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Part Three

 

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

 

Box Alignment - A Discussion

 

In Part Two we covered a lot of territory, this instalment will be a bit more brief and we will discuss the box alignment. First we will measure the electrical characteristics of the drivers. Of course, the Bass-Mid driver is the one we are primarily concerned with when considering box alignment, but it was easy to measure both at the same time, so here they are.

 

Bass-Mid:

 

Tweeter:

 

These measurements were made using Clio Version 7.0 acquired through Pat O’Brien of WAR Audio. A few comments on the above graphs; the impedance shows that the Bass-Mid is 8 Ohm approx and the Tweeter is 4 Ohm. You can tell these by the fact they show a pronounced peak, which corresponds to their “Free Air” resonance, also known as their “Fs”. The other curve is the electrical phase and the scale on the right side shows they are -/+ 48° approx for both drivers. Note that at the Fs the phase is zero degrees? That is no coincidence. In both cases the phase goes positive below the Fs and negative above the Fs. Eventually the phase goes positive about two octaves above Fs, this is a natural consequence of the inductance in the voice coils. When the phase is positive it is said to be ‘inductive’ and fairly benign. But when phase is negative it is said to be ‘capacitive’ and if excessive can become a severe load, especially only the amplifier.

 

The Tweeter’s negative phase will mainly be below the crossover frequency and such will not become a problem. But Bass-Mid’s negative phase of –40° at 80 Hertz approx would need more careful attention, but this will be modified by the box alignment and less likely to be an issue. Because we will use lossy crossover, otherwise known as Transient Perfect, I hope to have the overall phase response positive from 300 Hertz and up. Even if it should go negative, then it should do so moderately.

 

The key to provide an easy load is to avoid combining negative phase and low impedance at the same frequency. If we can keep the impedance higher than six ohm, even –40° at one frequency should not present a problem.

 

Since the impedance graph shows a minimum around 7 Ohm at 200 Hertz and –18° at that point, it will be logical that this will not be a speaker that creates any problem for any amplifier load wise.

 

We shall be able to maintain minimum impedance at 7 Ohm up to and near where we cross over to the Tweeter. But I suspect there will be some padding down (designer speak for reducing the output) of the Tweeter and this will be accomplished by adding some series resistance, which will likely up it to near 6 Ohm. If we can keep the system impedance above 6 Ohm, then this will qualify as a genuine 8 Ohm design and this will be our target. Even if we fall short in the top octave it should be positive phase above 10KHz. Needless to say, all this will need to be played out, but it all augurs well.

 

Thiele-Small Parameters

 

Now let us examine the electrical driver characteristics of the Bass-Mid. From these we can extract the Thiele-Small (also known as T-S) parameters. It is not generally understood that T-S measurements are electro-mechanical as opposed to acoustic. This separation of electrical and acoustic needs to be appreciated. Strange as it may seem, it is the electrical parameters that allows us to model the box alignment and the acoustic measurements comes later after the box has been constructed. Don’t worry if this confuses you now, but we will revisit this and will become clearer why.

 

The published T-S parameters are:

 

Fs        47.3 Hz

Qms     2.73

Qes      0.47

Qts       0.4

Sd        139 cm2

Vas      16.7 Litres

Sensitivity 87.8 dB

 

Next by adding a known amount of mass, such as Blue-Tack, to the cone, as well as supplying Sd (the effective cone area) Clio can now fill in the remaining parameters:

 

Fs        54.4 Hz

Qms     2.666

Qes      0.5012

Qts       0.4219

 

Next by adding a known amount of mass to the cone as well as supplying Sd (the effective cone area) Clio can now fill in the remaining parameters:

  

Vas      14.1 Litres

Sensitivity 88.5 dB

 

Now you can tell that they are not the same, so what gives? Don’t worry  this is not entirely unusual. Note that Fs has increased by 15% and Vas has decreased by 18%. This indicates that the suspension is still stiff and as the driver gets used the Fs will lower and Vas increase as the suspension becomes more compliant. Also the internal test method of Clio is very low power and the same test using increased signal level will also show lower Fs and higher Vas. Other parameters are within 5% but will also change slightly, so the published specs correlative reasonably well, just give it time. The interesting thing is that in the same box, the alignment does not change as much as you would think. See below:

Chances are the end results will lay between those two curves. But we do have a mechanism that will help us in the end. The above alignment is based on four drivers in series parallel and the box volume of 75 Litres and vent tuned to 40 Hertz.

In a sealed box the resonant tuned frequency Fc is determined by a combination of both driver parameters and the box volume. In a vented box the tuned frequency Fb is dependant on the box only.

This means that in the end when the box has been made we can tune the box frequency by varying the depth of the vent. Hence if we feel the box sounds a little too dry, we can shorten the vent and tune to a higher Fb and vice versa. Previous experience with the predecessor of this driver indicates that the above alignment should work nicely, but the individual user has the option to tweak Fb to suit them. Not an option available with sealed alignments.

