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Part Two


April 2011 - Elsinore Mk5 - some details below are obsolete.


Preliminary Design Details


In the first instalment, I did make a rather strong attack on the usual two-way speaker system that seems to be everywhere. It seems that I am not alone, as I have just picked up the November 2005 issue of Hi-Fi World. If you thought I was vociferous, then compare the two statements:


“I do have a problem with the general design of speakers, most of them are rather ho-hum, and sick of the usual two-way done to death, just another plain box with small flat baffle, a small port and fairly conventional crossover. Some of these are done rather better than others, but there is a bland sameness. They seem to follow too much a formulaic approach.”

Now for Hi-Fi World:

“Has the two-way loudspeaker had its day? Marrying a bass/midrange unit with a small dome tweeter can give good results but this idea has now become part of ingrained design mentality that is not only uninspiring, it’s limiting performance…a sea of mediocre two-ways.”

Has somebody been reading my mail? Find the issue and read Page 47. The main diagram shows the effect of the acoustic offsets when the drivers are mounted on a flat baffle. So while their solution is to change the crossover frequency to a lower point, they recognise just how difficult that is. So if Muhammad won’t go to the mountain, then the mountain must go to Muhammad. This will be our solution, namely to adjust the physical position so they are equidistant to where the listener sits.

Now let us get on with this month’s issues and last month we said we were going to discuss the following issues, be sure to read number 10 as it contains new original development:

1. Diffraction Loss

2. High sensitivity vs. electrical impedance/phase response

3. Flat off axis (and hence) total power response

4. Low thermally induced dynamic compression

5. Non-peaky crossovers, theoretical 'perfect transient'

6. Minimum phase disturbance over a wide bandwidth

7. Correct acoustic summing at crossover frequencies

8. Pipe Resonances in tall enclosures

9. Max acoustic power transference into room

10. ACOUSTIC IMPEDANCE DRIVER MATCHING <<< © Copyright Joe Rasmussen 2005

11. Quality Drivers


So let us start with the first one:


1. Diffraction Loss


Rarely do we hear much on this topic, but it is so crucial at the first stage of design. Basically, it has a profound effect on frequency response, overall sensitivity and tonal balance. In fact, the last two can be traded off against each other. Want more sensitivity, then simply use less compensation. But what is Diffraction Loss? Note here the accent should be on loss and something that needs compensation. Less compensation gives you higher sensitivity, but that will mean less bass because this is where the loss occurs. Diffraction Loss (DL) should not be confused with diffraction distortion (DD), which is not a loss but a series of acoustic discontinuities. I suspect we shall discuss that (too) later, but let us concentrate on diffraction loss, DL, and how to compensate for it.


Have you ever seen very nice frequency response graphs made available by driver/speaker manufacturers? When in comes to bass/mid drivers, they usually show a fairly smooth response at least up to 1KHz and hopefully beyond. But these responses are not recorded from drivers mounted in a box. No, it is usually on a large panel and in some cases a wall where the speaker facing into a room that is heavily absorbent like an anechoic chamber. But the most likely is the IEC Baffle:

So the driver is flushed mounted on a large surface area, nothing like the box it is going to end up in. Basically these panels (or chambers) have little or no DL above 100Hz. Note also the asymmetry of the mounting of the driver on an IEC Baffle, so that relative to edges and corners have no repeating dimensions. This means we are likely to get an even response not affected by Diffraction Loss above 100Hz (DL affects mostly 100Hz up to 1HKz max).


Mounting a driver in a box, the ideal desirable response might look like this:



Except for a few minor dips and peaks, that is what the overall response would look like if the driver were a good one. This is the response likely in an IEC Baffle, except the response would die off more rapidly below 100 Hz because of back-to-front cancellation (the baffle is open, so for the time being we can ignore that).


But what happens when we mount the driver in a box:




Okay. What is happening here? For a start, there is a gradual drop in response, which levels out at –6dB. Here we have a graphic example of DL. It always has a 6dB loss factor. This is the ideal curve for DL, but in fact it can be a lot more undulating that the smooth example here (see example further on). But the response will still start 6dB higher that it levels off on the bass. This is sometime also called the baffle’s ‘step response’ as it looks like a step in the response.


