The finished test unit comprises modified “Pre-Champ” and “Champ” units with components chosen to give a flat frequency response. The output from the preamp can be taken from either the 3.5mm mono jack or from the RCA socket. The hardware at the bottom of pic is a bathroom towel rail holder, used to hold the test unit on its stand as seen in the pic on the opposite page.
Accurate, commercial speaker measurement systems can cost tens of thousands of dollars
– way outside the budget of even the most dedicated audio enthusiast.
Now, with the advent of well developed PC “virtual instruments” and much-improved electret microphones, we are able to present an economic speaker measurement system capable of accurate and reliable results.
We have often seen enthusiastic loudspeaker experimenters take great care in selecting speaker drivers and mounting them in well-designed cabinets, only to find that the results don’t live up to their listening expectations.
More often than not, they can be let down by incorrectly designed crossover systems which cause large peaks (or worse still, deep troughs) or incorrect level adjustments for tweeters and midrange drivers.
This project removes the subjective errors which may result from adjustments made by using only listening tests. The operator will also have a facility to print all response curves.
The test set-up
Fig.1: apart from the modified Pre-Champ and Champ projects, the rest of the Analyser is simply input and output connections. The software that drives it all - fatpigdog - is powerful but quite cheap.
An audio sweep signal from 20Hz to 20kHz from the virtual instrument is amplified and fed through the speaker under test (SUT).
A wide-range electret microphone set very close to the speaker picks up the swept signal and its output is amplified and fed to a “virtual” spectrum analyser which then plots the amplitude of the speaker response on the vertical (Y) scale versus frequency on the horizontal (X) scale using a principal known as Fast Fourier Transform (FFT).
The result is a plot of the frequency response of the SUT. In this case we are using a “virtual spectrum analyser” which you can purchase and download from www.fatpigdog.com
The Author describes his Audio Spectrum Analyzer as suitable for “the Acoustic Specialist, Vibration Analyst, RF Engineer or True Geek”!
Even if you’re none of those, you’ll find the Audio Spectrum Analyzer easy to use and a very worthwhile program to own.
Best of all, at just $US39.99 the software is very reasonably priced but with the volatile Aussie dollar at the moment we won’t even hazard a guess at the $AU price; we imagine it will be fairly close to the $US price.
Figs.2&3: Pre-champ and Champ PCB component overlays with
the changed components (from the original projects) shown in red.
It also has a built in “tracking generator” (TG), which sweeps across the desired frequency range, in step with the analyser.
The audio sweep signal is fed to a “Champ” amplifier (SILICON CHIP, February 1994). This “oldie but a goodie” has been modified to give a flat frequency response and can drive an 8-ohm speaker to about half a watt.
This may not seem very much but you will be surprised how loud it can be and it is certainly adequate for frequency response testing. Of course, you could use any power amplifier which has as good or better response than the modified “Champ” which is ±0.2dB from 20Hz to 20kHz
The signal from the loudspeaker under test is picked up by a specially built microphone or a commercial calibrated microphone. We’ll have more details on these later in this article.
The electret then feeds our “Pre-Champ” preamplifier (SILICON CHIP, July 1994) which has also been modified for a flat response. The resultant signal is fed to the spectrum analyser for processing. You can save and print your response curves for further analysis. Both the “Pre-champ” and “Champ” are mounted in the same diecast box but each has a separate battery to minimise crosstalk and feedback.
Not only is the setup useful for measuring loudspeaker frequency response, it can also be used to plot the frequency response of an amplifier, pre-amplifier, audio filter or crossover network.
It is also handy as a general purpose portable microphone for public address systems or DJ work or even for good quality recording – just plug it into any line input or power amplifier.
Also, if you plug it into a frequency counter, you will be able to accurately tune instruments (assuming you know or find out what frequency equates to the notes in your particular instrument).
The virtual spectrum analyser will also be very useful as a training tool because it has been specifically designed to look and feel like a typical bench top analyser.
The new tracking audio generator included in the fatpigdog software is very useful too. It measured 0.0066% THD+N (at 1kHz when set at 635mV on “zero span”; measured on an Audio Precision test set!).
The THD+N is largely up to the quality of your sound card although ours was measured from the standard sound card in an Acer Aspire One which cost less than $500.
You can also use it all as a spectrum analyser and waterfall analyser and play around with various colour modes. It requires some skill and patience (just like a real benchtop spectrum analyser) but if you experiment, you will learn to master it all fairly quickly.