Microphone Sensitivity Ratings:
What does it all mean?

A microphone sensitivity specification tells how much electrical output (in thousandths of a volt or "millivolts") a microphone produces for a certain sound pressure input (in dB SPL). If two microphones are subject to the same sound pressure level and one puts out a stronger signal (higher voltage), that microphone is said to have higher sensitivity. However, keep in mind that a higher sensitivity rating does not necessarily make a microphone better than another microphone with a lower sensitivity rating.

The term "dB SPL " is a measurement of Sound Pressure Level (SPL) which is the force that acoustical sound waves apply to air particles. As a person talks or sings, SPL is strongest near the mouth and weakens as the acoustical waves move away from the person. As reference levels, 0 dB SPL is the quietest sound a human can normally hear and 1 dB is the smallest change in level that the human ear can detect. For comparison, at three feet, speech conversation level is about 60 dB SPL and a jackhammer's level is about 120 dB SPL.

Microphone manufacturers normally specify one of two dB SPL input levels: 74 dB SPL or 94 dB SPL. Shure uses 74 dB SPL unless indicated otherwise on the data sheet. How do these dB SPL values relate to the real world? 74 dB SPL is typical of the sound intensity twelve inches away from a talker. 94 dB SPL is typical of the sound intensity one inch away from the same talker. A microphone "hears" these sound intensities and converts the acoustic wave into an equivalent electrical signal. To determine which SPL is used for a sensitivity measurement, look at the microphone data sheet. Confusion can arise because of the different units by which SPL may be specified. For example, 94 dB SPL = 1 Pascal = 10 microbars = 10 dynes/cm2. And 74 dB SPL = 0.1 Pascal = 1 microbar = 1 dyne/cm2. Unfortunately for the microphone buyer, different manufacturers use different units to specify SPL. But by following these dB SPL unit conversions, you can determine how different dB SPL ratings relate to each other.

First, "open circuit" means the microphone is not connected to anything. That is, there is no electrical load on the microphone. The open circuit voltage rating indicates how much voltage appears at the microphone output when a certain SPL is introduced to the microphone diaphragm. A value for a typical dynamic mic is -75 dBV/microbar. The "V" in dBV indicates the microphone output level is referenced to 1 Volt. If there was a microphone with an output voltage of 1 volt, its level would be given as 0 dBV. The negative dBV output level ("-75") indicates that the mic output voltage is less than 1 volt (-75 dBV converts to .00018 volts). The "microbar" part indicates the microphone was tested with an input of 74 dB SPL. (Please reread the preceding section if the conversion from 1 microbar to 74 dB SPL is confusing.) To compare this typical dynamic microphone's sensitivity with a different microphone that was tested at 94 dB SPL or 1 Pascal, simply add 20 dB to the rating: -75 + 20 = -55 dBV/Pascal. Remember, to compare specifications from different manufacturers, make certain that each has been converted to the same input dB SPL level.

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Writing data to recordable compact disc is a complex process which demands a great deal from both hardware and software. Most of this complexity is hidden from the user by the power of the program, but you should be aware of these factors which influence the success of the operation.

The amount of data you are writing is less important than whether it contains a few large or many small files; in the latter case the system may have trouble locating and opening the files quickly enough to send them smoothly to the CD recorder.

By system we mean your computer and all the software running on it. ANY interruption is fatal to CD recording, so ensure that your CONFIG.SYS and AUTOEXEC.BAT do not load any TSR utilities which may periodically or otherwise interrupt operations. Screen savers, alarms or reminders, and incoming faxes will kill disc writing. You should also turn off network sharing so that no one tries to access the files youêre trying to write - this would also fatally interrupt writing.

To write a virtual image on the fly to compact disc, the hard disk from which you are writing MUST have a transfer rate fast enough to keep the memory buffer full in the CD recorder. This usually means an average hard disk access time of 13 milliseconds or better.

Having to search all over a very fragmented hard disk for the data to be written (whether on the fly or from a real ISO image) causes operations to slow down, possibly fatally. Defragment your hard disk!

All hard disks automatically periodically perform a thermal calibration to ensure proper functioning. Calibration interrupts hard disk operations for as much as one and a half seconds. Some hard disks force a calibration at fixed intervals, even if the disk is in use, causing interruptions fatal to real-time operations such as CD writing. This problem arises particularly when the amount of data to be written is large. Some hard disks calibrate intelligently, postponing it when the disk is being used. Fujitsu and many Micropolis hard disks work this way.

Many new CD recorders (and some old ones) are capable of writing at two (or even four) times the standard writing/playback speed of 150 kilobytes (75 sectors) per second. Ideally, it should be possible to choose the speed (and wherever the unit allows it, our drivers do give you the choice), because, although writing faster is a timesaver in some situations, in others it causes problems. Letês see why: CD recording occurs in real-time. Data is transferred from the computerês hard disk to the buffer memory of the CD recorder, and thence to the recording laser, which inscribes pits into the surface of the CD at a steady speed which cannot be altered or interrupted. The speed at which data must be transferred depends on the mode in which the data is written and on the speed selected:

When you write a real ISO image from hard disk to CD, speed is rarely a problem because the image is already one gigantic file in which the files and structures are already in order and divided into CD-ROM sectors, so it is only necessary to stream off the data to the recorder. When you write from a virtual image, things get trickier. Now, in order to write to CD, the program must consult its database to find out where each file should go in the image and where it actually is stored on hard disk. It must then open the file, divide it into CD-ROM sectors, all the while sending the data in a smooth, continuous stream to the recorder. Locating and opening the file is often the more time- consuming part of the process, which is why on-the-fly writing is more difficult when you have many small files.

The CD recorderês buffer memory helps to ensure that there is always data ready to be written, by storing extra data as it arrives from the computer. The size of the buffer is therefore critical to trouble-free writing: a slow-down or small interruption in the transfer of data from the computer will not interrupt writing so long as the buffer is not completely emptied. Obviously, the larger the buffer, the more safety margin you have in case of interruptions. If your CD recorder has a very small buffer and your hard disk is slow, you may find it difficult or impossible to write virtual images on- the-fly to CD. Your options are (a) make a real ISO image on hard disk and record from that, (b) buy a faster hard disk, or (c), if possible, get your recorder upgraded.

Given all of the above, if you want to write a virtual image on-the- fly to CD and you have a slow hard disk, it is generally safest to write at 1x speed. If you have a CD recorder which forces you to write at 2x, in many situations it will be difficult to write a virtual image straight to CD. Your only choices are (a) make a real ISO image on hard disk and record from that, or (b) if possible, get your CD recorder upgraded.