Its not easy being a loudspeaker designer and it likely take years to understand what to look for and in part to chose what to give up. I have been reading books and articles for a long time, but spite this, I sometimes feel stupider than when I started... LOL, and this time I want to focus on open baffle driver specifications. So lets look at what is commonly said about drivers.
One factor or indicator to look at is the EBP (Efficient Bandwidth Product) – The efficiency bandwidth product is a rough indicator measure. A common rule of thumb indicates that for when EBP >100, the driver is perhaps best used in a vented enclosure (ported, aperiodic and transmission line), while EBP<50 indicates a sealed enclosure. For EBP 50><100 either enclosure may be used effectively. Because a sealed cabinet offers cone or membrane support to a higher degree than in open air, the requirements of the suspension is different. The former can be looser so to speak. Lets check if this is correct
I am going to use two drivers from SB Audience because they perfectly encapsulate the issue at hand. Both drivers are 12". Driver #1 is the BIANCO-12OB150-01 which is marketed as an open baffle driver. Driver #2 is the BIANCO-12MW200 "non OB. According to the theory, the open baffle driver should have better suspension compliance and higher EPB values.
Driver - efficiency - Qms - Le - CMS - Rms - EPB
1) BIANCO-12OB150-01 (OB) - 96dB - 6.39 - 1.18mH - 0.25mm/N - 2.26kg/s - 64
2) BIANCO-12MW200 (non OB) - 99dB - 14.33 - 0.38mH - 0.15mm/N - 1.45kg/s - 100
Qms: A unitless measurement, characterizing the mechanical damping of the driver, that is, the losses in the suspension (surround and spider). It varies roughly between 0.5 and 10, with a typical value around 3. High Qms indicates lower mechanical losses, and low Qms indicates higher losses.
Cms: Measured in meter per newton (m/N). Describes the compliance (i.e., the inverse of stiffness) of the suspension. The more compliant a suspension system is, the lower its stiffness - The higher the force on the cone and suspension assembly the further it travels.
If we look at both Qms and Cms we can see that the are inverse to one another. As the Qms increases, Cms will decrease. An open baffle driver should therefore and theoretically have a high Qms and low Cms to be stiff enough to withstand external forces, compared to a driver mounted to a finite cabinet like mentioned before. Also, if we have a stiff suspension and high efficiency, all the better and together with a low Le, such a driver should be well suited for an open air operation or open baffle.
Q: Why then is the driver with less stiffness and less EBP (BIANCO-12OB150-01) recommended for open baffle ?
Sources:
1. https://en.wikipedia.org/wiki/Thiele/Small_parameters
2 http://www.sbaudience.com/index.php/products/woofers/bianco-12mw200/
3 http://www.sbaudience.com/index.php/products/open-baffle-drivers/bianco-12ob150/
One factor or indicator to look at is the EBP (Efficient Bandwidth Product) – The efficiency bandwidth product is a rough indicator measure. A common rule of thumb indicates that for when EBP >100, the driver is perhaps best used in a vented enclosure (ported, aperiodic and transmission line), while EBP<50 indicates a sealed enclosure. For EBP 50><100 either enclosure may be used effectively. Because a sealed cabinet offers cone or membrane support to a higher degree than in open air, the requirements of the suspension is different. The former can be looser so to speak. Lets check if this is correct
I am going to use two drivers from SB Audience because they perfectly encapsulate the issue at hand. Both drivers are 12". Driver #1 is the BIANCO-12OB150-01 which is marketed as an open baffle driver. Driver #2 is the BIANCO-12MW200 "non OB. According to the theory, the open baffle driver should have better suspension compliance and higher EPB values.
Driver - efficiency - Qms - Le - CMS - Rms - EPB
1) BIANCO-12OB150-01 (OB) - 96dB - 6.39 - 1.18mH - 0.25mm/N - 2.26kg/s - 64
2) BIANCO-12MW200 (non OB) - 99dB - 14.33 - 0.38mH - 0.15mm/N - 1.45kg/s - 100
Qms: A unitless measurement, characterizing the mechanical damping of the driver, that is, the losses in the suspension (surround and spider). It varies roughly between 0.5 and 10, with a typical value around 3. High Qms indicates lower mechanical losses, and low Qms indicates higher losses.
Cms: Measured in meter per newton (m/N). Describes the compliance (i.e., the inverse of stiffness) of the suspension. The more compliant a suspension system is, the lower its stiffness - The higher the force on the cone and suspension assembly the further it travels.
