Speaker protection

Lisa Lane-Collins

Lisa Lane-Collins

Sophomore
Dec 9, 2012
270
0
16
Adelaide, Australia
My amp is putting out 2100w at 4 ohms and I'm running that into a double 18 sub containing two speakers running in parallel 8 ohms 1000w peak.

I can use the DSP to limit the audio so the amp never runs into the red, but by my maths, that's still a little on the over powered side (if the 1000w speaker should ideally be paired with an amp that puts out around 700w peak, the halfway point between the speaker's RMS and peak rating). I could lower the limiting threshold further, but this will cost me volume, and arguably increase RMS in a bad way (if DJs drive it hard trying to get that missing volume back).

This set up blew one speaker in a double 18 before I got involved, and it would appear we've blown another one since I applied the limiting, although I think this speaker might have blown because the internal wiring turned out to be 4 pole, so one 1000w speaker was getting power meant for 2. Still, I'd like to say with more confidence to the owner of the PA "this will protect your system" so I am here to ask, where/how do I set the limiter? Is there even a way to protect against too much RMS? Is limiting all I need to do? How do I get maximum volume whilst still preserving the speakers?
 
Re: Speaker protection

The only way to guarantee not blowing up a speaker is to limit the power to half the RMS rating of the speaker. The reason the limited power has to be half, is that the RMS power is rated with a sine wave. If you drive hard into clipping, the wave becomes more of a square wave. A square wave of the same voltage as a sine wave is twice the actual power.

Now, if you've got to set a limiter THAT stringently for protection, then you've got some complete idiots working the equipment. Usually, you can get away with running program power, which is twice that of RMS. With most musical programs the dynamics of the music will mean the average overall power remains below RMS. However, again, if you put a limiter in and you push past the limiter, the average power shoots up, and stuff can blow.

Oh, and peak and program ratings are just numbers used by the marketing team to make things sound bigger than they are. RMS power is the only number you really need to know.
 
Re: Speaker protection

Most drivers are already into power compression by the time they are fed 1/2 their rated continuous power, so there's little if anything to be gained by feeding them more. How power compression comes into play on short-term* signals is more complex, but it seems to be common to have a non-zero attack on the system limiter. *"Transient" does not apply, as the rise time of any signal below 100Hz is comparatively long.
 
Re: Speaker protection

Brian jojade said:
T

...then you've got some complete idiots working the equipment....

Hello

I would say that for every reasonable person with some real knoledge, we have few fulltime "operators" in this business - perhaps they can sell their work or are good i.e. as dj, but as of understanding or even respecting the limits of any given system ....

I used to rent my stuff to all kind "technicians" - when things got wrong, there were two usual explanations:

- Well - I was thinking...
- Well - I was not thinking ...


What a wonderful set of exellent reasons ...

Lisa - make it safe - half power is only 3dB less than full power - nobody is going to hear any difference - as Tim mentioned, with power compression it is even less, perhaps only 0,5dB
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Re: Speaker protection

Tim Padrick said:
For better or worse, I run a "slow" (whatever that means) limiter set to the voltage that would be 1/2 of the speakers' rated continuous power into their nominal impedance spec.
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Even if set to half of continuous-loudspeakers can still be blown.

I have had it happen.

It depends on how hard they drive the system. IF they just keep turning up and up and keep on compressing the signal, the average level will get higher and higher.

And the speakers get hotter and hotter.
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Half power is a good idea for reasonable operators. For others-maybe 1/4 power is a better idea.

I know it sounds stupid but is the reality. There are plenty of people who just keep turning it up and up-ESPECIALLY if the gear belongs to somebody else.
 
Re: Speaker protection

Hi Lisa - Can I ask what 18's you are using? the driver itself and not just the maker of the box. The type of box design? The make and model of the amp? And the type of music you are playing? Just for perspective.
 
Re: Speaker protection

Dumb question maybe but how do I determine what half power is vis a vis the threshhold setting on the DSP? I have always wondered if there is a tool to measure amp out put, would be super duper helpful.

Another angle, would it be better to run the system at unity, and set the limiting threshold so it's just tickling that? (This assumes well matched amp and speaker I suppose).

Playing nightclubby dance/doof/latino music, I think they are EV speakers in home made boxes, double 18s, speakers rated 1000w peak, being driven by a lab gruppen fp 10000
 
Re: Speaker protection

Lisa Lane-Collins said:
Dumb question maybe but how do I determine what half power is vis a vis the threshhold setting on the DSP? I have always wondered if there is a tool to measure amp out put, would be super duper helpful.

Another angle, would it be better to run the system at unity, and set the limiting threshold so it's just tickling that? (This assumes well matched amp and speaker I suppose).

Playing nightclubby dance/doof/latino music, I think they are EV speakers in home made boxes, double 18s, speakers rated 1000w peak, being driven by a lab gruppen fp 10000
Click to expand...
Here is the math. If your speakers are 1000 watt peak-then that means 250 watt continuous. So half power would be 125 watts.

Assuming they are 8 ohms, that would be 31.6V. Basic ohms law.

So now that we know what voltage you are looking for, you need to set up the limiter.

Put a sine wave tone into the system (in the middle of the loudspeaker freq range) and DISCONNECT the speakers. Turn it up until it is above 31V-lets say 40V.

Now simply turn down the limiter threshold until your meter (AC voltmeter on the amplifier output terminals) reads 31V.

This is a VERY BASIC setup-and there are a number of variables that can affect the "proper" setting.

But it is a good start.
 
