Questions about driver and/for BLHs

[e-side]

Hello everyone,

I have a couple of questions about a 6.5" driver which I would like to use for a BLH.

The driver is from Jamo, type "EC 141 A". After searching on Internet I found that this driver is used in the "Jamo 68" speaker system (see attachment). I measured the T/S parameters myself:

Re = 9,4 Ohm
Fs = 54,7 Hz
Qms = 4,03
Qes = 0,89
Qts = 0,73
Vas ~ 13 L
Sd ~ 220 cm^2

As you can see on the attachment, the speaker system is vented. The dimensions are 44 x 24 x 24 cm (hxwxd), so the Vb of the system is less than 25 L.

Q1) For vented enclosures the driver should have quite low Qts, in this case the driver doesn't have. Also, Vb is very small compared to results from simulations in WinISD. Why does Jamo use a driver which isn't 'optimal' for this speaker system?

Q2) I'm not sure what type of enclosure will be suitable for this driver. Fs is too high for sealed, Qts is too high for vented, bandpass isn't an option, probably dipole won't work (low xmax, small driver), FLH isn't an option (no direct radiator). I think only TL and BLH are suitable, or am I wrong here?

For building a BLH, I got the following questions:

Q3) I'm not sure if a BLH will work well with this driver, because the horn will boost the low frequencies. Probably there will be too much bass compared to other, direct radiated frequencies. I think the bass will be 'overpowered'. Could I get problems with this?

Q4) For this driver Leach's math gives a throat of about 62 cm^2. This a compression ratio of 3,5 : 1. Is it too high and if so, what problems will I get because of this? I tried different throat surfaces and throat chamber sizes, but it seems that 62 cm^2 is the most optimal...

I hope somebody could answer these questions. Thanks in advance.

Regards

Erwin

[e-side]

I see I didn't provide the correct information. Sd should be less, so Q4 can be skipped.

Could anyone help me with the remaining questions?

[Tomac]

You could do a TL, but you should provide parameters for BL first.
Are you shure that you measured this parameters correctly? Qt is too high for vented box, but I wouldn't be suprised if they really did put such a high Q driver in vented box, it wouldn't be a first case in history
of loudspeaker making.

[e-side]

Originally posted by Tomac
You could do a TL, but you should provide parameters for BL first.
Are you shure that you measured this parameters correctly? Qt is too high for vented box, but I wouldn't be suprised if they really did put such a high Q driver in vented box, it wouldn't be a first case in history
of loudspeaker making.

I'm sure I measured the drivers correctly; I repeated the procedure four times, the results were the same.

Here's a complete list of parameters for this driver:

Re = 9,4 [Ohm]
Fs = 54,7 [Hz]
Qms = 4,03
Qes = 0,89
Qts = 0,73
Vas ~ 13 [L]
Sd ~ 140 [cm^2]

Cms = 4,72 * 10^(-4) [m/newton]
Mmd = 17 [g]
BL = 8,07 [tesla.m]
Rms = 1,53 [newton.sec/m]

[GM]

Greets!

Q1) Don't have a clue. Possibly it's severely underdamped to make its own acoustic BSC. Is it well stuffed, making it ~aperiodic, yielding a ~sealed alignment with gain? The vent size implies this.

Q2) Sounds good to me, though add aperiodic.

Q3) This depends on the horn design, but due to the high Qes, done right, it will be huge.

Q4) Way too high for a typical HIFI driver if FLH, with ~2:1 my limit, but this is a BLH, so we want to look at what sort of vent performance it has in a BR of whatever the filter chamber's Vb is. BTW, I get radically different specs, so what did you input?

Regardless, a MLTL looks pretty good and can be fine tuned with vent damping if need be:

L = 63.41"
zdriver = 21.56"
zport = 60"
SO/SL = 81.21"^2
rport = 1.75"
Lport = 0.75"

This puts the woofer up high enough to mount the HF below it and may not need any BSC once its Le midbass 'hump' and room effect is factored in.

GM

[e-side]

Thanks for your reply GM!

Q1) Don't have a clue. Possibly it's severely underdamped to make its own acoustic BSC. Is it well stuffed, making it ~aperiodic, yielding a ~sealed alignment with gain? The vent size implies this.

I don't know if the speaker system is stuffed, the only 'information' I've got so far is this picture...

Q3) This depends on the horn design, but due to the high Qes, done right, it will be huge.

I'm not an expert in horn-design... Could you tell me how to do this? Because usually I don't get a nice response when I change throat en throat chamber dimensions, which I calculated with Leach's math.

Q4) Way too high for a typical HIFI driver if FLH, with ~2:1 my limit, but this is a BLH, so we want to look at what sort of vent performance it has in a BR of whatever the filter chamber's Vb is. BTW, I get radically different specs, so what did you input?

The first attachment is from ML's program, which applies Leach's math. The second from HornResp.

I'll try your MLTL suggestion.

BTW, is it possible to calculate Le (when the impedance at 1 kHz is known) ?

thanks again.

[e-side]

And the attachment from HornResp

[Tomac]

You could do a classic TL with constant cross section area of 3 Sd, 200cm long.
For the same line length 3:1 taper is also possible, if you strive for a smaller box and litlle better impulse response. Presonally, I would choose a 3:1 taper over a straight line.
good luck

[GM]

Greets!

You're welcome!

Too much to type, but search for BLH in various forums for mine and other designer's thoughts. Your main problem though is Leach is calcing a FLH, so you have to juggle the variables until it calcs at least a large room sized Vb value. Think low flare frequency (Fc) and ~hyperbolic flare factor (M).

Le (mH) = [(Zxo^2-DCR^2)^0.5/(2*pi*Fxo)]*1000

Where:

Fxo = some frequency point away from Fs
Zxo = impedance at Fxo
Dcr = Re


GM

[e-side]

Originally posted by Tomac
You could do a classic TL with constant cross section area of 3 Sd, 200cm long.
For the same line length 3:1 taper is also possible, if you strive for a smaller box and litlle better impulse response. Presonally, I would choose a 3:1 taper over a straight line.
good luck

Thank you for the suggestion. I already experimented a bit with TL lengths and surfaces in MJK's spreadsheets, but there were quite a lot dips and peaks in the response..

Originally posted by GM

Too much to type, but search for BLH in various forums for mine and other designer's thoughts. Your main problem though is Leach is calcing a FLH, so you have to juggle the variables until it calcs at least a large room sized Vb value. Think low flare frequency (Fc) and ~hyperbolic flare factor (M).

For my current HornResp simulations I used the throat surface calculated with Leach's formula's, then I adjusted the throat chamber volume (VTC) until I got a decent response. The rear chamber volume (VRC) is 0 (for BLHs), which allows the use of the Back-Horn Tool when showing the SPL graph.

It seems 'impossible' to drop SPL below ~108 dB by changing throat surface and throat chamber volume in HornResp...

I added 2 screenshots of my current simulations (Fl = 60; Fh = 130).

Le (mH) = [(Zxo^2-DCR^2)^0.5/(2*pi*Fxo)]*1000

Where:

Fxo = some frequency point away from Fs
Zxo = impedance at Fxo
Dcr = Re

Thanks for sharing!

regards