But why does it happen and what modifies the response like that? Let me introduce you two 2Pi and 4Pi concepts. If the panel is infinitely large, then we have a 2Pi response at all frequencies. All the pressure created in front of the cone is pushed forward; none of it goes in reverse. With the IEC Baffle, which is rather less than infinitely sized baffle, the response is 2Pi above 100Hz. If the baffle was even larger, we could push the 2Pi response even further down in frequency. But what happens when the baffle area becomes smaller, then below the 2Pi response becomes 4Pi response. The pressure that was forced forward now can wrap around the baffle and hence the forward pressure is reduced roughly by half, which equals 6dB.



See that in the illustration, when the wavelength becomes longer it is now able to wrap around and as such the pressure, and hence volume level, is reduced to half the amplitude. In the crossover we can apply the following series compensation network.

Here R represents the driver load. The choke introduces a downward slope, is then arrested by the value of the parallel resistor, which value can be set to 6dB. This would look as follows:



So the top part of the graph is before compensation and lower is after compensation, and is now a flat response. This is the affect of compensation, we now have regained the required flat response but at what penalty? The system sensitivity has been reduced by 6dB and that means in real terms that it takes four times the power to maintain the same SPL, which is another way of saying volume (keep that in mind).


Small mini monitor speakers with their tiny baffle aligned with small cone, that is a recipe for low sensitivity and therefore in the 80-84dB range is common. They need a fair bit of pushing. Our target after compensation will be 88-90dB.


Unfortunately, there are complications with DL we must touch upon. The nice ‘step’ you see in the ideal DL slope rarely happens that way. In fact it, the pre-compensated response can look something like this:



Not pretty. So it seems that, while the driver may have a flat response, when mounted in a box we have both DL as well as ripples in the response as some edges of this rectangular baffle are closer than others, building up pressure at certain frequencies and not others. Still we can see that 6dB step is still there at the extremes of the graph. But we also experience some peaks and troughs in the response. So conceivably, if we can keep the baffle area reasonably large and rectangular (a round baffle is the worst), as well as using multiple drivers covering this area can smooth out the response. This is what we aim to do with the Elsinore.


If we can get the step down to a low frequency (because of largish baffle area), we may need to use only 4dB compensation. How much we end up using depends partly on how it will measure in box as well as listening. But hopefully we won’t loose the whole 6dB but we may have to be prepared to.


2. High sensitivity vs. electrical impedance/phase response


Selecting the driver affects the sensitivity of our total design, but so do other things. At the beginning I had in mind a speaker system with high sensitivity and yet also 8 Ohm. There are speakers that boosts the sensitivity by paralleling drivers, so two 8 Ohm drivers gives us an extra 6dB in sensitivity, but now being 4 Ohm together, draws twice the current. Now let us use our selected 88dB driver as an example. Put two in parallel, giving us 94dB. Take away 4dB for DL compensation, and we will have a final system sensitivity of 90dB and four Ohm. So in terms of sensitivity we have gained 6dB, but in terms efficiency, only 3dB since we have cheated an extra 3db by drawing twice the current.


But I want to gain the whole 6dB and to do that I must use four drivers is combined series and parallel that will give 8 Ohm for the system. I have now gained 6dB in both sensitivity and efficiency. This makes for a speaker system that will be easy to drive and also have very large power handling. It will be compatible with a huge number of amplifiers. Being an 8 Ohm system, we can also avoid the pitfalls of many expensive speakers, where we have frequencies where the impedance and the negative phase angle coincide and place great demands of the amplifier. Our projected system impedance should not drop below 6 Ohm at any frequency and hence even a moderate negative phase angle can easily be accommodated. And this is like most only to occur about 100-200Hz – this will be confirmed when we measure the final impedance/phase of our completed speaker system.


