100W box will be somewhere between 3 and 6dB louder than the 1kW system, because at such a low average power there will be little power compression in the loudspeaker(s)."

AcousticScience
Posts: 144
Joined: Tue Jan 20, 2015 4:37 pm

Re: Good article on SPL

#4 Post by AcousticScience » Fri Jun 05, 2020 11:27 am

I know this is old, but this is yet another myth. The coil on a 100 watt speaker fed 100 watts will run just as hot as the coil on 1000 watt speaker fed 1000 watts, so you can't know if the 1000 watt speaker power compresses any more than the 100 watt speaker. To be sure we need to know if the 1000 watt speaker gets its rating from better cooling the speaker coil, or that the coil can withstand higher temperatures, so we can work out the change in resistivity of the copper and thus the power compression.

User avatar
Bill Fitzmaurice
Site Admin
Posts: 27838
Joined: Tue May 02, 2006 5:59 pm

Re: Good article on SPL

#5 Post by Bill Fitzmaurice » Fri Jun 05, 2020 1:00 pm

Assuming 2% efficiency a 100w speaker fed 100w will dissipate 98w as heat, while a 1000w speaker fed 1000w will dissipate 980w as heat. That makes the latter coil potentially a lot hotter than the former, even if the latter employs better heat sinking and venting. This is just one of many reasons why a herd of lower powered speakers will usually give a better result than one that's higher powered.

AcousticScience
Posts: 144
Joined: Tue Jan 20, 2015 4:37 pm

Re: Good article on SPL

#6 Post by AcousticScience » Thu Dec 02, 2021 12:11 pm

Good points. Some voice coils can withstand a higher temperature to failure (usually the glue and insulation fails first), though if it's just a bigger coil that dissipates more watts at a given temperature, then power compression would be similar.
Now supplying 100 watts to 1000 capable speaker things run nice and cool. This is the holy grail.

User avatar
Bill Fitzmaurice
Site Admin
Posts: 27838
Joined: Tue May 02, 2006 5:59 pm

Re: Good article on SPL

#7 Post by Bill Fitzmaurice » Thu Dec 02, 2021 12:44 pm

Not necessarily. Higher power handling requires a larger coil wire gauge, which increases the moving parts mass, which all else being equal reduces sensitivity. You can never look at just one spec, you have to look at all of them.

User avatar
Seth
Posts: 1911
Joined: Thu Sep 10, 2009 8:06 pm
Location: Sacramento, California, USA
Contact:

Re: Good article on SPL

#8 Post by Seth » Thu Dec 02, 2021 1:24 pm

Interesting points. Great mind food.

Granted, the metallurgy and alloys could be different too... copper melts at 1984ºF/1085ºC. Whether it takes 100 or 1000 watts to get there and what the differences really are is an interesting inquisition. Hmmm.
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

AcousticScience
Posts: 144
Joined: Tue Jan 20, 2015 4:37 pm

Re: Good article on SPL

#9 Post by AcousticScience » Thu Dec 02, 2021 1:53 pm

You usually don't get to the melting point of the copper on a coil (apart from the case of a peak overload from several times too much power). What happens most often is you reach the melting point of the glue which causes the coil to come unwound into a jumbled mess of spaghetti, or another option is (in the case of an Ebay scammer selling a blown speaker) the pyrolysis of the insulation between coils. I know of Wayne Parham who did some experiments on the thermal power handling of Eminence Lab 12 drivers with and without his DIY heat sink design on the central pole piece.
Usually in a horn which is better mated to the air, the mass of the speaker moving parts is less significant to efficiency loss than with a direct radiator. This is why you can have Lab 12 drivers in a lot of these designs be almost as efficient as the kappalites (except obviously the Simplexx). I am not saying it makes no difference.
In guitar cabinets I have heard people's testimonies of losing treble response in higher powered drivers. Partly due to more mass and also due to coil inductance. The lower powered drivers will often do slightly more per watt especially in the top end when driven within their limits. The Eminence Wizard vs the higher powered Tonker come to mind.

User avatar
Seth
Posts: 1911
Joined: Thu Sep 10, 2009 8:06 pm
Location: Sacramento, California, USA
Contact:

Re: Good article on SPL

#10 Post by Seth » Thu Dec 02, 2021 7:57 pm

That's a good point.

I've experienced a few different sorts of failures over the years. I've had all 4 drivers in a sub system fail at full Xlim, pushed all the way out and stuck there... Twice, once each on two different systems years and years apart. I assume the coils melted in those cases. But, I really couldn't say for sure, I never disassembled the drivers to inspect and I don't recall whether I tested DC resistance or not either. Yes, overpowered, indeed.

And I've had a few do what you describe too. Sometimes they just short and put the amp in protect, and sometimes they certainly do unravel a bit. Gives a nice scratchy sound when pushing on the cone.

