I’m not sure I’ve ever seen a band rider that doesn’t state, “NO PASSIVE WEDGES”. Option B? “BIAMPED WEDGES”.

A passive speaker is one that has an internal crossover. With a biamped speaker you perform the crossover filtering before the amp channels.

In theory, a biamped speaker can provide superior sound. With a passive crossover, you’re losing power passing through the passive crossover network. With a biamped speaker, each transducer can be fed straight from the amp.

So biamped is more efficient as a concept but in this day and age of cheap & high quality power amps, I’m not sure efficiency is a concern.

I would guess that a biamped speaker can probably get louder than it’s passive counterpart but not sure that is actually the case.

Passive or Biamped?

Based on my experiences with biamped stage monitors, I would have to assume that very few sound companies know how to biamp a stage monitor correctly. If I had a dollar for every time a set of biamped speakers sounded completely different, I’d be retired by now.

The principles behind building a passive crossover network are well known & any decent speaker designer can do it.
This means that while a speaker with a passive crossover network might be limited in some ways, a batch of fully functional passive stage monitors should all sound the same. If the speakers haven’t been serviced incorrectly, the phase of all the transducers should be correct.

With biamped wedges there are many points along the signal path for things to go wrong. There are (4) conductors between the amps and the speaker. The active crossover points themselves can be wrong. The levels between the amp channels can be wrong which changes the crossover point & the sound of the stage monitor.

When I see a local monitor engineer adjusting the balance between the woofer & the tweeter at the amp, I know something is wrong.

For a stage monitor sitting on the floor, there should be no guess work involved with crossover points & transducer levels & they should all be the same. Any discrepancy should be adjusted somewhere upstream of the crossover & amp.

Prior to beginning to understand how speakers interact with each other & with the acoustic environment around them I thought that in a perfect situation, the perfect speaker would require no EQ.

Knowing that there is no perfect speaker or perfect speaker combination or perfect acoustic situation is vital to being able to accomplish a “best care scenario”.

For the sake of experiment, lets say we own a perfect speaker. A single transducer that can produce 20hz to 20k.

Perfect frequency response.
Perfect impulse response.
Perfect phase across the entire audible range.

Question:

Where?

If you design a speaker to be on the floor, raising it on a speaker stand means that it’s now low end deficient because it was relying on the coupling with the floor. If you design a speaker to be used on a speaker stand, putting it on the floor means it now has excessive low end. If you design a speaker to be listened to in free space, putting it up against a wall will mean that it’s low end changes. A corner? Even more low end. On the floor in a corner? Even more low end.

The solution that comes the furthest so far is a self powered speaker that has presets to compensate for acoustic loading.

I made a quantum step forward in my measurement pursuit today. I have been planning on measuring all monitor mixes before the band arrives for a long time but never made it before I ran out of time before.

For this gig, I use my Mac laptop for Qlab video & audio playback in addition to Spectra Foo Complete. We typically have either (1) center video screen / projector or (2) side video screens / projectors. I don’t know why I haven’t thought of this before but I realized that I could drag FOO onto the secondary video window & see it from the stage.
My original intent today was to roll my rig to the stage on a road box after I tuned the FOH rig. This way I would be able to see my screen as I worked on the band’s monitor mixes.

Being able to see a huge screen on the side wall was incredibly helpful as we could see the frequency response trace as we adjusted the crossover levels between lows & highs. I will need to figure out how to do recalibrate my delay offset remotely with my Iphone since moving the mic without doing so ruins the phase response trace.

Using an Aux on the FOH console, I sent pink noise generated in FOO down the talkback line to the monitor system. I had already extended my measurement mic cable to reach the stage plus some extra slack.

The stage monitors were a custom built 2 way design. 15″ / horn biamped wedges. Note to self. Don’t remove the X from a speaker without verifying it’s fully functional again. Even though I was told I could because it was fixed, I didn’t & it wasn’t…

Here is a photo of the setup for measuring Mix 1.

