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Q. What is meant by "rise time"? How does rise time pertain to the sound of an amplifier?
-Rawn Stafford, Gainesville, Florida
A. "Rise time" refers to the amount of time required for a pulse fed into an amplifier or other device to produce full amplitude at its output. Hopefully, this will happen as soon as the pulse enters the amplifier. In fact, however, there is a certain amount of time required for the various circuits to produce their outputs.
The faster the rise time, the better the equipment will reproduce transient sounds such as percussion instruments.
Q. What is "pink noise"? How is 1/3-octave pink noise used in testing equipment?
-Paul C. Lutz, Louisville, Kentucky
A. If one built a noise generator, chances are that the noise would produce signal over the entire audio spectrum. If one devised a filtering system however, this noise could be produced over a portion of the spectrum only. It is this reduced noise bandwidth which we refer to as "pink noise". The 1/3-octave "pink noise" is a special case of such pink noise, where the spectrum is divided into very narrow segments, each of which is 1/3-octave wide.
Such pink noise segments are often used to evaluate the performance of equipment because its waveform is often difficult for equipment to reproduce, especially with transducers, such as loudspeakers. The waveform produced by the speaker or other device is compared to the waveform of the "pink noise" which feeds into the device under test. The closer the input and output waveforms appear to be alike, the better is the equipment under test.
Listening rooms contain very sharp peaks and dips in their frequency response. They are often narrower than a 1/3-octave portion of the audio spectrum. However, these 1/3-octave segments represent a good compromise between performance and the complexity of the controls required on equalizers designed to correct for these peaks and dips of listening rooms. What happens is that signals are transmitted by means of calibrated transducers, and picked up at some other point in the room to be adjusted, by suitable, calibrated microphones. The signal consists of pink noise. A room equalizer, consisting of 1/3-octave boost and cut modules is used, and the controls are adjusted for best overall frequency response at some given listening point in the room. Note that the equalizer modules are made to match the segments of the pink noise used for calibration.
Connecting VU Meters to a Power Amplifier
Q. Could you tell me how I may attach two VU meters so that I can monitor the sound level from my stereo system? My equipment comprises a Dynaco PA T4 preamplifier, Dynaco Stereo 120 power amplifier and KLH 5 speakers, Sansui AM/FM tuner and a Miracord table.
-Phillip E. Kalanz, Victorville, California
A. VU meters are connected across the speaker terminals of each channel.
When calibrating, they should show you proper channel balance. Their calibration can be made to show when the amplifier is driven to maximum power output or can be set for a zero VU reading at some listening level which you would not want to exceed. If this listening level represents the lower power requirement, this is the best way to use the meters. I will assume that this is what you want to accomplish.
If you have a problem or question on audio, write to Mr. Joseph Giovanelli, at AUDIO, 134 North Thirteenth Street, Philadelphia, Pa. 19107. All letters are answered.
Please enclose a stamped, self-addressed envelope.
(Source: Audio magazine, Sept. 1974)
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