Even though the theory behind passive radiator loudspeakers –
i.e. drone cones - is persuasive and mathematically ordered, do they have a
reputation of producing rotten sounding low frequencies?
By: Ringo Bones
Despite having a history that dates back to the early days
of cone type loudspeakers, passive radiator equipped loudspeakers – i.e. drone
cone equipped loudspeakers – probably gained widespread popularity around 1972
when Polk Audio founder Matthew S. Polk released his Polk Monitor 7
loudspeakers back in 1972, and since then, the global audiophile community developed
a love / hate relationship around the idea behind drone cones / passive
radiators. But do drone cones / passive radiators truly deserve the reputation
for producing rotten sounding bass frequencies?
From a design standpoint, passive radiators are actually a
form of vent substitute and thus tend to follow vented loudspeaker design
methodology and design characteristics. Also known as “drone cones”, they have
two very important advantages over bas reflex vent designs. First, they
eliminate vent colorations – as in resonant pipe sounds – wind noises and the
internal high frequency sound reflected out of the vent. Second, passive
radiator / drone cone designs are practical for small enclosures – i.e.
bookshelf sized hi-fi loudspeakers – when the resulting design equations call
for vent lengths that exceed the internal box dimensions of the loudspeaker’s
enclosure. From a design standpoint, passive radiators are also simpler to deal
with, having fewer alignment algorithms that lead to calculation of losses. On
the downside, passive radiators have a steeper cutoff – and this offer less
transient stability – that vented / bass reflex designs (vented / bass reflex
loudspeaker design already roll-off their lowest frequencies at 24 db / octave),
a slightly higher cutoff frequency and greater overall losses (Q˪) than
vented / bass reflex designs.
A widely popular misconception about passive radiators /
drone cones is that they operate in the low frequency regions, mechanically
crossing over to the main / active bass driver at a higher frequency and extend
the bass of that driver. Actually, the passive radiator / drone cone operates
in conjunction with the main active driver – or woofer – at low frequencies,
sharing the acoustic load and reducing driver excursion, thus reducing the
total harmonic distortion produced by the main bass driver. Working as a
variant of a bass reflex vent, passive radiators / drone cones only add as much
as they subtract. From a design standpoint, this implies that passive radiators
have the same positive attributes as a bass reflex vent – such as higher power
handling and lower resulting harmonic distortion.
From a historical standpoint, passive radiator loudspeaker
designs were first described by Harry Olson in his patent “Loudspeaker and
Method of Propagating Sound”, issued back in January 1935. Except for an
article by Olson in 1954, very little was published about passive radiators /
drone cones until Nomura and Kitamura in their IEEE paper in October 1973 and
Richard Small’s Journal of the Audio Engineering Society paper in October 1974.
At around this time, Polk audio became the well known manufacturer of passive
radiator / drone cone equipped hi-fi loudspeakers when Matthew S. Polk released
the Polk Monitor 7 with its famed 10-inch passive radiator / drone cone. Scores
of other manufacturers have produced their own version of the passive radiator
/ drone cone hi-fi loudspeakers since then, like Bowers and Wilkins with their
DM620i, Boston Acoustics with their compact HD9 monitor loudspeakers, Technics’
SB-M300 loudspeaker and the British – German hi fi collaborative effort ALR / Jordan Note 7
whose passive radiators / drone cones can have various weights attached to them to match their low frequencies to the listening room’s dimensions they are
intended to be used. Given that, more or
less, hi fi loudspeakers with passive radiators / drone cones produced to the
years have more or less received favorable – and sometimes exemplar – reviews,
then why do passive radiator equipped loudspeaker designs harbor the reputation
of producing rotten, sluggish sounding bass?
Around the 1990s, almost all science enthusiasts –
especially science enthusiasts who are also audiophiles – have read in various
science journals, like Scientific American and their ilk, about how Mother
Nature abhors three things the most – i.e. a singularity, a vacuum and an audio
signal passing through a real-world capacitor. And given the steep cut-off of
passive radiator/ drone cone designs, this only spells bad news from a sound
quality perspective – especially in the pace, rhythm and timing department
despite of their ability to produce prodigious quantity of bass in comparison
to similarly sized hi fi loudspeakers.
I have an audio-buddy who has bought a second hand B&W
DM620i loudspeakers back in the mid 1990s, he notices how the speakers lacked
finesse when it comes to resolving subtle low bass to upper bass details. Even
though I found the B&W DM629i’s overall presentation and adequate enough
for casual listening of small-scale ensemble Classical Music like string
quartets and the like, this won’t be the loudspeaker I’ll be using if I want to
find out if a certain hair metal recording had its recording session engineer been
using a dynamic range compressor with a sidechain access to “clean up the
bottom end” by using the kickdrum as a “key” to limit the overall dynamics of
the bass guitar – as in the compressor pushes the volume of the bass guitar down whenever the kickdrum is hit.
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