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ARSD, Attack Transients and Head-End Ringing. What do these have in common? Read along for more about the life line of one sonic event . Enjoy this excerpt of a white paper by Art Noxon, PE Acoustical.

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What is a sonic event in music? It is the sound level variation of a single musical note. In music theory class a single musical event is defined as an ADSR event, a musical process that has 4 traditionally distinct stages: Attack, Decay, Sustain and Release. A musical line is a sequence of these musical events.

Typically this describes the lifeline of a plucked or hammered string instrument, a struck percussion or bell instrument, and blown wind instruments. In complex music, we have string of rapidly occurring sonic events, such as the rapidly plucked strings of a guitar.

For every attack transient, there is a complex set of harmonics involved. Speaking from the Fourier Transform or FFT perspective, as most audio specialists like to talk about, it takes a huge harmonic series in order to create the attack transient…that very fast rise in sound pressure at the onset of a sonic event.

There is more to attack transients. They are actually not a tone. They are just a very fast rise in pressure, a spike up in sound pressure. At first the speakers, woofer, mids and highs are standing still and suddenly, as if a huge voltage is snapped across the terminals, all speakers instantly jump forward, creating a rapid increase in sound pressure. After the seemingly simple rapid rise from no pressure to loud pressure, other things begin to happen to that sound. A tone appears for a short time or long time, and then it dies away, quickly or slowly.

Room Acoustics

But it’s the pressure spike (the Attack and the Decay) that is our present focus, not the afterglow elements (the Sustain and Release) of the sound. When the speakers jump forward, a pressure pulse is created in the front of the listening room, right around the loudspeakers. The pulse from the mids and highs are typically projected forward because of the size of the baffle board of the speaker. But as for the bass part of the pulse, what the 500 watt power dump into the woofer is doing, the instantaneous pressure pulse snaps out from the speaker with equal strength in all directions, in a full 4pi spherical wavefront.

Now, a third of this pulse snaps away in the front/back direction, a third in the lateral or left/right direction and the other third of the energy snaps in a vertical, up down direction. This means a third of the energy output is generally headed in the right direction, towards the listener, and it also means that two-thirds of the speaker’s energy is headed in the wrong direction, exactly perpendicular to the front-back direction.

The direct part of the wavefront snaps toward the listener at the speed of sound. The other 2/3rds of the wavefront snaps also at the speed of sound but in all directions perpendicular to the front back dimension. What happens to this powerful perpendicular shock wave? It gets trapped up in the front of the room. Bouncing back and forth, careening around all 4 surfaces, the two side walls, the floor and ceiling up in the front of the room. I have nicknamed this effect “head end ringing”.

While the remnants of the huge pressure spike are still bouncing around up in the front end of the room, the many-times-reflected (and now scrambled) sound begins to expand naturally into the rest of the room, down the room at about 1/10th the speed of sound, about 120 feet per second oozing right towards the listener. After about 1/12 second the head end ringing noise begins to significantly engulf the listener with a rising chaos that vaguely resembles the direct sound the listener just heard…except it is arriving a later and the various frequency components are completely scrambled up.

Sound Masking

The best sound masking sound is a sound that sounds just like the sound that it is supposed to mask, except that the masking sound is a time and phase scrambled version of the original sound. The worst masking sound sounds nothing like the sound that is supposed to be masked. How loud would a hiss sound have to be to mask the staccato tonal presence of a rapidly plucked bass guitar? Probably 40 dB louder than the guitar. If the guitar is being played at 50 dB,A and a steam pipe hiss is kicked on at about 90 dB,A, maybe, just maybe most of the guitar sound would be drowned out. But if the reverberant sound of the guitar itself was used along with a wild set of time delayed attack transients mixed back in, we could mask out the guitar with a sound masking sound level that equals the guitar level alone.

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