Ok, well seems to be some interest in SPL.
So lets get some technical fundamentals out of the way.
Hopefully wanting this thread to start at the fundamentals (waves, frequencies etc) and work its way into how it all relates to the car and into box design, port design, positioning etc. So try to keep it on topic and avoid the spam here please.

I personally have put very little thought into SPL, so should be a bit of a learning curve for myself as well. Feel free to question my logic at any time.

Anyway first things first. SPL (sound pressure level) competition aims to get the highest db measurement in a car.

This is measured in decibels (dB) which is a ratio of the sounds intensity or sound pressure.
The decibel is a dimensionless ratio (not a unit) between the quietest audible noise and the measured sound intensity. (1db is the smallest audible increase)

First things first, lets ignore subs and boxes...

SPL is measured with a microphone.

Anyone want to answer technically how a microphone does this? And how we can get the largest SPL reading from the microphone?
(using wave theory plz, at this stage we dont care about subs or amps or cars for that matter)

Peace

Tim

Well microphones work by converting pressure wave variations caused by the sound into electrical signals, where the intensity of the pressure variation (and hence 'loudness' of the sound) results in a more measurable electrical signal.

Presumably the conversion of that electrical signal into a dB value is some function of the sensitivity of the microphone (as a calibration factor) and the intensity of the electrical signal that resulted from measurement of the sound.

Sound ok?

Sounds good...

Now what are the key parts of a sound wave that will get us the highest measurement from this microphone?

I'd guess amplitude.. but that might depend on whether the microphone being used responds the same way to all frequencies. I don't know if microphone have a varying frequency response curve the same way speakers do.

But I suppose you want fact, not conjecture? :P lol

From what I read in a smaal article on this I think that the mike picks up all frequencies but when the SPL was stated the frequency was stated as well. But I'm not 100% sure of this.

I would have to say that it mesures the amplitude of a mechanical sinusoidal wave. The intensity of the wave is found by the amplitude squared and its units are in W / metre squared. It is found perpendicular to the wave velocity. Intensity decreases inversely with the square of the distance from the source. Glad i just had lunch. lol

Well there is always a specific freq that will attaint the max SPL for a particular car/equipment setup. Most guys will go along to someone who has a properly calibrated term pro and use a test CD with 1 Hz incriments to see what they can hit the highest number with. Its partially dependant on the best freq response your sub has, how efficient your amp may be, ports, boxes, positioning.....there is a heap of factors which once again will only be sorted out with good experimentation. You can put exactly the same system into another car and the result could be completely different. Some times you will see guys in SPL drags using 'sweeps' which is basically a sine wave that runs across a series of freq, other will only ever use one freq that they have tested as being loudest for their particular setup...

yes SPL meter will tell you both your PEAK db and the freq it was made at

thus to achieve hi SPL u need a termlab or some sort of SPL meter

cause u need to know what your car freq. peaks at also what you sub peaks at then u build you box to suit

Well there is always a specific freq that will attaint the max SPL for a particular car/equipment setup. Most guys will go along to someone who has a properly calibrated term pro and use a test CD with 1 Hz incriments to see what they can hit the highest number with. Its partially dependant on the best freq response your sub has, how efficient your amp may be, ports, boxes, positioning.....there is a heap of factors which once again will only be sorted out with good experimentation. You can put exactly the same system into another car and the result could be completely different. Some times you will see guys in SPL drags using 'sweeps' which is basically a sine wave that runs across a series of freq, other will only ever use one freq that they have tested as being loudest for their particular setup...

We are getting to that s_tim_ulate is trying to conduct a lesson on waves though first.

Resonance hmmm how to explain.. if a wave meets something that can oscillate back and forth as the wave passes, some of that waves energy will be transfered to the motion of the oscillator. When the waves Hz coincides with the oscillators natural Hz the waves energy will be transferred very effectivly to the oscillator. This energy transfer is called resonance. :D

the box is the main factor i dont know anyone that has NOT built more than 1 box of SPL drags

and its all trail and error

OK sorry guys i am jumping the gun

the wave is every important, this determines which way u fire the sub and the placement depending on hatch or sedan or van

OK well as far as waves go I think the most important thing to remember is that any wave that hits a solid surfaace will bounce back 180 degrees out of phase.....when this combines with a normal wave you can get phase cancellation......altho talking wave ets is usually much more important for sql etc (eg bouncing tweeters off windscreens etc), as opposed to SPL (sound pressure leve) which is pureply what sort of pressure differential you can create within the bounds of your cabin.....well thats my spin on it

On the right track Hunter, but this theory of cancellation is a major factor for SPL.
For anyone who hasnt studied physics.
Imagine two identical waves one out of phase and one in phase. (It is out of phase when the peak of one wave is at the trough of the other) When these waves are combined, they will cancel each other out to produce nothing.

Now if we have these two waves exactly in phase. What will happen?
Play with this...
http://www.measure.demon.co.uk/Acoustics_Software/anc.html

This will give you a good understanding of what we are trying to overcome.

I will get to where the second wave is coming from soon. (I'm pretty busy at work so anyone else feel free to explain that too Once we have established the basics.)

Superpositioning is when pulses or waves actually meet and there their amplitudes momentarily add together. If the same two waves have the same phase, i.e. there crests and troughs are prefectly aligned, the amplitude of the reuslting wave will be the sum of the amplitudes of component waves.

