MICROPHONES

North Suburban HAMMOND ORGAN Society

Finally we should look at microphone frequency response and also microphone hookups as well as high or low impedance and what it means as far as getting good results from a microphone. Frequency response is one of the important criteria to use when judging whether or not a microphone is good enough to be used for recording musical instruments or should it just be used for ordinary speech. Speech recording does not need to be high fidelity. If you can operate between 60 to 5000 Hz, you'll get the bottom end of male voices and also the sibilant sounds of speech which comprise the highest frequency components. The "ssss" sound of the letter S is a mixture of higher frequencies, but as long as you can get a significant part of that, you need not spend the money for a microphone to go up to 20 kHz if five- or six kHz is all you will need. But we're mainly interested in music recording here, and this demands something much better than a cheap speech mic.

We mentioned before a subject called coloration. Coloration here means what alterations to the original sound does the microphone contribute? The cheaper the mic, the more coloration. To get an idea of what coloration is, we need to look at the frequency response graph which shows how well a microphone responds to particular frequencies. Here's a coloration or frequency response graph for a cheap microphone.

graph, cheap mic frequency response

Figure 15. The frequency response of a cheap microphone is quite uneven. Notice also that it has lousy response at the low end; it really doesn't respond well until you get to around 100 Hz. Also, there is a noticeable emphasis around 2000Hz, after which the response begins to drop. It's already down about 8 dB at 5Khz, and virtually non-existent by 10Khz. Would you use this to record a musical instrument? Not if you wanted good results. This one would be good for a CB radio or a taxicab mic and that's about it!

graph, crystal mic frequency response

Figure 16. The frequency response of a crystal mic is a little better, both at the low end and also at the top. It is still not entirely even, and while it probably would be a good general-purpose microphone if you weren't too fussy, you wouldn't see this one being used at Carnegie Hall for recording any part of a performance.

graph, dynamic mic frequency response

Figure 17. The response of a general purpose dynamic microphone begins to look a lot better. The frequency response at the low end is significantly improved, and also the response right up to around 4kHz is reasonably flat. There is some drop-off at the high end, but it is a gradual and consistent drop and there is still some response even up to 20kHz. Because of the gradual drop-off at the high end, these mics tend to produce a good somewhat mellow sound. Some singers and recording engineers prefer these for recording singing, because if a singer's voice tends to be somewhat bright or reedy sounding, a dynamic mic's gradual roll off at the high end can reduce this effect. If you were recording a Hammond organ and using a Leslie speaker, this would be a good choice.

graph, ribbon mic frequency response

Figure 18. Here's the graph of a typical ribbon microphone. Like the one above, it also shows a very good response range. It has a better response at the low end, and also at the high end, because the ribbon is a very light, low-mass device. Notice also that between approximately 100 Hz to 10kHz, the response curve is fairly flat, without any very significant emphasis on any particular band of frequencies. Good ribbon mics are serious contenders for critical recording applications.

graph, condenser mic frequency response

Figure 19. As you might infer from the previous pages, the response of a good condenser microphone is generally the best of all. Notice that there is not much drop off at either the low end or the high end of the audio frequency band and also, throughout the major portion of the audio band, the frequency response is very even. Because condenser mics have excellent low frequency response, in some situations they can provide too much bass, particularly when used for speech or singing, where saying letters like "P" or "B" could otherwise produce loud thumps in a speaker system. Therefore, many condenser mics have a bass roll-off switch that, when turned on, provides an exaggerated drop off at the low end which varies inversely with the frequency. When the low cut-out switch is on, then the response of the microphone follows the dotted portion of the curve at the low end. This is the preferred microphone for all critical applications where the microphone should capture all sounds accurately without adding or subtracting anything to the audio signal. Variations on the basic design of the condenser microphone are used in industry and research, such as in the investigation of very loud sounds, and also very high-frequency sounds such as those made by some birds, bats, and mice among others. Special small diaphragm condenser mics can get audio frequencies of up to 100 kHz, and others with large diaphragms can accept sounds down in the single numbers of Hz.

These are general representations of the response curves of typical examples of microphones of different types. Among these classes, there is a lot of variation between individual units. Cheap is cheap; a low quality condenser microphone's response will not look much like that of the graph directly above, and likewise, an excellent quality crystal microphone may have a significantly better response curve that what you see in figure 16. Likewise, a really good dynamic microphone can give excellent results in music recording. All things considered, the above graphs represent typical examples in each category.

 

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