In this series of posts, I would like to share my way of using Adobe Audition to produce audio assets for my Captivate projects. In this particular post, I’ll cover the second step of my audio production workflow - the Equalization step
What is equalization?
According to Wikipedia,
Equalization (British: equalisation) is the process of adjusting the balance between frequency components within an electronic signal.
When applied to an audio signal, the equalization process lets us adjust the energy of particular frequencies or ranges of frequencies.
To achieve this adjustment, sound engineers use tools called Equalizers, and, because things are never as easy as we want them to be, there are different kinds of Equalizers available. The ones that we are going to use in this post are the parametric equalizer, the High Pass filter and the Low Pass filter.
The parametric equalizer
This particular kind of equalizer lets us control three parameters for each of the filter we want to use.
This first parameter let us decide the amount of boost or the amount of cut that we want to apply.
- Leave the gain at 0 if you don’t want any processing to occur.
- Give the gain a positive value (for example +6 db) if you want to apply a boost to a particular frequency or range of frequencies
- Give the gain a negative value (for example -6db) if you want to reduce (cut) the energy of a particular frequency or frequency range.
This second parameter let us choose the frequency at which we want to apply the gain or the cut described above. If you want to filter a range of frequencies instead of just one frequency, this setting represents the frequency that is at the center of the affected range.
The Q (quality) factor
This third parameter is less obvious. It let us choose the width of the frequency range affected by our boost or our cut.
If the value of the Q factor is high, a very narrow frequency range will be affected by our filter.
If the Q value is low, a wider frequency range will be affected by our filter.
The following image shows the three controls of a parametric equalizer as seen on a live mixing console. Some mixing consoles have a fixed Q and provide controls for the gain and the frequency only. This kind of equalizer is called a semi-parametric equalizer because it only gives us acces to part of its parameters
Using an equalizer
Correctly using an equalizer is not an easy task. It requires a lot of attentive listening and a bit of practice. To help you out, here are a few guidelines and use cases about how to use an equalizer.
Using an High Pass filter to reduce the proximity effect.
The proximity effect is a distortion of the audio signal that is due to the use of directional microphones during the sound recording. A directional microphone is a mike that picks up the sounds coming from one direction only. Such a mike is also called a cardioid microphone, because its polar pattern looks like the shape of an hart. Omni directional mikes (that pick up the sound coming from every direction) are therefore not affected by the proximity effect.
The proximity effect is characterised by a significant increase of the energy of the lower frequencies of the spectrum. The following audio recording has been made by a cardioid microphone (an AKG C3000). It is a cardioid mike and therefore, is subject to the proximity effect. The sound clip has been normalised using the techniques explained in the first post of this series, but no equalization has been applied. Pay close attention to the lower frequencies of this sound clip. Notice how ‘fat’ the sound is because of the proximity effect!
To combat the proximity effect, we will apply an High Pass filter to the audio clip. An high pass filter lets the higher frequencies pass through the filter and, consequently cuts the lower frequencies (Some people call this filter a Low Cut filter!)
All we need to do is to set the cutoff frequency. All the frequencies that are below the cutoff frequency will be reduced, while the frequencies that are above the cutoff frequency will be left untouched.
I have applied a high pass filter with a cutoff frequency set to 100 hz and a gain of -48 db /oct to the following audio file. Compare it with the previous audio file. Pay close attention to the lower frequencies of the signal.
Listen to that sound!
OK, the change is not (yet) obvious and the sound is still a bit fat, so let’s make it clearer.
Make the sound clearer
For a normal human ear, the audible audio frequencies span from 20 hz to 20.000 hz. The higher the frequency, the higher the pitch of the sound. The following table comes from Wikipedia and describes each range of frequency by the sensation it induce.
|Frequencies (in Hz)||Octaves||Description|
|16 to 32||1st||The human threshold of feeling, and the lowest pedal notes of a pipe organ|
|32 to 512||2nd to 5th||Rhythm frequencies, where the lower and upper bass notes lie|
|1024 to 2048||6th to 7th||Defines human speech intelligibility, gives a horn-like or tinny quality to sound|
|4096 to 8192||8th to 9th||Gives presence to speech, where labial and fricative sounds lie.|
|16384 to 32768||10th Brilliance||the sounds of bells and the ringing of cymbals. In speech, the sound of the letter "S" (8000–11000 Hz)|
The frequencies that made the voice intelligible are centered more or less around 1000 hz (or 1kHz).
Below that frequency, we have the medium bass and the basses. If they are present in excess, these frequencies made the sound ‘fat’. The following audio file is the same as above, but with a parametric filter applied. the filter has the following values
- a cut of -20 db
- A frequency of 165 Hz (Medium bass). For a female voice, this frequency will probably be higher.
- A reasonably low Q factor of 7,4, so a reasonable range of frequencies around 165 hz is affected by the cut.
Listen to that sound! Changes should be more obvious now!
Other adjustment are to your liking! They all depend on the microphone you used, on the voice you recorded (male or female) and on your personal taste. To help you out, these are a few tips that I discovered along the way for effective equalization. Don't hesitate to add your own in the comments section of this post!
- Ask yourself “What is there too much in this sound?” and not “What is missing in this sound?” That way, you will apply more cuts than boosts and the resulting sound will contain frequencies that were already present in the original sound and not frequencies that are artificially boosted.
- Keep in mind that in eLearning, most of your audience will listen to your audio files through cheap headphones. These headphones have a tendency to artificially boost the lower frequencies to cope with the requirement of the noisy an loud modern music!
- Also, keep in mind that you want your voice to be intelligible! Your primary focus should be on the voice intelligibility, not on the respect of the original tone of the voice.
- Test your equalized audio clip on an array of different headphones and speakers (your studio headphones, your iPod, ...). You ‘ll be amazed by the differences!
In the following audio clip, I’ve added a small boost of 6 db around 5500 Hz to enhance the sound brightness and presence a little bit. I have also added a Low Pass filter (also called a High Cut filter) with a cutoff frequency of 16000 hz (16 kHz). Frequencies higher than 16 000 are barely audible anyway!
Listen to this final sound! Don't hesitate to compare it with the original sound found at the beginning of this post!
Equalization is an essential part of the processing of your eLearning audio assets. Equalization will help you shape the sound in order to make it clearer, intelligible and optimized for the cheap computer headphones found in eLearning labs and computer stores.
Correctly equalizing an audio clip is not an easy task. Each speaker / microphone combination probably requires its own unique equalization settings.
Adobe Audition has a very nice parametric equalizer available. It also features a very convenient preset feature that enables you to save your equalization settings should you re-apply them on subsequent audio clips.
I hope this post will help you shape the sound of your own audio clips.
In the next post of this series we will talk about using the noise gate and the compressor to eliminate background noises and find the right audio level for the clip.
Until then, Happy eLearning!