Microphones

Microphones

In a PA-system or studio all audio equipment - from the microphone to the speakers - build a signal chain. It´s like football: the team is only as good as its weakest player! Therefore the components of a transmitter should be complementary to each other, especially the microphone which is usually the weakest link in the chain.

If you look at the transfer behaviour of each piece of equipment, you´ll notice slight differences. Actually it´s stricking that e.g. amplifiers, mixers or effects have a very good frequency. The deviation from the ideal straight line is around 0.5 to 1db which is at the brink of audibility. Microphones and speakers are different. Here the differences are much larger, whereas the speaker with its strong resonances and deep grooves in the frequency response comes off badly.

This is also confirmed in practice. In comparison to speakers and microphones you hardly hear an auditive difference between mixers. Although larger mixers normally have a very simple operation system, the only noteable difference in sound between a 12-channel mixer and an expensive mixer can be made by direct A/B-comparison. Of course, inexpensive mixers make more side noises.

Amplifiers also only show relativly small auditive differences. The main differences are in the on-hand capacity, in the impulse reaction in the deep frequencies or in the operating reliability - something to be concidered!

All this equipment have one thing in common: they only deal with electric signals - from the input to the output. The production of equipment that achieve this in high quality is no technical barrier in this day and age.

But this is not the case with microphones and speakers. A microphone recieves an acoustic input signal (sound wave) and transforms it into an electric output signal voltage. The speaker works exactly the other way round. It turns an electric signal into a sound wave. Microphones and speakers are therefore referred to as electroacoustic converters. Exact these transmitters are the soft spot in the whole signal chain. In comparison to pure electric-working equipment, converters have a quite wavy frequency, a high side noise factor and a weak impulse reaction.

Therefore microphone and speaker quality should be taken into account in accordance with each other. What´s it worth getting the most expensive mixer, if you have a low-grade microphone or bad positioning which disturbs the input signal? The only logical solution is, if you want to have fun with your PA, you should invest in good microphones and speakers.


Vocal Microphones

Vocal handmicrophones are microphones that are normally held in the hand during live-performances. Great demands are made on these kind of mics. They have to be insensitive towards overcontrol and acoustic feedback. Cardioid or super/hypercardioid polar patterns are their guideline features. They must be mechanically robust with full basses and "crispy" highs. In the majority of cases their frequency is deliberately hidden, as these mics are supposed to make "sound". Mostly types with near-speech-effect are used. By the way, you should make sure that pop or wind noises, and - most of all - picking noises are restrained.


Comb Filter Effects

Comb filter effects are not that bad, if you look at them seperately. They are a direct result of the room acoustic and therefore a "natural phenomenon". As we usually linger around in closed rooms - that means reflection afflicted rooms - we are permanently exposed to them. Even more: As our sense for locating is based on the existence of two signals (our two ears), what we don´t really acknowledge is that comb filter effects actually match this accustomed listening experience.

Comb filter effects in the auditive signals of a PA-system are in comparison to this highly undesired. And this is no contradiction, but easily explained: Firstly you do not want to transfer the stage acoustic. The signals that are sent from each of the stage mics wouldn´t be suitable to do so anyway and would also irritate our sense for locating. Secondly the individual mic-signals should acousticly be on a par with that what each musician is playing or singing. The signals must be as direct as possible.

If the signals that come from the mics already have comb filter effects, they cannot be removed - no matter how "intelligent" your equipment! The result is a frequnce that is defined by peaks and notches and has a "washed out", intransparent total sound.


How do comb filter effects emerge?

Example: A band is playing on stage. Like in the normal case there are more than ten microphones in usage. Every instrument is picked up with its own microphone. As mics have the awkward peculiarity not to be able to differ what they´re picking up and what not, it could happen that the bass guitar is picked up by the right mic, but alongside that the e-guitar-, drum-, and singing-mics also pick up the bass guitar. This crosstalk is a big problem.


Drum Mics

Picking up a drum mic is very complicated - not only on stage, but also in a studio. It´s complex! The choice of the mics and their positioning plays an important role. Maybe it´s a special charcteristic of this insrtrument, but if you have a tricky pick-up coming out of a cheap "shooting gallery", the PA-system still can conjure a beautiful sound.

For the bass drum pick up alongside the normal kick-mics we can recommend interface mics. If, for example, a mic is mounted to a plate and positioned inside or right infront of the bass drum, a crunchy bass drum kick unfolds.

Snares and toms need mics with cardioid features which can quash signal fractions coming from behind due to their directivity. As they convert high acoustic pressure, they are able to pick up these loud and rhythmic instruments.

There are two general ways to arrange the mics. If the mics are positioned on top of the tom only a few centimetres above the head, then the stick strikes can be heared very clearly. The sound is clear and does not disturb the resonances. A disadvantage is the crosstalk of neighboured acoustic sources.

To reduce the crosstalk, the mics can be placed inside the toms. Although the sound of the strike backdrops a little, the toms gain more sound character. It´s a matter of taste how you want to position the mics - just try out both possibilities!

The erroneous belief still exists that cmybals only have frequencies of 5kHz or more. Although in comparison to other instruments you cannot hear a clear, deep keynote in the sound mix, a broadband signal is created that can cover a frequency from 100 Hz to 20 kHz.

In order to cover the whole acoustic spectrum, a microphone is placed on the edge of the cymbal. If the cymbal is played more than once, then it can start to swing back and forth on its stand. This way the distance to the microphone is altered and priodical volume fluctuations appear. If the mic is positioned in the middle of the cymbal, either on the top or underneath, this problem will not occur. As the mic can be placed fairly close to the cymbal, the wanted signal is very loud so there will be no problems with crosstalking.

Concerning further frequency areas it is obvious to use a condence microphone for the cymbal pick-up. The desired soft sound is just not achieved with the dynamic model.

THe basic sound of the bass drum, depending on atmosphere and diametre is around 50Hz. The overtone concentration depends on the drum skin, material and conctruction of the stick as well as the cushioning. Articulated overtones are around max. 2 - 3 octaves above the keynote. The strike sound and overtones make for 5kHz of the total sound.

To prevnt the bass drum from "booming" in most cases it is cushioned. The appropriate degree of souns absorption has to be tested. The microphone pick up of the bass drum is quite uncomplicated. The best thing is to position the mic on a stand so that it looms through the hole in the resonances head into the drum. Besides given the drum a good sound it has the advantage that crosstalk is avoided.

The mic you use must have a good bass pick-up. As loud impulses arise inside the bass drum, sound absorbed condence mics shouldn´t be used. Most of the times dynamic mics are enough. If you use codence mics, you must make sure that these types of mics (especially concerning bass drums are more neutral than the dynamic ones. This way e.g. the AKG D112 gains a special bass emphasis.

The position of a mic inside a bass drum also plays an important role concerning sound. The exact positioning is maybe a matter of taste When using condence mics you should make sure that the strongest impulses come uo in the middle of the head and that the distortion is the largest at this spot.

The keynot of a 16" hanging tom has the frequenz of approx. 70Hz, that of a 12" hangign drum approx. 120 Hz. The overtones hardly reach 1kHz, the strikes approx. 6kHz. There are two possibilities to set up the mics. To place the mics only a few centimetres away from the tom top has the advantage that a strike will be very audible