Grandma Moon Speaks: Recording Sound

Topics Covered:

  • How Microphones Work
  • Polar Patterns
  • Microphone Specifications

We have two ears and one mouth so that we can listen twice as much as we speak – Epictetus

Ribbon Microphone

The job or ‘catching’ sound waves and saving them somewhere falls on the microphone. In simple terms, common microphones have the following parts:

  • Diaphragm (Plate, coil, ribbon, whatever)
  • Electromagnetic system (Coil, magnet)
  • Sampling System, analog or ADC

Together, these form what is called a Transducer (a device that changes energy from one form to another).

Sound waves shake the diaphragm due to pressure. This in turn causes the electromagnetic system to ‘dance’ accordingly. When it dances, the change in magnetic field causes a current in the coil, which results in a signal that can be sampled and recorded.

Based on the transduction principle applied, you can have many types of microphones – condenser, dynamic, ribbon, and so on. After all, more than one thing is capable of vibration! In fact, Professor Sampler might have already told you that everything is vibrating all the time!

This variety is a bane, too. You can design microphones to cater to every type of frequency or loudness range. Once this is done, you can sample sounds differently to further complicate matters. The number of microphone-types that can be created is infinite!

The useful aspect of this complexity is that you can design microphones for precise functions – like recording male tenor, airplane noise testing, dialogs, concerts and so on. The flip side is, over the years so many manufacturers have released models of microphones without precise specifications, and it is a herculean task to compare two based on specs alone.

The only way to compare two microphones is to test them against each other.

Directionality

A perfectly spherical microphone pattern is as follows:

Omni-directional polar pattern
Image Courtesy: Galak76

Transducers can be designed spherically, or to point to specific directions, like an antenna or dish. To plot where the sounds have to be to get its attention, one uses the Polar Pattern, as shown above for an omni-directional microphone.

00 represents the main direction, called on-axis. Since the distance from the microphone to the source influences the pressure or loudness of the sound, we see that represented as ‘waves’ with different dB ratings. As we have seen, 0dB is maximum loudness (at source). The further away the microphone, the quieter it gets.

The most common polar patterns are as follows:

Bi-directional polar patternCardoid Polar PatternHyper-Cardoid Polar PatternSuper-Cardoid Polar PatternShotgun Polar Pattern

 

 

 

 

The polar patterns are as follows: bi-directional, cardoid, hyper-cardoid, super-cardoid and shotgun. The basic idea is, to stay within the ‘zones’ to make best use of each microphone type. You don’t want to point a shotgun microphone the wrong way.

On paper, microphones can be compared in the following ways:

  • Frequency Response – decibels plotted over a frequency range (usually 20 Hz to 20 KHz), the larger the frequency range the more ‘universal’ the microphone, but not necessarily better.
  • Gain – in decibels, which shows the deviation of loudness between any two frequencies, ideally 0dB.
  • Self-noise level – the decibels produced by the microphone in the absence of sound, the lower the better, ideally 0dB.
  • Peak SPL – maximum decibels a microphone can record, the higher the better.
  • Harmonic Distortion – in Percentage (%), the distortion of sound from the ‘norm’. 0% is perfect.
  • Clippling level – in decibels, the point after which the signal reaches full amplitude – beyond the usable range.
  • Dynamic Range – in decibels, similar to how the dynamic range of the ear is measured, the difference between the lowest and highest dB levels. The higher the better.
  • Sensitivity – in decibels or millivolts per pascal (V/Pa), shows the efficiency of the microphone – how much of the pressure is actually used to create the voltage. The higher the better.
  • Impedence – in Ohms. Every electricomagnetic device has an impedence. When combining two dissimilar devices one has to take care to match the impedence, or risk a voltage mismatch. Small changes in voltage can have devastating repercussions on a circuit. Generally, the lower the better.

Take care not to believe manufacturer’s ratings completely, since many of these measurements can be carried out in different ways. Sometimes, two decibel ratings don’t mean the same thing.

Use these ratings only as a starting point. Your ears will tell you their true worth.

Takeaways:

  • Microphones are simple transducers, in which sound pressure is converted into electrical signals.
  • Don’t believe or adhere to manufacturer’s ratings. Perform your own tests.

Links for further study:

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