If we look at both Qms and Cms we can see that the are inverse to one another. As the Qms increases, Cms will decrease. An open baffle driver should therefore and theoretically have a high Qms and low Cms to be stiff enough to withstand external forces, compared to a driver mounted to a finite cabinet like mentioned before. Also, if we have a stiff suspension and high efficiency, all the better and together with a low Le, such a driver should be well suited for an open air operation or open baffle.
Q: Why then is the driver with less stiffness and less EBP (BIANCO-12OB150-01) recommended for open baffle ?
Sources:
1. https://en.wikipedia.org/wiki/Thiele/Small_parameters
2 http://www.sbaudience.com/index.php/products/woofers/bianco-12mw200/
3 http://www.sbaudience.com/index.php/products/open-baffle-drivers/bianco-12ob150/
In a 'nutshell', any driver is suited for OB since there's no box to control/modify it. You choose based on the desired in room polar response, speaker F6, Fb. As such, for a given Fs, the largest baffle sizes are best for the highest eff. (low Qt) drivers to boost its low end with low eff. (high Qt) drivers best suited for small baffles, which in effect damp its Fs peaking.
Note too that T/S theory peters out at its upper mass corner (Fhm):
Fhm = 2*Fs/Qts'
Qts': 2*Fs/Fhm
[Qts']: [Qts] + any added series resistance [Rs]: http://www.mh-audio.nl/Calculators/newqts.html
Note too that T/S theory peters out at its upper mass corner (Fhm):
Fhm = 2*Fs/Qts'
Qts': 2*Fs/Fhm
[Qts']: [Qts] + any added series resistance [Rs]: http://www.mh-audio.nl/Calculators/newqts.html
I don't agree on the any statement because the box and no box scenario dictates different approaches in regards to suspension stiffness. If you look at the drivers that are usually found in sealed or ported enclosures, we can see that their Qms is often around 4-6 and Cms is often 1-2mm/N or more, this indicate that they need the "air suspension" that a box offers - aka, aid in stiffening up the suspension.
So I give up transient response for a smaller baffle .... LOL
So I give up transient response for a smaller baffle .... LOL
Don't you mean HIGHER stiffness?
My guess is that through sophisticated modeling that the OB driver was done to achieve a bit more pressure below 100 Hz.
As for increased stiffness - this could well have been to lower non-linear distortion below 100 Hz (and specifically around the driver's Fs).
To me this driver makes the most *sense (with some added resistance from an inductor in series) for OB duty:
http://www.sbaudience.com/index.php/products/subwoofers/bianco-18sw450/
*value-wise.
Considering the wide range of driver's specs from ~0.2 - 2.86 Qt with pretty much as wide a range of Vas I've tested on OB, we'll have to agree to disagree. 😉
No, if you look at both drivers, the open baffle (BIANCO-12OB150-01) have less stiffness than the non open baffle (BIANCO-12MW200).
Another low EBP driver is the Eminence ALPHA-12A, there the EBP is 55.6 and its F6 is much higher than both the SBA drivers. Perhaps not 100% fair since both Sd and motor force is larger on the SBA... but still, this goes against all of my instincts. A low EBP indicate sealed enclosure and a high EBP indicate open air... but not here. I guess F6 from now on have to be included in my judgement ... LOL
The first speaker in vented box requires 450 liters of volume, and the second one can be vented and the bass will be more than in an open design.
So, the two SB Audience 12" drivers are telling me I have to chose between either better F6 or better transient performance.
A lower Qms means a HIGHER stiffness (and obviously a higher Qms means a lower stiffness).
"1) BIANCO-12OB150-01 (OB) - 96dB - 6.39
2) BIANCO-12MW200 (non OB) - 99dB - 14.33"
The BIANCO-12OB150-01 VAS (Volume of Air having the same acoustic compliance as driver Suspension) is 103.8 liters, the BIANCO-12MW200 VAS is 68.7 liters, a "stiffer" spring.
Larger VAS values mean lower stiffness.
Its the other way around. High Qms indicates lower mechanical losses and low Qms indicates higher losses. Look at Qms and Cms, they are inverse relationship to one another. As Qms increases the Cms value decreases.
Cms: Compliance of the driver's suspension, in meters per newton (the reciprocal of its 'stiffness') - The property of a material of undergoing elastic deformation or (of a gas) change in volume when subjected to an applied force. It is equal to the reciprocal of stiffness. Meaning: The less it deforms or comply to an external force the stiffer or non complaint it is. And less complaint it is, the lower the value.
The Qms 6.39 driver have a Cms of 0.25mm/N - meaning the suspension moves 0.25mm per applied Newton.
The Qms 14.33 driver have a Cms of 0.15mm/N - meaning the suspension moves 0.15mm per applied Newton --- ergo, this one is the stiffest of the two.