When to waves that are 180deg out of phase combine, the energy is actually transfered into heat. :D

Best experiment I have ever seen for this is to just get ur hifi system at home, wire one speaker pos 2 pos and neg to neg....and then wire the other on up in reverse.....if you position them correctly towards each other you will be able to turn them up and still hear nothing...its quite interesting, as when one cone moves out the other will move in, effectivly creating a signal 180 dgrees out of phase and causing phase cancellation

Basically all the means that, if 2 ways are travelling toward each other ( say your sub has fired one burp now the next burp is fired ) and they have the same or positive amplitude they will meet.. add together momentarily then continue on there merry way at there orginal value.

If one wave has positive amplitude and the other negative ( i.e. out of phase ) you will then need to subract the values for your result. Perfect 180deg out phase will equal 0.

Yes the amplitude of the resulting wave will increase. Two waves will never catch up with each other (from two burps) as the speed of them will remain constant. Instead the second wave comes from the port in our box.

Imagine a port in the front of the box, pointing the same side as the woofer itself.
Imagine a microphone 1 metre infront of the box. Now the waves from the sub are only travelling 1 metre to reach the microphone.

How do we ensure that the waves coming from the port itself are 'in phase' with those coming from the woofer at a certain frequency? Think about how far these waves travel. Calculations/diagrams anyone?

Peace

Tim

OK, what we hear is known as sound pressure, but you also have sound power, which is the fundamental descriptor of the soure, and sound intensity, which is the power spread over a particular area.

There are issues such as sound absorption and insulation (the amount of sound transmitted through and bounced back from an object respectively).

The basic way an SLM (sound level meter) measures noise depends on the frequency weightings which it is using at the time. Having it set to A-Weighting (what the human ear is modelled on) means that a lot of lower frequency noise is actually disregarded. If you have a look at a table of correction factors, you will see values of around -30 dB for octave centre frequenices of around 31.5Hz. However, a linear weighting treats all octave centre frequencies equally. This usually means that the overall level will be greater than when set to A-Weighting.

Also, statistical settings can be applied to the SLM depending on what needs to be measured in the first place, whether it be impulse noise, or something that takes a bit longer.

Would it have something to do with the sound having to reflect of the rear of the sub box to then exit the port? :D

Weighting originated from Fletcher-Munson in the 30's. It is trying to equalise to accomdate for the sensitivity to different frequencies of the average ear. Modern day weightings come from these curves. A,B and C wieghtings correspond to the 40, 70 and 100dB curve. http://www.webervst.com/fm.htm http://www.sfu.ca/sonic-studio/handbook/Equal_Loudness_Contours.html

Yay, uni is finally useful!

As these curves show and as we all know at low volumes we dont hear much bass i.e. loudness/bass boost buttons. C weighting is usually the best to calibrate to when using musical systems at decent levels.

this thread is making my brain all mushy, and its starting to hurt...... but interesting though, are we gonna discuss SQ?

SPL is a hard thing to explain and is a Very complicated thing...

S_tim_ulate, would it not be far easier to explain How to Achieve Maximum SPL???

SPL is a hard thing to explain and is a Very complicated thing...

S_tim_ulate, would it not be far easier to explain How to Achieve Maximum SPL???

hehe, i think so too. i don't care abou physics :P we all did it in high school, and sure as hell got sick of it at uni!! :rant:

but yeah, box designs, resonant frequencies, superposition, surface area, mechanical and electrical properties of sub, amp specs blah blah.

there's no real forumula for good spl, just lots of money time, and a lot of trial and error ;)

That's right Eek!!!

We alter the tuning of the port by changing its width and length!! :P

I'm happy to let the thread die... But I'll be damned if I change the topic halfway through. lol

Teach a man how to fish :)

Happy to do an SQ thread as well... I mean I dont know of any real SPL playas here anyway so was just doing this to spark some interest

Peace

Tim

That's right Eek!!!

We alter the tuning of the port by changing its width and length!! :P

I'm happy to let the thread die... But I'll be damned if I change the topic halfway through. lol

Teach a man how to fish :)

Happy to do an SQ thread as well... I mean I dont know of any real SPL playas here anyway so was just doing this to spark some interest

Peace

Tim

Yeah, no, u see, i thought it was cool to driver around with excessive amounts of bass and ratttles everywhere in my car... :doubt:

i say leave spl to the colt, mini and honda city drivers lol

too competitive

Well youd want a really efficient speaker with bloody big amp with a fs very close to the port frequency in an incredsibly solid car (concrete etc.) Well youd probably want to deaden the car first find the biggest standing wave/resonance tehn build the box around that with a choice of driver with the same fundamental resonance and then port it slightly lower with a big port(not plastic). Or use a fourth order bandpass box or a w horn.

Oh and really small cabin space :)

That model is great! Way better idea than ruining a real car!

Oh and really small cabin space :)

ahhaha, i spent like, 30 mins at uni reading thru that whole thread!! ITS INSANEE!!!

he used 10mm laminated glass or something for windows....its gonna be awesome once its finsihed!!

Yep... very cool

Remote controlled
1/3 rd scale Niki
1 DD 9510 dual 1 ohm coils
1 D3 @ 1/2 ohm = 3000rms
1 Odyssey 1500 battery

equals Goddamm! (http://www.caraudioaustralia.com/forums/showthread.php?t=52098&page=1&pp=20) I reckon 155's easy...

Peace

Tim

Yeah the only way I would enter is to see how loud my car is now, or put a DD1012 in a 6th order bandbass box in my dad mirage :bowrofl: Street A of course.