1. Crossover Trial Tweak Code
2. Crossover Trial Tweaks
3. Crossover Trial Tweak 2020
4. Crossover Trial Tweak Download

Ready-made crossover filters work fine and are easy to tweak. The ratio of costs between woofer, mid-tone driver, tweeter and crossover can be 2: 2: 2: 3. I choose a US$15 woofer and midrange driver. The tweeter was US$25. So I'm willing to pay around US$ 38 for my crossover.

• Novavax has added crossover arms to late-phase clinical trials of its COVID-19 vaccine. The action will enable participants in the placebo cohorts of the original trials to get vaccinated without.
• And I guess after doing auto calibration, I'm going to change/disagree with large or small, crossover, and levels (have to tweak manually after changing speaker size and crossover). So the only thing actually used from auto-cal is speaker distance which may disagree with actual distance.

Abstract

In the fourth piece of this series on research study designs, we look at interventional studies (clinical trials). These studies differ from observational studies in that the investigator decides whether or not a participant will receive the exposure (or intervention). In this article, we describe the key features and types of interventional studies.

INTRODUCTION

In previous articles in this series, we introduced the concept of study designs[] and have described in detail the observational study designs – descriptive[] as well as analytical.[] In this and another future piece, we will discuss the interventional study designs.

In observational studies, a researcher merely documents the presence of exposure(s) and outcome(s) as they occur, without trying to alter the course of natural events. By contrast, in interventional studies, the researcher actively interferes with nature – by performing an intervention in some or all study participants – to determine the effect of exposure to the intervention on the natural course of events. An example would be a study in which the investigator randomly assigns the participants to receive either aspirin or a placebo for a specific duration to determine whether the drug has an effect on the future risk of developing cerebrovascular events. In this example, aspirin (the “intervention”) is the “exposure,” and the risk of cerebrovascular events is the “outcome.” Interventional studies in humans are also commonly referred to as “trials.”

Interventional studies, by their very design, are prospective. This sometimes leads to confusion between interventional and prospective cohort study designs. For instance, the study design in the above example appears analogous to that of a prospective cohort study in which people attending a wellness clinic are asked whether they take aspirin regularly and then followed for a few years for occurrence of cerebrovascular events. The basic difference is that in the interventional study, it is the investigators who assign each person to take or not to take aspirin, whereas in the cohort study, this is determined by an extraneous factor.

Interventional studies can be divided broadly into two main types: (i) “controlled clinical trials” (or simply “clinical trials” or “trials”), in which individuals are assigned to one of two or more competing interventions, and (ii) “community trials” (or field trials), in which entire groups, e.g., villages, neighbourhoods, schools or districts, are assigned to different interventions.

The interventions can be quite varied; examples include administration of a drug or vaccine or dietary supplement, performance of a diagnostic or therapeutic procedure, and introduction of an educational tool. Depending on whether the intervention is aimed at preventing the occurrence of a disease (e.g., administration of a vaccine, boiling of water, distribution of condoms or of an educational pamphlet) or at providing relief to or curing patients with a disease (e.g., antiretroviral drugs in HIV-infected persons), a trial may also be referred to as “preventive trial” or “therapeutic trial”.

VARIOUS TYPES OF INTERVENTIONAL STUDY DESIGNS

Several variations of interventional study designs with varying complexity are possible, and each of these is described below. Of these, the most commonly used and possibly the strongest design is a randomized controlled trial (RCT).

Randomized controlled trials

In an RCT, a group of participants fulfilling certain inclusion and exclusion criteria is “randomly” assigned to two separate groups, each receiving a different intervention. Random assignment implies that each participant has an equal chance of being allocated to the two groups.

The use of randomization is a major distinguishing feature and strength of this study design. A well-implemented randomization procedure is expected to result in two groups that are comparable overall, when both measured and unmeasured factors are taken into account. Thus, theoretically, the two groups differ only in the intervention received, and any difference in outcomes between them is thus related to the effect of intervention.

