You need large diameter, high qts drivers with low Fs and high sensitivity.
Here are some explanations why this grouping of parameter dimensions is not necessarily the best for open baffle (taken from the website of a German driver distributor whose 18 incher R1830 in an open baffle just wide enough to accommodate its diameter works wonders in my OB;
http://www.hth-lautsprecher.de/theorie/dipol.htm; translated with DeepL)
You are here : HTH Homepage -> Overview Theory Pages -> Dipole
DIPOL, RIPOL, IPOL, OB, etc.
Frequently asked questions (FAQ) !
The same questions about these enclosure types often appear in consultations by email or on the phone and I regularly encounter nonsensical thought patterns, which I would like to dispel globally herewith. The widespread wishful thinking regarding loudspeaker parameters is closer to the real optimum than in the case of URPS.
Nevertheless, a few parameters remain where irrelevant or even disadvantageous values are desired !
"I am looking for a 38 with fs < 40Hz, Qts approx. 1, SPL > 102dB/W !"
Such unrealistic wishes are only fulfilled in fairy tales !
A 38cm bass can realistically have e.g. the following data :
Ex.1) fs = 40Hz, Qts = 0.7, SPL=96dB
If I want lower fs at the same Qts, the drive and SPL will be weaker (100dB won't work!):
Ex.2) fs = 20Hz, Qts = 0.7, SPL=approx. 93dB
If I want a higher Qts with the same fs, the drive and therefore the SPL will also be weaker (100dB will not work!):
Ex.3) fs = 40Hz, Qts = 1.2, SPL=approx.94dB
If I absolutely want high SPL, I have to accept either higher fs or lower Qts or both at the same time:
Ex.4) fs = 50Hz, Qts = 0.5, SPL=approx. 98dB
Of course there will always be manufacturers who state what is technically not feasible. But please do not believe everything and keep your brain switched on !
And if I want to "improve" one parameter in this context without worsening the others, I simply have to take one number more in diameter ...
"They say the Eminence Alpha drivers are good for open baffles ?"
Chassis like Eminence Alpha 12-15, which are mentioned again and again by OB fans, are quite badly suited for this ! Because apart from their ultra-weak drive they also have a lousy linear excursion ->
the high Qts due to the weak drive "consumes" a lot of stroke per input signal, but "available" is only a little ->
with a few watts it is at the limit and only distorts !
The only advantage is their hard suspension, which reduces the excursion in open cabinets. And if you still want the Eminence, I can sell it to you.
"Wouldn't fs = 20Hz and Qts = 0.7 be ideal for low bass in open baffles ?"
Another error in thinking is often in the imagined transfer curve from the TSP (Q-factor curves) : These are so achieved only in closed cabinet ! If I set myself the goal of selecting a flat curve as low as possible above it, I achieve shipwreck in the OB !
This is how many people imagine the difference between a chassis with fs = 20Hz and Qts = 0.7 and one with fs = 40Hz and Qts = 0.7.
After correction of thinking error 1 it looks like this:
With the same diameter, a lower tuned driver must necessarily be quieter !
And yet, even with the same input signal, it would consume more excursion and would therefore be less level stable, so that the difference in maximum volume would become even more obvious !
This is because the very low frequencies are becoming increasingly relevant for the displacement consumption.
Thinking error 2 : The previous only applies to closed cabinets, but the OB corresponds to an upstream 6dB high-pass filter with perhaps 150 (shown here) or 200 Hz lower cut-off frequency. Then it is irrelevant and acoustically indistinguishable whether further sloping bends are added at 20 Hz or at 40 Hz. At 40 Hz, both curves are already more than 10 dB below normal level. Everything that is even quieter is lost to the ear!
The disadvantage of the 20 Hz solution, however, due to the consumption of excursion and the lower level stability, remains fully intact.
As you can see, the desire for lower fs is rather disadvantageous. Perhaps even fs = 60 Hz would be better ?
Or, as a rule of thumb, as long as your baffle width remains below 2m, only your baffle is relevant for the lower cut-off frequency! The driver then only determines the efficiency !
"What is more important : low fs or high Qts ?"
Both parameters are equally important and must never be considered in isolation. Relevant for the reproduction is the quotient fs/Qts (also called EBP) :
This quotient is proportional to the strength of the resulting drive (in relation to the mass present) and drive is inversely proportional to draught ! Optimally, one should choose a compromise, e.g. for open baffles optimal fs/Qts = approx. 50.
Among the drivers that produce comparable results, e.g. 30Hz + Qts=0.6 or 40Hz + Qts=0.8 both result in fs/Qts of 50 and thus the same "bass fullness", I would prefer the 40Hz driver, as it would wobble less due to the harder suspension and would therefore be more level.
"So how big are the Rms (Mms) values ?"
Despite my very clear comment regarding B*L on the URPS page, still not everyone is able to make the transfer that this applies to all individual mechanical data.
It is just as silly to ask about B*L as it is for URPS fans to ask about Mms, because neither the numerator nor the denominator alone have any significance, but only the quotient of drive by mass ! And this quotient is proportional to EBP = fs/Qts, which you can read directly from the TSP without knowing Mms !
Mechanical primary data like Rms, Mms and B*L are not meaningful for the transmission behaviour of a loudspeaker. Only in combination with many other data do ratios arise from which one can draw conclusions about quality. This is why the Thiele-Small parameters were introduced, so that one can easily start and compare !
For example, a chassis with twice as much moving mass and twice as hard a suspension may also have twice as much Rms as an absolute value. The resulting ratios fs and Qm would then be exactly identical, just on a higher mechanical level. And only Qm allows a real qualitative statement about the mechanical losses, Rms on the other hand does not (think of the horsepower number of a truck !) !
The direct comparison of absolute mechanical primary data therefore leads to wrong decisions. That is why they are not included in my data sheets (any complete parameters are given indirectly via the equation systems anyway). Even when asked persistently, I will not give any values, but rather answer with a formula or a reference to a calculation programme. Because if you get such values all too easily, you won't think about it and will make the wrong decisions. But if you have typed in a calculation several times, you will get a feeling for the correlations and have learned something !!!
And another hint for the rethinkers : The quality Qm has an inversely proportional orientation like Rms. The higher Qm is, the lower are the mechanical losses !
"How much noise does this chassis make ?"
The noise is caused by air movements on / in the chassis, but is ultimately also a consequence of your individual baffle ! The same drivers work in other constructions to your complete satisfaction.
There is room for improvement in order to design a chassis with low noise levels during large strokes, because the cause is almost always the air compressed behind the dustcap and the spider (marked in red), which has to squeeze out somewhere.
The following ventilation measures (shown in green) can help:
- (1) perforated diaphragm neck : usually only small openings possible
- (2) rear-ventilated centre spider : possible on all sides and over a large area, optimal for these air cushions
- (3) Pole core drilling : simple and effective standard measure
- (4) Magnet holes : mainly for coil cooling, but better than pole core holes for the air behind the spider.
- (without) Phase plug : is also counterproductive, as the air outlet on the front of the diaphragm is out of phase.
But it is even better to solve the problem at the source and widen your baffle, e.g. by adding wings. The more you reduce their acoustic short-circuit, the less short-circuited reverberation you need for the same amount of audible sound. In this way, you reduce airborne noise at the same volume and improve your lower cut-off frequency at the same time.
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