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explain octaves, how they effect sound


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wildcatjay 
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Posted: November 30, 2007 at 6:59 AM / IP Logged  
please explian how octaves will effect the sound of my subs, my amp has the option for 12-24 octaves    whats the difference?
OK So Im new at this work with me
haemphyst 
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Posted: November 30, 2007 at 8:09 AM / IP Logged  
The audio spectrum is effectively divided into 8 parts - thus, the "Octave". Any doubling (say... 100Hz) or halving (say... 25Hz) of frequency from a selected frequency (in this case, 50Hz) is a one octave step from the reference frequency.
It's probably "dB/octave". It describes how fast the crossover works when filtering the higher frequencies from the signal.
24dB/octave will filter twice as fast as 12dB/octave, meaning that if you choose 50Hz as your crossover frequency, at 100Hz, your output will be 12dB lower than the selected crossover frequency, or 24dB lower than the selected crossover frequency, depending on the switch location.
There is not necessarily any "better", so DON'T ask that question... (just thought I'd get that in there, before you actually DID ask it. I could hear it coming explain octaves, how they effect sound - Last Post -- posted image.) It's for flexibility, and for options for blending your subs with your front stage. You will have to decide which works best in your system, with your sub and mains. No one will be able to answer that question if you ask it.
It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."
wildcatjay 
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Location: Indiana, United States
Posted: November 30, 2007 at 12:34 PM / IP Logged  
im going to read- and re-read your response till i have a grasp on the idea, ty for the help..   I guess now its just trial and error for personal preference.
OK So Im new at this work with me
stevdart 
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Posted: November 30, 2007 at 1:27 PM / IP Logged  

I'll refer you to this parts express page which might give you a better idea of how the sound spectrum is divided into octaves.  Looking at the first graph you will see the frequencies shown along the bottom (X axis).  You notice that every doubling of frequency is charted at the same space interval;  for instance, the distance between 50 Hz and 100 Hz is exactly the same distance apart as 1000 to 2000 Hz.  From that you see that sound is not linear as far as frequencies go;  we hear sound in octaves.  Every graph of the sound spectrum that you see will look like this, with frequencies divided into evenly spaced octaves.

Along the left side of the graph are decibels (this is the Y axis).  A typical graph will represent sound level from extreme quiet to extreme loud, as represented in this one that goes from a low of 70 db to a high of 120 db.  The response on that graph representing "custom crossover" shows a reponse where great care was given in the selection of many components, not least of which includes the orders of crossover slopes.  Re your original question, 12 db / oct. and 24 db / oct. are crossover orders, the former being 2nd order and the latter 4th order. 

The second graph on that page shows how a 18 db / octave slope rolls off the frequency response of a tweeter.  This particular response shows a true acoustic 3rd order slope (look at db difference in the octave range of 4K to 2K). 

The last graph on that page shows a woofer crossed with a tweeter.  You see how the woofer sharply slopes off at the high end while the tweeter takes over.  The overall reponse shows a summation of the two drivers at the crossover frequency.  The graph shows a summation that is flat.  

Your subwoofer has to blend seamlessly with the response of your mids and you want the best musical impact at the critical crossover region. 

The flat summation of driver responses at the crossover point is what you will be listening for as you try the various crossover controls your amplifier allows.  Try the 24db/oct setting first, with low pass set at 80 Hz.  Have the front speakers likewise set at 80 Hz for their highpass frequency.  You can either use recordings or test tones to find what settings sound best.  Test tones representing several frequencies in the crossover region will allow you to more quickly and accurately arrive at the best settings, but taking your time over a few days with several recordings will net you the results you're looking for as well.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
the12volt 
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Posted: November 30, 2007 at 1:42 PM / IP Logged  

Just a little clarification...

haemphyst wrote:
The audio spectrum is effectively divided into 8 parts - thus, the "Octave". Any doubling (say... 100Hz) or halving (say... 25Hz) of frequency from a selected frequency (in this case, 50Hz) is a one octave step from the reference frequency.  
It's only the audio spectrum between two points (notes or frequencies) that is divided into eight parts where the lower point is half the higher point. More specifically the interval of eight diatonic degrees between two musical tones or frequencies where the lower frequency is half of the higher frequency (or the higher frequency is double the lower frequency). Example: 100 Hz is one octave lower than 200 Hz. 200 Hz is one octave higher than 100 Hz. Two octaves above 100 Hz is 400 Hz. Three octaves above 100 Hz is 800 Hz. Four octaves above 100 Hz is 1600 Hz, and so on.  ....Ten octaves above 20 Hz is 20,480 Hz  :)

haemphyst wrote:
It's probably "dB/octave". It describes how fast the crossover works when filtering the higher frequencies from the signal.  
I agree, it's dB per octave. Decibels per octave refers to the slope at which the frequencies above a low pass filter or the frequencies below a high pass filter are attenuated. 

I wouldn't describe it as how fast the crossover works. The crossover works at the same speed regardless of the slope or crossover point. What I think you meant to say was the rate of attenuation increases (or is faster) at a higher slope ;)

wildcatjay, have a peek at our passive crossover section. Even though you're using the active crossover in your amplifier, this may help you understand what crossovers do and how different slopes can effect the interaction of multiple speakers.

explain octaves, how they effect sound - Last Post -- posted image. the12volt • Support the12volt.com
wildcatjay 
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Member spacespace
Joined: October 01, 2007
Location: Indiana, United States
Posted: December 02, 2007 at 11:56 PM / IP Logged  
so all and al 12 v.s. 24 will not effect how "hard" the drivers perform. It will just help blend high, mid, and low drivers for for more even sound quality correct?
OK So Im new at this work with me
stevdart 
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Posted: December 03, 2007 at 7:26 PM / IP Logged  

Yes, in a way it does affect how the speakers will hit.  If there is a dip in the response because the falloff curves of the two drivers isn't blended just right, you will have a decrease in the "hit".  The crossover area between sub and bass (or midbass) is deemed as a critical area because of this reason, which directly affects sound quality as well.  I use drums as a musical guide in tweaking this crossover region.  You want to feel a hard hit while in the driver's seat, which means that the front soundstage has to be set up well and plenty of damping of the sheet metal panels.  The mids of the front components must be able to handle the 80 to 120 Hz range with authority, and the sub's location should not be realized while listening (you can't localize it).  Both drivers will be handling a range of freqs at the same time while the blending is taking place, not too little and not too much.

It's a tweaking and listening thing...you know what sounds best when you hear it.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.
jfinks 
Copper - Posts: 76
Copper spacespace
Joined: May 31, 2007
Location: United States
Posted: December 03, 2007 at 11:34 PM / IP Logged  
I would like to point out the cross over point is that point where the sound has been attinuated (sp) by three dB at the start of the slop. So following what has been said so far for a High pass filter set to 80 Hz. If you go to the next octive down or 40Hz there will be 12dB less signal than at 80 Hz. A 24 dB crossover at 80 Hz would have 24 dB less singnal at 40 Hz. This can make a huge difference in the high pass signal to a midrange speaker in how hard it may "Hit" or distort because of how much bass signal is making it to the midrange below the crossover point.
jfinks 
Copper - Posts: 76
Copper spacespace
Joined: May 31, 2007
Location: United States
Posted: December 03, 2007 at 11:53 PM / IP Logged  

<--- Can I be a "sport" instead? or how about noe-Byte? OoO please please please...


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