Australian HIFI - - ON TEST -

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 IEC2685. 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

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