By measuring each monitor mix prior to the band’s arrival. I was able to learn a lot about how little I know about recognizing crossover points, phase, etc… I was also reminded of how absurd it is to use 31 band graphic eqs to process monitor mixes. Maybe some day that concept will be replaced with modern knowledge. For this gig, I didn’t have a lot of time to spend tuning the monitor mixes so I used the graphic eqs to flatten the peaks as much as possible.

Below are some Spectra Foo Complete transfer function screen captures to illustrate how the (3) different windows available within a transfer function relate to each other.

The top window shows frequency response (green) & coherence (red).
The middle window shows the phase response (green).
The bottom window shows the implied impulse response (green).

Here is an example of a perfect set of captures. This was made by measuring the internal pink noise generator inside the program against itself.

You’ll never see anything like the above screen capture in real life conditions. Instead you will see things like what is shown below.

An RTA shows the frequency response of an electronic or acoustic system in real time. The following screen captures are the various scaling options for Spectra Foo Complete’s “Spectra Graph”, their take on an RTA. As you split up the scale, you get finer & finer resolution. 1/3 scale being the industry standard since it matches the industry standard 1/3 graphic EQ.

1 octave scale
1/2 octave scale
1/3 octave scale
1/6 octave scale
1/12 octave scale
1/24 octave scale

Initially RTA devices was a stand alone units but modern FFT measurement software packages include an RTA function so stand alone units are some what obsolete now. There are many RTA apps for Ios devices. An RTA is useful for recognizing feedback frequencies but is not the right tool for PA tuning. The right tool for tuning a PA is the Transfer Function because you can see impulse, phase & frequency response again a known value.

Friends don’t let friend’s tune PA’s with an RTA.

In 2007 I spec-ed and installed a PA system in a small local church during their renovation. The space is very live with the back & side walls being painted block. Wooden columns break up the walls & the wooden ceiling. The rest of the space is textured sheet rock. The only soft surface is the carpet on the floor. Since the side walls are parallel & the front & back walls are parallel, speakers need to be aimed at the ears of the congregation & no where else to avoid reflections & feedback.

The main PA design originally consisted of (2) custom speakers made by Toby Speaker Corp called “Blasters” and (2) Toby “Sheriff” subs (hidden inside the duct work race ways on the side walls). Due to my own ignorance on 2007, it was decided to add a second set of “Blasters” right next to the original pair & simply jump from one to another. The concept at the time was to cover the side seats closest to the stage with the second set of speakers but without an additional amp channel & additional DSP, that was a poor design choice. At the time of the install, I actually had a Smaart 4 license but I had no idea how to really use it. Instead I used the built in RTA functions of the dbx Driverack 260. Another poor choice. At least I didn’t let the “dbx EQ wizard” function do the system tuning.

After learning some critical concepts from working with system designers & also reading Bob McCarthy’s books, I was pretty sure that the best thing I could do for that system was to disable 1/2 the speakers & after a road trip was preparing to make the call. Instead, I got a call from the church requesting a site visit to address some wireless issues they were having. I headed straight over to catch their Sunday service. Sometimes the best time to do something is right now. I scheduled more time next day to measure & troubleshoot their wireless gear further. After careful listening I am certain the second pair of speakers don’t help anything & cause all sorts of other issues. The PA would have sounded much better if I had used (2) main speakers like originally planned for. Amazing what over complicating a system can do to ruin it’s response.

This following trace was taken on axis of the inside speaker with both of speakers plugged in. In this position the mic is still measuring both speakers but is somewhat off axis of the outside speaker.

This trace was taken by moving the mic directly sideways to the side wall. Comb filtering?

Here are the same traces overlapping. Notice that the high frequency response of the inside speaker is worse than the outside speaker measurement. This is surely due to phase cancellation & comb filtering.

What happens if I unplugged the outside speaker leaving only (1) speaker on with the mic back at the on axis position of the inside speaker at the second row?

The result is kind of amazing actually. I wonder if that phase shift around 2k is due to the cross over?

Moving the measurement mic directly away from the same speaker toward the back of the space, note that the high end starts to roll off due to distance (see red trace)…

At the last row (see green trace), note that the high end has fallen off significantly. Roughly 75′ away from the main speaker pair. This space is a good candidate for a pair of delay speakers to add the missing high end back for the second half of the congregation. Especially considering that there is an isle right after the 7th row of seats. Literally splitting the seating area in 1/2 with about 6 feet of open area between them.

