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From Hi-Fi World - February 2008 issue

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Sizing Up

Small loudspeakers can sound surprisingly large - providing you set them up to get the best from their diminutive dimensions. Noel Keywood explains how...

 

Baby loudspeakers may represent the budget end of high fidelity, but they do a surprisingly good job all the same. Small cabinets, properly contoured can give wonderful soundstaging. They are also rigid, resonate little and lack the colouration of tall floorstanders, whilst small bass/midrange units give clean vocals. All the same, the breed has drawbacks. Here’s how to make the most of those strengths whilst minimising the weaknesses.


bw686

B&W supply a two-part bung to tune bass output of their small 686 loudspeaker;

insert bung for wall mounting. Leave it out for stand mounting. If you want to biwire,

remove the links, as shown at right. Remove the small plastic plugs too when

4mm plugs are used...


POWER AND SENSITIVITY

Contrary to what you might expect small loudspeakers need a lot of power to go loud. You’ll see in our group test that a baby loudspeaker like the ALR Jordan has a sensitivity of just 82dB from 1 Watt measured at 1m - and this is low.


The solution is to use a powerful amplifier, or so it might seem. Unfortunately, this is where you start to bump into some immutable laws of physics that frustrate every attempt to overcome them. Wind the volume up playing Slade's 'Merry Christmas Everybody' and that baby loudspeaker will start to sound strained. This is because the bass unit must move air. You’ll see the cone manfully covering big distances in its valiant attempt to so do, but this isn’t a good way to do it.


What you end up with is a lot of distortion, which is where the strain in the sound comes from. Powerful amplifiers might come small and cheap nowadays, but what seems like a simple, affordable solution carries penalties. You cannot therefore just pump a lot more power into a small loudspeaker without suffering strain and possibly a blown loudspeaker in the longer term. All that power is going into a coil of wire - the voice coil - that heats up like an electric fire. Driven too hard it just burns out... There’s no easy way around this. In a nutshell, small loudspeakers don’t like going loud. They are for small rooms and in this context manage well enough.



SIZE MATTERS

Testing loudspeakers shows us there is a considerable performance difference between miniature loudspeakers and medium sized stand mounters, by which I mean models standing around 30cms (12in) high, with a volume of 25 litres or so. They may loom a little larger in the room, but the payback is you get disproportionately more bass, as well as higher sensitivity and better power handling. So if you can, shoehorn larger cabinets into your listening room. Do not be seduced by small size.


If you put large standmounters on wall shelves, the bass is likely to be overpowering, but there is a trick that can overcome this little local difficulty. Most loudspeakers are ported; put a foam bung in the port. This reduces bass level and increases damping. It may just allow a larger cabinet to work satisfactorily on a wall shelf. Look at B&W's 686 loudspeaker pictured here, supplied with a two part foam bung for trimming bass.


Another approach worth considering I feel, is to use a bass tone control (argh - blasphemy!) to trim back bass. The success of this depends upon the control's trim characteristics. Modern hi-fi amplifiers are a lot better here than those of the not too distant past. Look at the bass cut introduced by setting the NAD C315BEE bass control to -1. This provides shelf cut below 200Hz without upsetting frequencies higher up - just what's needed. This allows bass to be trimmed back appropriately if a bigger loudspeaker is used close to, or against a wall. This way you get better power handling, less bass distortion and the benefit of higher sensitivity. You will still suffer rear wall interference effects though (see below).


Alternatively, use stand mounting. Put the loudspeakers on temporary stands, initially placed against the wall. Listen to them, then progressively move the stands with speakers out into the room, choosing the position that strikes the best balance between bass level and soundstage quality.


WALL MOUNTING

Really small loudspeakers are usually intended for shelf mounting, as a rear wall aids bass output. It isn’t an ideal position; reflections off the rear wall interfere with the forward sound, causing successive cancellations around 300Hz, if distance to the rear wall is around 12 inches for example.


There is no easy way to combat this. Stereo staging is degraded and the sound is not as smooth as possible. But it is the best position for strong bass. In a 12ft long room, for example, the lowest mode is 45Hz and you will drive it most effectively by wall mounting, ending up with a nice deep thud.


If you find wall mounting subjectively acceptable, then ensure the shelf is strong and rigid. B&Q sell suitable cast iron kitchen shelf brackets.

If the loudspeaker has a rear port a few inches will needed behind it to avoid chuffing sounds. Many small loudspeakers have front ports so they can be placed hard against a wall, the drawback here being that a front port often makes internal cabinet colouration audible.


It isn’t uncommon for manufacturers to reduce bass output a little in loudspeakers designed for wall mounting, to reduce possible room boom. In cases like this wall mounting is usually the only option. The handbook invariably makes this clear. You can use the bass control to trim output to taste, bearing in mind that turning bass up will cause strain and may not work too well.



