One of my best-received articles for FRONT of HOUSE detailed how to set up a cardioid subwoofer array (see FOH, Dec. 2011, page 28). In keeping with the “nuts and bolts” theme of that previous article, this month I’d like to present a straightforward set of guidelines to help the system tech or FOH engineer successfully tackle the challenge of providing a neutral sonic canvas for live sound events.
Since one can obtain a graduate degree in architectural acoustics, this article should in no way be considered a complete take on the topic. The goal instead is to produce an article that could be laminated for the workbox, or handed out to the crew for discussion in the wintery off-season. Everything here is based on solid physical principles and backed up by practical system tech experience. Interested readers can dig into the references at the end of the article to further explore the broader topics in room acoustics.
We’ll cover a few acoustics principles, and then move into how those principles influence the connection between sound system and room. Then we’ll provide some guidelines for coverage. Next we’ll talk about the processing tools we have to address the sound in space, and provide some practical guidelines for getting through “combat audio” in a smooth and speedy way.
Quick Acoustics Principles
• Acoustics are influenced by room geometry, dimensions and materials.
While this may appear self-evident, it remains important to internalize the root cause of room “sound.” Audio waves move in space and time (i.e., propagate) from the sound system to the various surfaces in the room, where those surfaces perform some mixture of reflecting, diffracting, and absorbing sound. The interaction of these effects determine the overall character of the room and sound system.
• Acoustics are influenced by the physical properties of air.
Sound requires a medium to propagate in, a way to get from the sound system to the architecture. For us that is a gas with fluid-like properties known as air, and composed mostly of nitrogen and oxygen. Temperature and humidity of air can have profound effects on how sound travels to the audience and about the room.
• Sound covers a huge range of wavelengths.
Wavelength is a measure of how far a periodic signal, like sound, travels in space before the wave repeats. In the range of human hearing, the shortest wavelengths are on the order of one inch, and the longest 60 feet! As a rough approximation, a 1 kHz tone has a wavelength of approximately one foot; a 20 Hz wave is more than 50 feet long. This huge range of wavelengths is the source of most issues in professional audio, because it’s hard to provide the same degree of control over both very short and very long wavelengths.
• Sound system-to-room interaction is frequency-dependent.
Most professional sound systems — even small “speakers on sticks” — provide good directional control at short wavelengths (i.e., high frequencies). A useful rule of thumb is that directional control is achieved for frequencies above 3 kiloHertz (3,000 Hz or 3 kHz). Above 3 kHz, the sound system’s physical aiming and nominal coverage pattern is typically representative of where the sound energy will end up.
• Room surfaces-to-sound interaction is frequency-dependent.
It would be convenient if room surfaces all absorbed sound in a consistent manner, but this isn’t the case. Whether brick or drywall, painted or unpainted, wood floors or carpet, the nature of each of these surfaces will change how the sound arriving from the speakers is returned back to the room.
• Room modes rarely behave like simple textbook examples.
Physics dictates that pressing down on different guitar frets produces various tones, and likewise, the geometry of any physical room setup will define modes that occur in any room. Room modes are a deep topic, and we’ve generally done disservice to them by portraying simple examples that don’t match experience in the field. One practical difference between textbooks and physical rooms is that modes are rarely as deep in real rooms. This is because the boundaries (i.e., walls) of real rooms are not as rigid as those in a textbook model.
• Human hearing picks up both direct and reflected sounds.
From birth, our ears are forced to make sense of a world full of both direct and reflected sounds. Our brains fuse a mixture of the direct sound from the speakers and the reflected sounds from the room. This core human behavior is why we need to consider effects of the room. Generally, our ears consider more direct sound at higher frequencies, and more reflected sound in the lower registers.
Basic Considerations
• The datasheet response curve is not what the audience hears!
Most manufacturers place a frequency response curve on their datasheet as a measure of the loudspeaker’s performance. In reality, that curve provides precious little useful information for the majority of the audience. Datasheet curves are typically taken directly on the main axis of the loudspeaker, and the frequency response they show represents the speaker’s response in the absence of any room influences. Back in the real world, your audience is standing inside a box of questionable acoustic pedigree, and only a small fraction of them are camped out near the speaker’s main response axis. Even outdoors, most of the audience are off-axis of the speaker system, and the air temperature, humidity and currents are substantially influencing the sound.
• Speakers interact most strongly with the room in a certain range of frequencies.
As loudspeakers can’t shape the directional response of all frequencies with the same degree of control, lower frequencies (i.e., longer wavelengths) will interact more strongly with the room. This is the first half of the speaker and room interaction. The second half of this equation is the absorbing behavior of the room features and surfaces. At high frequencies, most surfaces are strongly absorbing, and at very low frequencies, most are weakly absorbing. It is the absorbing character in the midrange (e.g., 150 Hz to 2 kHz) that most colors the sound in the room. Sometimes the most dominant effect on the room absorption is the audience!
