How sound travels and interacts in a room.
Have you ever wondered why you can’t understand what a singer is saying even though they’re loud as hell in the PA system? Why your guitar player was asked to turn down below his “rehearsal” volume? Why most PA systems are set up with two speaker stacks and a row of subwoofers in front of the stage? Look no further. This article explains the role of a PA system, and how to minimize the acoustic challenges that your live sound technician will face during your set.
Acoustics is the branch of physics that studies the behaviour of sound. This article will discuss what ‘sound’ is, how it is produced, and how it travels and interacts in a confined space (like a room). It will then introduce the concept of ‘sound reinforcement’, followed by some practical solutions to some of the common problems that bands and sound technicians face. The next article in this series will discuss sound reinforcement in more depth, focusing on the practical limitations of microphones, speakers and the human ear.
OVERVIEW OF SOUND:
Sound travels as pressure waves in the air around us. Although the physics of sound is a rather expansive topic, there are some concepts that are important to address in this series.
Air particles around us compress and rebound to transmit pressure waves of various frequencies. Different frequencies can be transmitted through air at the same time, which creates complex sounds. Some of these combinations of frequencies are perceived as part of the same sound, while others are distinct from each other. The sound of a distorted guitar, for example, is made up of many different frequencies, but it is perceived as a single sound. On the other hand, two different cymbals that are played at the same time can be distinguished from each other, even though they might sound quite similar.
The various frequencies that we perceive together as part of the same sound are referred to as “overtones”. These frequencies are the combination of resonances that are set into motion when an instrument is played, which are then transmitted into the air as sound waves. This applies both to acoustic instruments like cymbals, which interact directly with the air, as well as amplified instruments, which use a speaker as the interface.
A “pitched” instrument has a set of overtones that roughly conform to the harmonic series: these overtones have a simple mathematical relationship to the fundamental (or lowest) pitch produced by the sounding mechanism. This fundamental pitch is what we identify as the musical note being played, and the overtones are responsible for what we would describe as “tone”. Every guitar, for example, produces a similar harmonic series, however, different guitars will produce certain overtones more strongly than others. A really well made instrument will be designed to produce a combination of overtones that sounds subjectively good, and is consistent over the entire range of the instrument. Unpitched instruments, on the other hand, produce a series of overtones that have more complex mathematical relationships to the fundamental tone, and as a result, we don’t hear a strongly defined fundamental pitch, as we would with a pitched instrument.
Bear with me for a second here, this is going to get a bit mathy, but don’t worry – I made pictures… The harmonic series includes all of the frequencies that are integer multiples of the fundamental frequency. This sounds complicated, but it’s not: if the fundamental frequency is 100Hz (cycles per second), the first overtone in the harmonic series of that frequency is 2 times the fundamental frequency (200Hz), and the second overtone would be 3 times the fundamental frequency (300Hz), etc. No more math required. I promise.
At this point, it is useful to bring in a diagram of sound waves to get a little deeper into this discussion. A single pure tone containing only one frequency appears as a sine wave, that is, just a smooth wave with a regular rate of repetition. One full cycle, that is, one segment of the fundamental wave that travels both up and down one time, is the basic unit of any pitched sound (fig.1). Because all of the overtones in the harmonic series are multiples of the fundamental frequency, they all fit evenly into the same rate of repetition (ie. 200Hz fits twice into one cycle at 100Hz, 300Hz fits thrice, etc.) (fig.2). As we add overtones from the harmonic series to this wave, the wave’s shape becomes more complex, but the combined overtones simply change the shape of this basic repeating segment of the wave (fig.3). For pitched instruments, this repeating segment always corresponds to the fundamental frequency of the note.
Distortion can also change the shape of this basic unit of sound. If a signal is louder than an amplifier is able to reproduce it “accurately”, the wave is flattened out when it reaches the limit of the amplifier’s output capacity. Because this effect changes the shape of the basic wave segment, we can deduce that this adds overtones from the harmonic series of the fundamental frequency, which do not affect the perceived pitch (fundamental frequency) of the sound, but they can substantially change the tone. Depending on the type of amplifier (whether it is tube or solid state, in particular) this phenomenon can have widely varying characteristics. The following diagram shows how simple harmonic distortion occurs.
Interestingly, overtones are also an important attribute of the human voice. The different vowel sounds are produced by changing the shape of the mouth and nasal cavity to acoustically filter the overtones produced by vibrating vocal cords. Thus, each vowel sound is a distinctly different ‘tone’ of the voice. Consonants, on the other hand, are produced by a variety of mechanisms: they can be a percussive way of starting or stopping a sound (duh, ta, at), a way of dynamically changing how the overtone series is filtered in these resonant cavities (lo, wa), or just an unpitched sound (ssssslither like a sssssnake on a hhhhhot day). Because the vowel sounds make up the majority of speech, this allows sung words to be intelligible at different pitches. Because the voice relies on many different ‘tones’ to be understood, it is very complicated to amplify well. It requires more system resources than any other instrument, because it requires such a wide range of frequencies to be amplified accurately.
