Newsletter

Art Noxon, PE Acoustical is ASC’s founder & inventor of the TubeTrap. This week Art continues with part 4 of Bass Trapping in Your Recording Studio. Enjoy!

Sound “Cancelling” the Cold Spot

When sitting about the middle of the room at the “cold spot” while the first resonance is set up, the very curious effect of “sound cancelling” occurs. Here, the sound from the speaker is exactly out of phase with that of the room resonance at that location. Sound pressure may be cancelled, but nature does not give up so easily; acoustic energy is not cancelled. If sound (acoustic pressure) is “cancelled” in one part of the room, it has only been replaced with acoustic kinetic. Conversely, sound pressure will be found substantially louder elsewhere in the room at locations that have been stripped of acoustic kinetic. Acoustic energy is an interplay of acoustic pressure and acoustic kinetic. Ocean waves have a similar action–the water wave has height (pressure) and motion (kinetic) energy.

When we audition the one second tone burst here, we first hear clearly the initial sound from the speaker. But it becomes quieted as the buildup of the resonance in the room reaches full strength and cancels the direct sound at the listening position. When the speaker is turned off, suddenly we hear the sound of the reverberant field as it decays. The response of the burst is not the clean, crisp “boop” sound. It is more like a “bow-wow.”

In either case, and depending where one sits, the in phase or out of phase room resonance/speaker coupling effects dramatically rewrites musical dynamics and intonation. This illustrates why the engineer can hear magic and the producer on the talent couch still thinks it needs work–what you hear in the bottom end depends on where you sit.

Farfield playback monitors strongly couple to the room acoustic–that’s why they aren’t used very much except in well-designed downtown studios. It costs a lot to buy the monitors and a lot to fix the room to play them in. The move has been towards nearfield monitors that give strong direct signals and weak room resonance coupling.

The problem here is no bottom end–engineers have to just punt into the mix below 60 Hz. The next move up is to midfield monitors, a compromise, but still no bottom below 45 Hz. Another attempt is to add subwoofers into the system to get the bottom end back up.

Acoustic Coloration

So far, the distortion of amplitude modulation has been shown to result from room resonance. The mic or listening position has a tough time tracking the low frequency (LF) transients in musical passages. The fast tracking of a room is one important aspect of pro room acoustics. There remains another acoustic gremlin that impacts musical accuracy: coloration. By playing a tone burst into the room at a frequency just off a nearby resonant frequency, both the attack and the sustain of the burst develop a “vibrato” a beat frequency related to the difference between the applied tone and the nearby resonant frequency.

For example, if a 45 Hz note is played into a room with a resonance mode at 42 Hz, there would be a beating effect in the attack and sustain of a vibrato at the difference frequency of 3 Hz. A further coloration problem occurs when the speaker is shut off; the sound decays at the nearby room resonance of 42 Hz, and not with the sound of the musical note of 45 Hz. Essentially the note sours in decay. This effect, like the other resonance-controlled playback defects, remain clearly audible by means of an A/B headphone test.

Continue reading the rest of this article.