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by John L. Murphy

Subject: Using the ACI SV-12 Subwoofer in a large party room
(posted 23-25Jun99 to Bass List)

Steven wrote:

> I've been assigned the task to design two new subs for the party room at my
> fraternity. currently we have two old and dilapidated 15" subs. I've heard
> a lot of talk on the list about the SHIVA and NHT1259 subs and was
> wondering if after choosing between the two if running each sub with 2-4 of
> these in a closed box system in an array would work?
> I was thinking of using four per sub arrayed in a transmission line box.
> the sound doesn't need to be audiophile but it does need to fill a HUGE room.


Yes, use multiple drivers to achieve the SPL you need for your large room.
The hard part is predicting how many drivers are needed to produce the
desired SPL at the listening distances involved without running out of
either thermal or mechanical headroom.

As you start the project don't forget to look at Audio Concepts SV-12 at http://www.audioc.com.
You can also download a special promotional version of my WinSpeakerz
speaker simulator that works exclusively with the SV-12.  You can use it to
actually predict the responses of the various types of systems you are
considering.  WinSpeakerz will also help you predict the SPL at various
distances and input powers.

I recently started using an SV-12 subwoofer with my "Desktop" sound system
and have been quite impressed with the performance of this driver.

Regards,

John


Steven writes:

> if the room is 70-80feet long, 40-50 feet wide and a ceiling that is 2.5
> floors high what would be a good setup for using this driver? (we have room
> for two sub boxes, which are currently 3x3x5ft)

Since you are working with a fairly large room let's consider the SPL at a
10 meter (about 30 ft) listening distance.

Working with the WinSpeakerz SV-12 demo version I find the following:


No. Drivers     Total Input Power        SPL@1m       SPL@10m
===========     =================      ==========   ===========
     1                 1                87 dB SPL    67 dB SPL
     1               250               111           91
     4              1000               123          103
     8              2000               129          109
    16              4000               135          115


It looks like 4 to 8 SV-12's will deliver 100 to 110 dB SPL out in the room.

I'd put 4 SV-12's each in a sealed enclosure ranging from about 12 to 16
cubic feet.  These enclosures will provide bass coverage down to about 33 Hz
(- 3dB).  Allow about 1000 Watts per enclosure.  If you need more SPL
(loudness) add more enclosures.  At the indicated input powers the drivers
are within their excursion limits down to about 42 Hz.  Four of these
systems would provide excellent low bass coverage.

Note that these are relatively low efficiency systems but with awesome bass
extension to 33 Hz.   . . . and a -10 dB frequency of 18 Hz!   A more
traditional design might use pro sound drivers but would tend to tend to
have a significantly higher cutoff frequency and would probably require the
use of a vented enclosure to provide adequate bass extension.

Regards,

John


I previously posted:

>Working with the WinSpeakerz SV-12 demo version I find the following:
>No. Drivers     Total Input Power        SPL@1m       SPL@10m
>===========     =================      ==========   ===========
>     1                 1                87 dB SPL    67 dB SPL
>     1               250               111           91
>     4              1000               123          103
>     8              2000               129          109
>    16              4000               135          115

(thanks for cleaning up my chart Art)

Art asks several questions:
 
> Question #1:  That we're down a constant 20dB at 10m in every case,
> is this simply the inverse square law at work?

Yes, this is simply "spatial attenuation" or the inverse square law as it applies to point sources.  You lose 6 dB per doubling of distance or 20 dB for x10 as in this case.  This dB loss with distance (for a point source) is the same as for voltage.


> Question #2:  How much additional loss is there in the media air?  Is
> inverse square essentially good enough?

At these distances and frequencies we don't need to worry about atmospheric losses.


> Question #3: Didn't the bass-list just cover sealed rooms?  If the
> room were only 10x10x10m cube, what would be the SPL at 10m
> (at the far wall)?

This analysis is for half space and does not include the room.  In a room the SPL's would be somewhat higher as a result of reverberation, room modes, and cavity effect.  To accurately predict these effects would require a LOT more information and analysis.  The simple half space model gets us 90 % of the answer with only a small amount of work.  :-)


> Question #4:  If doubling power equates to 6dB as indicated above
> (going from 1 to 4 to 8 to 16 drivers), how do you get from 87dB to
> 111dB for the single driver?  (essentially 2 to eighth in power,
> 1 to 250)  (8 times 6dB would be 48dB more).  What am I missing?

We are increasing the input power from 1 Watt to 250 Watts.  First we will find the dB increase due to this increase in power, then we will add it to the 87 dB we start with.  We need to use the formula for dB as a function of a power ratio.

dB = Log (P2/P1)

dB Increase due to increased power = 10*Log(250/1) = 10*2.3979  = 23.979 ~= 24 dB

Add this to the starting dB SPL:   SPL = 87 + 24 = 111 dB SPL @ 250 W, 1m , 1 driver

I hope this is helpful.

Regards,

John

 
/////////////////////////////////////
John L. Murphy
Physicist/Audio Engineer
True Audio
https://www.trueaudio.com
Check out my new book "Introduction to Loudspeaker Design" at Amazon.com 


 

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