The Speaker Part 2: FOH
May 25, 2020
There are two things that make live sound different than studio sound or audiophile sound. These are: there is no retake and there are high volume loudspeakers, performers, and microphones all in the same “large” room. This issue with having speakers and microphones in the same room is feedback. Feedback limits how we use speakers.
For many live performances there are two sound stages, one for the audience or front of house (FOH) and one for the performers or the monitors. For now, I will limit the discussion to the FOH sound stage.
The purpose of the FOH speakers is to fill a large area with high quality sound at a respectable volume. Most often, the area in question is a room, but it can also be outside. Whether in a room or outside you always have one surface: the floor. Even outside you often have a second surface: the ceiling or the top of the tent, and even a third wall, the back of the stage. Inside a room you also have side walls, and a back of house wall. Room walls both isolate you from external noise (sounds) and also trap the inside sound inside. Walls give the trapped sound multipathing … echoes and reverberation. The speakers should fill the FOH with sound, and should never bleed sound back to the microphones.
There is something important about how FOH speakers are used. For the audiophile, or when using monitors, the listener is close to the speaker, and therefore in the near field for the lower mid range and bass frequencies. With FOH speakers the listener is in the far field for all but the sub-bass. A similar issue is the placement of the walls in most audiophile applications and many studio applications … the walls themselves are in the near field. Put more simply: a speaker sounds different if you are close to it than id does when you are far away from it. HiFi speakers are designed to sound their best when you are close to them. FOH speakers are designed to sound best when you are far from them.
We want speakers to reproduce sound in high fidelity. This means the speaker's amplitude response from 20 Hz to 20,000 Hz is flat and phase linear. We also want the fidelity of the speaker to be the same over the entire dynamic range of the speaker. These requirements apply to all speakers, be they stage monitors, studio monitors, FOH speakers, or those used by the most discriminating audiophile.
As the area we need to fill with sound is large, we also need the speaker to be able to output significant sound volume. This requirement differentiates FOH speakers from other speakers. The area we need to fill with sound is not omni-directional. Typically the audience space has a footprint that is quadrilateral, most often a rectangle but it can also be a trapezoid. We want to project the sound where the audience is, and not waist sound going to where the audience is not. There are a number of other reasons why we do not want the sound going everywhere other than efficiency and I will get to them. A speaker designed for FOH should be directional. There are a number of ways to accomplish this directionality from the shape of a compression horn, to using spatial arrays, or phased arrays of speakers.
You do not want to project sound up. If you are outdoors (not under a tent) it is a matter of sonic efficiency. If there is a ceiling, be it a tent or the ceiling of a room, the issue is sonic reflections off the ceiling. It is much the same case for the left and right and even the end walls. The floor is full of people, chairs, often a rug, and if outside, dirt and grass. Projecting sound to the floor is bad mostly because it is inefficient. Because the floor is both a good sonic diffuser and often a good sonic absorber, the sound hitting the floor is usually not an issue. The exception is a dance floor when there are relatively few dancers. In this case floor bound sound does cause multipathing issues
Generally live sound is from a limited location, the stage. Ideally, amplified sound should preserve the spatial characteristics of the sound source. The audience should have the illusion that the sound is coming from the performers on stage and not from the speakers.
Attempting to keep sound “focused” and having spatial depth over the entire range of 20 Hz to 20 KHz and over the entire audience space is virtually impossible. The problem is wavelength. When sound hits an object, since it is a wave, it can do three things, be reflected, be absorbed, or simply go around the object. A rule of thumb is: if the object is smaller than ¼ wavelength the sound does not strongly interact with the object. At 20 Hz the wavelength is ~50 ft, at 100Hz, ~10 ft, at 1000Hz ~1 ft, at 10KHz ~1.2 inches and at 20KHz about 0.6 inches.
Your ears are most sensitive in the frequency range of 1 to 5 KHz. You two ears are seperated by about 10 inches and thus their spatial discrimination is best at a few KHz. Most of what you hear (perceive) is in the frequency range of 300 to 8 KHz. The higher frequencies add “color and flavor” to the sound, however many people cannot hear above ~12 KHz. The bass and mid base (say 80 to 300 Hz) adds both “color and power” to the music. The sub-bass, below 80 Hz adds feeling, in the lower sub base, literally feeling and not hearing.
