Design Article
Introduction to the Six Basic Audio Measurements - Part 2
David Mathew, <A HREF="http://ap.com">Audio Precision</A>
11/21/2007 2:26 PM EST
Frequency Response
About Frequency Response measurements
A frequency response measurement reports the output levels of a DUT when stimulated with different frequencies of known level. The simplest of all frequency response measurements consists of only two or three tones, the first near the middle of a DUT's usable frequency range, and followed by a tone near the higher extreme of the range and sometimes a tone near the lower extreme. Assuming the tones are all generated at the same level, the DUT's output levels describe its response to these different frequencies.
Full-range frequency response measurements can be made by a number of different methods, the classic being a sweep of a sine wave from the lowest frequency in the range to the highest, the results plotted on a graph. A "flat" response describes the shape of a graph where the DUT responds equally at all frequencies, producing a trace with a slope of 0 and with minimal variations.
Making a Frequency Response measurement
As mentioned above, it is possible to make a basic response measurement using only two or three tones. However, the expectation is usually a full sweep across the audio spectrum, and that's what we'll do here.
We will set up a level versus frequency sweep and view a graph as the output.
First we have to decide on a level. We could make the sweep at a very low level, but we might see noise or other spurious signals in our response. We could make it at a very high level, but there is the possibility of amplifier distortion affecting the response. A common level for frequency response in a power amplifier is at the 1 W output level, and that's what we will use here.
Initial Setup
Start with the DUT and control software setup instructions in Section 2.
Make a Frequency Response sweep
For a more useful display of the same data, you can set the Analyzer dBr input reference to your 1 W level. This will display 0 dB at the center frequency of 1 kHz, if you set the Sweep units to dBrA. You can also adjust the graph axis scales for a more useful display. Since our DUT is quite flat, we can narrow the view to just a few decibels above and below the reference.
Here is the graph with units set to dBrA and range set to 1.5 dB:
THD+N
About THD+N measurements
THD+N stands for Total Harmonic Distortion plus Noise.
Harmonic distortion is the unwanted addition of new tones to the audio signal. These tones are harmonically related tones to the original signal. When the signal is one sine wave of frequency f1, harmonic tones are f2, f3, etc., integral multiples of the original tone. Total harmonic distortion is the sum of all of the harmonics measured in the DUT's bandwidth.
Why THD+N? Why not just measure THD (the distortion) and N (the noise) individually?
Well, at first glance it makes sense. However, in the pre-FFT days of audio measurement it was difficult to measure the THD by itself, without the noise, but it was relatively simple to measure the THD and the N together. So the accepted techniques handed down from years past specify THD+N, because that's what was practical. In addition, THD+N is a convenient and telling single-number mark of performance, widely understood and accepted.
Bandwidth and THD+N
The measured THD+N of a device will vary with the measurement bandwidth. You will almost always want to restrict the measurement bandwidth using high-pass and low-pass filters, and you must include the bandwidth used when you state the result. THD+N is typically measured and reported in a 20 Hz"20 kHz bandwidth.
Level and THD+N
The measured THD+N of a device will also vary with level and the frequency of the applied signal. Audio THD+N is typically measured and reported at a mid-range frequency (1 kHz or so) at the either the device's nominal operating level or at its maximum output level (MOL).
Making a THD+N measurement
We've already done this in Adjust DUT for 1% THD+N in the Level topic above, but this time we'll define the measurement bandwidth.
Initial Setup
Start with the DUT and control software setup instructions in Section 2. We have decided to make this THD+N measurement at the 1 W output level. The frequency setting for THD+N is typically 1 kHz, which is what we will use.
The method just described provides a quick, single-number result for THD+N, and is often the method of choice. However, other techniques can provide much more information about a device's distortion performance.
A THD+N versus frequency sweep or a THD+N versus amplitude sweeps will show how a DUT performs under varying stimulus. Frequency-domain FFTs or a dedicated Harmonic Distortion Analyzer (as found in Audio Precision's AP2700 and ATS) can reveal details of the individual distortion products.
Next: Phase
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