Using Hard Limiter

(In Adobe Audition 1.5)

At times there is a need to reduce the dynamic range (the difference between the loudest and softest sounds in a message) in order to provide a useful listening experience for those listening to these messages in a car, on headphones, or on their computer.

Most music CD’s are “mastered” where one of the processes is to boost the sound while making sure there is no clipping, and the dynamics are reduced.  Similarly radio stations employ a expander/gate/compressor/limiter to reduce dynamics and increase overall volume to the live stream and in preparing programs.

You must be careful as you do this reduction in dynamic range not to take so much out that the “normal” and intended variations in volume that are used for emphasis are reduced so much that the emphasis is lost.

We generally do this by bringing the lower peaks down and then “Normalizing” up to -1dB.  This new tool called “Hard Limiter” can be a great help where there are many high spike sounds but much of the message is much quieter.  Here is an example of a message that is a good candidate for the hard limiter.

Here is another example, perhaps a better one, of a file that needs the hard limiter:

And here is how it looked after being hard limited:

Pretty nice looking isn’t it!  And it only took the computer a couple minutes to do this while I poured myself a glass of milk!  Read further because you may be looking at those green areas that look “flat” at the -1dB line and think they are clipped.  They are NOT clipped!  It is just that there are many peaks of the waves at -1dB close together.  When I zoom in and look they are all very nice regular looking waves, and when I listen on very good noise cancelling headphones, they sound just fine!

Some files have even greater differences between the high volume portions and the low volume portions.   If there are any peaks that are clipped, the clip restore must first be used to restore these.

            The hard limiter button is just to the left of the normalize button in this screen shot.  It looks like an upside down green somewhat uneven “U”.

            Another consideration before boosting the volume on a file is to determine if the background noise is low enough to boost the sound.  Here is a screen shot of the background noise in this file before the hard limiter was used to boost the volumes and reduce the dynamics.

As the curser is looping around there is no sound that is much greater than -58dB and most of it in this section is less than -60dB!

OK, now I would select small portions of the file that show the highest and lowest volumes and test various settings.  I also check to make sure that the emphasis the speaker intended with their change in volume is not taken away by too much hard limiting which reduces the dynamics too much.  This takes a bit of trial and error.  I have used as little as 3dB boost and as much as 14dB boost.  It all depends on the file and the testing of small portions to see what sounds best.  Now also make sure to check the residual background noise on these tests.  If the end result is too high (greater than -42dB is my general rule of thumb) then do noise reduction BEFORE doing the hard limiting as this gives the best results.  At least it has in my testing. 

When I was done testing I then click in the file someplace to remove any highlights of small portions and then I actually set the settings as you see below:

            This file needed a great deal of boost to the sound so I chose 14dB boost in sound.  I wanted the max to be at -1dB so I set that.  All the other settings were the default settings which I did not change at all.

            Here is what the file looked like about 2 minutes later!

            Now before you panic, because it looks clipped, notice the highest sound is -1dB.  How the hard limiter works is that it takes EACH sample (44,100 of them every second!) and tries to add the boost I selected, in this case add 14dB of volume.  BUT, and this is the key, if the resulting volume would be above the maximum I set, it then recalculates how much volume to add to bring it just to -1dB.  It continues this sample point by sample point calculation through the entire file.  So if nothing was clipped before the hard limit, nothing will be clipped after!

Here is the background noise AFTER the hard limiter did its work on the same section of background noise we looked at before:

            Totally acceptable at -48dB!   This file is now ready for the rest of the “traditional” editing techniques to find the beginning and end and make them 1 second.  And then reduce any pauses of greater than 3 seconds. 

If this is to be edited to “Conference Editing” standards, you are done!

If this is to be edited to stricter standards like for radio, podcast, website etc, then you are ready to start listening through the message and fixing the other little things you have been led by the Spirit to fix.