Real World: The Building Block Approach
A straightforward look at test and measurement

By John Murray

Don’t be overwhelmed! A bit of study, combined with common sense, can go a long way to helping sort out the test and measurement maze. With that in mind, let’s start simple.

The primary basic audio measurements include:

• Amplitude (dB SPL)
• Frequency (Hz)
• Phase (degrees ref. @ Hz)
• Absolute and relative polarity (+/-)
• Delay (milliseconds)
• Impedance (Z, the complex resistive, capacitive, and inductive load)

The simplest measurement systems measure only one thing, in one dimension, like a Sound Pressure Level (SPL) meter or frequency counter. More sophisticated measurement systems measure things in two dimensions.

A Real-Time Analyzer (RTA) measures sound in both level, like an SPL meter, and in frequency, like a low-resolution frequency counter. A single-channel oscilloscope can measure sound in level and time, but frequency must be interpreted by the user.

Two-port measurement systems can measure sound in three dimensions, giving level and frequency, like an RTA, with time or phase information, like an oscilloscope, as well. For example, the TEF name indicates that it measures Time, Energy, and Frequency, the three measurement dimensions.

One-port measurement systems have only an input port for an incoming signal. Two-port measurement systems have a signal input port and a second port for either signal output or for a second reference signal input.

One-port systems, like an SPL meter or a standard RTA have only a signal input port and can only acquire limited information about that signal. Two-port systems can either output the reference signal used for testing, like the TEF-20, or can use the second port as a reference input that analyzes the test signal before it is sent through the system under test, like SMAART.

Either method allows the two-port measurement system to distinguish subtle differences between the signal before and after it has been sent through the sound system being analyzed This difference is known as the transfer function, and one-port measurement systems are not capable of deriving it.

What two-port, three-dimensional measurements give the user is the ability to analyze the signal in both the frequency and time domains. What this means is that you can see which reflective room surface is a problem, how well delayed speakers are synchronized with the main cluster, or whether or not drivers are aligned at crossover.

The technique generally used that enables this capability is the Fast Fourier Transform (FFT). This mathematical convolution converts a time-based impulse response, as would be seen on an oscilloscope, to a frequency-based frequency response. An Inverse FFT (IFFT) does the opposite, converting a frequency response into an impulse response.

Using FFT and IFFT, modern two-port, three-dimensional measurement systems can, for example, isolate and show the frequency response of a reflection, show the phase response of an equalizer, or the semi-anechoic response of a speaker without un-equalizable room effects. This is the primary advantage of these newer measurement systems over the old single-port, one- or two-dimensional standards of the past.

There is a multitude of measurement systems to choose from. Each has its strengths and weaknesses, so it‘s up to you to decide what combination of cost, versatility, portability, ease of use and accuracy is right for your approach to measuring and adjusting sound systems and their components.

Following are relatively brief overviews of some of the more popular and viable measurement systems currently in use. We’ve also included a chart that highlights the features of several measurement systems available today. Now, let’s get to work!

Jean-Baptiste Joseph Fourier: 1768 - 1830 Jean Baptiste Fourier showed mathematically that any signal or waveform could be duplicated by adding together a series of pure tones (sine waves) with appropriate amplitudes and phase/time relationships. In this way, even Beethoven's Ninth Symphony could be expressed as a very long, complex mathematical formula. Not a bad accomplishment for a guy working at a time that was more than a half century before the phonograph was even invented!

Live Sound Technical Editor John Murray is a 26-year industry veteran, working for EV, MediaMatrix and TOA. He has presented two AES papers, chaired three Syn-Aud-Con workshops and is a member of the TEF Advisory committee and ICIA adjunct faculty. We encourage you to send technical questions to John at jmurray@livesoundint.com

October 2003 Live Sound International