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9.4 Listening Qualities
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.
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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?
A relatively 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.
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Below 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.