Audio Amplification of Acoustic Instruments
The present trend towards the revival of acoustic instruments as opposed to so-called electric instruments is directing attention to the problems inherent in the amplification of non-electric instruments whilst retaining the acoustic quality of the sound.
This problem first arose in the “speak-easies” in America in the early days of the Blues and Jazz when musicians found that their acoustic instruments could not be heard over the hubbub in the venues they were playing. At that time the technology available allowed of only one solution. The instruments had to be fitted with steel strings so that the vibrations could be detected using a simple electromagnetic coil. The signal produced by this coil was then amplified and fed to a suitable loud-speaker. The resulting “electric” sound became a characteristic of amplified music. The guitar leant itself to this technique probably better than any other instrument and, eventually, the instrument became “solid-bodied” since the sound-board and cavity of the acoustic guitar played no significant part in the final sound The “acoustic” players disapproved of this sound to such an extent that “pick-ups” were not permitted at their gatherings. One well-known Blue-grass banjo player is reputed to have kept his instrument padlocked in the case, when he started using a pick-up, for fear of being banned!
Technology has now progressed to the point where it is possible to amplify acoustic instruments and retain their characteristic sound. The specific methods employed depend on the particular instrument and the basic principles are outlined below.
General
Amplifying a musical instrument would seem to be simply a matter of placing a “regular” microphone in front of the instrument. There are, however, many problems with this simple technique.
- At relatively modest levels of amplification “feed-back” or “howl-round” occurs which is difficult to control without sophisticated electronic equipment. This is especially troublesome if, as is generally the case, the musicians demand “on-stage monitoring” so that they can hear themselves play. In fact for optimum results the acoustic properties of each venue must be accommodated by equalising the room before attempting to “mic” the instruments. This can now be done using the latest automatic graphic analysers which are available at very reasonable prices. This approach will be the subject of another pamphlet.
- Traditionally groups of musicians, when playing together, have used instruments that are compatible in terms of the acoustic sound quality and volume, e.g. consorts of viols or recorders. The trend, for some time, has been for groups to use instruments that were never intended to be played together, e.g. one band uses saxophone, hurdy-gurdy, drums and electric bass guitar. Clearly it would be difficult to produce a balanced sound if these were to be played acoustically and it presents some problems to the sound engineer when the group is amplified.
Because the in-built acoustic qualities of the instruments do not result in a satisfactory blend, then placing one or more “regular microphones” in front of the group will not necessarily help the sound engineer to resolve this problem. What is required is some method of achieving acoustic separation between the instruments so that a satisfactory blend can be produced. It would appear that there is only one way to do this. A range of special “transducers” is required so that each instrument can have a dedicated transducer. This raises individual problems that are addressed below.
Specific Instruments
Acoustic Instruments can be divided into three categories according to how the sound vibrations are initiated and acoustically amplified.
Stringed Instruments.
The sound originates in the vibrations induced, by bowing, plucking or striking tensioned strings and is then amplified by means of some sort of sound-board. The main body of the sound emanates from the sound-board and the sound-hole but the characteristic sound of a particular instrument can depend, to some extent, on the presence of the higher frequencies that radiate directly from the strings. e.g. fiddle, guitar, piano, harp, dulcimer, kora.
Wind instruments
These are of three distinct kinds.
- The sound is produced by the vibration of a reed of some sort which can be controlled by the mouth. In this case the reed itself makes a significant contribution to the final sound which is due, mainly, to the column of air which is induced to vibrate in resonance with the reed e.g. clarinet, saxophone, oboe.
- The sound is produced by vibrating the lips, as in the trumpet, or from the eddy effect produced by blowing at the edge of an opening in the body of the instrument, as in the flute, recorder or pan-pipes. In both cases the characteristic sound, although not the tonal quality, is largely independent of the vibrating source and the main distinction is between open columns (flutes) and closed columns (pan-pipes)·
- The sound is produced by the vibration of so-called “free-reeds” mounted in banks and set in motion by forcing air past them. This is done by either blowing (mouth-organ, melodica) or pumping (accordion, concertina, bag-pipe) the air. The characteristic sound is due to a combination of the reed and the small chamber in which it is mounted.
Specialised Microphones for the above instruments.
Stringed Instruments.
Until recently all that was available for use with stringed instruments, apart from regular microphones, were the various forms of crystal ceramic contact micro-phones. These all suffered from the disadvantage that they receive signals from only a small area of the sound-board. One manufacturer even claimed that if its “bug” was placed on the “sweet spot” of the instrument then it would reproduce the acoustic sound of the instrument over all frequencies. An investigation of the modes of vibration of the sound-board of any stringed instrument will show that the patterns of vibration differ enormously according to the note being played. Unless, there-fore, the “bug” is of a peripatetic nature it will not have much chance to do this. More recently strip microphones, that receive signals from an extended area of the sound-board, have become available in both electret and piezo form. Either of these, in conjunction with an appropriately placed miniature microphone capsule, will give a very good reproduction of the instrument. More controllable results can be obtained by using a strip microphone alone but with a two band parametric equaliser. With the latter tuned to the respective frequencies of the sound-hole and the sound-board it is possible to reproduce the characteristic tonal balance of the instrument. This facility is particularly important for the guitar.
Wind Instruments.
- In these instruments the sound radiates from the reed itself, the bell end and also from the fingering holes. Ideally, therefore, the best solution would be to have a combination of microphones that will receive a signal from all three sources. This could be done using fixing devices to mount miniature condenser micro-phones at appropriate points.
- Apart from the absence of a reed these are similar to a). and the solution is a dedicated set of miniature microphones and mounts.