Here is the response of the single HL speaker at the second row position again with corrective EQ performed at the dbx driverack 260 (white trace). I would of addressed the extra bit of low mid energy around 250hz but the dbx Driverack 260 only allows for (4) parametric filters on the outputs & I had already used those to tame the extra high mid energy. I would of remeasured at the same middle & rear position as I was tuning but congregation members have already complained that it’s too loud up front & the cuts that were made at 7k, 4.5k, 2k & 1k will help solve that issue. Those sitting further back will have to suffer a bit until the delay speaker pair can be installed. In the meantime, everyone should benefit from unplugging the extra speakers.

Before & after EQ with the mic at the second row.

A recap. The extra pair of speakers are unplugged now & the system sounds & measures better than with (4) speakers improperly coupled.

The extra pair of speakers can be relocated 1/2 way back in the space to solve the loss of high frequencies due to distance from the mains. This will also allow for the main speakers to be turned down & also aimed down more which will help keep them from blowing on the back wall & side walls since they will be covering less.

Truly a win / win situation.

A few thoughts.

IF you need more coverage than one cabinet can provide, there is a right & wrong way to add a second cabinet.

1. The splay of the two cabinets need to be such that their coverage patterns overlap correctly. In an ideal situation you would use speakers that are designed specifically so that when the sides & fronts are touching, the coverage overlap is correct. One example of this type of speaker is the L’Acoustics L’Arc. The product like offers a spot & flood model. No matter how you mix & match them, their horn coverage pattern is the same as the outside cabinet angles so you get perfect overlap no matter how many of which cabinets you use. Smart!

If you don’t have the right type of cabinets but you need more coverage than a single cabinet provides, you need to do your own calculations. For example, if you have (2) cabinets with 80 degree conical horns (like the Toby Blasters), they should be splayed so that they are roughly 40 degrees apart from each other. Maybe a bit less, maybe a bit more. That is why we measure. To avoid guessing. At the same time you need to consider if having 110+ degrees of high frequency coverage is going to energize the walls and / or ceiling. If so, you might be better off with some coverage issues than with inherent phase issues that come with using a second cabinet. In the case of this church, small front fills would be the appropriate solution for covering the first row of seats.

2. If the cabinets you use aren’t designed to be perfectly coupled, you can still locate them side by side but in addition to setting the splay between the cabinets right, the delay offset between the two cabinets needs to be correctly set. Maybe one of those cabinets can be operated at a lower volume because it’s covering less & that alone will help minimize the bad stuff that can happen between adjacent cabinets.

3. Any time you couple full range cabinets together, you will see an addition in LF response of the system. Especially in the range where the cabinets become omni directional. This dictates that you should have a separate DSP & amp channel for each speaker. This way you can balance level, eq & delay to make the two cabinets working side by side as a complimentary pair.

It’s much easier to get (2) speakers widely spaced apart from each to sound good compared with (4) speakers coupled as adjacent pairs unless you have the right speakers or at least the right knowledge & gear. When you don’t have the tools to model an acoustic space prior to designing a PA, I would suggest that you experiment with only (2) speakers as a starting point & see what can be learned. If I’d spent enough time comparing (2) to (4) speakers in this space, I think I would of come to the same conclusion I reached today…Less is more.

When I work with Texas Ballet Theater we use stage monitors flown over the stage on batons. The TOBY speakers were designed specifically to fit perfectly between adjacent batons & are on yokes so they can be aimed downward & across the stage. The idea was to build a monitor that has the same profile as an ETC stage light. The one downside to the Toby “Blaster” is that is mechanically rolls off at around 100hz but since they’re always used in conjunction with a Main PA, this isn’t really a problem.

The stage monitors are flown instead of set on the floor to keep them out of the way & to make sure that dancers don’t stand in front of them blocking them for other dancers.

During a recent Swan Lake production I took some screenshots of my measurement graphs when I was performing corrective processing.

