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Cabinet Design

6/21/1999 10:20 PM
Mike	Cabinet Design What tonal differences are there in open, closed and 3/4 back cabinets? Also how close to a power amps output can I choose a speaker rating? For example with a 50 watt power amp would a 50 watt speaker be sufficient?

6/29/1999 1:11 PM
GFR	When a speaker vibrates, the cone makes the air move in waves. That's how the sound is propagated. When the cone is moving forward, the air in front of it is pushed while the air behind it is being pushed. The result is that the air in front of the speaker is being compressed while the air behind the speaker is being expanded - in other words, the waves in front and behind the speaker are OUT OF PHASE. If two out of phase waves are added they cancel each other. In an open back cabinet this cancellation is not total because the wave from behind has to travel some distance (the cabinet baflle dimensions) before it gets "mixed" with the "front" wave, so you cancel just the BASS. So an open back cabinet has less bass than a closed cabinet. The 3/4 cabinet is a way to make the "back" wave travel a greater distance before mixing with the "front" wave without having to build a bigger box. It's ecquivalent to have a larger baffle. The cancellations occur at lower frequencies and you have more bass. In a closed cabinet you have even more bass because there's no cancellation. Also, since the air behind the speaker is enclosed inside the cabinet, it will have a different behave when compressed or expanded (in comparison with open air). If the cabinet is very big you won't notice any difference. As the internal volume of the cabinet gets smaller you will notice the resonant frequency of the speaker increase and also the "Q" of the resonant peak will increase. In other words, if two closed cabinets use the same speakers, the smaller will have more "boominess".

6/29/1999 1:11 PM

GFR

When a speaker vibrates, the cone makes the air move in waves. That's how the sound is propagated. When the cone is moving forward, the air in front of it is pushed while the air behind it is being pushed. The result is that the air in front of the speaker is being compressed while the air behind the speaker is being expanded - in other words, the waves in front and behind the speaker are OUT OF PHASE.

If two out of phase waves are added they cancel each other. In an open back cabinet this cancellation is not total because the wave from behind has to travel some distance (the cabinet baflle dimensions) before it gets "mixed" with the "front" wave, so you cancel just the BASS. So an open back cabinet has less bass than a closed cabinet.

The 3/4 cabinet is a way to make the "back" wave travel a greater distance before mixing with the "front" wave without having to build a bigger box. It's ecquivalent to have a larger baffle. The cancellations occur at lower frequencies and you have more bass.

In a closed cabinet you have even more bass because there's no cancellation. Also, since the air behind the speaker is enclosed inside the cabinet, it will have a different behave when compressed or expanded (in comparison with open air). If the cabinet is very big you won't notice any difference. As the internal volume of the cabinet gets smaller you will notice the resonant frequency of the speaker increase and also the "Q" of the resonant peak will increase. In other words, if two closed cabinets use the same speakers, the smaller will have more "boominess".

6/29/1999 3:21 PM
Gary C	Interesting thoughts regarding the size of the box and its effects. I know that Naylor used to brag about the size of their cabs (compact) and the resulting tone. Have you seen their cabs? I would like to build a 2x10 to match my 1x12 combo...and ideas regarding design of a 2x10?

6/29/1999 6:09 PM
John K.	Are there formulas for specific dimensions? Thanks for the explanation. I've been wondering about this very subject lately since I've decided to buy some new quality speakers. The question is, how many in a cabinet and what kind of cabinet? How can I find out how to design an open back cabinet that will give me the most bass response? Will not the depth of the cabinet also be a factor? I would also like to know if you, Doc or anyone else has any knowledge pertaining to a cabinet/speaker configuration that does the best job of projecting clearly in a live performance situation. One night I went to hear a very good jazz/rock fusion band. I sat center stage, but all the way in the back of a fairly large nightclub. Both guitarists played strats. Each used an amp head-couldn't see the type. One guitarist's cabinet was one 15" speaker. The other had a cabinet with 6 10" speakers. Both volumes were well matched. The overall volume mix of the band was good. The 15" speaker came through loud and clear, nice and articulate, no matter the fx used. The six 10" speaker cabinet sounded like mush. The guitarist was a fine musician. I have noticed this same thing a number of times with different bands. This raises a number of questions for me. Do multiple speakers lose articulation or the ability to project any kind of distance/fill a room clearly? Does this have anything to do with whether the cabinet is open backed or closed? Does this have more to do with the speaker itself (quality) rather than the number of speakers in a cabinet? Anyone know what's going on here?

6/29/1999 6:09 PM

John K.

Are there formulas for specific dimensions?

