To EQ or Not to EQ?

To EQ or not to EQ... It would be a very lengthy process to cover the content of the Workshop and it is not the aim of this report, which is to give an outline of some of the points discussed concerning the EQ’ing of systems and the various philosophies behind them. Hopefully, this will give some food for thought ! First, the panel... This consisted of Pat Brown, John Murray of ProSonic Solutions, Dr. Timothy Leishman of Brigham Young University, Deward Timothy of Poll Sound, and representing the UK (and British humour), Peter Mapp of Peter Mapp Associates. The workshop covered a very broad range of EQ-related subjects, starting out with the definition of equalisation (as opposed to spectral shaping) and it’s history before going into the basic motivations for EQ and it’s objectives. From here the panel moved onto types of EQ, the sound practitioner’s viewpoint, prerequisites, filter types, what can and can’t be equalised, the various types of analysers and their strengths and weaknesses, an excursion into some fairly wild (and interesting) physics demonstrations of the nature of sound, array EQ and other fields virtually too numerous to mention ! There was a lot to go through and feedback from the floor was greatly encouraged. As stated, this will not be a potted version of the workshop but I will select some basic points from each of the presenters that are worthy of reflection. Pat Brown’s opening statement was that EQ is NOT the panacea for all audio ills - however much we might wish it ! - before stating what he considers the four main classifications of the types of EQ. Keeping these clearly maintained in the work process helps to clear up the muddle that often gets in the way of ’tuning’ systems and provides a logical working methodology. Corrective EQ is for the removal of a specific anomaly, such as a frequency peak in a transducer or a boundary loading effect. Technical EQ is for pass band restriction and direct field shaping, such as crossover networks and CD horn curves Spectral surgery is for the removal of troublesome frequencies that cause feedback or room modes by the use of notch filters. Spectral shaping for a preferred spectral balance or frequency response such as a ’smiley face’, house curves, etc. Why EQ ? As already mentioned, this report will not be a step-by-step account but a roundup of some of the many points raised. There are many definitions of EQ (as a quick trawl of the Web will confirm) but the general consensus is that EQ is the process by which a sound system will reproduce a signal as close as possible to it’s original form. For voice systems, the questions of clarity and intelligibility also come into play (certain airports, please note...) Filter types The variety of sound tools available to the audio engineer today is very varied and these include the tried and tested graphic equaliser (often with variable filter characteristics), the parametric equaliser, high and low pass filters, notch filters for ’narrow-band surgery’ and others. There are also types that we often do not think about being associated with EQ but are nonetheless, such as crossover networks or CD horn filters. The EQ process An interesting point that emerged is that there seems to be quite a difference in approach between the touring/temporary install community and those involved with fixed installations, where time is often a factor for the former and the ’here today, gone tomorrow’ aspect, whereas for the fixed installation it is here today and tomorrow and the day after... You either like processor-based loudspeaker systems or you don’t but one of the basic ideas is that the particular loudspeaker is equalised for optimum performance, so that system EQ, once it is installed, is to compensate for the venue or provide a desired result - or ’house curve’ - rather than trying to cure anomalies within the components of the system itself. The approach of equalising a system in the workshop before it goes on site was strongly recommended in order to provide a known basis from which to make the final adjustments. Large systems should, where possible, be broken down into zones or components, with each zone having it’s own equaliser and being set up individually as opposed to running the whole system. Typical zones within an array (of any type) would be for near field, midfield and far field coverage and thus be sub-systems in their own right. The idea of trying to equalise a complete array with just one equaliser was deemed as virtually a waste of time and a non-starter due to all of the interaction between the individual components. To illustrate the point, a simple experiment with just two small loudspeakers was carried out and the interaction between them as measured was staggering - there were large differences when one loudspeaker was just passive, i.e. not connected, and active. This just served to underline the problems faced by a sound engineer when using multiples of loudspeakers and these are not ones that can be solved by simply using EQ. (In fact, the more I go to these kind of classes, the more surprised I am that anything works !) Peter Mapp emphasised the importance of making sure that all is in order with a system before starting to EQ it. This includes checking that the installation has been properly installed and is free of hum and buzz, the components have been aimed correctly, there are no mechanical noises such as rattles and that the various loudspeaker assemblies have been tested and equalised for even response in the workshop (as stated above). This all seems pretty straightforward and common sense but experience shows that this logical approach is implemented far less than one might think. Dr. Tim Leishman gave an amusing selection of practical experiments to show the nature of sound - including a modulated array of gas flames ! - and gave some insights on current research on measurement tools for acoustics that is in development. The subject of measurement is a large one and the various methods currently in use were discussed, from RTA’s through to FFT-based tools and TEF. As with all things Syn-Aud-Con, ’it depends’ on what you want to measure and how you want to measure it, with all methods having their own particular strengths and weaknesses. In many instances, having a handheld RTA where you can walk the room will often be enough to measure what the system is doing and how it is behaving in an acoustic environment. In another situation, you may need to know what the transfer function is and measure the signal leaving the system without entering the room into the equation (e.g. Smaart, EASE or similar). Another point that was underlined and often forgotten (or not realised ?), was the question ’are you EQ’ing for one spot ?’ One often sees a measurement microphone placed in a single spot in a room and all measurement/EQ decisions are based on that one set of results. The truth of the matter is that the system will sound different from one place to another (whatever the marketing blurb may say) and that EQ decisions need to meet an overall average of measurements and not be limited to one place. John Murray took at look at EQ from an ’artistic’ point of view and discussed the question of house curves or tailored responses for a particular situation, as well as the experiences of Ronnie Rocker, the roadie, in EQ’ing systems (apparently dancing displays are good for impressing the girls...). An impromptu discussion with the floor confirmed that most of the class agreed that EQ’ing a system to a particular voice or instrument was a very bad idea and that the system should be as neutral as possible and audio ’surgery’ - or response shaping - should be carried out using the channel EQ for the signals concerned. (I had a case of this recently where I set up a system for a neutral response as far as was possible and the sound ’engineer’ with the band immediately changed the system EQ for a selective bass response which resulted in the whole band being swamped by the mud of the bass drum and bass guitar - and he didn’t even realise it ! You can’t win...) John Murray also gave some demonstrations of tuning for flat phase response (which is now possible with DSP tools) and many people remarked the increased clarity and neutrality in the vocal response.