Allan Hendry, a hi-fi enthusiast as a student, became an electronics engineer in aerospace. His work on advanced, phased-array, radar systems then made him realise there had to be a way of improving a long-established design process used for multi-driver hi-fi loudspeakers.
In summary. The onset of a real-life musical sound carries all its frequency components to the listener at the same time in its leading edge. It is these all-important leading edges which give our main sense of presence and direction.
But, multi-driver loudspeaker designs split up these leading-edge frequency components, and present them to the listener at different times. This established design process goes back over thirty years, and results in something significantly different from the way a real and live sound is presented to a listener.
So, in May 1993, by then having been elected as a Fellow of the Royal Aeronautical Society, Allan Hendry clearly defined his technical intentions by registering the brand name "MonoPulse", to cover an impulse-accurate hi-fi loudspeaker design he intended to create.
For a long time there seemed to be good reasons for the use of the standard design process and, although eventually working full-time on the project, it was to be almost eleven years later before the first of the new products overcame the problems and met his requirements. The result gave fantastic realism and accuracy of sound-stage. Since then, a range of these perfectionist-design, craftsmen made to each order, hi-fi loudspeakers, has been created.
Some overall conclusions?
The design has succeeded, but it does impose some limitations. The most important is enforced simplicity. This approach can only work by using just two high-quality drive units. So impressive arrays of multiple drivers, and sub-woofers are out.
Correct positioning of the HF unit also imposes certain cosmetic design limitations, and the crossover has to be a bit expensive and special.
But, if you are looking for the least compromise in your hi-fi, we believe the result is worth it. Accuracy of leading edge information is important - and can be heard in the realism and sound-stage produced.
Below, in three parts, we introduce you to the nature of the original problem, and to the MonoPulse solution:
Part 1. The Impulse Measurements. Here we demonstrate the absolute precision we achieve with the MonoPulse design. To show we have achieved something that is genuinely different, we also show measurements of some other top-end hi-fi designs.
Part 2. Phase Change, is extracted from an erudite article in the prestigious Hi-Fi News, by their Technical Consultant, Keith Howard. It explains part of the problem, and why so many hi-fi loudspeaker manufacturers internally connect drive-units out of phase as part of the standard design process. In Keith Howard's words it "is a bad habit the audio industry should break". The fundamental correctness of the MonoPulse design means it does not have to connect drive-units out of phase.
Part 3. The MonoPulse Design, provides more detail. It deals with the nature of our hearing, why typical hi-fi loudspeakers have problems producing true-to-life sounds, and what MonoPulse now does that is different.
Read on to Part 1 below.
Our ancestors had to sense danger, such as from the sound of a breaking twig, to survive in the world in which we evolved. Those sharp-edged noises arrive at our ears with all their frequency components together. That integrated leading impulse is all important - we sense direction by the difference in its arrival times at each of our ears.
This means that reproducing the integrity and timing of leading edges, is key to a loudspeaker's ability to position performers on a sound stage, and to create a sense of reality.
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 a microphone 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 82A. We are looking for the traces to be in exact synchronism.
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 more quickly once the leading edge of an impulse has passed.
MonoPulse Model 82A impulse response.
The horizontal scale here is important. Each gap between the vertical lines, is the time sound takes to travel 17 millimetres. In fact we are looking for 3mm accuracy on MonoPulse designs.
Do we need this accuracy? Yes. To know 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 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 the that, in response to an impulse which should have the two units in exact synchronism, the initial movements of the HF unit (blue) and the LF unit (red) are in opposite directions - the blue starts down, the red starts up.
In an erudite article for Hi-Fi News, (Extracts in Part 2 below) their Technical Consultant, Keith Howard, explains why so many loudspeaker manufacturers find it more convenient to do it this way. However, Keith Howard himself comments that it "is a bad habit the audio industry should break".
The second thing the measurement reveals is the lag of the LF unit behind the HF unit. This amounts to approximately 57 millimetres, or nearly 6 centimetres, of air path. What should have been a single integrated response, virtually becomes two different sounds travelling towards the listener 6cm apart.
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, but truly top-of-the-market, units.
It is mentioned in Keith Howard’s article, 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".
Below is another impulse response from a current best-selling top-end manufacturer. The delay between the HF response, and LF response, here is about 7 centimetres.
We are not saying that these are bad designs. They are all more expensive than the equivalent MonoPulse models and will have some aspects of their performance which may well be superior.
But it does seem to us that the impulse accuracy is an obvious, if inconvenient, measurement which reveals a key component of a speakers performance.
Let us present an extraordinary fact that illustrates just how significant impulse timing accuracy is.
It would be possible to cause an impulse inaccuracy in a MonoPulse speaker, equivalent to 4 centimetres of air path, as the above examples, by using incorrect cabling. It would require bi-wiring - with the LF cables about 18 miles longer than the HF cables - and connected the wrong way round. Do the arithmetic yourself - signals propagate in a cable about seven-hundred-thousand times faster than sound in air.
We mentioned in the introduction the constraints on appearance and styling imposed by the MonoPulse design. We can use only two drive units, closely spaced, and with the HF unit offset being very precisely defined. We often have the strange problem of a customer loving the sound, but wanting to spend more money, perhaps also on something looking more impressive - like with multiple drivers.
So what are the timing characteristics three-driver (or more) speaker setups? 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, with combined timing errors of about 25 centimetres of air path. This was true for all the makes we tried, and was even worse with more than three drivers.
Slightly related is the issue of electrostatic speakers. They are perfect for some users and material, being inherently impulse accurate - unless another driver is added to handle the bass. In which case the problem above applies. Without an extra drive unit there are limitations in power and bass for some content.
We believe that leading edge accuracy is a very important factor in the reproduction of realism, and for the creation of an accurate sound stage. Indeed, we think it is fairly difficult to argue that it isn't.
But maybe the most important thing is to listen.
Read below, the extracts from Keith Howard's article about out-of-phase drive unit connection.
Your speaker’s drive units may be connected out of phase. It isn’t faulty – it was designed that way. But, asks Keith Howard, 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 [see the March issue review of the MonoPulse 32] 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 when a sensor first reacts in response to that leading-edge onset pulse. 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 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 models 42A and 82A, 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