Expert review and test of the Castle Avon 4 Loudspeakers by Australian Hi-Fi Magazine. Free download.

When a company bills itself as ‘the world’s premier manufacturer of furniture quality hi-fi speakers’ it’s pretty obvious that it cares as much about the finish of its speakers as it does about their sound quality. So not only does Castle use real wood veneers, rather than faux ones, to build its speakers, it also uses crown cut veneers. And for all the greenies cringing at the thought of trees being cut down to create speaker cabinets, Castle sources its timber from managed forest resources. (And for those wondering what a ‘crown cut’ veneer is, it’s where the timber is sliced parallel to the growth rings, which results in a ‘crown’ figure, usually with a straight grain pattern on either side. The alternative method is where the timber is sliced at right angles to the growth rings, which results in a straight line pattern and is called a ‘quarter cut’.)

The ‘crown cut’ veneer means that no matter which finish you choose for your speakers—Natural Oak, Antique Oak, Black Oak, Walnut, Rosewood, Mahogany or Cherry—Castle’s Avon 4s will look absolutely gorgeous in your home. But how will they sound? That’s what we’re all here in order
to find out…

The Equipment

The Castle Avon 4 uses an extremely unusual technique to deliver deeper bass than would usually be possible from a speaker of its size that is using fairly small (130mm-diameter) bass drivers… even though it does use two of them to deliver the bass.
Instead of using a standard bass reflex or infinite baffle cabinet, the Castle Avon 4 uses a transmission line cabinet. The differences between the three cabinet types involve the way the energy from the rear side of the driver cone is handled, which is very important because the rear of the driver cone delivers around the same amount of acoustic energy as the front of the driver cone.

In an infinite baffle enclosure, the energy is simply retained inside the cabinet, so you never get to hear it at all. It’s wasted. In a bass reflex design, the acoustic energy from the rear of the driver excites the volume of air inside the cabinet, which is in turn used to send bass energy through a small opening (‘port’) in the cabinet to augment the bass from the front of the cone. In a transmission line design, the energy from the rear of the cone is funnelled through a very long tube inside the cabinet before being ‘transmitted’ into the listening room, usually via a slot.

Nearly all speaker designers agree that the transmission line delivers superior bass to the other two methods, delivering deeper bass (usually around one octave deeper) and with far less distortion, but another benefit of the transmission line is that it works well at all volume levels, whereas bass reflex designs tend to not work as well at very low and very high volume levels as they do at average volume levels.

So why are transmission line designs so rare? First, it makes the cabinet more expensive to build, because it has to contain internal ducting that’s critically damped with acoustic fill. Second, the type of bass driver used to energise a transmission line requires different parameters to those used for infinite baffle and bass reflex designs. This makes them more expensive to buy because since they’re only used in transmission line designs, they’re built in only small quantities, so manufacturers building TL speakers don’t get to take advantage of the cost benefits of mass production.

Now you know what a transmission line is, I have to point out that the Castle Avon 4 is what Castle says is a ‘twin drive transmission line’ which is a design it says it developed ‘after a three year investigation into the acoustics, mathematics and performance aspects of transmission line loudspeakers.’ The difference appears to be that whereas a traditional TL design is driven only at one end of the line, Castle adds the output from the rear of a second driver into the same line, but part-way along its length. 

According to Castle this technique delivers all the advantages of a transmission line but eliminates one of the disadvantages—namely the requirement to meticulously place damping material throughout the line—and enables higher sound pressure levels at the line’s output than usual, due to there being less energy lost inside the line.

As yet, Castle has yet to release detailed technical information about TDTL, but the lower bass driver is at the start of the transmission line, which runs up behind the front baffle, where it picks up the output from the rear of the upper bass driver before heading off down the line to the base of the speaker, where it’s turned around and sent back up to the exit on the front baffle. 

This means that, according to my ‘back of an envelope’ calculations, the transmission line in the Avon 4 is around 2.4 metres long with an average cross-section of around 140×110mm and an exit area of 57.5cm². The fabric covering the exit is 132×68mm.
As for the other aspects of the Castle Avon 4’s design, it’s a true three-way loudspeaker with the two bass drivers (C-037)crossing to a 115mm-diameter carbon-fibre-coned midrange driver (C-031) and the high frequencies handled by a true ribbon tweeter. Castle uses overall basket diameters to specify diameter. The Thiele/Small diameters of the bass drivers are 110mm and that of the midrange driver, 93mm.

As for that ribbon tweeter, unlike a dome tweeter, which has a relatively high mass and a suspension system to allow movement, a ribbon tweeter has an ultra-thin ‘ribbon’ of metal foil suspended in a magnetic field. Because the mass of the ribbon is negligible (usually around one-tenth that of a fabric dome) and there is no suspension to restrict the ribbon’s movement (it’s only tethered at the ends, which are also the electrical contact points), a ribbon’s response to electrical signals is virtually instantaneous, permitting its high-frequency response to be extraordinarily extended, without any unwanted resonant peaks in its pass band. However, because ribbon tweeters don’t work well at low frequencies, (so much so that a high-slope crossover is often required) they’re typically only found in three-way designs, such as this Avon 4.

