All In The Timing
Our ability to perceive sounds as separate events is not only dependent on there being a sufficient difference between them in frequency, but also on timing. This phenomenon is known as the “precedence effect”or the “Haas effect.”
These effects describe how when two identical sounds are presented in quick succession, they’re heard as a single sound. This perception occurs when the delay between the two sounds is between 1 to 5 ms for single click sounds, but up to 40 ms for more complex sounds such as piano music.
When the lag is longer, the second sound is heard as an echo. A single reflection arriving within 5 to 30 ms can be up to 10 dB louder than the direct sound without being perceived as a distinct event.
In 1951, Helmut Haas examined how the perception of speech is affected in the presence of a single reflection. He discovered that a reflection arriving later than 1 ms after the direct sound increases the perceived level and spaciousness (more precisely, the perceived width of the sound source), without being heard as a separate sound. This holds true up to about 20ms, at which point the sounds become distinguishable.
This can be an interesting experiment to try with a vocal mic and your IEMs. If you split the vocal mic down two channels, and delay one input somewhere between 1 and 20 ms, evaluate what you notice. Then try panning one input hard left and the other hard right to discover see how the vocal sounds thicker and creates a sense of width and space.
Play with the delay time, and you’ll see that if it’s too short the signal starts to phase; too long and you lose the illusion. This game does make the signal susceptible to comb-filtering if the inputs are summed back to mono, especially at shorter delay times, so be aware of that.
Once again I recommend extreme caution in using this in an IEM mix, as “tricking” a singer in this way can backfire!
However, it’s a useful principle to be aware of if you have the opportunity to get creative with other sounds, and I use it a lot when adding pre-delay to a reverb. No pre-delay creates a feeling of immediacy to the effect, but just 5 to 10ms creates a slight sense of space. If you’re after a little more breathiness and drama – “vampires swirling” as I once heard it described – try increasing the pre-delay up to 20 ms and listen to how it changes.
The Haas effect is also something to be very aware of when IEM mixing due to digital latency.
Every time we take a signal out of the console and send it somewhere else in the digital domain, a degree of minor time delay known as latency is introduced.
Different processing devices introduce different amounts of latency, and obviously the less, the better. The more devices we add, the more the latency stacks up.
While a few milliseconds of latency may be totally imperceptible for, say, a guitarist, it’s a different matter when it comes to vocals.
Singers will often be able to perceive something as being not quite right without being able to put their finger on it, because when we vocalize and have that signal returned to our ears, the discrepancy between what we hear at the moment of making the sound–and the moment of it returning–becomes heightened in our awareness. This is something to be vigilant about when dealing with any digital outboard, such as plugins, with singers.
