Tutorial & FAQ: Clipping, and Underpowering Speakers – Dispelling The Myths (And the Heat)

Introduction:
Too often people are under the impression that underpowering speakers is bad. This is true in a sense, but what do you think happens whenever you turn down the volume or, turn the gains down? You’re essentially underpowering the speakers. If speakers died whenever we turned down the volume, we would be shelling out for new ones every day.

Definition of Clipping:
Clipping occurs when an amplifier is overdriven, and attempts to increase voltage or current beyond its limits. When asked to create a signal greater than its maximum capacity, it will amplify the signal only up to that capacity, at which point the signal will be amplified no further. The amplifier then “clips” the top of the signal, thus outputting a square wave signal. The extra signal which is beyond the capability of the amplifier is simply cut off, resulting in a fixed signal. Like this:


When amplifiers ‘clip’, they can produce up to 2 times the rated power. The reasons for this can be seen when clipping is viewed on a CRO. There is more area underneath the clipped signal than the normal signal, due to being a square wave. This area equates to more power, and can be 2 times the amount of power output from a clean signal

So basically, if you have your amplifiers gains on maximum, and you headunits volume on maximum, then the amp will be being pushed to its limits, and start to output a clipped signal. This is why we don’t do it.

This is what a pure, unclipped sine wave should look like


Can Clipping Cause Damage?
There is a few different ways that a clipped signal can kill your speakers.

1. When an amp is clipping, it will output a DC signal. Speakers are not designed for DC, they are designed for alternating current (AC). When direct current passes through low resistance coils, they will produce a lot of heat. And because they have a DC current, they speaker coil/motor is not moving, and thus there is no airflow to dissipate the heat, which will melt Voice Coils. This leads me on to my next point, similar way of death for Voice Coils.

2. When a clean signal (imagine a clean sine wave for this) is being put through the speaker, the voice coil is constantly moving, and air is constantly being moved back and forth across the Voice Coil to dissipate any heat that may be generated. Now think of our clipped signal. There is a point there where full current is being thrown at the speakers/subs, but the voice coil isn’t moving. It stays at excursion, with plenty of current pumping through it. Current = Heat, but there isn’t any movement within the motor to pump air around. Not only this, but under a clipped signal, the motor isn’t moving the cone as far as a clean signal, which also means less airflow around the Voice Coils. Extended clipping means this will happen a lot, and over time, will burn the voice coils. It’s a very similar way of death the over exposure to DC current.

3. Simply overpowering the speaker. Let’s say for example I have a sub that can handle 400wrms, and an amp that is rated at 350 or 400wrms. Now if I push the amp into clipping then it may output 800wrms. My sub can only handle 400wrms, so I’ve obviously exceeded its limits, and the Voice Coil will burn up, causing premature death. But imagine if I had a sub that was rated at 1000wrms, you may think that it’s fine for this to happen, but incorrect. The other 2 effects of clipping still apply. So even though it’s able to handle to power, it may not be able to dissipate the heat induced by direct current, or move the cone sufficiently to dissipate it.

Will Underpowering Damage My Speakers?
Now you know the ways that clipping can kill a speaker, let’s look at the reasons why people think that underpowering a speaker will kill it.

Let’s say I’ve just got my very first sub and amp. Cheap stuff for a birthday present from mates who are “experts” in car audio. Say a 200wrms sub, and a 100wrms amp. I’ve got no idea, so I rig it up, crank the gains fully, and hit up some ying yang twins/ludacris/maximum bass test tones. Volume on 24/26. then start spewing when the sub smokes. Me being a n00b, I put this down to underpowering the sub..

NOT TRUE!!! My lack of knowledge in regards to gain setting led me to believe that if I turned it all the way up, it would be better. Not so. When I upped the gains, and the volume, then played bass heavy music, I forced the amp into clipping. My el cheapo subwoofer couldn’t handle to extra power, coupled with the extra heat that it couldn’t get rid of, so it up and died. That is why people think that underpowering speakers kills them.

It is infact a lack of knowledge about proper tuning of amps that kills speakers, so I suggest that you read this tute Amplifier Gain & Crossover Tute

and in the words of mad89:



shout out to the following people, who made this dream a reality

mad89
shiny_car
google
wikipedia
bcae

Nice write up there trism.
There is only one thing that i think it needs. Try and find a picture of a CRO displaying a pure, unclipped sine wave, as some beginners might not know what it looks like compared to the picture of a clipped signal you have.

done

good tute for n00bs...

maybe include definition of thermal power limit vs. exceeding Xmax to kill a sub...

This is not really true. The clipped part of the wave is not DC, there is no such thing as DC in an alternating wave. If there was DC in the signal, then it would be seen as DC offset. For the speaker to instantly stop moving when it hits the 'flat' bit of the clipped wave, the speaker would have to have a flat response to infinity hz. This is because it does not consist of DC, but harmonics of the fundamental tone. The 'wasted' power goes into those harmonics. (But there will also be significantly more power at the fundamental frequency when you clip an amplifier too, this is why moderate clipping is beneficial for SPL scores)

Of course in the real world, speakers have inductance which means they will inherently overhang at higher frequencies. If you gave a subwoofer a 50hz square wave signal, and then analyzed the wave that subwoofer reproduced, you would find that the subwoofer actually rounded off the edges/and had overhang at the higher frequencies.

Also remember that even though the cone doesn't move as far at higher frequencies, it cycles more times per second. But regardless, the cooling effect due to the movement of the cone is not as signficant as you may think.
The risk with clipping is the extra power sent to the speaker. And the simplest way to avoid clipping is to make sure the gains are set correctly and simply turn down the volume if/when you hear distortion.

Plus with a square wave signal, the signal may be flat, but the cone doesnt actually stop moving....


***Edit, No wait, I think thats what Bassaholic means by overhang....***

so do different amps have ways to stop this 'clipping'.....or not stop just limit the amount of output power

Some have a clipping indicator, others turn off/turn down when they get too hot. Not sure if any turn themselves down/off if they are sending a clipped signal.

Leigh

There are a few amps out there which turn themselves down/shut off if they are driven too hard. (even when cold)