When you begin the measurement process, your first task is to set the delay correctly between the source & response so you’re phase window is presenting useful information.  As you capture the delay time between the source & the response, you also get a graph of the theoretical impulse response of the system being measured. Your measurement app inserts a delay on which ever signal needs to be delayed to line the two signals back up.

In this case, there is (1) speaker and one mic.

In this case, there are (2) speakers and one mic. The first transient is closer & louder. Both speakers are reproducing the same sound.

In this case, there are (4) speakers and one mic. The first transient is closer & louder. The other (3) speakers are reproducing the same sound but at a lower volume because they are farther away from the mic. If I went to the trouble to find the exact center of all the speakers, these separate transients would possibly become one. It would be fairly easy to achieve this with (2) speakers just by moving the measurement mic toward & away from one or the other speakers & based on a new impulse response measurement, narrowing the position. With (4) speakers, I’m not sure one would ever get there. This is why it’s important to measure single speakers if at all possible. Whether that means unplugging or muting at the DSP.

For this production there were (4) overhead monitor speakers. A pair downstage & a pair upstage. During the first performance, some of the musicians in the pit commented on hearing the stage monitors & what they described as feedback. I took this information as indication that the stage monitors were:

A. blowing into the pit (a mistake of placement on my part)

B. unbalanced in frequency response (a mistake on my part for not prioritizing tuning them before the first performance)

So what did my measurements reveal?

This is a measurement taking with the mic on a stand in between the downstage pair of speakers.  Classic comb filtering & excessive low & high mid content.

This is a measurement made of the downstage overhead pair after I performed corrective EQ with the mic placed over the pit. It shows that the highs are rolling off but the mid range is still quite present. The comb filtering caused by having (2) speakers cross firing is still visible.

A common practice in live audio is to use a pair (or more) of speakers located in the wings to cover the stage for monitors. What is wrong with this? An better question is, “what isn’t wrong with this practice”?

The results of a recent measurement session at the Winspear Opera House for a dance production are a good example of how cross firing speakers interact causing massive comb filtering:

Wiki – Comb Filtering

Comb filtering results from combing multiple signals (audio &/or acoustic) that are out of time. The amount of comb filtering & the frequency at which the filtering starts is determined by the amount of time offset.

WIKI – Phase of Waves

The measurements also reveal how placing speakers near a boundary (the floor, a wall, the ceiling) creates excessive low & low mid frequency content which causes an unnatural balance for the performers & any audience members who are able to hear the stage monitors.

A speaker that measures flat when in free space will have extra low & low mid content when placed against or near a boundary. If a speaker / speakers is placed near multiple boundaries, the low / low mid content will be increased further. The same is true of mic position. think of mics & speakers as cousins that both have the same grand parents. Mics are small & capture sound. Speakers are large & reproduce sound. Both respond to the laws of physics in the same manner.

Measurement rig: Spectra Foo Complete
Mic: Earthworks TC30k
I/O: Metric Halo ULN2
Source: (2) Renkus Heinz two way self powered speakers (15″ woofer)

Ideally you want to measure speakers one at a time but for this purpose, I knew the speakers were going to be cross firing so I purposely measured both speakers at the same time from a central location.

So the first thing you always do is take a time measurement to let your measurement app figure which signal is early & which one is late. In this case I’m measuring the internal signal generator in Spectra Foo Complete again an Earthworks TC30K so obviously the signal from the mic will be last.

Here is what you get when you measurement (2) sources that aren’t time aligned (impossible with cross firing deck speakers). This graph is called an “impulse response”. It is a glimpse into the time domain. If there was only (1) speaker & there were no immediate boundaries to cause secondary reflections, you would see one excitation. Instead with (2) speakers cross firing & not evenly spaced, you see (2). If the mic were placed exactly between them, you would see a single excitation but it would be more pronounced.

Here is what the resultant response looks like. Note all the comb filtering (extreme dips) across the entire audio range.

That is caused by having (2) speakers out of time with each other firing at each other. A sort of worst case scenario. Of special interest is the excessive low & low mid information. Roughly 9db at the peaks between 63hz and 2.5khz. Trust me. You don’t want 9db of extra low and low mid energy on stage. That sort of spectral tilt at the state will easily overpower the mains for those up close to the stage.

