Frequency response measures the output level of a device at different frequencies. Results are displayed on a graph showing level vs. frequency.
Usually we’re looking to see if the response is flat, that is, that the device doesn’t unexpectedly change its output level at any particular frequencies. Of course, some variation in level might be intentional: for example we might be looking at the center of a graphic equalizer’s frequency band or its “Q” width, or confirming that a weighting filter has been implemented correctly.
Because Frequency Response is essentially just a series of level measurements made at different frequencies, the units are the same as level: volts r-m-s, dBV and dBu are the most common.
Traditional analyzers measure frequency response by hitting the device under test (or D-U-T) with a series of tones at different frequencies. Two or three tones will give you a very basic idea of frequency response, but 60 or more points aren’t uncommon. Which frequencies to choose depend on what you’re looking for. For a general frequency response, most analyzers default to a “1/3 octave” sweep, dividing each octave of the bandwidth you want to measure into three frequencies. “1/6 octave” is also common. There are 10 octaves between 20 hertz and 20 kilohertz, so at three tones per octave, a 1/3 octave sweep would require 31 points.
Of course, the more frequencies you want to use, the longer the measurement will take. A sweep with one-second steps will take over a minute for a 61 steps.
To get around this speed limit, Audio Precision has added a new frequency response measurement that uses the continuous sweep method. Continuous sweep is also known as a “log-swept sine” or “chirp.” Instead of using a series of discrete tones, a continuous sweep signal is swept smoothly from the start frequency to the stop frequency in a short burst, providing a much more comprehensive measurement in a fraction of the time.
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