3
Diffraction
Diffraction causes frequency response and time response errors and therefore a reduction in tonal, spatial, and transient fidelity.
Diffraction occurs when some of the energy radiated by the drivers is re-radiated from the cabinet edges at a later time. For musical signals
that remain constant for a few milli-seconds, diffraction causes, by constructive and destructive interference, an excess of energy to the
listener at some frequencies and a deficient amount of energy to the listener at other frequencies. Diffraction also causes all transient
signals to be radiated to the listener a second (and possibly a third) time, smearing transient impact and distorting spatial cues.
To reduce diffraction the CS1.5 employs a grille board that fits around (rather than on) the
baffle and one that is curved at the edges so energy radiated along the baffle can continue into
the room without encountering abrupt cabinet edges.
Results
The end result of reducing diffraction
and diaphragm resonances is a speaker
with very accurate tonal characteristics.
Figure 3 shows the on-axis frequency
response of the CS1.5. It is uniform
within
±
3 dB from 43 Hz to 22 KHz.
Subjectively even more important is the
octave-averaged frequency response.
Figure 4 shows this response to be
within
±
1 dB from 60 Hz to 10 KHz
indicating extremely accurate overall
tonal balance. Furthermore, as a result of
gradual crossover slopes, the off-axis
frequency response of the speaker system
is also smooth and well balanced. This
unusual performance is important for
producing a uniform amount of ambient
energy at all frequencies, necessary for
natural spatial reproduction. Figure 5
shows this octave-averaged, 30
°
off-axis
response to be within
±
1.5 dB from 45
Hz to 15 KHz, showing very uniform
dispersion of energy at all frequencies.
TIME RESPONSE
In most loudspeakers the sound from each driver reaches the listener at different times causing the loss of much spatial information.
One problem caused by different arrival times from each driver is that the only dependable locational clue is the relative loudness of each
speaker which causes the sound stage to exist only between the speakers. In contrast to this loudness type of imaging information, the ear–
brain interprets real life sounds by using timing information to locate the position of a sound.
The ear perceives a natural sound as coming from the left mainly because the left ear hears it
first. That it may also sound louder to the left ear is of secondary importance.
Another problem is that for realistic reproduction, it is important that the attack, or start, of
every sound be clearly focused in time. Because more than one driver is involved in the
reproduction of the several harmonics of any single sound, the drivers must be heard in unison
to preserve the structure of the sound. Since, in most speakers, the tweeter is closer to the
listener’s ear, the initial attack of the upper harmonics arrives a millisecond or so before the
body of the sound. This delay results in a noticeable reduction in the realism of the reproduced
sound.
To eliminate both these problems the CS1.5 drivers are mounted on a sloped baffle to
position them so the sound from each reaches the listener at the same time. This arrangement
can work perfectly for only one listening position. However, because the drivers are positioned
in a vertical line the error introduced by a listener to the side of the speaker is very small. Also,
the error introduced by changes in
listener height are small within the
range of normal seated listening
heights provided the listener is 8 feet
or more from the speakers.
Figure 6 shows the group delay,
the measure of time error, of the CS1.5
from 200 Hz to 20 KHz. For all
frequencies above 400 Hz the delay is
less than 0.5 ms.
Cabinet-edge diffraction
tweeter
10K
Frequency
1K
25
20
15
10
5
0
-5
-10
100
20
20K
Amplitude — dB
Figure 3 On-axis frequency response
10K
Frequency
1K
25
20
15
10
5
0
-5
-10
100
20
20K
Amplitude — dB
Figure 4 On-axis Octave averaged frequency response
10K
Frequency
1K
25
20
15
10
5
0
-5
-10
100
20
20K
Amplitude — dB
Figure 5 30
°
off axis Octave averaged frequency response
Time correction
Figure 6 Time response
10
Frequency - KHz
1
2.5
2.0
1.5
1.0
0.5
0.0
-0.5
-1.0
-1.5
-2.0
-2.5
Transfer Function Group Delay - msecs