The term “controlled” refers to the presence of a concurrent control or comparator group. These studies have two or more groups – treatment and control. The control group receives no intervention or another intervention that resembles the test intervention in some ways but lacks its activity (e.g., placebo or sham procedure, referred to also as “placebo-controlled” or “sham-controlled” trials) or another active treatment (e.g., the current standard of care). The outcomes are then compared between the intervention and the comparator groups.

If an effort is made to ensure that other factors are similar across groups, then the availability of data from the comparator group allows a stronger inference about the effect of the intervention being tested than is possible in studies that lack a control group.

Some additional methodological features are often added to this study design to further improve the validity of a trial. These include allocation concealment, blinding, intention-to-treat analysis, measurement of compliance, minimizing the dropouts, and ensuring appropriate sample size. These will be discussed in the next piece.

Nonrandomized controlled clinical trials

In this design, participants are assigned to different intervention arms without following a “random” procedure. For instance, this may be based on the investigator's convenience or whether the participant can afford a particular drug or not. Although such a design can suggest a possible relationship between the intervention and the outcome, it is susceptible to bias – with patients in the two groups being potentially dissimilar – and hence validity of the results obtained is low.

Interventional studies without concurrent controls

When a new intervention, e.g., a new drug, becomes available, it is possible to a researcher to assign a group of persons to receive it and compare the outcome in them to that in a similar group of persons followed up in the past without this treatment (”historical controls”). This is liable to a high risk of bias, e.g., through differences in the severity of disease or other factors in the two groups or through improvement over time in the available supportive care.

Before–after (pre–post) studies

In this design, a variable of interest is measured before and after an intervention in the same participants. Examples include measurement of glycated hemoglobin of a group of persons before and after administration of a new drug (in a particular dose schedule and at a particular time in relation to it) or number of traffic accident deaths in a city before and after implementation of a policy of mandatory helmet use for two-wheeler drivers.

Such studies have a single arm and lack a comparator arm. The only basis of deriving a conclusion from these studies is the temporal relationship of the measurements to the intervention. However, the outcome can instead be related to other changes that occurred around the same time as the intervention, e.g., change in diet or implementation of alcohol use restrictions, respectively, in the above examples. The change can also represent a natural variation (e.g., diurnal or seasonal) in the variable of interest or a change in the instrument used to measure it. Thus, the outcomes observed in such studies cannot be reliably attributed to the specific intervention, making this a weaker design than RCT.

Some believe that the before-after design is comparable to observational design and that only studies with a “comparator” group, as discussed above, are truly interventional studies.

Factorial study design

If two (or more) interventions are available for a particular disease condition, the relevant question is not only whether each drug is efficacious but also whether a combination of the two is more efficacious than either of them alone.

The simplest factorial design is a 2 × 2 factorial design. Let us think of two interventions: A and B. The participants are randomly allocated to one of four combinations of these interventions – A alone, B alone, both A and B, and neither A nor B (control). This design allows (i) comparison of each intervention with the control group, (ii) comparison of the two interventions with each other, and (iii) investigation of possible interactions between the two treatments (whether the effect of the combination differs from the sum of effects of A and B when given separately). As an example, in a recent study, infants in South India being administered a rotavirus vaccine were randomly assigned to receive a zinc supplement and a probiotic, only probiotic (with zinc placebo), only zinc supplement (with probiotic placebo), or neither (probiotic placebo and zinc placebo).[] Neither zinc nor probiotic led to any change in the immunogenicity of the vaccine, but the group receiving the zinc-probiotic combination had a modest improvement.

This design allows the study of two interventions in the same trial without unduly increasing the required number of participants, as also the study of interaction between the two treatments.

