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Keeping it Real II: In-Ear Monitoring And The Acoustic Reflex Threshold

Considering some of the complex mechanisms performed by the human brain that affect localization and our perception of loudness.

Editor’s note: This is part two of a three-part series. Part one was featured in LSI October 2018 and can also be found on ProSoundWeb.

Have you ever noticed how you and the band can take a break from rehearsing, come back half an hour later, and when put in your in-ear monitors (IEMs), everything feels louder? And then how after a few moments it settles down and feels normal again?

It’s because of a reflex action of the stapedius muscle in the middle ear. When this little muscle contracts, it pulls the stapes or “stirrup bone” slightly away from the oval window of the cochlea, against which it normally vibrates to transmit pressure waves to be converted into nerve impulses. This action, which is a response to sounds between 70 to 100dB SPL, effectively creates a compression effect that results in about a 20dB reduction in what we hear.

However, the muscle can’t stay fully contracted for long periods, so after a few seconds, the tension drops to around 50 percent of the maximum. While the initial reaction, at 150 milliseconds (ms), is not fast enough to fully protect the ear against very loud and sudden transient sounds, it helps in reducing hearing fatigue over longer periods.

The middle ear, including the location of the stapedius muscle.

Interestingly, this reflex also occurs when a person vocalizes, which helps to explain why a singer’s IEM mix of the band might sound loud enough in isolation, but when they start singing they find that they need more instrumentation.

It happens in conjunction with the fact they are hearing themselves not only via the mix but through the bone conductivity of their skull. It’s well worth trying to sing along to an IEM mix that you’ve prepared for a singer to experience what this feels like for them because it’s a very different sensation from simply shouting down the microphone to EQ it.

The acoustic reflex threshold also means that transients appear quieter than sustained sounds of the same level, and it’s the thinking behind a compression trick that is often used in studios and film production.

When you compress the decay of a short sound such as a drum hit, it fools the brain into thinking the drum hit as a whole is significantly louder and punchier than it is, although the peak level – the transient – has not changed.

I advocate caution if you’re going to try this in a monitor mix – drummers need to hear what their drums actually sound like, and getting things such as drum tuning and mic placement correct at the source are vital – but it’s an interesting thing to be aware of.

Read More
#5: Finding The Sweet Spot: Getting The Desired Vocal Sound In IEM Mixes
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