Such accuracy is, above all, what the unique MonoPulse design sets out to achieve.
It is widely accepted that pure electrostatic hi-fi loudspeakers are inherently accurate. This is because different frequency elements are never split up.
They present sound in a way that is true to real life, where all frequency components arrive at the listener together. This is particularly important with the leading edge impulses in sounds, because these give our main sense of presence and direction. (More detail in part 3 below)
Conventional loudspeaker designs, with multiple drivers, although offering bass and volume levels which are unachievable with pure electrostatics, inherently split up these frequency components, and then present them to the listener with these important leading edge impulses out of phase.
When studying for his electronic engineering degree, Allan Hendry, MonoPulse's designer, was an enthusiast of electrostatics. It was to be much later though that his work on phased-array radar systems made him realise there had to be a way of achieving leading-edge impulse accuracy using two moving-coil drivers - and getting the best of both systems.
So, in May 1993, by then a Fellow of the Royal Aeronautical Society, he clearly defined his objective by registering the brand name "MonoPulse", to cover an impulse-accurate hi-fi loudspeaker design he intended to create.
It was to be almost eleven years before he had produced a design which met his requirements. The result was the realism and accuracy of sound-stage he was seeking. This is now offered in a unique range of perfectionist-design, craftsmen made to each order, hi-fi loudspeakers.
If you are looking for no compromise in your hi-fi, the eleven years were worth it.
Cost. The design has to use just two drivers. With any more, impulse accuracy is impossible. High quality drivers to cover the full frequency range and deliver the required power are expensive. A special crossover is also needed. And, if the design goes to such lengths to get one aspect of performance correct, it ought to do the same with the others.
Physical constraints. The electronic design defines the exact position of the drive units, so form has to follow function.
Directionality. These are inherently "sweet spot" loudspeakers - for frequency side/side, and impulse accuracy up/down. The optimum zone is about 2.5 metres wide, and between 0.8 metres and 1.2 metres high, but this is OK for 2-3 people, and covers anything from a low sofa to an upright chair. This directionality is inevitable in achieving impulse alignment, giving hi-fi speakers to be listened to - not for background.
Can the MonoPulse difference be proven? Yes, very clearly. This, and its significance, are shown and explained below in three parts.
To measure a speaker's response accuracy to an impulse or leading edge, we use a function generator to input a step function to an amplifier driving the loudspeaker. Like this:
Impulse input.
The speaker's resulting response to this input impulse, is then measured with microphones in front of each drive-unit, exactly the same distance from the listening position.
The two traces below are the responses of the HF unit (blue), and the LF unit (red), of a MonoPulse Model A. We are looking for the traces to be in exact synchronism.
MonoPulse Model A impulse response.
As you can see, the onset of the responses are almost indistinguishable.
The rapid bounce-back of the HF unit is normal, and will be seen on all traces. HF units respond only to high frequencies and their cones or diaphragms move back quickly once the leading edge of an impulse has passed.
The horizontal scale here is important. Each gap between the vertical lines, is the time sound takes to travel 17 millimetres. We are looking for 3mm accuracy on MonoPulse designs.
Do we need this accuracy? Yes. To sense a sound's direction, we detect the different impulse arrival times at each of our ears. To get that direction accurate to ten degrees, we need to resolve the travel paths of the impulses to within about that 3 millimetres.
So how can we show that this MonoPulse precision is genuinely different?
All we can do is show the responses of some other top-end loudspeakers.
Typical impulse response of a prestigious two-way hi-fi loudspeaker.
(Ignore the "wiggle" on the LF trace. It is due to background high-frequency electrical noise in the dealer's premises where these measurements were taken.)
Two points are immediately obvious:
The first, is that the initial movements of the HF unit (blue) and the LF unit (red) are actually in opposite directions - the blue starts down, the red starts up.
The second is that the response of the LF unit lags behind the HF unit, by nearly 6 centimetres of air path.
What should have been a single sound, becomes two different sounds traveling towards the listener 6cm apart, and out of phase.
Here is the trace from a particularly expensive model. Again it shows the units setting off in anti-phase, and with the LF unit lagging the HF unit, this time by about 4 centimetres of air path.
We emphasise that we did not specially select any loudspeakers to measure - they are very typical top-of-the-market, units, and mostly more expensive than the equivalent MonoPulse models.
It is mentioned in Keith Howard’s article for Hi-Fi News, below, that some hi-fi loudspeaker manufacturers have now, like MonoPulse, adopted in-phase driver connection. This change is confirmed by one of our measurements (below), showing that the initial movements of the two units are in the same direction.
But, unfortunately, again the LF unit (red) lags, again by 4cm. The activity in the HF unit is all but over before the LF unit responds to the same input. This is despite the fact that the HF unit is separately mounted on top, and allegedly "time aligned".
The situation becomes even worse with three-way designs. These have the same timing discrepancy between their HF and MF units. Then there is a further delay of the LF unit behind the MF unit, typically up to 20 centimetres of air path, and in the wrong direction.
It ends up with two of the three units initially going in one direction, and the other the opposite way, and with large timing errors. This was true for all the makes of three-way units we tried, and was even worse for those with more than three drivers.
Read on to Keith Howard's Hi-Fi News article below.
Your speaker’s drive units may be connected out of phase. It isn’t faulty – it was designed that way. But, asks Keith Howard, our technical consultant, is this really a good idea?
If you are familiar with the design of loudspeaker crossovers, you will know it is common practice to internally connect up drive units with opposite polarities.