Resonances have an enormous effect on sounds travelling in an enclosed space like a room or a speaker enclosure. Because the speed of sound in air is constant, every frequency has a different length of wave. This is important when considering how sounds travel in air, and how they interact in a closed room. Low frequency waves are quite long, and high frequency waves are much shorter. The bass frequencies (think of the resonance of a kick drum, or the lowest note on a 5-string bass) are actually long enough that there is only enough space to fit one length of the wave in an entire room. Frequencies whose wavelength matches the length of the room create ‘standing waves’. These frequencies tend to resonate, meaning that they maintain their energy and continue to vibrate instead of dissipating normally like other non-resonant frequencies. Every fundamental standing wave that exists in a room also gives rise to a harmonic series of resonant frequencies. As a result, every enclosed, or partially enclosed space (room, speaker cabinet, stage enclosure, etc.) has it’s own resonant frequencies which must be compensated for. In many cases these can be beneficial:
- The resonances within a guitar speaker cabinet are partially responsible for the characteristic chugging sound that defines many ‘heavy’ rhythm guitar parts.
- I often emphasize the room’s resonant frequencies in the kick drum signal, to give it a weightier sound without requiring any extra power from the amplifiers. This is useful in creating an appropriate kick sound for reggae, while leaving plenty of power for big phat bass-lines, in particular.
These room resonances can also make the low end of a mix very muddy if they are not treated carefully. They tend to fill up the room, regardless of the source, and can be very difficult to keep under control.
Because low frequencies have such long waves, they can also behave strangely if the same signal is being produced by several different sources. When two identical signals cross paths while travelling through air, they can either amplify or cancel each other out, depending on the distance of the listening point from each of the sound sources (see diagram below). This phenomenon is called phase cancellation. For this reason, PA systems are often designed to produce bass frequencies from a single source. This is why large concert sound systems often use a row of subwoofers that are placed together in front of the stage. In some (usually smaller) sound systems, this ‘ideal’ setup is not possible, so the technician must compensate for this effect if it presents any problems.
If you recall from the previous article (Live Sound Explained: 3. The PA System), explaining the components of a PA system, I mentioned that DI boxes often include a phase inversion switch. This feature is commonly used to correct for the phase cancellation which can occur between the sound generated by a bass guitar amplifier and the FOH subwoofers. If a sound engineer suspects that this is a problem while sound checking the bass, he/she might ask the bass player to locate and flip this switch on the DI box. Once flipped, the signal that is transmitted to the PA system will be inverted; where the old signal went up, the new one goes down; where the bass amp was previously cancelling the PA output, it will now be contributing to a louder signal.
On the other end of the spectrum, high frequencies tend to be very directional. Instead of filling the room and resonating, they tend to radiate in a straight path from their source, and either reflect off a wall or absorb into a body (or furniture). When they are reflected repeatedly, bouncing around within a room, they can create an ‘echoing’ effect. When this occurs, it creates acoustic reverberance, which can be just as troublesome as a standing-wave bass resonance. Synthetic reverberance, generated by an effects processor (“reverb”), can be a useful effect to add body to a particular signal in a mix, however, in some acoustic settings, reverberance can completely bury the mix. On the other hand, high frequency sounds’ tendency to be absorbed by bodies can also create problems, especially if a stage or PA speakers are not elevated enough to project the stage sound evenly over the audience.
Because of this highly directional behaviour, the PA system is often used to selectively amplify frequencies in directions that they do not disperse to on their own. For example, a guitar amplifier may not produce a full sound throughout the entire room, especially if it’s not facing straight down the middle of the audience. In this case, the sound tech would likely use an equalizer to emphasize the high frequencies in the guitar signal, and pan in such a way as to evenly distribute that part of the guitar’s sound (the lower frequencies will usually disperse pretty evenly on their own).
Feedback occurs when a signal that is amplified loud enough that it is able to return to the input device where it originated to be reamplified. This creates a loop that makes the signal louder and louder. The familiar feedback sound tends to occur at frequencies that are naturally resonant and/or reverberant in the room. Feedback is best known for the familiar howls and screeches that most people associate with the word, however, it can also behave in more subtle ways. PA systems use many different amplified signals with different EQ settings, routed in separate mixes to the FOH and monitor speakers, These sounds combine with those being produced by instrument amplifiers on stage, and will then interact with a variety of input devices, located in different locations, subject to unique acoustic resonances. Needless to say, the root cause of a feedback issue can be very complex, and even feedback that is not obvious to the average listener can have an enormous effect of the fidelity of the FOH mix. If your sound engineer seems perplexed and keeps muttering something about a feedback problem that you can’t hear, it’s very likely that you have lucked into an experienced and competent sound tech. ;-)
Sound reinforcement is the guiding principle for a live sound technician. Constrained by the concepts outlined above, operating a sound system in a confined space often requires a delicate balancing act. The factors that limit the volume and fidelity of the mix are often complex, interconnected webs of acoustic interactions. A good live sound technician will focus on exploiting the desirable qualities of the PA system and room to reinforce a band’s stage sound both efficiently and tastefully.