POSITIONING

Loudspeakers are usually placed 8ft or so apart, at ear height and toed in to point straight at the listener. Common wisdom has it that this is ideal. However, our measurements show that sometimes loudspeakers are designed to face straight down the room, giving too much treble if pointed directly at the listener. Stereo imaging is a little less sharp when pointed straight ahead, but the listening position is less critical. It’s always worth experimenting here; in particular coaxial loudspeakers like Tannoy Dual Concentrics and KEF Uni-Qs usually sound a little better listened to off axis.


If you use stands, fill hollow ones with sand or shot. This makes the stand stable and acoustically inert. It is common to spike a stand into a wooden floor, often through carpet. With bare wood floors you soon end up with what looks like a concerted woodworm attack, these days guaranteed to knock so many grand off anything other than a cowshed that you may consider using Blu-tak as a less damaging alternative. You can also buy small spike 'cups' with indentations on their upper surface to accept and anchor the spike, or you can use a coin under each. The general idea is to ensure the loudspeaker is firmly anchored, as this gives better defined transient attack and cleaner dynamics. Of course, the loudspeaker must sit firmly atop the stand too, so the whole assembly doesn’t move. Top spikes are often used for this purpose. Be careful though, as top spikes, as well as floor spikes, can inflict nasty wounds. They are best not used where children are around.


CABLES

Always use dedicated loudspeaker cables. We make up measurement cabling using heavy duty Maplin Oxygen Free Copper (OFC) Shark Wire, silver soldered into quality connectors and it sounds quite hazy against even a budget loudspeaker cable from Chord Company or similar. Good cabling really does help, especially when the cables are long.


Biwiring helps in long runs too; often the bass conductor comprises many fine strands, whilst treble is carried in a few thicker strands. Most loudspeakers nowadays are designed for biwiring, having removable links. The impedance characteristics of bass and treble crossover sections within the loudspeaker serve to ensure each cable conducts current only over its frequency of operation, which offers benefit to the sound.


Another handy feature of biwireable loudspeakers is that, if they sound too bright, they can be tamed by placing a resistor in series with the treble section, starting out with 1 Ohm and increased in 1 Ohm increments. Buy a pack of five or ten 1 Ohm resistors for this, 1W Carbon Film type.


AMPLIFIER MATCHING

Small loudspeakers are little different to large ones when it comes to amplifier matching. Nowadays some effort is made by most loudspeaker designers to ensure the electrical load presented to an amplifier is reasonably ‘friendly’ so matching isn’t a problem. Transistor amplifiers produce more distortion into 4 Ohm than 8 Ohm loudspeakers, but the increase isn’t usually great, from something like 0.018% to 0.022% for example - not enough to say that 4 Ohm loudspeakers are a problem.



nadc315bee


nad-315bee-bass-cut


Turning bass down to -1 on the NAD315BEE introduces shelf bass cut. This neatly compensates

for wall mounting a large-ish loudspeaker, a scheme with many benefits. Stand mounting is

always the best choice though.



A majority of modern loudspeakers use 4 Ohm bass units in order to maximise sensitivity. Whilst amplifiers may have an easier time with 8 Ohm loudspeakers, much of their inherent capacity to deliver power, such as a big mains transformer and beefy heatsinks, goes wasted. So on balance it's probably best to choose 4 or 6 Ohm rated loudspeakers, especially when they are small, as it improves sensitivity, and modern amplifiers are designed to cope.


Small loudspeakers are all about getting good sound, usually at a low price, in a small room. It can be done - and very well. Choose your components carefully and spend a little time and cash on extras like stands and cables and you can get a fantastic result, without breaking the bank.

 
Comments (1)
little help then...?
1Tuesday, 06 September 2011 15:45
Marko Saric
Hello,
First of all, this all is very helpful to someone like me, who is in the process of searching the right speakers for NAD C315bee.
I used Monitor Audio BR5 for quite some time, but had to sell them. Now searching for bookshelf speakers. Considering rs1, rx1, rx2...
What would be the right choice?
Thank you

Hi Marco. THe BR5 is a very good loudspeaker and a fine match for the NAD 315DEE so you are right to stay with this brand.

Of the Monitor Audio loudspeakers you list the RS1 is obsolete. The RX1 is small shelf mounter and the RX2 much larger, suitable for a large shelf or a stand. Look at their dimensions on the Monitor Audio website at - http://www.monitoraudio.co.uk/products/silver-rx/rx1/#/specification. If the room is small, less than 12ft / 4m maximum - a large loudspeaker on a shelf will give boomy bass. If you were happy with bass from the BR5 then choose the RX2, as large shelf mounters go louder and deeper than small ones. Be aware though that floorstanders like the BR5 are more sensitive than shelf mounters so you are taking a step backwards here. Get another floorstander if you can.

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