The later is also more efficient, so easier to drive. Generally speaking, drivers with high efficiency and low Cms is better for transient performance since they tend to come to rest quicker than higher Cms (low Qms drivers). Take Troels Gravesen JA8008-HMQ 8" driver which he uses for some of his OB projects, this driver have a Qms of 10 and a Cms of 0.66mm/N and an EBP of 176... which is very decent, it is clearly designed for open baffle. See, the EBP vent up
http://www.troelsgravesen.dk/JA8008-HMQ.htm
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There was a design by Martin King using an Eminence Alpha driver, where he described that the weaker motor system, which allows a higher Q resonance, made it simpler to equalise when compensating the dipole slope. Since then, this has been talked about. One of the considerations was to use as few components as necessary.
Is it possible that stereotypes are being drawn when they aren't necessary? Ok, a strong motor may not be strictly necessary with an open baffle but when you consider that you are going to be equalising anyway does it matter so much?
Is it possible that stereotypes are being drawn when they aren't necessary? Ok, a strong motor may not be strictly necessary with an open baffle but when you consider that you are going to be equalising anyway does it matter so much?
Um, you do realize that you are now in agreement with me?
(Increased Stiffness = Increased Loss)
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Generally speaking one doesn't need as strong of a motor for OB, I agree on that. The 12OB150-01 have very decent spec and a low Cms, so I do like it. For its intended usage, say 150Hz and bellow, its a nice contender.
Also, I am interested in focusing my low end around a cardioid / super-cardiod pattern rather than dipole (see RiPole).
This is what you wrote in post #9: A lower Qms means a HIGHER stiffness (and obviously a higher Qms means a lower stiffness) - which is opposite of the correct relationship. That is why I took some extra time to explain it in debt, that.
You can verify this, the Qms and Cms relationship by looking at T/S definitions. As Qms goes up Cms goes down and Cms is directly related to compliance - the less complaint a driver is, the stiffer it is.
If this thread is to go by, you have to pick your poison. Like with the two SB Audience drivers used as an example, one is returns a better transient performance by giving up some low end output and the other produces better low end (by some 3dB or so) but is ever so slightly less transient. Both are good drivers.
Also, I am interested in focusing my low end around a cardioid / super-cardiod pattern rather than dipole (see RiPole).
Um, you do realize that you are now in agreement with me?
This is what you wrote in post #9: A lower Qms means a HIGHER stiffness (and obviously a higher Qms means a lower stiffness) - which is opposite of the correct relationship. That is why I took some extra time to explain it in debt, that.
You can verify this, the Qms and Cms relationship by looking at T/S definitions. As Qms goes up Cms goes down and Cms is directly related to compliance - the less complaint a driver is, the stiffer it is.
If this thread is to go by, you have to pick your poison. Like with the two SB Audience drivers used as an example, one is returns a better transient performance by giving up some low end output and the other produces better low end (by some 3dB or so) but is ever so slightly less transient. Both are good drivers.
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@ Scott: In the OP, there you can look at the losses (kg/s) and you can see there as well that the less compliant suspension is also the one with less losses (Rms).
A higher number for Rms indicates HIGHER losses - at 2.26 Rms the OB driver has more resistance (mostly derived from spider stiffness) than the non-OB driver at 1.45 Rms.
Qms (Mechanical Loss Factor) is derived from both Cms and Rms (but Rms is by far the most pertinent parameter). Qms is an *inverse measure of losses (mostly in the suspension though it includes things like air-load, and as it relates to the driver's resonance).
*low Qms means high losses and high Qms means low losses. Again, those losses are mostly derived from the spider's stiffness. Again, higher losses = higher stiffness: a low Qms driver (because of its inverse measure) has higher stiffness.
BTW, there are several threads devoted to Qms (and Rms) on this forum.
Qms (Mechanical Loss Factor) is derived from both Cms and Rms (but Rms is by far the most pertinent parameter). Qms is an *inverse measure of losses (mostly in the suspension though it includes things like air-load, and as it relates to the driver's resonance).
*low Qms means high losses and high Qms means low losses. Again, those losses are mostly derived from the spider's stiffness. Again, higher losses = higher stiffness: a low Qms driver (because of its inverse measure) has higher stiffness.
BTW, there are several threads devoted to Qms (and Rms) on this forum.
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I don't think we are concerned with the return force by the (mostly) spider generated stiffness with respect to mechanical losses as a lumped parameter at driver resonance (designated as Qms or "Mechanical Loss Factor").
Qms is represented by three components.
Mms and Cms are the storage components of the resonance, and here Cms represents stiffness. Rms represents losses.
Mms and Cms are the storage components of the resonance, and here Cms represents stiffness. Rms represents losses.