This is a capture of how the response looked once we did some parametric eq correction. Note that the comb filter is still present. I knew going into this measurement that I was dealing with a poor acoustic condition that wouldn’t change. Some times you pick your battles & this time I was just hoping to reduce the LF build up on stage.

Overlay the two graphs & you get this:

After these measurements were made we copied the same eq & pasted it onto the upstage pair of speakers. In theory, turning on (4) speakers created more low and low mid content even though each pair of speakers was flattened out a bit but I didn’t get a chance to measure with all (4) speakers on at the same time. I know that they would of interacted with each other so the comb filtering would of been worse.

The moral of the story is that you want to avoid cross firing speakers when at all possible. By raising speakers off the floor you avoid excessive low / low mid build up. Anything to avoid (2) speakers aimed right at each other & outputting the same exact signal. But it’s a common practice to put speakers offstage firing at each other so it’s good to understand what to expect, what can be done to minimize the affects & to know when to take dinner…

I had the good fortune of landing a job with SHOWCO in 1992. The shop was off Regal Row in Dallas. I showed up for work on my first day & a staff member (I don’t recall his name) took me around, introduced me to various department heads & then ushered me to my area. Cable. Cable for miles. My new job was to catch all types of cable as it came in the door from being on tour & test it & then put it back where it belonged. I showed up the next morning & the guy I’d met was gone out on the road. I was obviously “trained” and “qualified” in managements mind so I went back to work sorting & testing cable. A few days later a gentleman named John Blasutta came up to me & asked, “Do you have a passport?”, “yes sir!” I replied. “We’re going to Mexico…” he said.

I think we left about 2 weeks later so I got just enough time in the shop to get really excited. Billy Joel’s rig came back from the road while I was there. Chicago too maybe. A few tours were getting prepped while I was there to go back out. Lots of speaker testing in the area next to cable.

I did a very small tour of Mexico with SHOWCO & The Beach Boys in the Summer of 1994. I learned later that The Beach Boys was a sort of test site for new SHOWCO guys & I must of done OK since I was sent on to my next adventure.

OZZY – No More Tours Tour 1992

This gig would ultimately prove to be the end of my SHOWCO career. There must of been an unspoken rule that new (green) SHOWCO employees had to be broken down in spirit because I endured the equivalent of hazing by the monitor guy GUNK & then his assistant TATERS. I didn’t get hired because I knew everything there was to know about audio & touring. I got hired because I was a warm body, was inside the system and was eager and willing.

I was given the nickname “EB” by GUNK soon after my arrival at the rehearsal complex, a reference to the “Green Acres” character & it caught on. Soon the whole crew was calling me that…

http://en.wikipedia.org/wiki/Tom_Lester

At the time I was making $600 a week & doing 4 or 5 shows on average per week & show days were as long as 18 hours each. You do the math. Come to find out later that SHOWCO charged $1500 a week for my services.

I’ll save the gory details but the tour was due a short pause & Sharon (Ozzy’s wife) became ill so he canceled the next leg of the tour. I went back home & worked a bit more at SHOWCO in the shop but soon that was over. I don’t even recall what happened. I was just glad to be away from GUNK & TATERs. While there were a lot of bad things about that tour, I still learned a lot of important things. How to work without sleep, how to skip meals, how to get yelled over the PA in front of a packed arena, etc… Good stuff that prepared me for my next job:)

What a long & winded way of introducing visitors to the SHOWCO PRISM system.

At the time I toured with SHOWCO, the Prism rig was considered to be the Cadillac of speakers ans was shrouded in secrecy. Only once did I have one of the grills off. To take it off you needed a special oval shaped bit to match the oval shaped heads on the grill screws. I was told, “Go in that room, lock the door and remove the screws with this drill. Once the grill is loose, call me. I called and I left the room and the other SHOWCO tech went in and locked the door.

I’m going to see if I can find some information about the Prism rig now that it’s long since been discontinued and replaced by newer options never to return to the prime time slot it was once awarded.

More soon…