Hi,
I was trying to understand the requirements for good high frequency performance for drivers.
Fullrangers have good high frequency extension and they manage with quite low BL/Mms.
Domes also have good high frequency extension but they have better BL/Mms.
Compression drivers have very high BLs, the Mms is not known but looking at just the BL we can assume that it has a very high BL/Mms.
So my questions,
1)For a diy experiment I want to mass load a tweeter then how much should the BL be increased to have a flat out to 20Khz as before mass loading? My mass load could be about 10gms.
2) If BL is increased sufficiently (such that BL/Mms is unchanged) then does such a mass increase have poor transient response or energy storage issues when used as a tweeter?
Thanks,
WA
I was trying to understand the requirements for good high frequency performance for drivers.
Fullrangers have good high frequency extension and they manage with quite low BL/Mms.
Domes also have good high frequency extension but they have better BL/Mms.
Compression drivers have very high BLs, the Mms is not known but looking at just the BL we can assume that it has a very high BL/Mms.
So my questions,
1)For a diy experiment I want to mass load a tweeter then how much should the BL be increased to have a flat out to 20Khz as before mass loading? My mass load could be about 10gms.
2) If BL is increased sufficiently (such that BL/Mms is unchanged) then does such a mass increase have poor transient response or energy storage issues when used as a tweeter?
Thanks,
WA
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Assume that a flat piston(of 10g) is put on the voice coil, not a dome.
The question is can we increase the BL to bring the original high frequency extension back, without any audible issues?
The ratio of B*L[*i] to Mms has no direct correlation to HF extension in a tweeter, wideband, or any other moving coil drive unit.
-If you are assuming pistonic conditions (which are inapplicable to wideband drive units except over a relatively narrow BW) then all adding mass will do is reduce efficiency. It will not limit HF extension.
-If you are assuming resonant conditions (which is how moving coil wideband drivers produce most of their BW, and a lot of soft dome tweeters &c. the very top end of their output) then if you add mass to the emitting surfaces without any other effects in play, you'll physically damp their resonant behaviour, which will reduce HF output. Again, nothing to do with nominal magnetic strength in the VC gap for a given current, or its ratio to the moving mass[es].
-If you are assuming pistonic conditions (which are inapplicable to wideband drive units except over a relatively narrow BW) then all adding mass will do is reduce efficiency. It will not limit HF extension.
-If you are assuming resonant conditions (which is how moving coil wideband drivers produce most of their BW, and a lot of soft dome tweeters &c. the very top end of their output) then if you add mass to the emitting surfaces without any other effects in play, you'll physically damp their resonant behaviour, which will reduce HF output. Again, nothing to do with nominal magnetic strength in the VC gap for a given current, or its ratio to the moving mass[es].
The same tweeter tailored for different impedance has different BL. What counts is the mechanical impedance of the electromagnet, which is BL²/Re . The mass acts in electrical equivalent (voltage=velocity, current=acceleration) as a capacitor . A resistor and a capacitor form a mechanical low pass function , some call this the fundamental frequency of the speaker , because if you add a suspension ,it becomes an electro mechanical resonator whose Qes is the ratio of F resonance/F fundamental. By this , you can get the fundamental frequency by Fr/Qes . If the diaphragm is a piston , half of the transient power at this frequency is lost and it sounds as if recorded far from microphone.
By adding an extra mass on the voice coil , yes , you need to increase the BL² in proportion to get the same F fundamental.
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I can't talk about 'sounds as if recorded far from the microphone' which has absolutely no technical meaning whatsoever, but I can point out that the question was about HF extension, not the fundamental resonant frequency. And HF extension has no corrolation to the ratio of mass to B*L[*i]. As noted above:
-Assuming a given MC drive unit and assuming that drive unit functions under pistonic conditions across its BW, then increasing moving mass (with all other factors held constant) will reduce efficiency but not reduce the upper HF limit.
-If HF is produced by resonant action, then, again holding all other factors constant, you will reduce efficiency, but you may also reduce HF response since this may be damped by the higher mass (since the emitting surfaces of such drivers are specifically designed to resonate, and high levels of mass or mechanical damping are not typically condusive to this). This has nothing to do with the magnetic force in the VC gap however.
-Assuming a given MC drive unit and assuming that drive unit functions under pistonic conditions across its BW, then increasing moving mass (with all other factors held constant) will reduce efficiency but not reduce the upper HF limit.
-If HF is produced by resonant action, then, again holding all other factors constant, you will reduce efficiency, but you may also reduce HF response since this may be damped by the higher mass (since the emitting surfaces of such drivers are specifically designed to resonate, and high levels of mass or mechanical damping are not typically condusive to this). This has nothing to do with the magnetic force in the VC gap however.
So, what you are saying is that the added mass lowers efficiency across the operating BW? (it may lower Fs but lets not get that into picture at the moment)
So, now if we just increase BL we should get back to the original efficiency.
Am I understanding correct?
Assuming pistonic conditions (note that caveat), yes.
We have to, because if you just chuck extra mass onto an existing drive unit, you will change more than just that variable since the effective suspension compliance and other matters will be altered.
Not necessarily, since we are dealing with practical conditions and you can't just add mass to an existing drive unit without changing other aspects of behaviour in ways which are not automatically predicable. And as noted, this falls completely to pieces the second you depart from purely pistonic conditions in the functioning BW. Since we are talking high frequencies, that means compression drivers, and some (as in 'some', not 'automatically all') quality aluminium / aluminium alloy, titanium, ceramic, magnesium, beryllium & diamond dome tweeters.
I believe a stronger motor, stiffer and beefier suspension similar to compression drivers should help get there.
Any kind of corrugated structure should be enough to reach 20khz irrespective of the material. Take for example the Thiel CS3.7 speaker, it has a 4.5" midrange that has a pistonic response to 20khz. Quoting sterephile
"This allows us to use a very thin diaphragm that operates pistonically—very close to ideally," said Thiel. "It's linear up to around 20kHz, which is unheard of." That's not simply because of the driver's complex shape, however. The diaphragm is driven by a massive 3" voice-coil and a powerful neodymium magnet, which give it "exceptional circumferential strength."
Thiel CS3.7 loudspeaker | Stereophile.com
The corrugation looks so big in that midrange. A smaller (tighter ) corrugation could be much stiffer.
As I already explained , you do not consider BL/Mms ratio but (BL)²/(Re x Mms) ratio . If you devide it by 2pi ,you get the fundumental frequency which is equal to Fr/Qes that you can find on most datasheets. The efficiency of a driver is AirZ x MechnicalZ x (Sd/Mms)² . That is
5.445x10-⁴ x (BL)²/Re x (Sd/Mms)²
5.445x10-⁴ x (BL)²/Re x (Sd/Mms)²
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