- These instruments have either bass and treble, or left and right sides. To control the desired balance it is essential to mount two or more microphones at a suitable distance from the reeds. This is critical so that the physical spread of the reeds is covered but without the microphones being so close that the sound pressure level, which can be very high, overloads them
Miniature condenser microphones are, generally, omni-directional. This introduces an incipient feed-back problem the solution to which requires, in the first instance, correction for the room resonance at any particular venue. This will be the subject of a further pamphlet.
Feedback or Howlround
The overriding problem in amplifying acoustic instruments, especially when they are used in conjunction with so-called electric instruments, is feedback. Feedback is produced by the repeated amplification of resonant peak frequencies arriving at the microphones from the loudspeaker either directly or after reflections from the walls, ceiling or floor.
Causes of feedback.
The onset of feedback is due to the resonant frequencies in the overall system which can be due to a number of reasons
1. The natural resonant frequencies of the room.
2. The natural resonant frequencies of the microphone.
3. The natural resonant frequencies of the Loudspeaker.
4. The peak frequencies of the electronic system.
5. The natural resonant frequencies of the particular musical instrument; e.g., the resonant frequency of a Guitar sound-board or of the cone of a National Steel guitar
Minimising Feedback.
Although the situation looks very complicated there are some steps that can be taken that will produce a significant improvement.
- Use directional loudspeakers and microphones to reduce the amount of interaction. Judicious positioning and the use of a curtain on the back wall of the stage will produce a noticeably higher volume before feedback. The drawback is that good quality loudspeakers that are also directional are very expensive. This also applies to microphones where an omni-directional microphone always gives far better sound quality than a uni-directional microphone of similar price.
- The room resonances can usually be corrected for by using a good quality graphic equaliser. This can be done ‘by ear’ if you have enough experience or, as will be explained later, there automatic equaliser/analysers that will do it for you.
- AUTOMATIC FEEDBACK ELIMINATION
The latest device for automatic feedback elimination combines the action of an automatic analyser/equaliser with a feedback frequency find and suppresser. This device has nine active filters which are easily set to various configurations according to the requirement of the band and the venue. The method of setting up the device means that the room resonances are first corrected leaving sufficient filters to cope with feedback frequencies.
Peaks and Notches.
Whereas a peak adds a small band of frequencies to the overall sound a notch is the exact opposite in that it removes a small band of frequencies. It can be seen therefore that if a peak is causing feedback in the system then it can be countered by introducing into the system a notch of the same size. Fig. 1 shows how this can be achieved and also indicates how this results in a higher gain before feedback. It is important to note that for effective cancellation it is necessary that the midfrequency of the notch must be the same as that of the peak. In addition, the frequency width and height of both must be as near the same as possible. If this is not the case then optimum cancellation will not be achieved.
If the notch is wider than the peak then more musical information is taken out than is necessary and if the notch is narrower than the peak then this will result in two peaks in place of one - Fig. 2 illustrates this point.
Graphic Equaliser
The graphic equaliser is so-called because it has a set of volume controls which are operated by sliders. Each Slider controls the volume over a narrow fixed band of frequencies and can increase the volume, a peak, or reduce the volume, a notch. It is called a ‘graphic’ because the positions of the sliders indicate graphically the adjustments made.
Audio Spectrum
31-62 : 62- l25 : 125-250 : 50·500 : 500-1000 : lK-2K : 2K-4K : 4K-8K: 8K-16K
The commonest graphic equaliser is therefore one with five bands in which each band covers two octaves. This is clearly of little use in dealing with frequency peaks because it removes far too much music information. The ten bank graphic which is found on many mixers and amplifiers has a control for each octave but is still of limited use for controlling feedback because it is highly unlikely that the centre of a band will coincide with the peak frequency that is causing the feedback. To have any effect on feedback then a thirty band graphic equaliser is required where each band covers only one third of an octave. This device is commonly called a one third octave equaliser. Even in this case a peak will not always coincide with the centre point of a band but using two adjacent bands will usually give a measure of control without having too severe an effect on the overall balances.
Reducing Feedback
The technique is one of trial and error. The overall gain of the system is raised until feedback is taking place. If it sounds like a dogs howl then try around 250 - 555 Hz; if it’s more like a high singing tone then try 1000 Hz upwards. Reduce the gain at this particular frequency until the feedback stops. Probably at this point the second highest peak will set off a new feedback oscillation and the process will have to be repeated. It may be necessary to go back to the first peaks because dealing with the subsequent peaks may show that the original adjust
Parametric Equaliser
It can be seen from the above that the graphic; whilst of some use, suffers from the disadvantage that the bands each have a set peak frequency and also a set band width. Ideally it would be preferable to have control of both the centre frequency and the band width. Such a device is possible where the peak frequency of each band can be varied over, say the range of two adjacent bands and the band width varied over a range of two band-widths. This device is called a Para-graphic and is very expensive. It is found, in practice that two bands of parametric equalisation will often be sufficient although three, four and six band barometric equalisers are in common use. A parametric equaliser that can reduce gain but not increase it is commonly known as a variable frequency notch filter.
The technique for eliminating feedback using a parametric equaliser is exactly the same as with a graphic equaliser except that now the exact peak of the offending frequency can be located. To do this set the overall gain at just below feedback threshold. Set the gain of a parametric band at about half maximum and sweep the frequency control. This will usually identify the feedback frequency which can be controlled by adjusting the band gain. Note that it may be necessary to start with the band width narrow, say one tenth on an octave, and gradually increase the width to achieve the best result. This procedure will not be possible with a variable frequency notch filter and it is then necessary to start with the system in continuous oscillation and then sweep the frequency control to find the feedback frequency.
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