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Loudspeakers
--- What should they do? --- Design of loudspeakers --- Loudspeaker evaluation --- Accurate
stereo --- Whatever happened?
---
--- Room acoustics --- Sound field control --- Other designs --- System
design --- Dipole vs. monopole
woofer ---
--- Room
reflections misunderstood? --- Optimum
radiation pattern? ---
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Baffle Diffraction Versus
Neutral Sound
Dispersion
Presentation at Burning Amp - 2017
(YouTube)
(PDF) |
Loudspeakers - What should they do?
| The
best loudspeakers ...
The best loudspeakers for stereo sound
reproduction are those that disappear chameleon-like from the listening
room and simultaneously withdraw attention from the room. What remains is
an acoustic scene of phantom sources and spaces in front of the listener;
an illusion that the brain creates from the naturalness of the sonic cues
imbedded in the recording, which the two loudspeakers reproduce. Output
volume and dynamic range of the loudspeakers has to mimic the live event
for the illusion to be believable.
Few loudspeakers are capable of this magician
trick. A single driver, full range loudspeaker adds too many distortion
cues to disappear from the scene. A 2-way loudspeaker may come close,
except for struggling in the bass and with output volume. 3-way or 4-way
loudspeakers can be practical solutions to the output volume and frequency
range requirements. Since they become physically large they usually suffer
from a non-uniform radiation pattern with changing frequency. Next to a
flat on-axis frequency response in free-space, a frequency-independent
off-axis or constant power response is the most important loudspeaker
parameter. If a loudspeaker is directional, then it should be directional
in the same way for all frequencies and at least in the horizontal
plane.
Few
loudspeakers are capable of this magician trick
Except for omni-directional loudspeakers few are
designed with a uniform off-axis response in mind. Electrostatic or
magnetic panel loudspeakers meet the polar response requirement at low
frequencies, but become multi-directional at high frequencies and suffer
from insufficient dynamic range. Loudspeakers that use electro-dynamic
drivers on an open baffle overcome these shortcomings. Compared to
omni-directional loudspeakers such dipolar loudspeakers cause fewer room
modes and wall reflections, which helps them in hiding the room at greater
listening distances. The absence of an enclosure to retain the rear
radiation is a major advantage. It avoids frequency selective and resonant
re-radiation of acoustical and mechanical energy that is transmitted into
the enclosure walls and not converted into heat. Instead the rear
radiation is productively used to establish the sound field in the room.
The typical box loudspeakers with a vent for low
frequency extension suffer from resonant bass, delayed panel radiation and
non-uniform polar response to varying degrees, but they can be built to
meet the acoustic output volume needs. They are not suited to realize the
full imaging and illusion potential that is inherent in stereo, because
they create sonic artifacts which are distracting. Many audiophiles listen
for the presence or absence of such artifacts and use them as
differentiators between loudspeakers.
The best loudspeakers are able to deliver in a
normal living room a believable illusion of a live acoustic event.
Accurate
Stereo performance
tests
SL - October 2009 |
A commonly shared experience may help us understand which loudspeaker characteristics are most important for live-like
sound reproduction:
When we hear the sound of instruments or voices,
yet the source is blocked from our view, we are still able to tell in most cases
whether the sound comes from a loudspeaker or is live. What allows us to
recognize the difference? What cues might we work from? What forms of distortion
affect the loudspeaker reproduction?
- Most likely it is not the on-axis frequency
response of the speaker, because if the source is in some other room or
building, the frequency spectrum that reaches us may be rolled off at the
high end or modified in some unpredictable way. It could be the power
response, the radiation in all directions that is often different between a
speaker and a live source, that gives us a clue. Most speakers have a power
response that drops 10 dB to 20 dB from low to high frequencies.
- Most likely it is not the start up transient
response of the speaker, because by the time we hear the signal, it has
undergone many reflections and the waveform fidelity has been lost. It could
well be, though, the slow decay of transients due to energy storage in
resonant mechanical and acoustical structures of the speaker which we
recognize as typical for a loudspeaker and missing in the corresponding live
event.
- Most likely it is not simply the dynamic range
between loud and soft that gives us a cue, because we can usually tell the
nature of even a very faint sound that comes from a far distance. It could
be, though, that for loud sounds we recognize the change in sound character
that is caused by intermodulation in the loudspeaker and which creates new
spectral components that are foreign to the live source.
We normally listen to a speaker relatively close
up and the above observations do not apply completely, but I have found that an
excellent test of a speaker is to listen to it from the next room or from down
the hallway. Consistent with the above observations, I have found a set of
priorities for loudspeaker design that must be followed to obtain an outstanding
product.
- Low non-linear distortion, e.g. drivers that
can move sufficient amounts of air linearly in all parts of the frequency
range but especially at the low end.
- Minimal excitation of room resonances,
particularly at low frequencies. This requires low frequency directional
speakers such as dipoles.
- Low amounts of stored energy in drivers,
cabinet, air cavities and filters for fast transient decays.
- Smooth, extended frequency response from 20 Hz
on up and without exaggerated high frequencies, both on-axis and off-axis.
Minimal roll-off in power response.
There are additional requirements, such as an
acoustic center for the speaker at ear height, vertical extension of the source,
etc, etc, - but I consider the above four as most important in their given
order.
Many speaker designs fall short of these
priorities and try to use psycho-acoustic effects to overcome their sonic
deficiencies. A lack of bass due to drivers and cabinets which are too small can
be compensated to some extend by a boost in the 100 Hz to 200 Hz region. Hearing
the overtones of the bass notes our mind fills in the missing fundamentals. Or,
the particular form of distortion of a given small driver can give the
impression of a lot of "bass punch" and "greater output"
than is physically possible. Yet, we can recognize this as loudspeaker sound and
are not fooled into believing that we listen to the real thing.
Sound reproduction is like trying to create an
auditory illusion in our mind, similar to the visual illusion that a magician
creates on stage. Just as the magician has to present us with the right cues, so
has a loudspeaker. It is most important to avoid giving the wrong cues as in the
case of the small, distorting driver. It is likewise very important that the
correct cues from the loudspeaker are not masked by the acoustic properties of
the listening room.
When all this is taken into consideration it is
indeed possible - despite the multitude of examples to the contrary - to build
loudspeakers that will do a respectable job of reproducing complex sonic events,
provided they have been recorded with a minimum of acoustic and electronic
processing.
Loudspeaker evaluation is not like wine tasting,
though a majority of audio sales persons and magazine reviewers treat it like that. Unlike
in wine tasting, you have an absolute reference in naturally occurring sounds.
Familiarize yourself with a wide range of un-amplified sounds and keep them in mind when you
try to judge the accuracy of a loudspeaker.
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