We have seen how many speaker systems store resonant bass energy, resulting in ringing and poor transient accuracy. Now we will turn our attention to the listening experience, and describe how these measurable properties correlate with our subjective impressions.
There are two main factors which affect subjective low-frequency accuracy, frequency response and transient response. At low frequencies, these two descriptions are different aspects of the same event.5 Nevertheless, for the purposes of this discussion, we will treat these two topics separately as much as is possible.
Frequency Response Effects
As you listen to music, images of the instruments that created the sounds are elicited. For instance, one can tell the approximate size of a drum from the sound it produces. On a high-resolution playback system, finer details can be heard; i.e., is the head made of plastic or calfskin? Is the player using light sticks or heavy ones?
Arelatively broad-band emphasis (or de-emphasis) of a given frequency range can tend to exaggerate (or diminish) the relative size of the instruments playing in that range. A useful tool for evaluating these distortions of size is a recording of a small group of unamplified acoustic instruments made with a simple microphone set-up.
Listening for Size Distortions
Play a recording of this type, with the volume adjusted to achieve a natural playback level. As you listen, create a mental image of the players based on sounds being recreated. Then ask yourself, "Does this sonic image correspond to the musical instruments that generated these sounds?"
Is the portrait a natural one, or are certain elements distorted? Does a stand-up bass sound like the correct size, or is it exaggerated, sounding like it is ten feet tall, or as if the strings are the size of ropes? A speaker with excessive in-room bass response can create these effects. On the other hand, a speaker system with rolled-off bass can shrink the size of instruments, turning the same stand-up bass into a cello-sized instrument.
5Below about 200 Hz, virtually every woofer operates as a minimum-phase device. This means that the responses in the time-domain and the frequency-domain are inextricably linked, and that the one generates the other. Thus, two woofers with the same frequency response will necessarily exhibit the same time (phase) response. However, many crossover networks display non-minimum phase response, and will thereby alter the phase response of the woofer in the speaker system.