The ribbon tweeter in the Avon 4 is made by Chinese manufacturer Fountek (its NeoX 1.0 model) and the reason it’s called a ‘true’ ribbon is because it is. (It has to be called a ‘true’ ribbon, because some manufacturers call their planar magnetic designs ‘ribbons’ which they’re not.) The problem with a ‘true’ ribbon tweeter is that it’s very amplifier-unfriendly, and therefore must be driven via a transformer, rather than directly by the amplifier. The three coned drivers in the Avon 4 are made by Castle’s owner, International Audio Group, which also owns Audiolab, Ekco, Luxman, Mission, Quad and Wharfedale. The Castle Avon 4s are manufactured at IAG’s plant in Shenzhen, China.

All this manufacturing information is shown on the rear terminal plate, which is home to two sets of absolutely massive multi-way gold-plated terminals so you can separately access the LF and HF sections of the crossover. If you choose not to do this, Castle supplies two huge plate-like gold-plated buss bars in order that you can link the two sections. Internally, the crossover is also split, with one section (with a large iron-cored inductor, a 47μV bipolar capacitor and a 10-watt cermet resistor) located behind the lower of the two bass drivers, and the other section, with four ‘Spirit’-branded MKP capacitors, one bipolar electrolytic, two cermet 5-watt resistors and a pair of inductors (one air-cored, one iron-cored), properly cross-mounted on the PCB located close to the tweeter.
Physically, the Castle Avon 4 cabinet is 955mm high, 180mm wide and 310mm deep, and weighs 17.6kg.

In Use and Listening Sessions

As with many modern speaker designs, the Castle Avon 4 is fairly tall and narrow, so it can be fairly easily toppled if given a hard shove, particularly from the side. Castle does supply a bottom plate that increases stability (it attaches via four large screws) but it increases stability only marginally. For maximum stability you could replace it with your own baseplate, using Castle’s original as both a template and a style guide.

Once positioned, I found the ribbon tweeter was a little too low on the baffle to deliver best sound at my listening position, so I used the adjustable spikes that are provided by Castle to tilt the speakers backwards slightly so the tweeters were aimed exactly at my seated ear height. This solved one of the known failings of ribbon tweeters, which is that although their horizontal dispersion is exemplary, the same can’t be said of their vertical dispersion, so it’s always best to have them aimed at your ears to achieve the correct frequency balance.

It didn’t take more than a few bars of listening to Bach’s Passacaglia and Fugue in C Minor to realise that the bass of the Castle Avon 4 was extended well beyond what I’d expect given the size of the bass drivers and the volume of the cabinet. But it took a jazz recording (James Muller’s ‘Neurotica’) to realise how well the transmission line integrated that extremely low bass with the upper bass. Whereas the bass of loudspeakers with sealed cabinets can sometimes sound too constricted across the midbass, and that from bass reflex speakers can be too flabby, the sound from the Avon 4 was ‘just right’… and by just right, I mean effortlessly naturalistic.

The term ‘effortlessly naturalistic’ also described the Castle Avon 4’s sound when it was reproducing midrange sounds, which are the most important in music, not least because this is where human hearing is most sensitive. Because the Avon 4 has its own dedicated midrange driver operating from its own sealed enclosure within the main enclosure, the performance of that driver is totally unaffected by that of the bass drivers. I heard this immediately when listening to Emma Swift’s voice on her self-titled album. Her voice came through as if she were standing between the speakers, singing live: It was that real. In the last verse of Bittersweet Swift inadvertently moves closer to the microphone singing that very word, and when listening to the Castle Avon 4s I involuntarily moved my head backward when she sang it because she was so obviously moving forward. Wow! This album is also a beautiful demonstration of the smoothness and extension of the transmission line principle, due to the unusual combination (on King of America) of baritone and pedal steel guitars, against bass… and of course against Swift’s own vocal.

If you haven’t heard a true ribbon tweeter before, you’re in for a treat with this one, because even though it’s only a small one, it delivers beautifully sweet and uncoloured high frequencies that you don’t have to be sitting in the sweet spot to hear perfectly. And the high-frequency extension is as extraordinary as the clarity—you can really hear the ‘shimmer’ in the sound of cymbals and the air around the harmonics of higher-pitched instruments is palpable. This was demonstrated to me perfectly when I listened to Rickie Lee Jones’ classic Chuck E’s in Love.

For me, the Castle Avon 4’s greatest strength was its performance at normal and lower listening levels. It’s no coincidence that most dealer and hi-fi show demos are at fairly loud levels, because this is where most speakers sound their best, yet when you’re listening at home, it would be rare to listen at these levels. Bear this in mind when you’re comparing the Castle Avon 4s against the competition, and ask not only for the listening levels to be matched, but also for the levels to be about the same as you’d use in your own home.

