An ongoing struggle facing all live sound engineers is the impact of poor room acoustics on their mix. All too often, we find ourselves mixing music in spaces not intended for live music, or spaces designed for live music with minimal concern for acoustics. Sometimes we mix music in theaters that, once upon a time, were acoustically designed for unamplified music, the favorable properties of which go out the window once a PA is brought in-house.
Some of the more common issues with room acoustics include excessive reverb time, irregular bass response, poor distribution of sound and reflection of sound from the house back to the stage. Identifying the problems can help us formulate solutions so that we can create a better mix for a larger portion of the audience.
There Is Reverb Left Over from the 1980s
Long reverb times decrease clarity and intelligibility by interfering with sound being produced by the PA system. Excessive reverb is difficult to fight and may force you to mix louder in an effort to “overcome” the reflections of the room with the PA. Unfortunately, this approach is generally counterproductive, because the louder you mix, the more energy bounces off the walls.
Low-frequency room anomalies can drive you crazy. These can make the kick drum and bass instruments vary in level dependent upon listener location. We’ve all experienced this: you’re standing behind the desk and the mix sounds great. When you have a moment to leave your station, you walk around the room listening. As you move, the kick drum disappears — or maybe it gets louder than it was at the mix position.
This is often due to “standing waves.” A standing wave is set up when the wavelength of a sound is a multiple of a room dimension [Note that this is an oversimplification. A thorough discussion would reveal that standing waves also occur when the room dimension is a multiple of the wavelength, or when more complex rooms modes are excited]. When this happens, the sound wave repeatedly bounces back and forth between two opposite boundaries (e.g., two walls or floor and ceiling).
As an analogy, imagine you’re trying on a new suit and there are mirrors in front of and behind you. If the two mirrors are parallel, your image bounces between them infinitely. This is the visual equivalent of a standing wave: the sound waves bounces back and forth, creating areas of interference called nodes and antinodes. A node is an area of destructive interference: the reflected wave is out of phase with the original and they cancel. An antinode is an area of constructive interference: the reflected wave is in-phase with the original and they are summed. When you move to an area where the kick drum disappears, you are experiencing a node. When you are in an area where the kick drum gets really loud, that’s an antinode.
Now For the Nutz and Boltz
A variety of acoustic treatments can counter the effects of poor room acoustics, generally falling into three categories: absorbers, reflectors or diffusors. Note that we are talking about room treatment, not isolation. An acoustic absorber traps sound that hits it, turning sound energy into heat. You’ll be happy to hear that tons of sound energy equals very little heat energy, so no need to call the fire department. Diffusors reflect sound back into the room at random angles, reducing standing waves. An acoustic reflector does exactly what it’s name implies, though I think you’d be hard-pressed to find a typical performance venue requires an increase in reflected sound, although these are sometimes used in classical music venues, both in the house (for example, to redirect sound away from the ceiling and back toward the audience) and on onstage, in the case of orchestral shells.
Absorbers
Acoustic absorbers come in three basic flavors: porous, diaphragmatic and Helmholtz resonators. Examples of porous absorbers include carpet, heavy curtains, glass fiber insulation and the familiar acoustic foam, which typically are useful for treating high- and upper mid-frequency reflections, such as flutter echoes. A flutter echo is a high-frequency standing wave. When you clap your hands in a locker room (no jokes, please) and hear a “boiinnnggg,” that’s a flutter echo.
In a live music venue, absorbers are our best friends. They help reduce reverb time, improve intelligibility and generally “tighten up” the sound of a room. It’s important to realize that most materials that are effective for absorbing high frequencies are poor at controlling low frequencies — so if a room suffers from “one-note bass,” plastering the walls with acoustic foam is useless.
On the other hand, diaphragmatic absorbers (a.k.a. bass traps) are very useful for reducing low-frequency standing waves. A bass trap is essentially a box with a thin flexible diaphragm on one side and dampening material (mineral fiber, glass fiber insulation, etc.) inside. Low-frequency sound waves pass through the diaphragm into the container, where the energy is absorbed and not reflected back into the room. Bass trapped.
