Drone Cones: Rotten Bass?

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.

Loudspeakers With Rear-Firing Tweeters: The Least Room Fussy Hi-Fi Loudspeakers?

For the budget audiophile skeptical about acoustically treating your listening room, are loudspeakers with rear-firing tweeters the best value-for-money solution?

By: Ringo Bones

Budget-conscious audiophiles had always been skeptical about “pricey” acoustic room treatments. They argue that their listening rooms …” is neither a recording studio, nor a mastering or recording post-production studio…” And both anecdotal and empirical evidence seems to reinforce this very idea by the time they manage to afford those relatively pricey second hand Quad electrostatic loudspeakers either the original ESL 57 from the 1950s or the newer one and far more common ESL 63 from the early 1980s. In practice, loudspeakers with rear-firing tweeters and their more elaborate counterparts – i.e. bipolar loudspeakers that radiate sound in both the front and the back – excel at sounding as if the very recorded acoustic event is actually happening in your listening room reproduced with timbral accuracy that conventional monopolar loudspeakers (i.e. those that radiate sound only in the frontal 180-degree arc) seem to lack. But in all things audiophile, not all bipolar / dipolar / loudspeakers with rear firing tweeters are not created equal.

Back when the late, great Audio Hall Of Fame speaker designer Arnie Nudell released the Eosone RSS series back in the early 1990s, many budget-conscious audiophiles got their first exposure of a budget loudspeaker that can reproduce the timbral accuracy of the cymbals in a basic rock combo drum-kit. But there are bigger, pricier ones that are found to be wanting from this time period.

Despite of the brownie points earned for a clean, full sound in a majority of hi fi magazine reviews, the Snell Type B loudspeaker was criticized by more than a few audiophiles as having its rear-firing tweeter for being a tad too loud. Some even suggested gluing a penny over the rear tweeter in order to make it sound better. Maybe a better solution is for the designer (s) of the Snell Type B to incorporate an l-pad attenuator / rheostat in the first place to attenuate the rear tweeter to make it around 12 dB SPL quieter than the front tweeter while preserving its high-pass filter crossover point or maybe include an on-off switch for the rear tweeter in “worst-case scenarios” for an overly bright listening room.

Bipolar loudspeakers – especially those manufactured by Mirage as in their Bipolar series - gained popularity back in the 1990s due to their rather “psychedelic phasiness” that they introduce to electric guitar recordings. Though criticized for not exactly accurate, they could take this effect too far by making those early Eric Clapton recordings when he’s playing through a small-ish tweed guitar amp, most bipolar loudspeakers tend to make tweed guitar amps sound acoustically small despite of the rich harmonics of the tone being produced. And sometimes bipolar loudspeakers exaggerate the “acoustical size” of some famous electric guitar recordings – i.e. that era when Eric Clapton preferred Gibson Explorers played through Marshall full stacks.

Open baffled loudspeakers – especially ones with an open baffle midrange configuration -  are praised for reproducing female vocals with a timbral accuracy as if there was no microphone and onstage monitor loudspeakers - in short, the timbral accuracy of an unamplified female voice. Despite of their virtues, open baffle loudspeakers have one glaring shortcoming, they could sound devoid of any extraneous reverb bas in they sound drier than “Prohibition era Georgia” when paired with a very low jitter digital front end. This effect is very noticeable on electric guitar recordings that are injected directly into the mixing board with distortion and tonal effects only in the preamplifier level – as in the timbral roar of the 1980s era hair metal guitar solos championed by MTV.