Thanks for the explanation. I've been wondering about this very subject lately since I've decided to buy some new quality speakers. The question is, how many in a cabinet and what kind of cabinet? How can I find out how to design an open back cabinet that will give me the most bass response? Will not the depth of the cabinet also be a factor?

I would also like to know if you, Doc or anyone else has any knowledge pertaining to a cabinet/speaker configuration that does the best job of projecting clearly in a live performance situation. One night I went to hear a very good jazz/rock fusion band. I sat center stage, but all the way in the back of a fairly large nightclub. Both guitarists played strats. Each used an amp head-couldn't see the type. One guitarist's cabinet was one 15" speaker. The other had a cabinet with 6 10" speakers. Both volumes were well matched. The overall volume mix of the band was good. The 15" speaker came through loud and clear, nice and articulate, no matter the fx used. The six 10" speaker cabinet sounded like mush. The guitarist was a fine musician. I have noticed this same thing a number of times with different bands. This raises a number of questions for me. Do multiple speakers lose articulation or the ability to project any kind of distance/fill a room clearly? Does this have anything to do with whether the cabinet is open backed or closed? Does this have more to do with the speaker itself (quality) rather than the number of speakers in a cabinet? Anyone know what's going on here?

6/29/1999 6:37 PM
GFR	A deeper open box will have more bass because it's like the baffle was bigger. I think open back cabinets will be more directional than closed back ones (because of the cancellations). Multi speaker cabinets will have less articulation because the damping factor is lessened. Closed back cabinets may have less articulation because of the boominess (resonant peak). Multi speaker cabinets can either be more or less directional, depending on how they're designed. This is very complex.

6/29/1999 6:37 PM

GFR

A deeper open box will have more bass because it's like the baffle was bigger.

I think open back cabinets will be more directional than closed back ones (because of the cancellations).

Multi speaker cabinets will have less articulation because the damping factor is lessened. Closed back cabinets may have less articulation because of the boominess (resonant peak).

Multi speaker cabinets can either be more or less directional, depending on how they're designed. This is very complex.

6/29/1999 6:48 PM
John K.	Very helpful. It sounds like...... Open backed cabinets are easier to design? Is there any rule of thumb when it comes to the size and shape of the opening in the back?

6/30/1999 12:13 PM
GFR	Let's imagine a box like this (viewed form the side): <----D-------> ______________ \| ^ \| \| \| H/2 / \| / \| / speaker v \| \| \| -------------- Where D is the depth of the box and H the height. You can consider like the waves from the back of the speaker have to travel H/2+D to leave the box and the D+H/2 to reach where the waves from the front of the speaker originates. This gives a total distance of H+2D. If after this distance the waves are out of phase, cancellation occurs. This means that this distance equals half the wavelength of the wave. The frequency equals the speed of sound divided by the wavelength. Say you have a box 60cm high by 20cm deep. This gives H+2D=1m. Wavelength=2m and F=340/2=170Hz. As a rule of thumb, this box will have a 6dB/octave bass rolloff (like in a simple RC filter) with this frequency as the corner freq. Note that the amount of bass you can get by increasing the box is limited by the resonant freq of the speaker - the speaker itself will have a 12dB/octave rolloff below this frequency. If the box is very big and the rolloff of the speaker is what's limiting the freq response you've got what's called a "infinite baffle" cabinet.

6/30/1999 12:13 PM

GFR

Let's imagine a box like this (viewed form the side):

 

 
          <----D------->
 

 
          ______________          
 

 
                        |           ^
 

 
                        |           | 
 

 
                        |          H/2 
 

 
                       /            |
 

 
                      /             |
 

 
                     / speaker      v
 

 
                     
 

 
                      
 

 
                       
 

 
                        |
 

 
                        |
 

 
                        |
 

 
          --------------

Where D is the depth of the box and H the height. You can consider like the waves from the back of the speaker have to travel H/2+D to leave the box and the D+H/2 to reach where the waves from the front of the speaker originates. This gives a total distance of H+2D. If after this distance the waves are out of phase, cancellation occurs. This means that this distance equals half the wavelength of the wave. The frequency equals the speed of sound divided by the wavelength.

Say you have a box 60cm high by 20cm deep. This gives H+2D=1m. Wavelength=2m and F=340/2=170Hz.

As a rule of thumb, this box will have a 6dB/octave bass rolloff (like in a simple RC filter) with this frequency as the corner freq.

Note that the amount of bass you can get by increasing the box is limited by the resonant freq of the speaker - the speaker itself will have a 12dB/octave rolloff below this frequency. If the box is very big and the rolloff of the speaker is what's limiting the freq response you've got what's called a "infinite baffle" cabinet.

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