Helmholtz resonators work in a slightly different manner. A Helmholtz resonator is a slotted or vented enclosure, the interior volume of which determines its resonant frequency. When a sound wave hits the slot, air is pushed through, causing the interior to resonate. The sound wave is reflected back out of the box, out-of-phase with the original, causing destructive interference. Helmholtz resonators and bass traps can be tuned to specific problem frequencies and can be configured in a variety of shapes and sizes.
Diffusors
Diffusors are used to disperse reflections, breaking up standing waves (and thus removing echoes) without altering the reverb time of a room. Employing diffusion as opposed to absorption allows your listeners to retain a sense of space while improving intelligibility and clarity. Diffusors come in a huge variety of flavors, including wood and molded plastic. An Internet search will yield plans for a multitude of DIY designs as well as commercial offerings.
In rooms where a balcony overhangs floor seating, reflection of sound from the face of the balcony to the stage can create a slap echo that the front of house engineer may never hear—but the band certainly will. This is a good case for treating the face of the balcony with absorptive materials. Acoustic panels often cure the itch because it’s the mid and high frequencies that tend to bounce back to the stage (the low frequencies simply bend around the balcony construction).
Hear, Here
In an ideal world, every live music venue would boast well-designed acoustics, but even simple acoustic tweaks, such as breaking up flat balcony faces and rear walls with diffusion materials, and the addition of bass traps and absorptive materials to the walls to control reverb and excessive low-frequency energy, can go a long way to improve the sound of a problematic venue. Such measures may not make a venue perfect, but sometimes, any improvement is better than none.
The issue that many of us encounter when arriving at a problem venue is that the show is hours away and the chances for acoustic enhancements happening before show time is highly unlikely. This is a situation we’ve all faced, but what can do you? Here are a few ideas:
• Consider the dispersion pattern of the main speakers and re-aim or tilt them so that the horns are directed at the audience, not the walls or ceiling.
• Reduce — or attempt to reduce — the stage volume level. (Let me know how that works out!) Along that thought, try to get the band to use in-ears, which will reduce spill from the stage.
• If the PA is producing “one-note” bass, try cutting a few dB at 160 Hz and 315 Hz. Cutting a few dB at 630 Hz can also help reduce mud in the mix.
• Room EQ can’t fix everything, but it can help with certain problems. If the room has excessive low-frequency buildup, try using a high-pass filter on the mains at 100 Hz. You may sacrifice bandwidth, but you’ll gain clarity. In rooms with excessive high-frequency splash, use a graphic EQ and cut 4 or 5 dB at 20 kHz, 16 kHz and even a bit at 12.5 kHz.
• Confer with the venue’s house engineer to find out if these issues diminish when the house is full. Listen to the results of each alteration so that when the audience arrives, you can quickly locate areas where you may have applied excessive EQ.
Mix Tips for Dealing with Poor Acoustics
Your mix itself can either help or hinder the overall sound, and this is especially important when working in acoustically-challenged venues. Here are a few suggestions that can make a difference in such situations.
• Minimize the number of open mics on stage
• If the room is small, let the on-stage bass rig do some work for you and reduce
the amount of bass in the PA.
• Avoid over-using effects, especially reverb.
• Use a high-pass filter on all vocal channels. Listen to each vocal and roll up the
high-pass frequency until you barely hear the vocal thin out. Then back the filter
down a hair.
• A small amount of EQ boost (2 to 3 dB) in the upper-mids around 2 kHz, 4 kHz or
5 kHz can improve vocal clarity. I generally stay away boosting from 1.6k and
3.15k Hz, because I find they add harshness to vocals.
• Pull the lower-mids out of the kick drum for increased clarity. Frequency will
depend upon the tuning of the kick but cutting somewhere between 250 and
600 Hz helps makes space for the other instruments.
• You can apply the above concept to bass guitar as well, keeping in mind that some
venues only have so much room in the low- frequency hotel. You may need to
favor either the kick or the bass guitar to carry the low energy of your mix so that
they don’t fight each other. A boost in the upper mids around 2.5 or 3.15 kHz helps
the kick drum “smack” cut through the mix so you can provide the illusion that
it’s loud.
• Use that high-pass filter on the bass channels and kick drum channels to make sure
you’re filtering out low frequencies that you can’t hear