Re: Speaker protection

Ivan Beaver said:
Put a sine wave tone into the system (in the middle of the loudspeaker freq range) and DISCONNECT the speakers. Turn it up until it is above 31V-lets say 40V.

Now simply turn down the limiter threshold until your meter (AC voltmeter on the amplifier output terminals) reads 31V.
Click to expand...

Better make sure your voltmeter measures properly in the passband of your loudspeaker, as many aren't all that accurate above ~400Hz (or even lower in some cases). Note also that a true-RMS voltmeter will measure the equivalent heating voltage of the clipped square wave, while a non true-RMS meter will assume a sine wave (at lower power for the same peak-peak voltage)
 
Re: Speaker protection

Rob Timmerman said:
Better make sure your voltmeter measures properly in the passband of your loudspeaker, as many aren't all that accurate above ~400Hz (or even lower in some cases). Note also that a true-RMS voltmeter will measure the equivalent heating voltage of the clipped square wave, while a non true-RMS meter will assume a sine wave (at lower power for the same peak-peak voltage)
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If the waveform is clipped, then no matter what the voltage is-it is to high.

In no case should the amp be clipping

You can always change the passband of the amp as needed-since the limiter is after any sort of eq or filters
 
Re: Speaker protection

Sorry, in late here. But I'm surprised no me asked how you set your high pass filters. You are much more likely to damage a sub by putting in a signal that is too low frequency than by putting in too much power. Stupid accidents aside, if it sounds ok while it is playing you are probably not overpowering it. But too low of a frequency will cause mechanical damage to the driver. You should set high pass filters to the -3dB frequency in the speaker specs and use an 18-24 dB/Oct high pass filter.

From your our post I have no idea what the real specs are for your speakers. What specific model and make are they?
 
Re: Speaker protection

Lisa Lane-Collins said:
My amp is putting out 2100w at 4 ohms and I'm running that into a double 18 sub containing two speakers running in parallel 8 ohms 1000w peak.

I can use the DSP to limit the audio so the amp never runs into the red, but by my maths, that's still a little on the over powered side (if the 1000w speaker should ideally be paired with an amp that puts out around 700w peak, the halfway point between the speaker's RMS and peak rating). I could lower the limiting threshold further, but this will cost me volume, and arguably increase RMS in a bad way (if DJs drive it hard trying to get that missing volume back).

This set up blew one speaker in a double 18 before I got involved, and it would appear we've blown another one since I applied the limiting, although I think this speaker might have blown because the internal wiring turned out to be 4 pole, so one 1000w speaker was getting power meant for 2. Still, I'd like to say with more confidence to the owner of the PA "this will protect your system" so I am here to ask, where/how do I set the limiter? Is there even a way to protect against too much RMS? Is limiting all I need to do? How do I get maximum volume whilst still preserving the speakers?
Click to expand...

I know this post is old, but it may help others in the future.

You may want to first make certain that the 4 pole plug (NL4) has both drivers wired in parallel on pins +1 and -1. Then make certain that the drivers are in phase with one another. You may also want to double check the at-rest electrical resistance (ohms) of the cabinet without the amp being connected. (There are plenty of youtube vids showing how to do this.) And just to be safe, check that the amp channels are not bridged.

Once you know exactly what numbers you are working with, the above advice may become more productive.
 
Re: Speaker protection

Lisa Lane-Collins said:
This set up blew one speaker in a double 18 before I got involved, and it would appear we've blown another one since I applied the limiting, although I think this speaker might have blown because the internal wiring turned out to be 4 pole, so one 1000w speaker was getting power meant for 2.
Click to expand...
Actually it was NOT "getting power meant for 2".

The ONLY thing you can apply is voltage-NOT wattage. The wattage is the RESULT of the voltage across a certain impedance load.

Let's look at your "numbers". You say 1000 watts peak for 2 drivers that are 8 ohms each-so that is a 8 ohm load and 250watts continuous per driver or 500 watts @ 4 ohms total cabinet rating.

IF this seems wrong-then maybe I am not understanding your "specs".

But follow along anyway for an example.

So for a wattage of 500 watts@ 4 ohms, that is 44.7 volts.

So if each driver is 250 watts @ 8 ohms, that is ALSO 44.7 volts.

So if you were only driving one driver with 44.7V, then you ARE NOT applying any more power than the speaker is rated at.

HOWEVER-HERE is the problem with only driving one driver in a cabinet.

First of all you are not getting the combined output of BOTH drivers when being driven . This results in a 6dB lower output.

BUT-it gets worse. Since the other driver is not being driven, it will actually be CANCELLING the sound from the driven loudspeaker. How much? It depends on the design and construction of the cabinet. Are the drivers sharing a common chamber? or are they in separate chambers? In the first case you will be detuning the overall response. In the second case you will have notches in the response-making it quieter at some freq.

But the POWER will not be changing (yes there will be some slight differences in impedance-but I am ignoring that for now, for the sake of simplicity).

But with the second driver not being driven, the system is likely to be pushed harder-which will increase the average VOLTAGE going to the speaker-which results in more power being dissipated and burning it up.

So if you are NOT driving the power for 2 into a single driver. They both would be getting the same voltage.

I hope this makes sense and helps.
 
Re: Speaker protection

Sorry for the slooow reply, I have been lurking (and will definitely be referencing this thread next time I have to do this). For now, haven't heard anything back from the company that employed me and according to bar staff the rig sounds fine and runs well. So, that's good news. :) Still looking forward to being more precise next time.
 
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