The Colloms Curve & Average Room

Remember our earlier discussion re Diffraction Loss. Recall that this creates a step in the response because sound wraps around the front baffle below a certain frequency causing a theoretical loss of 6dB? What happens to that sound below this step? It is of course still in the room and is constrained by the room boundaries. When the wrap-around-energy meets the rear wall, ceiling, side walls and floor, something remarkable happens. Take a close look at the following graph, courtesy of Martin Colloms and his book High Performance Loudspeakers:

 

This graph is only a guide. But it indicates a rising response at low frequencies. As speakers descend in frequency they become increasingly omni-directional. This means they become, gradually, a spherical radiator. The increased energy then gets bounces back into the room. The above shows the average room corrected response. This now needs to be superimposed on a standard 2Pi response, and we get the following results.

 

 

Our in room target will be –6dB at 30 Hertz as represented by the cross-hairs. There are two alignments shown:

 

1.   Vb-1. This is a computer generated optimally flat 2Pi response. It is clear that if average room correction was applied, there would be a significant peak around 50-60 Hertz. This 4th Order Butterworth design has pour transient response relative to Bessel Alignment. Our driver is not suitable for Bessel, but a similar amplitude response can be chosen that is similar and that will have improved transient response.

2.   Vb-2. This is our chosen alignment. Note that the initial roll-off is more like that of a sealed box. It has much better transient performance.

3.   Vb-2 (Colloms Adjusted). This is the average room response. Note that we are only –1dB at 40 Hertz relative to the zero line. Our in room response target of better than –6dB at 30 Hertz is also met. The 24dB/Octave Line shows that the ports maintain output down to about 27 Hertz before reaching maximum slope.

 

We have chosen an alignment,  Vb-2, that can be classed as veering towards the dynamic, whereas as Vb-1 is a static type alignment. The dynamic type alignment better integrates with the room. Nearly all speakers have an in room elevation between 60-80 Hertz. Even though we have chosen a dynamic alignment, this elevation is kept under 2dB. The static alignment would have an even larger and potential a peak in the response, at least ours is reduced and flat. For those with a different taste, we can tune the box to a higher Fb and go for a static alignment (not recommended) or a compromise between the two. This nicely introduces the next appropriate subject:

 

Another Golden Ratio?

 

Let me introduce you to a concept that is rarely mentioned or understood. In the end we use this to fine tune the box, and as this is DIY, even for our own room and taste.

 

I refer to the ratio between the Fb and F3 in vented boxes. Here Fb represents the frequency the box is tuned at via the use of a vent (remember this is independent of the driver) and F3 represents the frequency where the response is 3dB down. Look at the Vb-2 curve, note that it is tuned at 40 Hertz, so where is the F3? About 53 Hertz. Now divide F3 by 40 equals 1.325 and thus:

 

F3/Fb = 1.325   (53/40 = 1.325)

 

In a 4th Order Butterworth as shown in Vb-1, the F3/Fb = 1, yes one and then we have a static alignment. When F3 and Fb are the same the transient response is not optimum (and neither is frequency response, as we shall see later. If we increase the F3/Fb ratio to 1.2 things improve significantly and about 1.3, a more dynamic type alignment,  which is what our target will be initially. Going too much above 1.3 and we start to over damp and the bass will start to become too dry and the roll-off is too great for bass extension.

 

When the speaker is listened to, we can revisit the box tuning. By increasing Fb up to 42-44 Hertz (just shorten the vent) we can try F3/Fb = 1.2 and see if we like that as a compromise. Maybe in larger rooms the latter may work better whereas in most rooms 1.3 will be about right. The beauty is, should you choose to take up the challenge and build this speaker system, don’t permanently fix the port (vent), but try shortening it gradually while listening and fine-tune it to your requirements. You could never do this with a sealed box alignment as only with vented alignments the box tuning is independent of the driver. The ratio F3/Fb is a great tool for the DIY buffs, even if it is a bit useless for a manufacturer who has to settle for a fixed point.

 

What Next?

 

The box drawings have been complete for a while. Bernard Chambers (the sponsor who has paid for the drivers) has just picked up the drawings and it is hoped over the next month or so they will be built. We can then mount the drivers in the box and start the next phase.

 

Earlier I said we must start with the electrical characteristics and how they help us to extract the Thiele-Small Parameters, which in turn allows us to design the box alignment. Then the drawings are prepared and the boxes built. Once the drivers are mounted we can get onto the interesting acoustic measurement. Combined with Thiele-Small Parameters we can use the collective data and computer model the crossover.

 

These acoustic measurements must be done in situ. It is not possible to measure the drivers’ response unless they are in the exact location that they are used in. Only electrical measurements can be performed outside the box.

 

But now we are also faced with something else, the box will now also modify those parameters. It is important then to distinguish between raw date and in box data. When doing computer modelling, we use the raw data for driver parameters, but impedance and acoustic measurements we use in box data.

 

I think that will do for now. Just need to get the boxes done and press on.

 

A date has been fixed and the idea at this stage is to have the speakers ready for the July 2006 ASoN meeting. That is three months away as off now and is a can-do, still a lot of work to do.

 

Next: Construction Drawings and Notes

 

 

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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?