Crossover study design

This is a special type of interventional study design, in which study participants intentionally “crossover” to the other intervention arm. Each participant first receives one intervention (usually by random allocation, as described above). At the end of this “ first” intervention, each participant is switched over to the other intervention. Most often, the two interventions are separated by a washout period to get rid of the effect of the first intervention and to allow each participant to return to the baseline state. For example, in a recent study, obese participants underwent two 5-day inpatient stays – with a 1-month washout period between them, during which they consumed a smoothie containing 48-g walnuts or a macronutrient-matched placebo smoothie without nuts and underwent measurement of several blood analytes, hemodynamics, and gut microbiota.[]

This design has the advantages of (i) each participant serving as his/her own control, thereby reducing the effect of interindividual variability, and (ii) needing fewer participants than a parallel-arm RCT. However, this design can be used only for disease conditions which are stable and cannot be cured, and where interventions provide only transient relief. For instance, this design would be highly useful for comparing the effect of two anti-inflammatory drugs on symptoms in patients with long-standing rheumatoid arthritis.

Cluster randomized trials

Sometimes, an intervention cannot be easily administered to individuals but can be applied to groups. In such cases, a trial can be done by assigning “clusters” – some logical groups of participants – to receive or not receive the intervention.

As an example, a study in Greece looked at the effect of providing meals in schools on household food security.[] The 51 schools in this study were randomly allocated to provide or not provide a healthy meal every day to students; schools in both the groups provided an educational intervention.

However, such studies need a somewhat larger sample size than individual-randomized studies and the use of special statistical tools for data analysis.

Conflicts of interest

There are no conflicts of interest.

REFERENCES

Do Better Quality Parts Really Matter?

In part one of 'Crossover, Brain of your speaker system', we introduced the concepts of inductance, capacitance, and resistance. We then examined how these three basic passive elements relate and combine to create frequency selective networks called High pass and Low pass sections, the building blocks of the crossover network. We also considered in part one, the effect of real loudspeaker impedance, and how, unlike a resistor, its amplitude and phase vary with frequency to complicate and frustrate the function of constant resistance type crossover networks. These real loudspeaker impedance variations result in frequency and phase responses which end up being very different than what our textbook equations would have us expect, because they assume a speaker behaves like a simple resistor. We also made the assumption that the parts used in our crossover networks were theoretically perfect and without flaws. In part two, we will discuss how in the real world, capacitors, inductors and resistors exhibit behavior which is neither ideal nor perfect. We will determine if better quality parts truly yields better performance.

Some of my more recent efforts

Real world parts, the kind you will actually find in your own crossovers, suffer from many flaws. In part two, we will discuss and illustrate the effects of some of these. We will also examine how simple mistakes, like the physical orientation and location of inductors on the crossover board can result in non-ideal behavior like cross-talk. This article will allow the reader to gain some insight into the kinds of mistakes made by amateur and professional crossover designers alike, and allow us to recognize compromises in crossovers by simply looking at the networks. We also hope to gain some understanding into flaws which are not quite so easy to see with the naked eye. While this article is not going to be an exhaustive study of crossover component parts, it will touch on most of the major flaws present in the three basic components used in all real world crossovers, resistors, capacitors, and inductors.
I am hopeful this light shed on crossover networks will make you all better and somewhat more cynical consumers, ones who understand the importance of the passive crossover parts used in their speaker system. Reading some of the more ardent audiophile press, one can be left with the opinion that there is all sort of magic going on in this network. In fact the enemies of these passive components are basically the same as the enemies of all electronic parts; hysteresis, loss, tolerance, insufficient power handling capacity, insufficient space, and compromises made on behalf of cost.

Resistors & Tolerances

Let’s start by considering the simplest of the three electrical components used in our crossover, the resistor. It will, in combination with inductors and capacitors create time constants used in frequency selective circuits, although by itself the resistor does nothing other than to consume power. In a crossover network, resistors are usually used in combination with other components to control either impedance magnitudes or the relative levels between different drivers in a system. Resistors are most often used in 'padding' a tweeter which is more efficient than the woofer, so the overall system frequency response will be flat. The resistor, in series or parallel with capacitors and/or inductors, is often used as part of a Zobel or impedance compensation network.

