AF’s Weblog

October 14, 2009

Basics of Acoustics: Time (I)

Filed under: Instructional articles — Tags: , , , , , — audiofanzine @ 6:01 am

Time (1)

With stopwatch in hand, our perception of time seems straightforward. But in everyday life we’re not always watching the clock, and everyone knows that the passage of time is relative. It differs from one person to another and especially from one activity to another: An hour spent watching a great movie doesn’t feel as long as an hour in traffic.

SignalFig.1a : Sound signal with reverb. Note the progressive attenuation of the sound level

Scientists may conceive time in seconds, but most musicians feel it in a more fluctuating manner: either in the speeding up of a tempo or the slightly off-pitch note due to stress. In fact, pitch, which is defined by frequency, is a value linked to time and depends on our perception of a second. If it seems longer or shorter, the note can seem sharp or flat.

It’s said that during the middle ages, long before the invention of the metronome in 1816, a person’s pulse was used as the reference. It was therefore better to choose a musician who was calm.

Signal reverseFig.1b : The same signal, through a reverse effect Reverse : maximum gain is at the end of the signal

An Experiment

For those of you who can remember magnetic tape, a piano note played in reverse doesn’t sound at all like a piano, and a verse of Shakespeare in reverse sounds strangely like…Swedish. In fact, what our ears perceive as a single homogenous sound is really like a small train made up of four different cars: if we watch it as it moves forwards or backwards, the order of arrival won’t be the same and therefore our perception of the sound will be different.

It’s this idea that’s expressed through the notion of the A.D.S.R curve, also called envelope curve. A « reverse » preset found on some reverbs manipulates nothing but the reverb envelope. It will probably be a decreasing sound and look like figure 1. If it’s played in reverse, the end will therefore be played before the beginning (figure 1b).

Compression

The second case in which a musician-technician might find themselves confronted with having to manipulate an envelope generator: a compressor. A compressor usually has envelope adjustments that change the action time of the compression effect (fig. 5)

Depending on the gear, you’ll usually find an Attack adjustment, which corresponds to the time the compression kicks in once the signal reaches the limit of compression. By putting this setting on slow, the compression will be much more discrete and lets you assure a certain amount of compression without it being too sensitive (for classical music for example). But on the other hand, all sudden peaks corresponding to short attacks will escape treatment. A short Attack adjustment will allow the compressor to react instantly , but that typical compressed « punchy » sound will be heard. In today’s music this can be a desired and interesting effect, if used with moderation. You can also modify the Release which adjusts the time it takes the compressor to bring the level back to its initial level. As with the attack settings, a middle setting will be more discrete and will be more delicate in bringing the level back down. The opposite, a release set to zero can, if the compressor intervenes often, give a disagreeable wave effect.

Figure 5Fig 5a: Cubase’s standard compressor

Figure 5Fig 5b: TC Electronics TC CL1B plug-in which models a tube compressor

To read the full detailed article see:  Basics of Acoustics: Time

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