Calculating VAS for Corals 4a-70

[Jameslc]

hi all just wondering how to calculate the vas from the qts,qes,qms and fs, Here are the specs i have found

cheers james

FULL RANGE SPEAKER
INPUT POWER 40 W MAX.
* Manufacturer: CORAL
* Model: 4A - 70
* f(s)= 121.10 Hz
* R(e)= 6.90 Ohms
* Z(max)= 27.10 Ohms
* Q(ms)= 2.455
* Q(es)= 0.838
* Q(ts)= 0.625
* L(e)= 0.19 mH

[GM]

Greets!

Need to know at least its ‘Sd’ or effective piston diameter to calculate it, though ideally its measured ‘BL’, ‘Mms’ and ‘po’, ‘c’ the machine used to measure with also if the person that measured them bothered to save these. Makes me wonder if his were dried out from age.

FWIW, some seriously different/better Fs, Qts specs published here: Vintage SpectrumAudio

GM

[Jameslc]

Hi GM thanks for the reply tried to find the piston size but was unable this is all the dimensions i could find. I might have to do it the old way by building a box and putting the signal generator on it and working it out that way but i have already built boxes for the corals here is the pic's with the box dimensions Picasa Web Albums - arustyrednail - coral 4A-70 p... but i wanted to try a back loaded horn design but don't know how to design one of them yet

[GM]

Greets!

You’re welcome!

OK, not that big a deal, you can measure [Dd] close enough by measuring the diameter across the diaphragm where it meets the surround [d1] and the diameter across the surround’s outer limit [d2], then [d1*d2]^0.5 = [Dd] and of course effective piston area [Sd] = Dd^2*pi/4.

WRT the speed of sound value [c] and air mass [po], I’ll use an average of the default values of some popular design programs.

Been there, done that many times and I date back to when the typical DIYer ‘designed’ based on a formula that only used Dd, Fs to generate a net box volume [Vb] and since this was back when amps typically had a very high output impedance and drivers had a huge Vas spec to keep efficiency high, the cabs were huge by today’s standards.

GM

[Jameslc]

cheers GM i will work that out next week because i am going away but when i get the piston diameter could you post a link or just write a formula for working out the vas it would be most appreciated and i am guessing pi =pie symbol

james

[GM]

Yes, but never mind, it’s been long enough since I messed with Corals that I’d forgotten I had a bunch of misc. data including its Sd, which is 62.9 cm^2, ergo its max calculated Vas for these specs and a published 2.7 gr. Mms is ~3.61 L.

GM

[Jameslc]

thanks GM i am very greatful that you managed to dig out the sd and the vas

cheers james

[GM]

You're welcome! Unfortunately, short of a relatively large BLH, you'll need at least a mid-bass system to properly cover a human's singing BW with this driver.

GM

[Jameslc]

yeah it does lack bass but i have got some corals 12L-25B maybe do some sort of 2 way system but i would have to think about the volume of the speaker because WINisd said that it want's 400 litres which is a bit too big

[GM]

True, but this assumes a maximally flat response which will tend to sound [very] 'boomy'/'one note' in-room, so better to make it around its ~110 L Vas spec and tune it to Fs if driven with a high output impedance, otherwise down around 30-32 Hz and use internal damping to quell any perceived 'ringing' it may have due to it being technically a somewhat under-damped alignment.

GM