A good Meter is the best way to 'Trust, but verify'

Crossover Trial Tweak Code

Of all flaws with which we must deal, the simplest to understand is tolerance; the allowable variation of the components value, whether that component is a resistor, inductor, or capacitor. No surprises here, everyone can understand how a part with a small 1% tolerance will lead to a more uniform and reliable frequency or amplitude response performance than a part with a 10% tolerance. The tolerance issue, while seeming obvious, becomes more critical as we increase the order of the network. Remember, a first order network has one part with tolerance, while a third order network is going to have 3 which vary with tolerance. It is for this reason that the higher order the network, the greater the need for a tight component tolerance. Said another way, for a given amount of allowable variation in response, (plus or minus 1 db for example), a second order network requires tighter tolerances from its components than does a first order network, and a third order network requires tighter tolerances than a second order network. As we increase the complexity (order)of the network, the sensitivity of the network to component tolerance increases. So, as we increase the network order, not only do we add additional parts, for a given crossover frequency, we require both larger size (value) components and tighter tolerance in those components in order to keep the frequency response window tolerance the same as the simpler network. This is often a hidden and un-calculated cost in using higher order networks. This exponential rise in part size and cost should explain why crossover networks are almost never found in complexity above fourth order.

Resistors normally deviate from their design values within a window of anywhere from 0.1% to 1%. If you buy a 5% 10 ohm resistor from an electronic store, you might go back there complaining you measured it with your meter and found is was only 9.5 ohms, but if you get a refund it is because they are hoping you don't return to buy more stuff. You will find neither the highest or lowest tolerance parts in most crossover networks, as the typical tolerance specification is either 10% or 5 %. The letters (K) and (J) on the part will indicate if it is 10% or 5% respectively. The effect of this variation is one of magnitude and is important to hold close enough so that there is not much variation from one speaker system to the next. Lets consider an example.

We have an 8 ohm tweeter which is 6 db hotter than the woofer in the system. If we put an 8 ohm resistor in series with the tweeter, the combination of the 8 ohm series resistor and the 8 ohm tweeter presents 16 ohms as a load to the amplifier. Since power = V2 /R and since we have doubled R, we have halved the power the entire network (resistor plus speaker) consumes from the amplifier. The amp is delivering 1/2 the current to the loudspeaker load. Now half of the power that is delivered is consumed in the series resistor, the 8 ohm resistor in series with the tweeter. So, we have cut the power in half twice, and therefore get (- 3 db) + (- 3 db) = ( 6 db) attenuation. Let's say we pick a 10% tolerance for our resistor. This generally defines a 20% allowed variance, since the specification is +/- 10%. We are making a stereo pair, and the two resistors we use are 8.8 ohms, and 7.2 ohms respectively, both within our 10% tolerance window. In the first case, (8.8 ohms in series) we attenuate 6.44 db, and in the second case (7.2 ohms) we attenuate the signal 5.57 db. This means we have a mismatch between our pair of 0.87 db, and a definitely audible difference. This same magnitude variation (tolerance) when the part is used in conjunction with a capacitance or inductance will also cause a shift in the frequency corner of the network.

Another very well documented issue with resistors is inductance. While high impedance, small wattage resistors are most often made from a metal film, higher wattage parts of low impedances (the kind most likely to be used in crossover networks) are often wire-wound parts. (If you have never noticed before, the symbol for the resistor is a bunch of wire scrunched up). Some old wire-wound types have a inductance high enough to cause issues in a crossover at very high frequencies. You may often find wire-wound resistors being referred to as 'non inductive' to let the buyer or engineer know these parts have eliminated this potential flaw. Modern day parts are often wound with a serpentine pattern so the windings have self canceling inductance without having an effect on their intrinsic resistance.

The largest real world problem you will run into with resistors, is a universal problem for all components, heat sensitivity. As we read the specifications for any component, we must bear in mind that these specifications are only met within a certain allowable window of temperature.