SImply stated, the room modifies the sound coming from the speakers and different wavelengths are modified differently. One way to compensate is to minimize the room effects: limit the amount of sound interacting with walls using speaker placement and speaker directionality. The first rule is simple: keep speakers away from walls, say 18” for mid and high frequencies, and more distance for bass frequencies.
One good way to project sound where you want it is to use a spatial array of speakers. For a spatial array of speakers to work as an array, the length of the array needs to be at least 1 or 2 wavelengths. Thus a spatial array has to be very large to work at low frequencies. Simply put: the lower the frequency, the shorter the range a spatial array will work.
Another method to “focus” sound is to use a horn, but again, for a compression horn to work at low frequencies it has to be big. In this case, not all frequencies are focused the same so as you get near the edge of the sonic beam you will also get frequency distortion.
We have two fairly good ways to project mid and high frequency sound where we want it, but these schemes don’t work so well for bass frequencies.
A phased array can work very nicely at bass and sub-bass frequencies. However a phase array only works well at a specific frequency and you need at least two drivers (sound sources) usually spaced ¼ wavelength apart. Because low frequency phased arrays have a large footprint and because they are a bit of a pain to set up, they are seldom used except when the audience space is very large.
The typical setup for portable live sound is: two sets of speakers flanking and in front of the stage. The “main” speaker is often a combination of a tweeter, horn for the mid range, and a cone speaker for the mid and low bass. There is also a subwoofer for the sub bass. The crossovers between the drivers need to be smooth in terms of dynamics and phase. There is another issue with having four discrete drivers (tweeter, mid range, bass, and subwoofer) … the different frequencies of sound are coming from different physical locations. If you are close to the speakers, such as in a home HiFi room or even stage monitors, this can be a problem. However once you are a few yards (meters) from the speakers this issue is extremely minor and so is not an issue for FOH
As there are two speakers, left and right, the spatial depth of the live sound is more or less preserved. There is a problem with audience members sitting too close to the left or right speaker … they only hear the speaker closest to them, and for them the sound is not coming from the performers on stage. To help solve this problem, we add a center speaker, usually smaller than either of the mains. This pulls the sound back towards center stage for those sitting in the very front of the house. We have also done one more thing for the main speakers … raised them up so the main “beam” of sound is going over the top of the people and chairs. This array of objects (people, chairs, rugs, etc) would absorb too much mid to high frequency sound if the sound was pointed directly at them.
We want speakers to be directional and pointed to the audience and not back to the stage. The problem with sound being pointed at the stage is that is where the mics are. Many (but not all) mics used in live sound are fairly deaf to sound behind them. However, most often, there is a wall behind the stage that can echo sound back to the mic, causing feedback. It is a very good idea, both for acoustics and aesthetics, to have a curtain behind the stage. Curtains both absorb and diffuse sound, however curtains, even very heavy curtains, do not work well for bass frequencies. Thus it is best to keep the FOH sound away from the stage.
Feedback is the enemy of live sound. Simply put, if there is a mic and a sound source in the same room there is potential for feedback. Contrary to what you may think, it does not matter how loud the original sound source is, be that a singer or an acoustic instrument. What matters is the loop gain: how much gain is in your speaker system and how well the speaker / mic is coupled. This coupling is very frequency dependent. The better you can isolate the mic from the speaker, the louder you can play the music. This is a primary reason why for live sound both speakers and, often, mics are directional. Directionality does hurt HiFi. Live sound is a compromise.
Two problems remain. Taming the echoes off the walls and ceilings, and keeping the sound volume even as you move to the rear of the house.
A room can tunnel sound acting like a wave guide. I often do sound for a dance in a good sized very lively room. If you take a dB meter and walk from the very front of the house to the very back of the house the overall sound level only drops 2 or 3 dB. Overall this room is an efficient waveguide, but it is a multi-node waveguide (the sound coherence is lost) and there are some frequency specific peaks and dips in this waveguide. Put more simply, this room does not require much sound power, but by the time the sound gets to the back of the room it is muddy and there are some frequency specific resonances.
