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GarageBand Tutorial: Built-in Audio Unit Effects

Underlying GarageBand's simple interface is a host of complex functionality and features that offer some serious power in modifying your audio inputs. Many of these features are inherited from FinalCut Pro/Soundtrack, or from Apple's other audio products such as the eMagic line, including Logic. As users become more comfortable with GarageBand and want to stretch their recordings further and further, mastering all of the powerful features of the program becomes a desirable goal. In this tutorial, we'll take you deep into the bowels of GarageBand's high-end Audio Unit filters and explain how they can change the sound of your recordings.
Audio Unit Filter Basics

Audio Units are special audio program plug-ins that conform to a certain specification. The Audio Unit standard was created by Apple and is used in a number of their audo applications (including Logic, GarageBand, Soundtrack, etc.), however Audio Units are also supported in many non-Apple audio applications as well. Not all Audio Unit plugins will necessarily work with GarageBand, however the program ships with several default Audio Unit plugins to expand its functionality. We'll show you where these AudioUnits show up in GarageBand and what each of them can do for you.

Finding the Audio Unit Filters in GarageBand

If you double click any track on the far left of the GarageBand interface, you'll be presented with the settings for that track. From there, click on the triangle next to "Details..." and you'll see something similar to the image below:

This is where you'll find a slew of powerful effects you can apply to any track you use in GarageBand. We'll be covering all of these settings in other tutorials, but for now let's focus in on the Audio Unit filters. These can be accessed in one of the two drop-lists directly below the "Equalizer" option. If you click and hold one of these drop-lists, you'll see the following menu of effects:

At the top of this menu are the GarageBand instrument-related effects, which are frequently used in electric guitar and bass instrument pre-settings. Again, we'll cover these in a later tutorial, but for now we want to look at the second half of the pop-up menu shown above. All of the items listed under the grayed-out text "Audio Unit Effects" are effects that map to Audio Unit plugins that come with GarageBand.

Audio Unit Effects Settings

If you don't happen to have audio engineering experience, the names of the Audio Unit Effects available to you are likely quite daunting. Never fear! We'll explain what each of these filters can do for you and have you talking audio filter trash at your next family reunion. Off the bat, we note that each of the Audio Unit Effects has settings that can be modified to custom tailor the results of the effects on your track. This is really where the power of these effects comes through and where GarageBand gives you an unbelievable amount of versatility to use in your recordings. To access the settings for a particular Audio Unit Effect, just select the effect you want and then click the little square button with a diagonal pencil next to the effect.

A Sound Basics Crash Course

Since all of the Audio Unit effects that are included with GarageBand are really audio "filters" that do one thing or another to modify the sound waves produced by your track, it is worth it to make sure we're all on the same page with respect to certain sound concepts. If you're keen with basic physics or principles of sound, you can skip this part. If not, you may even find this interesting.

Sound is created when air is vibrated by objects and our ears (or microphones) detect these changes in the air. A bell that has been rung, or the pulsating driver of a speaker or your headphones, vibrate the air in this way. The waves created in the air by these vibrating objects can have different frequencies -- the waves can fluctuate slowly and produce low pitched sounds, or they can fluctuate rapidly and produce high pitched sounds. Think of a rock dropped in a lake: near the place very close to where the rock was dropped, the waves created are very close together (they have high frequency), but further out, the waves are further apart (they have low frequency). Sound waves are very similar in nature to the waves created in a lake by objects that disturb the water (or other fluids).

Another feature of sound waves is that they have amplitude or strength. For sound waves, this is determined by the level of air pressure produced by the vibrating object. If you were looking at a two dimensional expression of a soundwave, the amplitude would be the height of the wave (and the frequency would be the length of the wave, i.e., wavelength).

These very basic principles should give you a glimpse at what the Audio Unit Effects in GarageBand do: they manimpulate the sound waves of the track you've selected. One final point to consider: a "track" or any expression/recording of sound is really a collection of information that can re-produce (through your audio output devices like speakers and headphones) a pattern of sound waves. A recording of a piano or guitar will include information to produce a number of soundwaves in varying frequencies and amplitudes, all at the same time.

Now, let's look at each of the Audio Unit Effects one by one.


A band pass filter simply allows frequencies within a certain range to be heard, while filtering out all frequencies above or below that range. In practice, most of the "filtered out" frequencies are not actually eliminated entirely, but are de-emphasized in the resulting sounds.

With a band pass filter you can accentuate those frequencies of a track that you want to be heard more than others. Using the band pass filter to emphasize the high, mid, and low range frequencies of a track will all result in different sounding tracks. The settings of the AUBandPass filter allow you to control two things: (1) bandwidth -- the size of the "band" or range of frequencies you want to emphasize, and (2) center frequency -- where the center point of that range falls on the range of frequencies in a track. Below is an image of the settings panel for the AUBandPass filter:


AUDelay is a standard delay filter that repeatedly replays sounds from a track to create an echo effect. The delay filter is very similar to the basic echo effect that GarageBand provides for each track, but the level of control offered by the AUDelay plugin is much, much richer. The settings for AUDelay are shown below:

Each of the settings for AUDelay above control some aspect of the repeated playing of track sounds. Dry/wet mix controls how intertwined the repetitious sounds are -- a wetter mix will produce a more crowded sound where a number of reptitions play over each other at the same time. The delay time controls the amount of time between each repetition. Feedback controls the number of repetitions that are played -- 0% means no repetitions are played, and any value lower or higher increases the number. If you move the slider all the way to the right or left, play a track, and then stop it, you'll notice the repeated sounds going on for a very long time. Finally, the lowpass cutoff frequency simply removes certain frequencies from passing through the delay filter.


