OB15'S SPECT AND FREQ RESPONSE GRAPH

[mikegannon]

hi has anybody got the ob15's spec and a drawing of the frequency response curve
thanks mike

[simon5]

I will try to ask John. I think this was posted on a forum somewhere but not here...

[simon5]

Finally found the specs on the audiocircle forum... AFAIK, price is now 149$ per driver or 500$ for 4, same as IB15.

OB15
Fs: 29hz
Qms: 4.6
VasL 257L
Cms: .25mm/N
Mms: 120g
Rms: 4.71 kg/s
Sd: 851sqcm
Vd: 3.07L
Qes: .75
Re: 5.32ohm
Le: .25mh
Z: 8ohm
Pe: 300W
Qts: .64
1W/1m 91.4dB

[NiToNi]

Simon,

Could you please confirm with John that the specs you found on AudioCircle truly are the correct ones.

The reason being that I came across the following specs myself on the very same forum:

AESpeakers OB15 *note this is the LO15 not OB15*
Fs: 33.8Hz
Qms: 19.4
Qes 0.86
Qts 0.82
Vas 311 Liters
Cms 0.3 mm/N
Mms 74 grams
Sd 855 cm2
Rms .81 Kg/S
Bl: 7.5 T/m
Re: 3.1ohms
Z 4 ohms
Le 0.0375 mH
Pe (max) 100 Watts
Pe (transient) 250 Watts
1WSpl: 93.5 dB
2.83V: 97.6dB
Linear Xmax 9 mm (peak)
Mech Xsus 12 mm (peak)

As you can see, they differ quite significantly.

Also, if your specs are the correct ones, what are Xmax and Xmech?

[simon5]

Those specs are LO15 specs or "TD15-Dipole" which was the prototype name for Lynn Olson.

You can see it in that post just a few posts below, those specs you just listed match with that of TD15-Dipole : http://www.audiocircle.com/index.php?topic=59619.msg547723#msg547723

[NiToNi]

Thanks Simon.

Do you know the specs for Xmax and Xmech?

Also, what power is required to reach Xmax at Fs on an open baffle (of 60 cm width) and what SPL can be expected (at 1 m taking both the OB15's sensitivity and efficiency into account)?

[simon5]

I'm quite sure Xmax is similar to IB15 so that would be 18.5 mm one-way Xmax, with 25 mm one-way Xmech.

IB15 and OB15 are nearly twins, but OB15 has a few tricks to achieve better high frequency response and lower mms to achive higher Qts and higher efficiency.

I'll try to reach John to confirm.

At Fs, SPL is totally Xmax dependant... efficiency and sensivity doesn't count. Not much power would be needed, probably 100W or less.

Max SPL in a closed box would be 116.6 dB at Fs. I'm not really used to open baffle systems, if you know how much dB down your 60 cm wide baffle is at 29 Hz, then you will know the final number. There's probably a formula.

[NiToNi]

Thanks Simon. Looking forward to your confirmation of Xmax and Xmech.

I've just placed an order for four OB15 with Beth.

So back to the SPL, yes there is a formula to calculate the baffle roll-off (typically 6dB/octave below a certain frequency as determined by the baffle size) but it deducts from the max SPL of a driver on an infinite baffle, not from a sealed box. SO I guess I am still a bit unsure about what SPL levels to expect.

I hear you also on the limited power needed at Fs. What I am trying to get a grip on though is how much power I would need to drive the OB15's to Xmax (max SPL) over their passband (30-120 Hz) in my open baffle application, i.e. optimal wattage to reach maximum SPL, while at the same time making sure I'm not overdriving the drivers with too much juice (up to 80% of Xmech??). Basically, I guess I just want to know what amplifier to get

EDIT: I've played around a bit in The Edge and I think it is safe to say that the 6dB/octave roll-off will apply to the full intended operating range (30-120 Hz) of the woofers on my baffle, so I will be applying steep EQ roll-off below 30 Hz (for protection), no EQ cut at 30 Hz, -6dB cut at 60 Hz and -12 db cut at 120 Hz).

I will use two OB15's per side, using active crossover and EQ (PCXO/AudioLense). Each pair of OB15's will matched with a Supravox 215 Bicone Signature (120-15k Hz) for each side. If the potential output of the woofers are loud enough, I would like a house curve whereas the woofers are about 3-5 dB higher in level than the full-range driver.

[simon5]

If you simulate in a HUGE closed box (AT LEAST 10 times the Vas) you approximate very closely an IB enclosure. That's how I found out your 116 dB if I remember correctly. If you use WinISD Pro Alpha, do not put a too big number because it has troubles with huge numbers.

You could simulate what power is needed to reach Xmax at Fs by using a huge box approximation also. I can't do that for now because I'm a bit busy... I'm sure it would be around 100W or so, but better to simulate to be sure.

One woofer might have a hard time to follow your fullrange, but with a pair I think you will have no problems. Again, simulating could help you check that.

[simon5]

John just confirmed that Xmax and Xmech on OB15 is the same as IB15, so that would be 18.5 mm one-way Xmax, with 25 mm one-way Xmech.

[NiToNi]

Thanks again Simon for the confirmation of the Xmax and the Xmech.

I found this post by John in which he discusses the OB15, its SPL capability and power handling at Fs. From his post, it seems that a single OB15 in an infinite baffle is not reaching its Xmax (18.5 mm) at Fs (or 30 Hz instead of 29 Hz to be exact) until input power of 400W and at a level of 114.4dB.

So if my baffle is down 12 dB from 120 Hz, the single driver should be able to put out 102.4 dB. Two drivers another 3 dB, so 105.4 dB. Then if I sit 8 feet away, I lose 7.7 db due to dispersion/distance, so a total of 97.7 dB. This is without taking into account the room boundaries of course. Nevertheless, if +95 dB is a true approximation of what this system will be able to deliver, I will be happy.

[simon5]

You're right that you gain 3 dB by using two drivers at 400 W, but if you can deliver 400 W to each driver, you will gain another 3 dB, for a total of 6 dB.

If your goal is only 95 dB, I think you will have no problems !