When you analyse all their advantages, it should be no surprise the Castle Avon 4s performed so well. The true three-way design, with the bass benefiting from the fluidity and extension that characterises the transmission line enclosure and the treble benefiting from being delivered by a true ribbon tweeter make for an aurally potent combination.  Daniel Stiel

POSTCRIPT: If you intend to use a subwoofer in conjunction with the Castle Avon 4 speakers you will need to ensure correct integration of the subwoofer's output with that of the Avon 4s by setting the subwoofer's volume, phase and crossover frequency controls correctly. You can read an article on a simple, effective method of how to do that HERE

Laboratory Test Results

Newport Test Labs measured the frequency response of the Castle Avon 4 Loudspeakers as 40Hz to 30kHz ±3.5dB which, although it’s just slightly outside the dB tolerance of Castle’s specification (which is 45Hz~20kHz ±3dB) more than compensates by extending both lower and higher than Castle’s claimed extension.

Looking at the response itself (shown in Graph 1) you can see that were it not for the tweeter rolling off a little above 15kHz, the ±dB tolerance could have been much tighter, because the response between 45Hz and 16kHz is within ±2.5dB. (Note that the trace shown on this graph is the result of two different measurements being spliced together at 1kHz. The response below 1kHz is the averaged result of nine individual frequency sweeps measured at three metres using a pink noise test stimulus. The response above 1kHz was measured using a gated sinus technique that simulates the response that would be obtained in an anechoic chamber.)

The director of Acoustic Design at IAG Group Ltd, Peter Comeau, and his team have managed to get the frequency response of the Castle Avon 4 extraordinarily flat across the midrange, so that from 150Hz up to 4kHz, the frequency response is within ±1dB. As for that roll-off above 15kHz shown in Graph 1 (and in Graph 2), that could have been a microphone/speaker positioning issue, because ribbon tweeters are highly directional. However, the roll-off occurs above the limit of hearing for most adult listeners in any case.

The high-frequency response of the Castle Avon 4 is shown in Graph 2 both with the grille fitted (red trace) and without it (black trace). You can see that the grille is truly acoustically transparent. The response is very slightly more linear without the grille, but the differences between the two traces are so slight that I’d recommend listening with the grilles on all the time, to ensure additional mechanical protection for the drivers.

The low-frequency response of the Castle Avon 4 is shown in Graph 3. Note that the level of the transmission line’s output (red trace) has not been adjusted to compensate for the difference in radiating area between it and the bass driver. You can see that it starts delivering bass at 150Hz, and keeps delivering bass within 5dB of this level right down to 30Hz and is 7.5dB down at 20Hz. This makes it fairly obvious where the Avon 4’s bass extension comes from, and seems proof enough that Castle’s ‘twin drive transmission line’ is a genuine advance in transmission line technology.

The dual bass drivers have a very smooth response across their operating range, and their low-frequency output is extended, holding at almost reference level down to 70Hz before rolling off at around 12dB/octave. From 70Hz up to 260Hz the response is within ±1.5dB. The roll-off at 260Hz is due to the crossover point being at 280Hz. You can see the midrange driver kicking in at this frequency, so the electrical and acoustic crossover points coincide very neatly, suggesting that the drivers were either designed specifically for this design, or just extremely well-chosen.

Newport Test Labs’ measurement of the Castle Avon 4’s impedance is shown in Graph 4. As you’d expect, the impedance curve at low frequencies is atypical, due to the transmission line loading, with a single resonant peak at around 72Hz (12Ω) with a small ‘shelf’ at 28Hz at around 6Ω, after which the impedance drops to its minimum of 4Ω at 10Hz. In fact, except for the resonant peak, the impedance stays well below 8Ω right across the audio band until at around 12kHz it rises above this value and continues to rise thereafter—which makes the speaker very ‘amplifier-friendly’. This rise in impedance is due to the transformer required for the ribbon tweeter, so you can see the ribbon starts working at around 5kHz. Although Castle specifies the ‘nominal’ impedance at 8Ω, it’s able to do this because it also specs the minimum impedance (which it puts at 3.5Ω), which is kind of a loophole in IEC268-5. Newport Test Labs’ tests put the minimum impedance at 4Ω and I would personally regard the nominal impedance of this design as being more like 4Ω, than 8Ω.

Newport Test Labs’ measurements put the sensitivity of the Castle Avon 4 loudspeakers at 87dBSPL at one metre using its standard test methodology. This is 2dB lower than Castle’s own specification (of 89dBSPL), but Newport Test Lab’s technique is tougher on speakers than the method used by speaker manufacturers and almost always results in lower figures, so 87dBSPL is a very good result and means you will be able to achieve very respectable sound pressure levels in your listening room without requiring too much amplifier power.

The Castle Avon 4s didn’t put a foot wrong in Newport Test Labs’ measurements, returning a superbly flat and extended frequency response, good sensitivity, and an easily manageable impedance modulus. 

Overall, an outstanding example of good speaker design.  Steve Holding