The most common problems that affect a band’s live sound occur because their stage sound is difficult to reinforce. These issues are pretty easy to identify and fix, and they often cause the most significant problems with a mix. Bands that tweak their sound to be easily reinforced will sound great on any stage, whether they’re blessed with a competent sound technician or not.
- A good balance between the instruments (and their amplifiers) on stage is very important. If your sound is pretty close to a good mix right off the stage, the technician can focus the PA System’s power on making everything sound larger than life. If things are out of whack, the monitors and front of house speakers will have to compensate, leaving less power for the technician to use at will.
- To solve this issue before sound check, have each musician play alone with the drummer and have a buddy stand in front of the stage to gauge the volume of their amp. Every instrument’s sound should match the drummer’s normal playing volume.
- The location of each amp and direction that it is pointed affect how they will interact with the rest of the room. Ideally, you should make amplifiers spread evenly into the room so that it won’t have to be panned excessively in the front of house system.
- If there is only one guitar/keyboard amp, put it as far back on the stage as you can, facing the middle of the audience. Avoid pointing it directly at the drum kit, if possible. Any amplifiers that bleed excessively into the drum mics will interfere with noise gates on the drums and make the amplifiers sound washy. If the amp is not centred on the audience, the tech will have to pan the amplified signal to compensate, and may have to pan other signals in the mix to compensate for that. Everything is interconnected in a mix, and PA systems run more efficiently if you can balance the demands on all of the speakers evenly.
- If there are two or more guitar/keyboard amps that are running at a similar volumes, again, put them as far back as possible. In this case, however, you can angle them to cross a little bit if you want each one to sound more distinct. If you want, you can turn them 90° to the audience, facing inward toward the drums (a ‘side washed’ configuration). If you choose to do this, set the amps at a slightly lower volume to reduce the bleed into the drum mics. Side washing amps is a useful technique if you want hard-panned guitars in the front of house mix. The louder stage sound of the guitars will also leave more room in the monitors for vocals and acoustic instruments.
- Bass amps can usually go anywhere on stage, but will sound increasingly muddy as they approach walls and corners. If you use an electric bass with distortion or a twangy string sound (punk rockers…), or if you play an upright bass, try to point it to the centre of the audience. The lows aren’t directional, but the high frequency string sounds and distorted harmonics are. Choose a location that sounds right to you and your band mates before sound check.
- If your amplifiers are tall, tilted back, or elevated (especially “full stacks”:  4×12″ cabinets), try to point them away from vocal mics. The guitar sound that is amplified through the vocal mic will sound washy through FOH speakers and can sometimes overpower the voice. If the vocal microphone is picking up more guitar than voice, turning up that channel just turns up the guitar. This is one of the most common common reasons that “the singer is too quiet.” Beware!
- Don’t block the monitors with amplifier cabinets. Remember – most of the frequencies that make your singer’s voice intelligible are highly directional. It may still seem pretty loud, but you will have trouble understanding anything. If you absolutely need to block the monitors, just don’t complain about the monitor mix. Please.
- Every room is different. Tune and arrange your equipment to sound good in the room that you’re performing in.
- Check the tone of drums and amplifiers when you set up. Listen for the muddy resonances that the room brings out in the amplifier’s tone and have a buddy check your instrument’s tone for harshness directly in front of the stage. Even the best technician in the world can’t fix something that’s too loud to begin with. If in doubt, either turn down or ask the tech for advice. In my experience, I found that a lot of bands were surprised at how much they could turn up, and many others that were unaware that they really needed to turn down.
- Move floor monitors to where you need them most. Coordinate your monitor needs with the rest of the band before the show. If monitors are limited, make sure that you accommodate the musicians who will need them most within the group. Often, the most important signals in the monitor mix rely on the higher and very directional frequencies, so try to point them directly at the performer that they are intended for.
- Singers and some acoustic instruments don’t have an amplifier. They should always have a dedicated floor monitor for their own sound.
- Drummers are almost always out of the range of amps. They need their own floor monitor unless there are side fills. If neither is available, side wash the guitar amps, and let the tech worry about front of house. Ideally, try to isolate the drum mics as much as possible from guitar/bass/keyboard amplifiers. If you do, the tech will have the ability to adjust the drummer’s monitor mix if any unwanted signals are bleeding into the drum mics and causing any serious problems.
- If you do move monitors, always put them back where you found them when your set is over. It only takes a minute, and it makes sound technicians really happy.