3. Flat off axis (and hence) total power response


Most are familiar with the concept of ‘frequency response.’ But what is not appreciated is that it is a singular discrete measurement. You look at a frequency response and you’d have to ask where the microphone was situated, the height, the distance and whether it on axis or not etc. The frequency response graph only shows what the response is where the microphone is located. So in fact the speaker has many frequency responses, because the response will change when you change the position of the microphone. So we need to understand the concept of ‘power’ response. Van Dickason of Loudspeaker CookBook fame, suggested that it is the off axis frequency response that indicates whether the power response is flat or not. While that may take a bit to wrap one’s head around, it does in fact make a lot of sense. But John Atkinson of Stereophile has taken this idea a step further: The real frequency response of a loudspeaker is the averaged out response of the speaker over a 50° arch at the height of the listener (and usually the height of the tweeter). This way we are able to reconcile both frequency and power responses.


It has been shown that a speaker that measures flat on axis but has a droop in the response when measured off axis (often the midrange start to beam before tweeter takes over and the total off axis response dies) sounds dull in the upper midrange and likely sounds like two drivers that do not gel properly. Image placement and other audible shortcomings issue from poor integrated response in the power domain.


So how do we deal with this? We must have a strategy before we can even start. I propose the following, using low order crossover the drivers must be physically related to each other at the listening position. Or to put it more plainly, the distance between the acoustic centres of each driver must be equal distance to the listener. Our lower crossover is likely to be 300-500Hz where the wavelength is nearly a meter. Provided the difference is driver offsets are within 10% of the wavelength of the crossover frequency, we can mount them on the same flat baffle. So this is no problem with our lower crossover. The long wavelength solves that one. But if our upper crossover is likely to be as high as 4KHz, where our wavelength is 85mm, then a few millimetres become critical, and our error offset should now be well below 10mm. Now the problem that Hi-Fi World highlighted has been solved. The way to deal with it is to mount the tweeter further into the cabinet. This is because the acoustic centres of the 6.5” drivers are much further back than that of the tweeters; this is what we call their offsets. So we need to compensate for the difference in offsets and that difference equals how deep the tweeter needs to be mounted. If you go back to the main box illustration, you can see the tweeter recess, this needs to be filled with felt or foam pieces, carefully cut to size minimise diffraction distortion that can be caused by sharp corners around the recess. This technique was common in Duntech speakers, so much so that they patented it if using felt in particular. But the patent was was contested in the market place (not court) and was not defended.


Since we have now aligned the acoustic centres of all the drivers, we can now construct crossovers that have complimentary phase (not the same as absolute phase) and optimise both our on and off axis responses. The beauty of this method is that it causes almost zero acoustic interference between drivers (seen as ripples in the acoustic response as a series of cancellations and summings leading to a series of peaks and troughs either side of the crossover point) and also get a nice blending of the two. When we come to constructing the crossovers, we shall revisit the actual details of this. But the simple fact is that the summing and blending of the two drivers becomes a great deal easier this way, the audible driver integration is hugely enhanced and tweaking the crossover point becomes much more simple when using the ‘sum and null’ technique explained later.


4. Low thermally induced dynamic compression


One of the great enemies of loudspeaker technology is heat. It goes to the obvious that speakers are rarely more than 1% efficient, then what happens to the 99%? It gets expended as heat, and heat in the voice coil has to be dissipated. More than that, the rise in temperature increases the DC resistance of the voice coil conductor, whether copper or something more exotic. When a transient is being fed there will be a sudden increase in that effective resistance which then erodes the Qe (electrical Q) of the driver, which in turn lowers efficiency at that moment. So on major transients we now have a mechanism that causes dynamic compression. How audible is it? It is a lack of responsiveness and expressiveness. Many like highly efficient speakers like Lowthers and horns because they have wonderful get-up-and-go quality. They may not be the ultimate in transparency but they may be the ultimate in immediacy. They need less power and they suffer less from dynamic compression.


In the Elsinore project we are using drivers that are about 88dB @ 1 meter sensitivity and 8 Ohm. If we were then to compensate for DL, we would end up with 83dB approx sensitivity and even though they are a reasonable 8 Ohm, they will need to be driven hard. Not a solution that excites me.


But here is where the germ of the main idea of the Elsinore comes in. Four drivers will gives us 94dB instead of 88dB and still the same 8 Ohm. Hence the 6dB increase is genuine as it is 6dB increase in efficiency. We can trade that against DL and hence we have lost nothing.