As already noted, curtains can help tame echoes. A better approach is to add diffusers, but for portable life sound this is impractical. Keep speakers away from walls and pointing them toed in a bit so echoes off the walls, particularly off the rear wall, are more scattered.
The sound you hear is a combination of the sound from the speakers and how the room affects the sound. Bit there is a big exception: portable sound, be it live sound at an event or even a pool party … speakers come first and somehow fit them to match the room the best you can.
For non-portable sound, speakers can be carefully placed to even out the sound in the entire audience space and to tame unwanted echoes. However there is a new problem. You are in the audience. You see the performers on stage. Your vision is overriding your ears as to where the sound is coming from. Now close your eyes. Suddenly the illusion of the sound coming from the stage is missing … it is coming from the speakers over there or above me. The sonic imaging is missing! For me there is a worse problem. Even with two identical speakers, due to the acoustic shape of the room, different frequencies will sound louder from different speakers. Consider a singer reaching a high note and suddenly her voice shifts from one speaker to another speaker. Having HiFi sound and good sonic imaging in a large room is a trade-off.
The other problem in the FOH is to have the sound loud enough (but not muddy) in the rear of the house while keeping the sound to safe limits in the very front of the house. Unfortunately the “solution” often used is to just crank up the sound level so high that it is damaging the hearing for those in the very front of the house. Sound level falls off as the square of the distance from the source or every time you double the distance, by 6 dB. There is an exception to this law … a spatial array creates a slab of sound that does not expand in the vertical direction and so it only falls off at 3 dB with distance. However this only works when the length of the array is more than say two wavelengths of the sound of interest. Put another way, a good solution for mid and high frequencies, not so good for bass. While arrays hung from towers can have good length, most portable arrays are 3 feet or less in length and I only know of one portable array column that has a length of 6 ft.
There is another solution to keeping the sound level even …. distributed sound ... using more speakers, where some of the speakers are in the house. The problem is you have to match both the volume and the delay of the in-house speakers. Consider a speaker placed 55 ft from the front main speaker. The sound from the main speaker takes approximately 50 mS to arrive at the location and so you need to delay the signal to the remote speaker by 50 mS. You also want to preserve the illusion the sound is coming from the stage and not from this speaker in the middle of the house. If you set this speaker's volume too high it will become the dominant sound. So you also have to adjust the volume so it blends and does not dominate.
Speakers can be passive (needing an external power amplifier) or active (having an internal power amp). There is one more issue to be solved when using passive speakers. A speaker amp combination has something called a damping factor. A speaker system with a low damping factor is very much like a car with bad shocks … it will ring (bounce). You want a damping factor of at least 20. The oversimplified definition of the damping factor is resistance of the speaker (usually 8 or 4 ohm) divided by the output impedance of the amplifier plus that of the power cable to the speaker. For modern solid state amplifiers the output impedance is negligible. Thus the damping factor really is the speaker impedance divided by the total (out and back) cable resistance. For short runs, like in a home stereo or in a studio, 12 or 14 gauge cable is just fine. Audiophiles often give preference to 4 ohm speakers. However, when you have speakers maybe 50 ft from the power amp you will need 10 gauge wire for 4 ohm speakers and 12 gauge wire for 8 ohm speakers.
There is a related problem with active speakers … the AC power cord. To keep the sound dynamics crisp you want to maintain the AC power voltage at the speaker stable … no dips when a loud note demands a lot of power. So again, for long runs, heavy duty power cords … for 120 VAC, a 50 ft cord should be 10 or 12 gauge … if the speaker is over 500 W or so, 10 gauge is recommended.
In summary: FOH speakers are different from HiFi speakers or monitors in that they need to be capable of high sound pressure output, directional, and use heavy power cables. Directionality and high sound pressure favor compression horns and / or arrays. For portable sound it helps if they are relatively light and small. If you have a large truck and young people with strong backs this last requirement can be waived.