Hopefully my blown driver days a well behind me. :D
Build in process - 2 WH6, one Alpha 6a loaded, one PRV Audio 6MB250-NDY loaded

Two 2x6 shorty SLA Pro's
One T39, 16", 3012LF loaded
Tall AutoTuba, 20" wide, 2x 8" MCM 55-2421
TruckTuba, 8½" wide, 2x 8" MCM 55-2421

User avatar
Bill Fitzmaurice
Site Admin
Posts: 27838
Joined: Tue May 02, 2006 5:59 pm

Re: Good article on SPL

#11 Post by Bill Fitzmaurice » Thu Dec 02, 2021 9:26 pm

AcousticScience wrote:
Thu Dec 02, 2021 1:53 pm
You usually don't get to the melting point of the copper on a coil.
The most common source of voice coil failure is burnt insulation, which causes coils to short to each other, which lowers the impedance, which pulls more current, which heats the wire even more, which burns more insulation. Then it's a race to which happens first, melted windings or a toasted amp. Sometimes it's a tie. :(

Parham's LAB 12 heat sink was created to address a design fault of the LAB Sub. The rear chamber was too small. In fact it was half the size that it should have been, the result of the cab having been designed by committee. The small chamber air volume couldn't dissipate the heat of the drivers. The LAB Sub fix was to put aluminum covers on the access holes instead of plywood. Parham's fix with his enclosure worked too, but a larger chamber was all that was needed.

AcousticScience
Posts: 144
Joined: Tue Jan 20, 2015 4:37 pm

Re: Good article on SPL

#12 Post by AcousticScience » Wed Dec 15, 2021 8:28 pm

I read Wayne Parham's full article on cooling the Lab drivers. For those who like a good read, https://audioroundtable.com/PiSpeakers/ ... 17646.html

His findings were that the central pole piece (the cylinder of metal conducting the field from one side of the magnet such that there is one pole within the voice coil and one outside) was getting very hot, and preventing the voice coil from cooling down. Whilst rear chamber size gives more air that takes longer to heat (and also increases speaker impedance if the driver motion was previously restricted by an undersize rear chamber), Wayne found even lab drivers in the open air could eventually fail with a 40 volt sine wave on for 50% of the time and off for 50% of the time. With his heat sink design, this increased to a steady 60 volt continuous (there was significant power compression but the heat sink could extract heat fast enough that the speaker never failed). I am unsure of the speaker impedance at the frequencies he tested in order to work out the maximum stable heat watts that can be evicted in each setup.

Personally I would like to test out channels running through the pole pieces which can accept a fluid (distilled water or oil) and be pumped to an external radiator - in much the same way a car engine is cooled. Being able to pump a fluid through the coil itself would be amazing but impractical due to the narrow wire diameter. Tweeter coils can be bathed in ferrofluid, but the excursion of bass drivers means the fluid would likely splash and not sit nicely in the gap.

The original heat sink on the lab horn was not connected to the speaker in any way and was a simple panel as steel or aluminium is a better conductor of heat than plywood. Also, stacked subs will have their speaker chambers huddled together like a row of terraced houses. There is less wall area open to the outside to lose heat.
The issue of heat build up in a sealed chamber will lower the temperature gradient and thus cause a higher operating temperature of speaker components, but there is also the issue of getting the heat out of the speaker components and into the air in the first place.

In the case of a subwoofer I blew (an SPL10R on a Peavey IPR1600), it was being used in a back pack as a tactile transducer and took too much continuous power, but to be honest I turned it off as soon as it started buzzing and yes, a bit of smoke came out the pole piece. I suppose if you IGNORE this and carry on, then the coil insulation might burn completely and two or more coil windings touch creating a "thick low resistance wire". But melting is not usually the first mode of failure unless your amps are vastly overpowered and your system is unlimited and your DJ is drunk. The coil on the 18 I bought from an ebay scammer looked pretty toasty - like the residue on a barbecue grille that's done many chicken wings without a clean.

User avatar
Bill Fitzmaurice
Site Admin
Posts: 27838
Joined: Tue May 02, 2006 5:59 pm

Re: Good article on SPL

#13 Post by Bill Fitzmaurice » Wed Dec 15, 2021 11:04 pm

With regard to Parhams's heat sink, it was something he came up with for use with his 12 Pi sub, a design he created specifically to beat the LAB Sub. That goal was fueled by his longstanding feud with Tom Danley, although it was very much a one sided feud, Danley never gave it much thought. The heatsink may work well, but the overriding reason for it was that if one wanted to build a 12 Pi they had to buy the drivers from Parham. Since selling drivers for his designs was his main business it wasn't a bad idea. However, judging the claims of how effective the heatsink is should take that into consideration.

AcousticScience
Posts: 144
Joined: Tue Jan 20, 2015 4:37 pm

Re: Good article on SPL

#14 Post by AcousticScience » Thu Dec 16, 2021 7:03 pm

Sometimes great discoveries happen during petty feuds. Nikola Tesla vs Thomas Edison comes to mind (Edison thought to be the "aggressor" and his barbaric execution of an elephant with AC to "slander" Tesla). If I recall, Wayne is not selling drivers, only the cooling kits, which can also be fitted to the standard lab 12 be it in a LabSub, 12pi, or Tuba.

Post Reply