Calculating SPL

[Ben Lawrance]

Hi All,

 

I have a project that I am working on at the moment, which is nearly complete, but I just need a bit of help.

 

I have just finished building some cabinets, and I know what drivers I am going to put in them, but I don't know how to calculate the resultant SPL.

 

The cabinets are (hopefully) in the picture below. 2x Subs with 2x 15", and 2x Mid/tops with 2x 12" and a conical horn.

 

http://img320.imageshack.us/img320/6955/emptycabsuq6.jpg

 

I intend to load the subs with 2x Beyma 15LX60 per cab, and load the mid/tops with 2x Beyma 12LX60 and 1x Beyma CP755ti Comp per cab.

 

 

Can somebody show me the maths I need to use to work out what SPL I will get when the are all loaded up.

 

Kind Regards

Ben

[Mr.Si]

I don't know how to calculate what you'd have, probably because I'm not thinking at the moment and partly because I don't build speakers.

 

But how about once having loaded them, why don't you hire/borrow/steal/buy a decent sound level meter and measure it in different settings?

Lazy maybe, but that's what alot of people would do, I'm sure...

 

When I've had a think, I might post back. Meanwhile, I am wondering if Simon Lewis with his acoustic knowledge hat on will post a more useful message.

[Simon Lewis]

Ben,

 

In theory, the SPL from one driver will be:

 

 

SPL = reference sensitivity (@1W @1m) + 10log applied power.

 

Calculating the total SPL is harder. a) the devices so not all operate across the audio spectrum b) you do not state if these cabs are run active or passive (therefore we do not know if there is a crossover insertion loss) c) we do not know the power compression and d) it is very likely that complex cancellation and addition will occur, making the real SPL at a given point different from a predicted one.

 

For a rough guess, calculate the theoretical SPL for the bins, the mids and the tops, convert this figure back to individual acoustic pressures, sum them, then calculate the resultant SPL.

 

Simon

[The Magus]

I'm intrigued by your design: I hope you will let us know how the speakers perform once you have completed them. What made you decide to use an axisymmetric horn? Have you read this paper by Keith Holland from the AES archive? Are you planning to use an 'off the peg' horn, or are you creating your own?

 

I hope you're pleased with the finished boxes.

[Chris Hinds]

As Simon Lewis says, you can theoretically calculate the SPL from one driver. You should get +6dB over that from the sub for having two drivers within 1/2 a wavelength of each other in the same box. Depending on your intended crossover frequency to the compression driver in the Mid/Hi this should also apply to the 12" drivers. However, there will be a resonant frequency of the cabinets due to their volume and porting. If that falls in a range where it is going to provide massive boosts or massive dips then no matter how much driver you throw your SPL readings will be way out from what you expect.

 

How are you planning to process/amplify the system?

 

Regards

 

Chris

[Ben Lawrance]

Hi All,

 

I did not design the boxes. I bought them off a company who designed them and didn't get round to making them.

 

The Processing section will be handled by a Berry DCX (for the time being anyway - let's not start that debate again)

 

The amps will be QSC RMX series amps. RMX850 on the Comps, RMX4050HD on the 12's, and a QSC5050 on the 15's

 

Active system run 3 way. Crossed at around 130Hz & 1.2KHz. These are not final yet as I haven't got the drivers in the boxes, to be able to play around with it.

 

 

Regards

Ben

[Bobbsy]

I'm not even going to try to answer this...I'm okay with SPL maths for USING speakers, but I've never designed a cabinet in my life!

 

However, Simon Lewis' post now has me curious. Is it possible to do a calculation based just on the specs for the drivers, or do you need to also take into account the cabinet design. Instinct (always dangerous!) tells me that the cabinets could have quite a large affect on how efficiently the output of the drivers translates to the full system output. Things like ported/unported, bass relex units, cabinet volume and shape, and so on ought to effect the SPL I'd think...or am I completely off base here?

 

Cheers,

 

Bob

[Chris Hinds]

However, Simon Lewis' post now has me curious. Is it possible to do a calculation based just on the specs for the drivers, or do you need to also take into account the cabinet design. Instinct (always dangerous!) tells me that the cabinets could have quite a large affect on how efficiently the output of the drivers translates to the full system output. Things like ported/unported, bass relex units, cabinet volume and shape, and so on ought to effect the SPL I'd think...or am I completely off base here?

Note: This is about as much acoustics as I know, but yes you do have to take the box into consideration. Porting affects where the speaker will have a resonant frequency and where maximum output will be (where the ported output and the main output combine constructively). Volume and shape will also affect tuning and frewuency response. It's all a huge jumble together which gets even worse once you throw horn loading into the equation for good measure.

 

Regards

 

Chris

[Chris Beesley]

Im not a wizard on box calculations - would it be easier to get hold of one of the speaker design CD's and enter the thiele small parameters etc?

[Simon Lewis]

or am I completely off base here?

 

No, you're quite right... trying to predict the total SPL is something of a difficult task, for all of the reasons stated above (and a few more that I won't go into).

 

The calculation I gave is OK for getting a good estimate.

 

Simon