Instrumental Microphone Techniques:
Traditionally, Dynamic or Condenser Microphones offer the most accurate sound reproduction at a reasonable price – however they can be problematic. To provide good sound reproduction they must operate within certain conditions which are not always met during performance.
As discussed in my previous blog the most common types of microphone display the ‘cardioid pick-up’ characteristic which unfortunately allows sound to leak into the ‘pick-up zone’ from the surrounding areas.
Microphones are shaped in a way which suggests that sound is picked up from the front only (0 degrees) but the reality is far from this assumption.
This chart shows the sensitivity (in decibels ‘db’) of a microphone at different angles of incidence, the microphone shown left is aligned with the chart to demonstrate.
The chart and microphone show the horizontal plane but this response is also true of the vertical plane, the resulting three dimensional ‘pick-up zone’ might look like an apple without a dimple at the bottom!
So, at right angles to the front of the microphone – rejection to background noise is only a matter of -7db, which over a ‘normal’ level of say 60-70db – is only 10%.
This means that most traditional microphones are almost as sensitive at the sides as they are at the front. Where musicians in small venues are relatively close together and foldback or ‘on-stage’ monitors are used, ‘feedback’ could become a problem.
Is the term for sound produced by speakers entering the microphone, back into the speakers again and so on… in a perpetual loop until a particular frequency is heard at maximum volume!
Any musician who has experienced this will know how distracting and impossible to handle during a performance it can be. Unfortunately many factors are responsible for feedback such as;
– size and shape of the room
– other microphones on stage
– placement of speakers
So finding a universal solution for every situation is unlikely.
‘Close miking’ an instrument is the only way to ensure that gain applied to a microphone is at the minimum level. When gain is at a minimum level this reduces the amount of background noise entering the microphone. We can visualise the objective of ‘close miking’ in the following way:
Less gain = less microphone sensitivity = Less likelihood of feedback
Unfortunately, when you reduce the gain of a microphone you also reduce the effective area of its pickup. This becomes a problem where instruments are linear and a full tone is not heard from the instrument until the listener is at least one meter or so away.
As the listener approaches the instrument the observed tone appears more specific; higher frequencies heard near the top of the instrument and lower frequencies nearer the bottom – the distribution being complex and uneven.
A way around this is to overlap the ‘pick-up zones’. With instruments such as Soprano saxophone, Bass Clarinet or Bassoon we can use several microphones to capture the complete tonal picture accurately. Now it is possible to apply only a little gain to each microphone and in doing so significantly reject background noise. However, this solution can be complex and costly!
Here is an example of close miking, albeit used creatively!
Pick up microphones:
Sometimes known as ‘piezoelectric’ microphones are different as they capture sound by converting vibrations into an electrical signal via a pressure sensitive material. This makes the ‘pick-up’ microphone less sensitive to extremes of the frequency spectrum and must be placed very close to the sound source to give an accurate sound image.
With woodwind instruments the optimum place for this is just after the reed, before the sound has had a chance to be affected by body resonance. Generally this requires accessing the sound directly by forming a small hole in the crook and mounting the microphone flush with the inside of the bore of the neck to minimise any acoustical effects on the instrument.
Having experimented with a generic saxophone neck on my own instrument I was unable to detect intonation irregularities brought about by the installation of the ‘pick-up’ and the tone was fairly full given that a lot of the sound of an instrument is brought about by body resonance.
Being inside the instrument the microphone is completely isolated from any feedback effects as none of the direct background noise is able to compete with the sound level inside the instrument so for all intents and purposes may be discounted.
So in conclusion, a pick up microphone is able to offer full frequency sound reproduction with no feedback problems, no need to consider microphone placement and at a fraction of the cost of an instrument specific acoustic microphone equivalent.
See my recordings for a demonstration.
Also check out Andrew Ostler using the K1X in live act ‘Darkroom’…..