An American Made Resistor Bank - 3 times 300 Watts

Temperature dependence of resistance

Crossover Trial Tweaks

The electrical resistance of a metal is approximately proportional to its temperature over a limited range. Resistivity of materials is usually specified at normal room temperature, 20 degrees C (68 degrees F). If one knows the resistivity of the material at room temperature, and the rate at which it changes, we can calculate the resistivity at other temperatures with the following formula:

Where:

R = Resistance at temperature T

Ro = Resistance at temperature To

To = Temperature at Reference T (usually 20 degrees C)

alpha (the Greek letter with the bracket and outside parenthesis) = Percentage change in Resistivity per unit temperature

Let’s work one example. Suppose we put an 8 ohm resistor into a crossover network, and use it to drop

the sensitivity of the tweeter so it will match the woofer. Lets say we are driving the speaker system pretty hard, and we are heating the resistors so that they rise to be 200 degrees F, (a not uncommon operating temperature for a resistor in a crossover network). 200 degrees F is equal to 93.33 degrees Celsius. (Celsius and Centigrade are the same). If the resistor is wound from Copper wire, the temperature coefficient would be (3.9 * 10-3)) / deg C. Since the resistor was 8 ohms at 20 degrees C, and has now heated up to 93 degrees C, the new value of resistance would be:

8 * [(3.9 * 10-3 / oC(93.33C - 20C) + 1] = 1.286 * 8 = 10.288 Ohms

We can plainly see this increase of 25% is more significant than the component tolerance of 10%. Suppose we do not use copper in the wire-wound resistor. If we use a material called Nichrome, which is basically an alloy of nickel and chromium, our change in resistance will be considerably less. Nichrome has a low resistance variation (alpha) with temperature, alpha = 0.4 e-3/ (deg C) Using this material to make the resistor gets us an eventual resistance of 8.23 ohms at the elevated temperature of 93 degrees C. This is an increase of only 3%. This is no doubt, a small increase compared to the change of resistance in the voice coil which is either copper, aluminum, or a combination of those two.

As the power dissipated in a resistor increases, so does its temperature. As the temperature increases, so does the parts resistance (as illustrated above). As the resistor is heated, its ability to absorb power is compromised, and its value in the circuit is not as designed. That resistors get hot, and can burn out is a well known phenomena. What is not as well known is that running high power into resistors at or near their limits brings with it audible effects on your music.

Crossover Trial Tweak 2020

Since the resistor manufacturer has no control over how the part is mounted to the printed circuit board (PCB), or its orientation, proximity to other parts on the PCB, they cannot predict with accuracy at what point the power through the circuit will cause the resistor to go outside of its allowable range. Take a look at the resistor in the photographs below:

The 20 ohm resistor shown above (white rectangular part) is mounted directly to the printed circuit board (PCB) which is an effective insulator of heat. The path for the heat generated inside the resistor to escape has lost 1 of 4 large sides. Now lets take a look at another PCB which has eliminated this issue.

On this PCB we see the terminals of the resistor are designed to stand the part off the board. This has the disadvantage of making the part taller, but the advantage of creating space between the hot resistor (whose only job is to dissipate power) and the PCB (whose only job is to connect the different components without routing the heat from one part to another). With the resistor mounted off the PCB, the hot air can circulate around the part more easily, dissipating the resistors power more efficiently.

Crossover Trial Tweak Download

Suppose both of these parts shown above are 20 watt resistors. Which one burns out first? Now suppose one of the boards is mounted to the bottom of the cabinet so the heat from the resistor can rise into the entire cabinet volume, and escape through a nearby port. Suppose the other PCB is mounted upside down on the top of the cabinet, inside a sealed speaker box, and under the cabinet stuffing which is fiberglass? Although both parts are 20 watt resistors, the one in the heat containing environment is going to burn out faster than the one with good ventilation when it needs to handle all 20 watts. Power ratings on resistors are NOT independent of the way in which they are mounted to a PCB.