Average hi-fi users, with the familiar warning about connecting speakers to the amplifier with consistent phase (red terminal to red terminal, black to black) ringing in their ears, will find this odd. There are good reasons why it’s done, but it turns out that the knee-jerk reaction to consider it strange may be the right one.
Some loudspeaker designers have come to the conclusion that it is a bad habit the audio industry should break.
Talking to a succession of speaker designers in recent months, they have mentioned a factor which isn’t often heard about – loudspeaker impulse response - and the impulse response of a typical multi-way loudspeaker is not a pretty sight.
MonoPulse speakers have been designed with impulse response firmly at the top of the agenda.
Allan Hendry justifies his unusual choice of crossover filters on the basis that it allows the bass-mid unit and the tweeter to be connected in phase.
And I had a telephone conversation with Steve Roe of B&W whose latest 800 Series deliberately avoids anti-phase driver connection. And why? For the best possible reason: it sounds better.
The article then explains why anti-phase driver connection has been used for decades in hi-fi loudspeakers, since Siegfried Linkwitz and Russ Riley in 1976, and their second-order crossover design. The article also includes the comment...
But if you look back through audio’s annals you will find occasional voices raised in unease at it. Over 20 years ago, Richard Greiner. To paraphrase what Greiner was saying, is that if you put an impulse into a speaker with opposed driver polarities, then as one diaphragm moves forward, the other will move back – an intuitively undesirable situation, particularly given the established significance of leading-edge transients in music.
Read below, of the way we hear sounds, and why the MonoPulse design is so significant.
This section explains what has driven the MonoPulse design, and how time-domain impulse accuracy, with in-phase driver connection, makes them so realistic.
It first needs some facts about our hearing, and about how hi-fi systems, and hi-fi loudspeakers in particular, have problems in handling some components of sounds the way we need them to.
So let’s start from the beginning...
Our ancestors had to sense danger, such as from the sound of a breaking twig, to survive in the world in which we evolved. We still have this ability to instantaneously and very accurately know the direction those sharp-edged noises come from. But how we do this is not at first obvious.
Any continuous sound, however complex and harsh, is in fact a mixture of many pure tones – as proven many years ago by the mathematician Fourier. In each of our ears these sounds are detected by about 3,000 tiny sensors, each 'tuned to', or picking up a resonance at, a different frequency. When we listen to a particular noise, a mix of tones, just those sensors for that mix of pure tones will react – and each one sends a signal to the brain. Our brains analyse and recognise these different combinations of frequencies – maybe as the shriek of the wind, the howl of a wolf – or the cry of a child.
And interpret this tone mix as a particular sound. But, once the sound has started, these sensors have no sense of phase - in which direction the incoming air pressure is moving at any moment - just that it is moving. So, if we are not sensitive to phase in sound waves – that it means sound-systems which mess it up are OK? No.
The problem is revealed when you consider how we so accurately sense a sound’s direction. For this, we use the sharp-edged impulses at the leading edges of sounds. One bit of phase our brains can detect very precisely, is the exact moment that a leading-edge pulse arrives. Depending on the direction they came from, these pulses arrive at each of our ears at different moments. And, sensing the difference in those arrival times, we work out the direction. To position the source of a sound to within ten degrees, needs an accuracy of about 3 millimetres in air-path detection. A bit of simple geometry can show this. Smoke detector alarms don’t produce many impulses – that's why it's not always easy to tell where they are. But, if we close our eyes in front of a group or orchestra, we don't just want to know what instruments are playing, but where they are. This is the sound stage, and without it there is no proper sense of realism.
In the natural world, an impulse starts at one moment in time from its source and arrives, unchanged apart from volume, at our ears as a single wave-front. There is impulse integrity - all frequencies within that sound travelling and arriving together.
An impulse, like any component of sound, is a mixture of frequencies. In most loudspeakers these are split up by crossovers, or by digital processors, and sent to different drive-units, with acoustic-centres at different distances from the listener. These differences are usually well over that crucial 3 millimetres - in fact typically more than 4 centimetres different - and there are sometimes three or more different distances. The result? What should be a single wave-front, has divided frequency components arriving at our ears at a mixture of different times. This was shown by the measurements in part 1 above. So our brains, which sense so much from the arrival timing of a real-life integrated impulse, know that we are not in a natural sound stage.
An erudite article in Hi-Fi News in July 2005, "Phase change", by their technical editor Keith Howard (extracts in part 2 above), revealed that nearly every multi-way hi-fi design in the last forty years, presented with an impulse, has one speaker cone with its initial movement in one direction, and the other(s) going in the opposite direction. What does our highly evolved auditory system make of that? Conclude that this is not a sound made by a real event? Almost certainly.
Keith Howard himself comments, "it is a bad habit the audio industry should break."
Our holy grail is impulse integrity. We use only two high-quality drive units, closely spaced, correctly offset to millimetre accuracy, and with special crossover electronics, to give a single impulse wave-front, accurate to within the 3mm detectable by our ears. On the Model A, we also acoustically isolate the HF units separately on the quarter-inch rolled-steel surrounds, preventing intermodulation from the main cabinet, and giving further purity of response.
MonoPulse hi-fi loudspeakers can improve any system. The impulse accuracy means that they can create a sound stage even if placed wide apart. It can be a dramatic effect - and shows that if positioned conventionally, the sound stage will be superior. Everything is better – but unplugged style recordings are the most changed. In a musical sense, the human voice is the instrument we are most familiar with – and full of sharp-edged impulses from our imperfect vocal cords. We can tell if that sound has been messed around – or notice the difference if it has not.
Try it!!
Allan Hendry, BScEng, AKC, FRAeS