But more than that, if we run them together up to at least 300Hz, then above that we can use the two to cover the mids while gradually rolling off the bottom two drivers. In effect the top two bass/mids and tweeters become a D’Appolito MTM array.


As these take over, DL will diminish, making the top two bass/mids working into 2Pi and compensated 6dB for flat response, while all drivers are combining below 600Hz and so will compensate for DL and add the required 6dB acoustically where it is needed. Bingo, a wonderfully simple mechanism to deal with two key problems needing solutions. So by combining DL and a simple crossover techniques into multiple drivers, we should, worse case scenario get 88dB @ 8 Ohm and likely 90dB.


What about dynamic compression, is this being reduced? Absolutely.


Since the gain of 6dB equals four times less power required for same SPL (volume) and that four drivers share the thermal load, each driver has to cope with only one sixteenth the power.


The heat and temperature in the voice coil is several orders lower and dynamic compression dramatically reduced. This speaker will have excellent dynamics and highly expressive, but without sacrificing other good things like transparency and image placement.zH


5. Non-peaky crossovers, theoretical 'perfect transient’


This topic will be short. Some crossovers store energy and then release it a moment later. They also cause smearing in time. So why are they used? For instance, in 3-way crossovers, the losses in a bandpass filter can be a real problem, so using peaky crossover we can boost the output. Nice technique on paper, but to be avoided at all cost. So what are theoretical ‘perfect transient’ crossovers? Rather than react they give, which is to say they are lossy. Think of throwing a ball at a wall, it will bounce back. That is reactive. Now a ball that is able to absorb or loose the impact, hits the wall and drops straight down.


Generally crossovers that are as resistive as possible are ‘perfect transient.’ This includes 1st order and also carefully designed quasi-2nd order crossovers. We must be able to inject a voltage into the crossover and the voltage must never be greater on the other side at any frequency, in fact usually less. This is lossy as opposed to reactive. There are of course higher order crossovers that achieve the something similar, but these are basically in the ‘critically damped’ crossover camp, these include 2nd and 4th order L-R types. But the chosen crossover we have chosen here complements how we will deal with DL and power response solutions. High order crossovers prevent these solutions. We have also carefully chosen the drivers that are capable of low order crossovers that creates these possibilities.


6. Minimum phase disturbance over a wide bandwidth


There is an obsession with some to achieve completely flat phase response, a laudable objective, but maybe more academic than real world. I will aim at something more practicable that is that phase should be continuous and gradual. No sudden changes but just a smoothness transition, especially through the two crossover points. Also the crossover must be minimum phase rather than absolute phase. This is a much more practical and workable solution. What is especially good about it is: You know it is going to work even before you start. It is better to be with a safe bank and have peace of mind rather than chasing elusive gains.


8. Correct acoustic summing at crossover frequencies


We shall be using a ‘sum and null’ technique that allows us to tell whether the two drivers will sum correctly at the crossover frequency. Here one has to understand the two concepts of summing and nulling. The two are opposites, but they can each tell things about the other. By nulling we can tell how well the two sums. Confused? OK, we need an illustration:



We can see the two separate drivers responses and the overlap. But the dip at the crossover point, theoretically –6dB, should disappear when the two drivers sum together, this is next shown as the darker line:



If summed perfectly the response should completely flat as shown by the darker line, there should be no ripples that may look like this:



Note that the ripple goes both above the ideal summed line and also below, hence it looks like a ‘ripple’ in the response. It is caused by poor phase integration and this kind of effect is common in low order filters. Of course high order filters, especially 4th, largely avoids this, as the Crossover Overlap is so small that there is not enough energy either side of the crossover frequency to cause significant ripple. The only way to get perfect summing is to stagger the drive units so that the acoustic offsets and acoustic centres are equidistant at the listening position. Only this way and careful adjustment of the electrical crossover can achieve virtually perfect summing throughout the Crossover Overlap. But when that is achieved, then two important results are superb driver integration and blending, and consistent off-axis response that result in flat power response through the Crossover Overlap. When this has been achieved, then as a test we can reverse the optimum electrical phase and if we have a cleanly summed response we will get a deep null point:



Note that this clean and deep null response should occur ‘off axis.’


