February 28, 2012

Akabak vs Hornresp vs Real measurements

Here is a chart that shows the results from Hornresp vs Akabak when compared to the measurements of the actual real thing:


Black is the measured response of the 4" closed back midrange on S1. Red is Hornresp, green is Akabak.

S1 is a 60 x 60 point source horn:


Note: Pyle drivers shown are not what is being investigated here.

Here are the Hornresp parameters:


Included at the end of this post is the script for Akabak. I exported the script from Hornresp, so it was automatically generated.

You will notice an offet of the ports larger than built - 9.2cm was entered as it matches the first dip. The ports were located 8.6cm from the throat - a 7% difference. This could be related to the compression driver exit tube coming into play, or it could be related also to atmospheric conditions and the speed of sound which is  used to determine the port offset.

In this example it's interesting to see that Hornresp is actually more accurate where it matters most. In particular I'm interested in the top and bottom end roll off behaviour as it assists in choosing the parameters. Hornresp is closer with both. It estimates the first dip well enough to guide design decisions and then becomes very inaccurate at higher frequencies where the crossover takes over. In the bottom end, both fail to predict the peak and dip.


Akabak script generated by Hornresp:


|DATA EXPORTED FROM HORNRESP - RESONANCES NOT MASKED

|COMMENT: Celestion 4" single on S1

|========================================================================================================

|REQUIRED AKABAK SETTINGS:

|File > Preferences > Physical system constants:

|Sound velocity c = 344m/s
|Medium density rho = 1.205kg/m3

|Sum > Acoustic power:

|Frequency range = 10Hz to 20kHz
|Points = 533
|Input voltage = 2.83V rms
|Integration = 2Pi-sr
|Integration steps = 1 degree ... 1 degree
|Integration method = Cross

|========================================================================================================

Def_Const |Hornresp Input Parameter Values
{
|Length, area and volume values converted to metres, square metres and cubic metres:

S1  =     4.90e-4;  |Horn segment 1 throat area (sq cm)
S2  =   100.00e-4;  |Horn segment 1 mouth area and horn segment 2 throat area (sq cm)
S3  =  2500.00e-4;  |Horn segment 2 mouth area (sq cm)

L12 =     9.20e-2;  |Horn segment 1 axial length (cm)
L23 =    33.00e-2;  |Horn segment 2 axial length (cm)

Ap  =     2.50e-4;  |Chamber port cross-sectional area (sq cm)
Lpt =     1.80e-2;  |Chamber port tube length (cm)

Vtc =    34.00e-6;  |Throat chamber volume (cc)
Atc =    55.00e-4;  |Throat chamber cross-sectional area (sq cm)

|Parameter Conversions:

Sd  = 60.00e-4;  |Diaphragm area (sq cm)

Ltc = Vtc / Atc;
}
|========================================================================================================

|Network node numbers for this offset driver horn system:

|       0-Voltage-1
|                 |
|Radiator(1)-5-Driver-6-Chamber-7-Port-
|                                      |
|                            8-Segment-9-Segment-10-Radiator(2)

|========================================================================================================

Def_Driver 'Driver'

  Sd=60.00cm2
  Bl=6.49Tm
  Cms=3.01E-05m/N
  Rms=0.98Ns/m
  fs=444.4946Hz  |Mmd = 4.00g not recognised by AkAbak, fs calculated and used instead
  Le=0.60mH
  Re=5.69ohm
  ExpoLe=1

System 'System'

  Driver Def='Driver''Driver'
    Node=1=0=5=6

  Radiator 'Diaphragm'
    Node=5
    SD={Sd}
    Label=1

  Duct 'Throat chamber'
    Node=6=7
    SD={Atc}
    Len={Ltc}
    Visc=0

  Duct 'Port'
    Node=7=9
    SD={Ap}
    Len={Lpt}
    Visc=0

  Waveguide 'Horn segment 1'
    Node=8=9
    STh={S1}
    SMo={S2}
    Len={L12}
    Conical

  Waveguide 'Horn segment 2'
    Node=9=10
    STh={S2}
    SMo={S3}
    Len={L23}
    Conical

  Radiator 'Horn mouth'
    Node=10
    SD={S3}
    Label=2

10 comments:

  1. Paul, on the Hornresp screenshot, do you know the significance of the "Cir>1 for S2" warning? I get that too, when I tried to sim, but the Help files don't really tell me if it is something I need to worry about.

    ReplyDelete
  2. Hi Paul,
    Another question -- I tried entering your sim values in and can only get the curve you show when I have the 'Chamber' selected to be 'Throat Ported' (i.e., when the 'Ap' value you have is instead 'Ap1'). Can't really find a good description of what "Throat Ported" means, though. Reduction of the chamber area through a narrower port? But it seems that would be already handled via Sd....

    ReplyDelete
  3. It means you are pushing the limits of Hornresp!

    Start from the very first default parameters, click add. Input all the settings, pressing C after entering lengths to get conical, then go into "driver arrangement" and select offset driver. It works fine.

    ReplyDelete
  4. Hi Paul,

    I think you're on the money with Hornresp more than you think - with "throat ported" selected, the sim lines up much better down low as dubb discovered.

    Do Vtc and Atc refer to the enclosed rear of the driver, or chamber between the driver cone and horn wall it's bolted to?

    The driver is ported, but it seems like throat rather than rear ported, though I'm new to this so could me getting easily confused...

    cheers, Richard

    Dubb : I'm still learning the terminology too, but found a couple of good pages that helped me get my head around it :
    http://www.diyaudio.com/forums/subwoofers/119854-hornresp-147.html post 1464 for an explanation of ap vs ap1
    http://www.avsforum.com/avs-vb/showthread.php?t=1211856 for a good hornresp thread with lots of diagrams explainging what all those variables actually mean in the "real" world :)

    ReplyDelete
  5. The throat chamber is the air between the cone and horn walls

    ReplyDelete
  6. How do I read hornresp VTC because this is CC, and the ATC is square cm, VTC looks like it is chamber between speaker cone and baffle with port in it, and if so is quite a low CC and so much space there with give a troublesome outcome.

    thanks.

    kees

    ReplyDelete
    Replies
    1. VTC is the throat chamber, which in this case is the area under the cone - with a 4" driver obviously this will be a small number

      Delete
  7. @Paul

    yes you are right, but I have a synergy woofer section with two 8 inch woofers who have 488 cubic cm on a baffle of 1 cm thick to prevent cone from hit the horn port itselfs, this I have to include I presume and is 251.6 cubic cm this means for hornresp, I did try one speaker and see response is not much changed who means hornresp multiply the number of woofers itself.

    When I include the baffle cutout with the speaker cone surface I get a bad response, so fill up that cone earea but this is not very easy with synergy horns.

    I hope I get somwhere some cearance about this, it is confusing.

    thanks

    Then atc is cross sectional cm2

    ReplyDelete
  8. VTC is the total volume in front of all drivers. You can find this kind of info in the help file.

    ReplyDelete
  9. Hi Paul

    I have found now that no use of a plate between speaker and the baffle for clearance conus and in stead of that the use of a mill to mill out clearance in the baffle itselfs will fill up VTC such a way that ATC can be small will give a solution for the problems of to much volume in throat chamber.

    regards

    kees

    ReplyDelete

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