The Hi Pass filter, and several other filters we'll see, are really just filters that work very similarly to Band Pass, but are directed to a certain frequency band rather than allowing the users to select any part of the frequency spectrum for manipulation.

The Hi Pass filter reduces the emphasis of lower frequency sounds, allowing higher frequency sounds to pass through. You would use this filter if you wanted to get rid of low-frequency sounds in a track, for example if you were recording vocals in a room where there is a low rumble throughout the mix (e.g., if there is air conditioning or some other noisy interference occuring). The settings for Hi Pass allow you to determine where the cutoff occurs:


The high shelf filter cuts off high end frequencies and allows lower frequencies to pass through. You can set the "gain" or sound boost that is given to the low frequency sounds that pass through the filter. The settings for this filter are shown below:


The peak limiter is a filter that modifies sounds where the amplitude is comparatively high -- when there are spikes in the loudness of the track. This is helpful if you are concerned about various instruments being so loud that they will cause interference in sound output devices like speakers, amplifiers, and the like. The settings for the peak limiter are shown below:

The attack and release time indicate how quickly the filter modifies a peak level and how quickly that modification comes to an end. When we say "modify," what we really mean is that the amplitude of the signal is lowered so the sound is quieter. Thus, a larger attack time means a longer period of time is used to lower the volume of the peaking signal, and a higher release time means that a longer time is taken for the track to resume its normal amplitude or volume level. The limiting amount refers to how much the amplitude is decreased when a peak happens.

One final note on the peak limiter is that this filter is not simply lowering your track volume when there is a period of high volume. Remember that a given track is actually producing sound in a wide range of frequencies. When ONE of those frequencies peaks, then that soundwave in particular is limited, and not the rest of the frequencies. What this results in is a sound that does not seem like an entire track has been lowered in volume. In this way, you can boost the overall volume level of a track without worrying about spikes in certain frequencies causing harmful distortion to your output devices. This is quite useful if you have something like a live microphone or instrument recording that has erratic volume levels and must be "cranked" in order to sound acceptable during most of the track, but at times is very, very loud.


The low pass filter is the exact opposit of the hi pass Hi Pass filter from above. The low pass filter reduces the emphasis of high end frequency while allowing low end frequencies to remain untouched. Just like the high pass filter, the low pass filter can be used to weed out unwanted sounds at the relevant end of the spectrum. While the hi pass filter was used to weed out low, rumbling sounds, the low pass filter would be used to weed out high pitched annoyances like hiss, whine, or machine noise. The controls for the low pass filter are the same as for the hi pass, and look like this:

AULowShelfFilter As above, the low shelf filter is the mirror image of the high shelf filter. It will cut off the low end of the frequency range and allow high end frequencies to pass through. The gain setting allows you to control the level of boost added to the high range frequencies. Note that this filter is very similar in effect to the AUHiPass filter from above. The controls for the low shelf filter look like this:


The multiband compressor is very powerful and complext tool. Audio compressors serve the purpose of reducing the dynamic range of an audio signal so that both the low and high amplitude sound waves are "compressed" closser to a particular ideal level somewhere in between. The AUMultibandCompressor allows you to select the compression settings for multiple bands of frequencies, rather than just one. However, if you've taken a look at the settings panel for GarageBand's multiband compressor, you're likely to feel in over your head. In that case, you might want to stick to Apple's simplified compressor effects, which are available from the main settings window for any track within GarageBand. Another option would be to use one of the factory presets for the AUMultibandCompressor, which are shown in the AUMultibandCompressor settings below:

Of course, if you've used a hardware audio compressor or are otherwise inclined to play with the many options for GarageBand's Audio Unit compressor, then you should feel free to play with the options above. Some of the controls common to most audio compressors are: (1) Attack and Release: the quickness with which the compressor takes effect on sufficiently low or high amplitude signals, and correspondingly, the quickness with which the compressor's effects dissipate (2) Threshold: the range above or below which the compressor begins to kick in, and (3) Amount: the amount of compression or signal adjustment that is applied when the singal moves beyond the threshold.


Once again, this Audio Unit plugin is somewhat repetitive of functionality that GarageBand makes available elsewhere in a simpler format. Of course, if you were happy with what is easily made available in the way of filters and effects, you probably wouldn't be reading this. So, we'll show you how to take GarageBand's reverb effects outside the box.

The matrix reverb plugin provides a healthy amount of custom control over the reverb effects used on a track. Due to a lack of documentation, most of our knowledge of the matrix reverb plugin comes from experimentation. Without describing what each of the various settings can do for your reverb mix, suffice it to say that a healthy number of different reverb qualities can be obtained by adjusting the settings until you find your desired effect. Generally speaking, the higher each setting level is made, the longer or more pronounced your reverb effect will be. We were able to get some very large-hall sounding reverb by playing around a bit, and you can likely easily find what you're looking for as well. The control panel for matrix reverb looks like the following:

One tip for exprimenting with this or any other Audio Unit plugin: GarageBand very conveniently allows you to preview the effect of your settings changes in real time. Thus, if you play your instrument right after (or even while) making a change, you'll be able to detect the effect of that change in your subsequent playing. This makes toying around with various settings quite productive when searching for a particular sound without knowing quite what settings you should change.


The final built-in Audio Unit Effect that comes with GarageBand is the parametric EQ. This fairly boring effect is actually not very interesting. Simply put, the parametric EQ allows you to boost or decrease the amplitude of a signal within a certain frequency range. This is not particularly exciting since GarageBand gives you a full spectrum EQ elsewhere in the program. The settings for the parametric EQ are shown below:

That's a Wrap!

And that concludes our look at the built-in Audio Unit effects for Apple's GarageBand software. We'll be taking you in depth with many other features of GarageBand here at MacJams.com, so stay tuned and keep on jammin'!

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