Methods: This randomized controlled crossover clinical trial was conducted on 53 patients with type 2 diabetes. The participants were randomly divided into groups of control (weight maintenance diet) or treatment (weight maintenance diet +50 g/day honey) for 8 weeks. VWAP/MVWAP/EMA CROSSOVER. Only difference in this one is that a EMA is used which should give quicker signals but theres a chance for more false signals as per usual use TA and other indicators to confirm positions. So far I've been liking the Vortex Indicator. When the VWAP (Thick Yellow) AND the two EMA (Orange and thin Yellow) cross above.
The Objective:
In the forum postings, I have often read of the trials of aspiring speaker builders overcoming the pitfalls of
could be devised that would take into account some of the variables implicit in such a design, such as BSC
and tweeters in a simple enclosure and still provide an acceptable on axis FR (frequency response) of within +/-
The Assumptions:
Of course no crossover can be truly universal, so certain limitations and assumptions must be made to define
For the purposes of this study, it is assumed the published information is accurate, and that the crossover
crossover point. On one hand you have the Fs of the tweeters, which should be at least one octave below the
4KHz. With this in mind, as well as the driver spacing limitations, I chose 2350 Hz as the target crossover point.
> Baffle configuration:
One consideration necessary in order to achieve flat frequency response is to account for the diffraction effects
MT designs and a 9 in. by 28 in. for MTMs. For ease of construction, no chamfering or rounding was modeled.
woofer was centered 10 inches from the top of the baffle. On the MTM, the top woofer was modeled centered
woofer centered 18 inches from the top of the baffle. The center-to-center spacing between the mid/woofer and
maximum size of the mid/woofer to approximately 8 inches.
> Acoustic centers:
A difference in signal propagation time will occur if the distances from the acoustic centers of the woofer and
and cause frequency response abnormalities in the crossover region. In the MT crossover, no attempt was made
tilt the baffle to align the centers. This can be done either by sloping the front baffle, or tilting the entire
assumed 6 ohms and 1.0 mh. For the tweeter, 5 ohms and 0.05 mh.
> Driver FR:
The drivers should have a flat FR throughout the pass band, and ideally at least one octave past the crossover
The Design Process:
To compensate for diffraction effects, I first determined a target response for the system. This is required as
and driver orientation using Paul Verdone's BDS program, and determined the woofer diffraction effects (gain)
caused by the baffle. This curve was inverted and exported to Ingemar Johansson'sLspCAD to provide the
target response, or attenuation required to obtain a flat on axis frequency response. Different target responses
response and the transfer functions of the crossovers selected. My initial investigation was to see if an acoustic
the crossover frequency. Importing various driver FR files however, indicated insufficient attenuation with some
the attenuation was still insufficient in some cases. I then modeled an acoustic 4th order LR, and found the
attenuation was adequate to mitigate the response irregularities of most drivers, including some with some
4100 Hz. This peak was attenuated acceptably using this modeled crossover. A zobel circuit was used to allow
provided acceptable results with a variety of tweeters. Phase tracking was good, assuming proper alignment of
target curve for the MTM baffle, and modified
the assumption of the tweeter and woofers relative acoustic centers, but due to the sharp roll off rates of the
impedance curves for the MTM remained above 4 ohms for all the drivers I modeled.
The Circuits:
MT woofer networkMT tweeter networkMTM woofer networkMTM tweeter network
The circuit topology turned out to be quite simple and is the same for both designs. Any resistors with values
For the low pass network, a third order electrical filter with zorbel was used to achieve the BSC and the 4th
a 4th order acoustic slope. The two resistors after the network represent an L-pad, or fixed resistors for tweeter
The Tweaks:
For most of the drivers I modeled, the frequency response criteria of +/- 3 dB could be achieved by merely
design, I would suggest purchasing a pair of L-pads for each speaker to do the initial voicing adjustments. The
resistance with respect to the middle terminal when the L-pad is turned fully clockwise. On the ones I tested,
but abruptly went open circuit at the full clockwise position.. To obtain the best response, additional resistance
will reduce the resistance, and reduce the woofer output from roughly 700 Hz to 2 KHz. Note that while the
or lowering both L-pads simultaneously.
Additional Tweaks:
While the modeled frequency response of most drivers fell acceptably within my +/- 3 db target using only the
system.
>Low pass section:
Reducing the value of L1 will have the effect of lessening the baffle step. Note that this will also raise the
change in the crossover frequency, at the expense of making the system slightly less efficient.
Increasing the value of C1 will lower the crossover frequency and affect the FR around the crossover
crossover frequency.
Modeled Results:
After determining the final values and topology for this circuit, I wondered if it would model well with a woofer
I selected the response curves of a SEAS L17REP, a 6.5 inch aluminum cone driver to model with my TM
accompanying snapshot. The blue curve is the SEAS woofer without the
sufficiently far down in the pass band as to mitigate their audible effects. The red curve is an SS9500 with
the black band is the combined response with the tweeter out of phase, showing good phase tracking through
Conclusions:
This was a design study. I have not used this crossover in an actual design. However, the modeling of various
results. A reasonably flat modeled frequency response is accomplished in most instances with no adjustments
actual drivers in the intended enclosure with an optimized crossover design unique for that specific application.
equipment or design software, I suggest the topologies presented here might be used as a template for further
Erratum:
While I made every attempt to insure the validity of my procedures, data, and results. I do not profess to
constructive comments or suggestions you may have.
Copyright 2002 by Curt Campbell
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
Crossover Trial Tweak Code
Crossover Trial Tweak Download
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