Indeed during the crossover modelling/construction phase, we can tweak the crossover components while switching backward and forward between ‘null’ and ‘sum’ and then adjust for flattest summed response and deepest and cleanest null. If at the same time we can adjust the acoustic offsets at the same time, this prevents the ripples and a perfect integration is possible. And if we are really clever, we will do this with the whole speaker system off axis at the same time. This will bias the whole procedure towards power response rather than frequency response.


Basically we must actually model the crossovers twice. So at the time we capture/measure the acoustic response of the drivers, we must have two measurements, one off axis and the other on axis. By making changes to the crossover modelling we can now observe how the changing values affect both off and on axis responses.


Also, we must weigh the off axis response as twice as important as the on axis. We can now also model a third power response by combining (the technical term is averaging) these three, 2 times off axis plus one times on axis, and this allows us also to model final power response. Now we can see why computers are such a powerful tool, but like all tools needs to be used intelligently.


8. Pipe Resonances in tall enclosures


Whew! We are getting nearer the end of this monster instalment. Pipe resonances? Yes, all cavities will show a pipe resonance when one long dimension dominates. So they are common in tallish and slim cabinets. Pipe resonances actually show up clearly in the impedance (Z) graph. Whenever I see a tall speaker, I cannot wait till I seen the Z plot, and sure enough there will be a wriggle in around a few hundred Hertz. Here is an example scanned from a magazine describing a tall slim box driven from a bass/mid in the upper part:



The wriggle in the impedance plot around 150 Hertz is classic pipe resonance. It cannot be ignored. Since our chosen box is rather tall and that the height is the dominant dimension, it is also prone to have a major pipe resonance in a similar manner. But it is hoped that it will not as we have several drivers activating the column of air inside the box from four points of the length of the column. It will be interesting if this will suppress the effect. Boxes that follow the Golden Ratio, where the height is 1.6 times the width or depth of the enclosure and the third dimension 1.6th of that, I have samples showing that boxes following this Golden Ratio have virtually no vertical resonance. I have spoken to Brad Serhan about this effect especially as a number of his Orpheus Loudspeaker models are tall slim floor standers. He is well aware of the effect and also stressed that it was a mistake to overly damp this resonance as it will kill the sound, so careful but not excessive damping, largely tuned by ear, is the way to go. In our case, the effect from being driven from multiple points (four) should mean that any over damping of the enclosure should easily be avoided.


9. Max acoustic power transference into room


This is something I became more acutely aware about twenty years ago. We had an unusual two-way speaker system based on a Ted Jordan design. The crossover was a low 150Hz and the two-inch driver covered 150Hz to 20KHz. This was quite a remarkable feat and it certainly worked. But something was also perceived as lacking. When the crossover was lifted to around 500Hz it improved dramatically. There were good reasons for this. The median of the power range of music is 300Hz approx. That is to say that half the energy of music is above 300Hz and the other half is below 300Hz. This means that the maximum power generated is centred on the same 300Hz. So the octave below and above, 150Hz to 600Hz is where most of the energy is emanating from your speaker. To effectively couple this energy to the room and air we need one rather basic commodity, a large radiating surface area. Otherwise it may sound weak and anaemic.


It really is rather simple, the surface and interfacing with air is a poor acoustic transformer and a larger surface area means we have a more efficient mechanism for coupling the energy spectrum and thus the full impact reaching the listener. The audible benefits are obvious if the drivers chosen do their job (quality is no substitute) and better defined dynamics. There are of course some interesting by-products such as smoother transition in terms of the DL.


The combined four 6.5” drivers all outputs into these two power octaves and have a total radiating area greater than a single 12” driver. That should do nicely.


The next topic is also related to maximising power transference into room.




Please take a good look at the photo of the finished speaker. It looks like the Tweeter is horn loaded, right? Well, yes and no. What will be called the Diffraction Wedge (or waveguide) has two purposes. One, to control the Diffraction Distortion caused by the deep recess the Tweeter in mounted in. This recess adjust the acoustic centres of the top three drivers to be very close relative to the listening position. Severe peaks and troughs in the response would appear without the Diffraction Wedge.


So it is not there for horn loading? Yes it is, but as will be discussed in detail later when measurements and modelling is described, the horn loading takes place in the two octaves below the crossover frequency.


