Wednesday, 17 August 2016

Guide To A Professional Audio Practices: Loudness In PA And Sound Reinforcement Systems.

Loudness is a word that describes our experience of sound. It is related to sound pressure level (SPL), but is not exactly the same: we sometimes experience one sound as being louder than another when the SPL is actually the same level (or in some cases lower in level).

Where the only variation is in SPL, two otherwise identical sounds will seem to differ in loudness where their sound pressure levels differ by more than around 3dB. While smaller differences (down to 1dB and below) can be perceived, they are less readily noticeable.

However, other factors interfere with comparison where two sounds are not identical. The main interfering factors are:
• Distortion
• Average level
• Frequency content
• Exposure
One of the things that tells our ears that a sound is loud is distortion. When amplifier circuits are driven into distortion, they create high-frequency harmonics of the original sound. The extra high-frequency content makes the sound seem louder (see Frequency content, below). 

Also, when an amplifier reaches distortion, its dynamic range is reduced: although the quietest sounds it reproduces are increased in level, the loudest sounds it can reproduce are limited by its output capability. This will also make it seem louder (see Average level, below). These two factors combined can make an amplifier with limited power driven to distortion seem louder than an amplifier with greater output operated within its limits.

Many inexperienced guitarists will set the level of an "overdriven" channel at a lower level than the "clean" channel. Without any other cues, the distortion of the "overdriven" sound makes it seem louder, where a meter shows it is in fact lower in level.

Average level
The ear can deal with a substantial range of sound pressure levels, from silence to the threshold of pain (the threshold of pain typically falls somewhere between 100dB and 140dB SPL, depending on frequency and varying between individuals). 

Where a sound is relatively constant in level, it may seem louder than a sound with greater variation in level, even though the sound with greater variation has a higher peak level. Sound pressure levels ranging between 100 dB and 104 dB (SPL) with an average level of 103 dB will generally seem a lot louder than levels ranging from 80 dB to 110 dB with an average level of 96dB, even though the peak level of the "quieter" sound is a full 6 dB (the equivalent of four times more amplifier power) higher. 

For this reason compressors can make a mix seem louder - by increasing the level of the quietest sounds (and increasing the average level) - without increasing the peak level. Note, however, that it is average levels which overheat speakers: increasing the average level increases thermal stress on speaker drivers. Compressors don't necessarily make your system safer.

Frequency content
Our ears do not hear all frequencies equally. They are most sensitive at around 3-4kHz, and much less sensitive at the extremes of frequency (in most concert systems the extremes will lie at around 40Hz and 16kHz). At the threshold of hearing the ear is around 60dB less sensitive at 40Hz than it is at 3.15kHz. Although the difference is less at high sound pressure levels (about 35dB at levels of 120dB SPL), it is still substantial. 110dB at 40Hz is loud, but 110dB at 3.15kHz is above the threshold of pain for many people.

A system with limited frequency response that has high output from 2-4kHz can seem uncomfortably loud, where a system with broader frequency response (and substantially higher acoustic energy overall) may not. Similarly, boosting the midrange EQ on a vocal channel can make a voice sound louder than it would by simply increasing its overall level. The loudness in that case would come at the expense of naturalness (and, arguably, pleasantness), although, conversely, it might improve intelligibility.

In contrast, boosting the lower and higher frequencies (or making comparative cuts in the upper midrange) can make a sound seem "bigger" without making it "louder".

High frequencies attenuate in air much faster than low frequencies, and our ears compensate for this (we "expect" a distant sound to have less high frequency content than a close sound). This is the basis of the "presence" control on older PA systems: if the high-frequency content is boosted, the sound seems closer, as well as louder.

Reflections from room surfaces increase as the overall sound pressure level rises, and can interfere substantially with intelligibility, as well as changing the overall frequency content (walls reflect some frequencies more than others). While it may seem natural to turn it up if you can't hear the vocals clearly, it can sometimes actually improve things if you turn it down.

Our ears try to protect themselves from very loud sounds (hearing damage caused by loud noises - also known as Permanent Threshold Shift - is cumulative and incurable). 

There are two principal protective mechanisms: the bones in the ear shift, reducing the mechanical conduction of vibration to the inner ear; and the blood vessels contract, reducing the blood supply to the organ of Corti (the part of the ear that translates physical vibrations into nerve impulses), and protecting the sensitive hairs that deal with the smallest sounds and uppermost frequencies.

These mechanisms reduce the sensitivity of the ear, with more effect at the very highest frequencies. Once the sound has ceased, the ear will gradually return to normal (usually within 24 hours or so, although it can take as long as a week). In the meantime, there is some loss of hearing (also known as Temporary Threshold Shift). Repeated exposure to sounds that cause Temporary Threshold Shift can lead to Permanent Threshold Shift.

Effectively, your own ears will turn down the volume and mute the damaging highest frequencies if the concert is too loud. Do it long enough and/or often enough, however, and you will damage them anyway.

If you think good music needs to be deafening, it will eventually deafen you (as well as your audience).
Your systems must have broad frequency response, good dynamic range, and are clean (undistorted) at high sound pressure levels. Consequently, they may not seem to be as "loud" at high volumes as systems with measurably lower acoustic output.

Thanks Guys and always remember to keep the creativity perfected.
... Steve Aluko

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