LIVE SOUND
Peter Janis is President of Radial Engineering Ltd ., a manufacturer of professional audio products used for live touring and in recording studios around the world , including the
PZ-DI direct box for acoustic and orchestral instruments . Special thanks to Brad Madix for his added input on this article .
By Peter Janis
Amplifying Orchestral Instruments at Rock Concert Levels
One of the most challenging tasks ever confronted by an audio engineer is amplifying orchestral instruments on a loud stage . Problems abound , including bleed , resonance , feedback , and frustration ! To solve the problem , one must first understand the environment and then deal with the challenges .
When in a “ classical ” concert hall , orchestral instruments such as the violin , cello , or upright bass are usually miked using an omni-directional condenser microphone . Omnis are particularly effective at producing a natural sound as they do not focus their attention on a particular area of the instrument , but capture a larger area that includes the bow , strings , F-holes , and so on . During classical concerts , feedback problems are usually not a concern as the PA system is only used for “ sound reinforcement ” and sound pressure levels rarely exceed 90dB .
The Issues Problems set in when the rock band hits the stage . Drums , electric guitars , and bass generate significant SPLs that in turn must be compensated for by turning up wedge monitors . The sound generated by the orchestral instruments is lost . To compensate , one can either try close miking the instrument using a directional cardioid microphone that attaches to the instrument or some form of piezo pickup . The cardioid microphone can work reasonably well but is not without issues . A directional mic only captures the sound from a specific area , which may or may not sound right . And because it still is “ a microphone ,” it will inevitably pick up sounds from adjacent instruments , the PA system , and the fold-back monitors . In order to hear themselves on stage , the violins ask for more sound from the wedge monitors and next thing you know , feedback problems set in . Things can get even worse when playing outdoors : feedback due to room acoustics is replaced by wind noise , sound pressures are increased due to lack of room acoustics , and this often pushes engineers to use alternatives such as piezo electric transducers .
Brad Madix , FOH engineer for Rush , experienced this problem a few years back : “ For the 2013 Clockwork Angels Tour , the production design called for setting up four violins and four cellos directly behind Neil Peart ’ s drums . We therefore ruled out miking the strings pretty early on . We did experiment a little with small mics in proximity of the drums and decided we were going to get as much snare in the mic as violin . All of the players were on IEMs , so even if we did mic the strings in order to provide a proper mix to the band ( not to mention the audience ), we would have to use contact mics additionally . A combination of mics and pickups might be a good solution , but in the end we decided to go strictly with pickups .”
The Solution A piezo is a contact pickup that captures the vibration of the instrument . It is typically connected to a preamp of sorts and the signal is processed like a microphone . But anyone who has tried a piezo pickup will tell you that , for the most part , they do not sound all that great . They tend to sound peaky , and with violin , they can sound shrill . The problem is not so much the piezo transducer , but the way it is loaded .
During our research , we discovered that when you apply the typical load of a mixing console – say 10 k-ohms – on a piezo , it causes the bass and high frequencies to roll off , narrowing the response , and generates peaks in the mid-range . As you increase the load , it begins to flatten out . For years , electronic manufacturers have employed a onesize-fits-all 1 m-ohm input impedance as a means to satisfy as many sources as possible . As the impedance rises above 4 m-ohms , the response extends and flattens out further and seems to really sound great at around 10 meg-ohms .
Madix continues : “ We knew that impedance matching was going to be a problem right from the get-go . In the world of DIs , there are really not a lot of options for 10 m-ohm transducers . The preamps that the pickup manufacturers offer have too much tonal control for our purposes . We just wanted to get the signal into the console without mucking it up by mismatching the impedances . I thought we were going to have to have something custom made but it so happened that there was a solution in the works . Our first tests with the PZ-DI were impressive . The pickups required some EQ ( I would say “ the usual ”) but the frequency response was great . Good frequency response , good dynamic range , good signal to noise ... none of the issues that become apparent when impedances are mismatched .”
Properly loading the piezo magically transforms the device into a truly functional transducer . In fact , it works so well , a new problem sets in : you get so much bottom end that you have to keep it under control . By inserting a variable high-pass filter into the signal chain , you can eliminate excessive bass and size the instrument to suit . Cleaning up excessive resonance eliminates low frequency modulation and enables the various instruments to better sit in the mix , making it easier to balance them as a whole .
So whether you are mixing a Celtic band in a club , a 50-piece pop orchestra in a park , or an eight-piece violin section next to the biggest drum kit in the world , piezo pickups can prove to be an excellent choice .
60 • CANADIAN MUSICIAN