From time to time, some speaker researchers and commentators have pointed to the obvious fact that when we cross over to a small speaker we are crossing over from a cone (usually) of much larger area. Loudspeakers are really a kind of multi-transformer, converting electrical energy into mechanical energy and then to acoustic energy. Both processes are quite inefficient. We have covered the first part but the latter, converting motion into acoustic energy, is the focus here.


Clearly a large cone area has a different 'air load' than a tiny (usual 1 inch dome). It is this air load that determines the acoustic impedance. There is a sudden discontinuity in that acoustic impedance at the crossover point, especially below the crossover frequency. The off axis response of the larger cone suffers off axis and if we are using low order


What if we could increase the air load on the Tweeter at around and below the crossover frequency? We could construct a horn with just the right dimensions where above the crossover frequency, the horn loading is non-existing. You want the Tweeter response to be flat here and no horn loading. Coincide the horn loading to the off axis (as well as overall) response of the Midrange driver. This means the electrical crossover must be adjusted hand in hand as well. It becomes a precise integrated whole. Many hours were spent getting this right - but experiments showed this partial horn loading was possible in real life and not just a theory.


The horn loading of the Tweeter increase air load and hence increases the effective are of the radiator.


Changing the air load is the same as increasing the effective cone/radiating area. We are now adopting the air to do the the job instead of actual cone area. 


Get the ingredients just right and the sonic benefits are substantial. The physical alignment of drivers, forming a point source, the horn loading of a specific portion of the Tweeters output, improves driver integration. The crossover becomes seamless. The kind of benefits claimed for full-range single driver (Lowther etc) which does not suffer from these discontinuities (but suffers elsewhere).


Most other conventional multi-way systems sound thin or lacking a certain substance in the upper midrange, the Elsinore sounds full and hearty and has this substance where others become largely vague.


Just quickly. Re horn loading, it is not readily understood that this is a bandwidth related characteristic. There is always a frequency below and above where the loading ceases. Also, front loading increases efficiency, but rear loading does not. We use a short front horn that has a narrow and precise bandwidth exactly where we need it. The physical dimensions of the Diffraction wedge sets that bandwidth, not the driver itself. Extreme horns also colour the sound IMO, but this is avoided here as the angle of the horn does not even approach 45°. Cup your hand before your mouth more than 45° and you will hear the effect whilst speaking. Lessen to less than 45° and it become virtually inaudible.


11. Quality Drivers


Let us not put a too fine a point on this: There is no substitute for quality drivers. Yes, even the more expensive kinds may have difficult problems but they usually are also of greater potential. The drivers we have chosen are of Danish origin and that would have to be for a speaker bearing the name of Elsinore, a name so famous in Danish folklore and history.


The four Peerless HDS drivers in the chosen box/alignment is expected to be flat down to 30 Hertz in room. This is their latest version but we have already have experience with the earlier ones and know the behaviour of them well.


The Tweeter is the venerable Vifa XT-25 and I make no apologies for being a fan. Michael Lenehan (of Lenehan Acoustics) and myself were the first in Australia to use them, in fact acquired the first box of ten brought in by Mass Technologies (Oz importer).


There is a trick to getting ultra-low distortion from this tweeter. Read on and you will find out in a later instalment.




That is enough to absorb for now. There will be a few months before the next issue is out. Next instalment shall discuss the bass/box alignment in greater detail. The way a speaker interacts with the room as well as the dynamic versus static performance choices. It is little appreciated that the above-mentioned 4Pi response actually reverts to 2Pi plus response at very low frequencies with a potential 10dB boost (that much is rarely realised, but 7-8dB is certainly reachable). There are a few dangers lurking here if we are not careful, we can end up with a mid bass peak and a trough in the response between 100-200Hz. Yikes! Who said that speaker design was supposed to be easy?


Loudspeaker design involves so many choices, potential vices and optional solutions and few decisions comes without penalty – I can only think of designing valve output transformers as a more masochistic art form. Yet it is fascinating to those who have caught the bug.


Next: Box Alignment - A Discussion




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Last modified: Monday June 08, 2015

Just had a terrible thought. If "intelligent design" is unscientific, then who will design our audio equipment?