Frequency Response Curves
The Frequency Response Curve, FRC is the most common test in audio. Every processor, amp, speaker, interconnect and listening room gets a frequency-sweep test.
Hidden far below the FRC lies a band of background noise, transformer hum, tube and transistor hiss. Sometimes this is measured and added to the FRC to show how low the background noise floor is below the level of the FRC. Another noise usually comes up whenever a signal is being passed through a piece of gear. This self-generated noise is usually harmonic distortion on the electronic side of the audio chain, and it is acoustic distortion on the acoustic side of the audio chain.
What the FRC does not reveal is the presence and level of these bands of self-induced noise that are being added to the signal path. However, on the electronic side there are harmonic distortion analyzers, sweep-tone and multi-tone analyzers that map out these types of self-induced distortion bands.
On the acoustic side, it’s a different story. Acoustic distortion is created in the space between the speaker and the listener, in the listening room. But what is acoustic distortion and how is it measured?
There are acoustic tests beside FRC. The RTA real time analyzer shows the spectrum of sound at any given moment. The ETF energy time frequency shows the waterfall spectral decay of sound. But these don’t define or measure acoustic distortion.
Music is not a steady state sound. It is a sound that rapidly varies in frequency and loudness. A simplified version of music can be boiled down to a series of tone/chord bursts. Musical quality is assessed by comparing what is heard at the listening position to what the speaker delivered to the room.
There are two parts to musicality, one is hearing each of the complex tones and the other is hearing the rapidity of their coming and going, essentially the variation in loudness of the tones being heard, dynamic level fluctuation. When a speaker plays a sound, that sound is quickly heard all over the room.
The issue of musicality is not that we don’t hear the musical sound as it was originally sounded. It’s when we can’t hear the low-level leading edge of attack transients and the subsequent musical dynamics of the sound as it was originally played. Acoustic distortion in the playback environment is not about tones it’s about not keeping up with the rapid variation in the loudness of the tones, dynamic distortion.
The test for accurate variation in loudness, amplitude modulation, is an acoustic modulation test. This then is also the test for dynamic distortion in playback rooms. But first let’s review the basic electronic tests for harmonic distortion.