Dear friends,
I’ve got a question that in need of your help because I’ve only limited knowledge of physics of acoustics.
Forgive me, as I don’t know if this had been discussed somewhere in time…
Recently, i’m preparing to DIY a pair of full range cabinets and I’ve got a pair of drivers and its data sheet.
With some calculation with the data given, I was able to get almost exactly the same speaker driver impedance curve as provided by the manufacturer data sheet.
Upon calculating the SPL curve (only interested in Low Frequency range below 200Hz), I calculated the acoustical output power curve in front of the driver piston mounted in an infinite baffle. It is a generic curve given by the mid to low frequency efficiency expressed in SPL unit as shown below.
I plotted this curve below (Green) in superposition with the supplier’s SPL curve (Black).
My question is that why, at low frequency, is the data sheet SPL curve (Black), where driver mounted in an infinite baffle, exhibits roughly a 40dB/dec slope roll off at low frequency rather than the 20dB/dec slope roll off of sound pressure level (Green) from the volume velocity driving source of the vibrating diaphragm?
Have I missed anything? Please help!
Thanks in advance.
Yours sincerely,
KW
I’ve got a question that in need of your help because I’ve only limited knowledge of physics of acoustics.
Forgive me, as I don’t know if this had been discussed somewhere in time…
Recently, i’m preparing to DIY a pair of full range cabinets and I’ve got a pair of drivers and its data sheet.
With some calculation with the data given, I was able to get almost exactly the same speaker driver impedance curve as provided by the manufacturer data sheet.
Upon calculating the SPL curve (only interested in Low Frequency range below 200Hz), I calculated the acoustical output power curve in front of the driver piston mounted in an infinite baffle. It is a generic curve given by the mid to low frequency efficiency expressed in SPL unit as shown below.
I plotted this curve below (Green) in superposition with the supplier’s SPL curve (Black).
My question is that why, at low frequency, is the data sheet SPL curve (Black), where driver mounted in an infinite baffle, exhibits roughly a 40dB/dec slope roll off at low frequency rather than the 20dB/dec slope roll off of sound pressure level (Green) from the volume velocity driving source of the vibrating diaphragm?
Have I missed anything? Please help!
Thanks in advance.
Yours sincerely,
KW
Without knowing what the manufactuer & driver are, it's pretty much speculation. However, it may not be an infinite baffle, or nominal equivalent thereof. It may be an IEC (or equivalent) open baffle. That's not unknown, especially amongst smaller manufacturers who can't afford a very large anechoic chamber. Markaudio for e.g. uses essentially the measurement method Seas used to employ up to about 2003, which was just that: a large open baffle. Seas then drew in the LF IB response derived from the T/S parameters on the same graph so you had a visual representation. They have since shifted to small sealed test boxes, but still draw in the mathematically derived IB response onto the graph. Personally I prefer their previous method with the larger baffle, but swings & roundabouts.
Hi Scottmoose,
Thanks a lot for reminding me that the SPL curve given by the manufactuer may not be performed in an infinite baffle.
When I searched the Seas data sheets, as you mentioned, I see similar plots of measured free field sound pressure and calculated infinite baffle curves as shown below for their X1-04 Exotic F8,
The steeper roll-off at low frequency is caused by the modified closed box system compliance and resistance loss hence quality factor.
This remined me that the SPL curve given by the manufactuer of my drivers may indeed not be tested in an infinite baffle.
So, I had tried to modify the calculated SPL curve from an infinite baffle to one with the box volume ratio factor Vas/Vab as below (Orange),
Which is the advertised resistance of the driver unit.
It seems that the above could, indeed, explain the reason why the system resonance frequency shifted by the series acoustic compliance of the box and the added 15 Ohms generator output test resistance accounted for the increased in system quality factor.
As you said, it is all pretty much speculation, but at least I’ve got your idea now.
Once again, really thank you for your kindly advise!
Have a nice day…
Yours,
KW
P.S. BTW, attached below is manufactuer data sheet of the drivers I’m going to use.
Thanks a lot for reminding me that the SPL curve given by the manufactuer may not be performed in an infinite baffle.
When I searched the Seas data sheets, as you mentioned, I see similar plots of measured free field sound pressure and calculated infinite baffle curves as shown below for their X1-04 Exotic F8,
The steeper roll-off at low frequency is caused by the modified closed box system compliance and resistance loss hence quality factor.
This remined me that the SPL curve given by the manufactuer of my drivers may indeed not be tested in an infinite baffle.
So, I had tried to modify the calculated SPL curve from an infinite baffle to one with the box volume ratio factor Vas/Vab as below (Orange),
Which is the advertised resistance of the driver unit.
It seems that the above could, indeed, explain the reason why the system resonance frequency shifted by the series acoustic compliance of the box and the added 15 Ohms generator output test resistance accounted for the increased in system quality factor.
As you said, it is all pretty much speculation, but at least I’ve got your idea now.
Once again, really thank you for your kindly advise!
Have a nice day…
Yours,
KW
P.S. BTW, attached below is manufactuer data sheet of the drivers I’m going to use.
Attachments
No problem.
That's one way. It may also be (and I suspect more likely, as I suggested above) loading / edge diffraction from a large IEC or equivalent test baffle e.g. IEC 268-5 (53in x 68in x-y axis). For e.g., see the attached. This is the Seas H1142 using their old IEC 268-5 baffle & measurement protocol. I've also attached a basic simulation of that baffle's anechoic load / diffraction characteristics with a flat piston of the rated Lowther 163.5mm diameter, offset by golden ratio on both the x & y axis, microphone axial to the driver & at a nominal 1m / 3.28ft distance.
Couple of caveats there: this is an approximation, since the Lowther is a cone driver & therefore not a perfect piston, & the offsets are pure speculation, though common enough with these test baffles. You can see the rise between characteristic LF rise centred on approximately 150Hz & diffraction through the midband. Hence the reason Seas used to draw in the mathematical LF IB response derived from the rated T/S parameters, while the impedance was taken in free air. YMMV as always of course.
That's one way. It may also be (and I suspect more likely, as I suggested above) loading / edge diffraction from a large IEC or equivalent test baffle e.g. IEC 268-5 (53in x 68in x-y axis). For e.g., see the attached. This is the Seas H1142 using their old IEC 268-5 baffle & measurement protocol. I've also attached a basic simulation of that baffle's anechoic load / diffraction characteristics with a flat piston of the rated Lowther 163.5mm diameter, offset by golden ratio on both the x & y axis, microphone axial to the driver & at a nominal 1m / 3.28ft distance.
Couple of caveats there: this is an approximation, since the Lowther is a cone driver & therefore not a perfect piston, & the offsets are pure speculation, though common enough with these test baffles. You can see the rise between characteristic LF rise centred on approximately 150Hz & diffraction through the midband. Hence the reason Seas used to draw in the mathematical LF IB response derived from the rated T/S parameters, while the impedance was taken in free air. YMMV as always of course.