• Multiple speakers used together radiate sound differently than individual speakers!
When the datasheet shows the coverage pattern of a single speaker, and one is using multiple speakers for coverage and/or output, the aggregate output of multiple speakers is not simply a wider version of the single speaker. At some frequencies, the coverage will be wider, but at other frequencies, it will be narrower.
Rules of Coverage
• Aim the speakers at the audience!
The single best way to improve problems with room acoustics is to insure as much of the sound hits the audience, and is therefore absorbed by them, as possible.
• Use geometry to improve even coverage.
For typical point-source loudspeaker setups, a straightforward way to insure the audience has even coverage is to minimize the differences in distance between the loudspeaker and each audience member. Usually it’s not practical to make these values the same for all audience members, but smaller differences usually result in more even coverage. In most spaces, this dictates a high placement of the loudspeaker, so that trigonometry allows similar distances to the first row and the last row.
• Use the minimum amount of loudspeakers possible.
Every additional loudspeaker is another source to dump extra acoustic energy into the room, and those additional loudspeakers can interact with each other and the room in complicated ways. Since even very large loudspeakers have limited directional control below 300 Hz, using too many acoustic sources is a recipe for murky, muddy low-mid character.
Then Processing
For the FOH mixer especially, the coverage of the loudspeaker system is often outside of one’s control. This leaves the processing tools either already at FOH, or carried to the gig by the mixer. You can EQ the system, but unfortunately, there is no such thing as “equalizing the room,” although tonal shaping of the loudspeaker’s direct coverage can be surprisingly effective in influencing the overall perceived balance of the combined room + loudspeaker response. Here are some general principles on the application and limitations of equalization.
• EQ cannot solve spatially-dependent issues.
If the loudspeaker either over- or under-covers the audience, equalization cannot mitigate how the loudspeakers are aimed.
• EQ cannot solve a speaker’s changing directional coverage.
Loudspeakers provide good directional control at high frequencies and poor directional control at low frequencies. Equalization controls the total amount of energy at a particular frequency, but cannot modify the loudspeaker’s coverage pattern.
• EQ can help balance a room’s combined direct/reflected sound.
When the combined output of the room and loudspeaker produces too much energy in a specific range of frequencies, equalization can be effectively used to bring the overall sound level of that range back into balance.
• EQ at high frequencies can often ignore the room.
At high frequencies (e.g., above 3 kHz) one can shape the equalization almost without considering the room. This is a combination of three effects. First, most surfaces are absorptive at higher frequencies. Second, the directional coverage of the loudspeakers is well controlled. Third, human hearing primarily considers the direct response, ignoring most reflections, at higher frequencies.
• Midrange EQ decisions should account for both the room and the speakers.
Because so much of the sound from the loudspeaker spills around the sides and back of the speaker, the aggregate response of speaker and room must be considered. The aggregate, pleasing response curve for the majority of the audience is not likely to be representative of a flat line for the loudspeaker’s axial response with no room present.
• Measurement systems used to make EQ decisions need control over the amount of reflected sound included in the measurement.
Practically speaking, this means that the classic real time analyzer (RTA) is not very useful for making calculated equalization decisions. However, any of the transfer function-based measurement platforms (e.g. SMAART, SysTune, SIM 3, ARTA, etc.) are useful for these decisions. This author strongly encourages training to learn how to take valid measurements for equalization.
Conclusion
At the end of the day, equalization is the most common approach to tackle problems that would generally be best solved in another manner. Despite this reality, equalization can be a powerful tool to balance the overall response of the sound system for the audience. Equalization through the midrange frequencies where the room and loudspeaker are interacting together can improve coverage for much of the audience, and equalization at the highest frequencies will influence the overall tonal character of the speaker’s direct sound.
As the direct sound is dominant at high frequencies, the equalization at high frequencies goes a long way towards one’s impression of the loudspeaker system. This effect can be further magnified as the humidity changes. Wet air absorbs high frequencies much less than dry, so a room that sounded fine empty at sound check can grow very bright and unpleasant when full of hot, sweaty audience members.
But in any case, a well-placed, well-designed system still remains the most important step in dealing with any acoustic space. Once those fundamentals are in place, a knowledge of some basic acoustical principles can go a long way towards creating a listening environment that provides smooth, even coverage for the entire audience.
Some Suggested Reading
The Master Handbook of Acoustics by Everest and Pohlmann is an accessible introduction to room acoustics.
Acoustics by Leo Beranek is a common reference textbook on architectural acoustics, though it is now somewhat dated.
Sound Reproduction: Loudspeakers and Rooms by Floyd Toole is a recent, comprehensive synthesis of how to produce sound, observe how it interacts in space and consider our ears’ final interpretation thereof. It is somewhat targeted at recording studios and theaters. Find it at plsnbookshelf.com.
Phil Graham is the senior engineering consultant of PASSBAND, llc (passbandllc.com). Email him at: pgraham@fohonline.com.
