Internal bracing vs SPL

[lndm]

You mentioned the word travel. This is an interesting term that can be played with.

Conditions aside (blah blah), sound travels at a certain set velocity regardless of frequency. If the frequency is high enough that a wave cycles from compression to rarefaction before the compression travels across the inside of the enclosure, it could be said to travel as it appears to do so.

On the other hand if the sound cycles slowly, the entire box fills with the compression and the rarefaction independently and in turn. Thus it is like blowing up and releasing a balloon, and I suspect this is the distinction your friend is making when he refers to low frequencies.

Anyhow, dealing specifically with this low frequency effect is just as BassAwdyO says where you tie the centres of the panel together to stop them 'breathing'.

Some thoughts. So perhaps since the low frequency effects are all encompassing within the cabinet, I feel it's hard to say that the waves will meet the braces like obstacles as pressure equalises on all sides of the braces. OTOH, I do feel that higher frequencies may dislike the braces more (consider diffraction and reflection).

In practice, braces can be made non-intrusive by rounding, keeping a low profile, and adding damping material. Still though, I use external braces in the locations where I can get away with it.

In closing I'd like to mention a point that Rademakers made. Bracing can be more effective than thicker panels. Though this has a practical limit (don't use cardboard ), a heavy panel can store more energy than a light one. Lack of mass is good, and stiffness is good, or more to the point, it's the ratio of mass to stiffness. This is largely why bracing is good.

BTW, dense hardwood makes the best braces in my opinion.

[paulspencer]

Bracing isn't likely to have an impact on SPL, except in the most extreme situations. It's purpose is to make the box more solid and reduce the requirement of thick walls. It will only impact SPL to a significant amount if the walls are FAR too thin.

More the frequency is low, more it is hard for the waves to travel in the enclosure and especialy by the ports. So if you put internal braces, you can restrict more the air flow created by the woofer and the port (or making waves a harder time to travel in the enclosure) in the enclosure at lower freqencies, thus, reducing SPL. This is why this is better to use external braces in order to avoid this problem.

Sounds like nonsense to me.

Air flow does not occur inside a subwoofer box. Sound waves are a pressure disturbance where air particles jostle each other and pass on vibration like a chinese whisper. Picture a cork floating on water when a wave comes along. It moves in a circular motion but does not flow with the wave. The wave moves along, that is the disturbance. But there is no flow.

Bass won't travel as a wave inside a subwoofer box. This occurs in the midrange, but in a subwoofer box, the wavelength is so much bigger than the box that it is not possible for reflections to occur. What happens instead is that the box is pressurised. A 40 Hz sound wave is 8.6m long, much bigger than a subwoofer box. An 80 Hz tone is 4.3m long, again much bigger.

If your friend is talking about a subwoofer box that isn't insanely huge, then the problem is imaginary.

Using external braces makes the enclosure stiffer, and you dont have anymore the problems of possible air restriction in the enclosure, more especially with lower frequencies.

Actually I think you will find the opposite is true. Internal bracing is far more effective in making a box rigid. The ideal bracing system is matrix bracing where braces in three planes each touch four walls of the box. As a result, all the walls are tied together and form a solid structure. This matrix system makes a lot more sense than external braces, which would need to be far bigger to achieve the same stiffness. If you want to look into it further, have a look at the B&W website. It shows some research in this area.

For a box where midrange is present, the sonic impact of the bracing becomes an issue. As I recall, B&W use foam in the holes in the bracing to mitigate this, but this is irrelevant to subwoofers.

You should avoid holes in the bracing that are too small as you don't want to add resistance to the pressure moving through the box.

If the box is vented, then aerodynamic factors come into play, especailly around the ends of the vent, both internal and external. You need internal clearance around the vent, otherwise the tuning may be affected in ways you can't predict resulting in a less than optimal response, and you may get some turbulence reaching audible levels. It gets more difficult to design the bracing where you have to accomodate a vent. If you have a small box this gets more difficult, although the requirement for bracing then is less and you can focus on thicker walls more. But in this case, with a high excursion driver it will be very difficult to fit in a vent that won't chuff like mad.

[BassAwdyO]

Paul and Indm hit the nail on the head...

[firepinch]

WOW! Thanx a lot for these clarifications! Im gonna see what he will have to say against those answers! Maybe he will acknowledge that he is wrong

[firepinch]

Oh! Im coming with other points. Im gonna talk at this place with 3 points:

First point (retake again what has been talked before): In a subwoofer, there is airflow. In other words, when the woofer moves, there is airflow in the box. I am conscious that bass waves are very long waves, therefore, reflection and refraction doesnt occur, but only pression. However, when we talk about pression variation, we talk about air movement in the box. So braces can restrict air movement in the box, thus, reducing somewhat the maximum SPL.

Second point: Basically, with a port, there are shock waves which travel in the box and more the ports are able to sense them, more they are effective.

Third point: If the port is not placed at the opposite side of the woofer (for example, woofer in the front and the port in the back), the port becomes more leaky and the ported box doesnt have as much SPL.

It seems to me that the third point doesnt make any sense, I have never seen this kind of affirmation before! What do you think about all of this?

[lndm]

Originally posted by firepinch
In a subwoofer, there is airflow.

As paulspencer said.

Air flow does not occur inside a subwoofer box. Sound waves are a pressure disturbance where air particles jostle each other and pass on vibration like a chinese whisper. Picture a cork floating on water when a wave comes along. It moves in a circular motion but does not flow with the wave. The wave moves along, that is the disturbance. But there is no flow.

The word flow suggests that air moves along. However, in a speaker cabinet any air that moves, moves back (in theory, and largely so in practice), just like the water only moves up and down. Similarly, the cone moves back and forth by the same amount, proven by the fact that it stays centred rather than making its way slowly across the room (hoping this is helpful )

Originally posted by firepinch
Second point: Basically, with a port, there are shock waves which travel in the box and more the ports are able to sense them, more they are effective.

Considering low frequencies with properties as discussed here, I might ask you: If you connect a garden hose to a tap, and seal the other end, then turn on the tap so the hose becomes hard with pressure, would more water escape if you pierced the hose near the tap or near the closed end?

[paulspencer]

In a subwoofer, there is airflow.

This might sound like splitting hairs but there is no airflow. If you think in terms of airflow you will come up with ideas that are incorrect. Sound waves are a pressure disturbance as previously discussed.

So braces can restrict air movement in the box, thus, reducing somewhat the maximum SPL.

Bracing does not affect SPL. In a sealed box the function of the box is to retain sound radiated from the rear of cone - in this case the sound involved does not make a significant contribution to SPL. In a vented box the issue regarding SPL is the vent, which should have sufficient clearance around the inside opening. If there is not enough clearance, the first thing that happens is that the walls that are too close become an extension of the vent, which lowers tuning, and reduces the efficiency of the vent - this will affect SPL. If the lack of clearance is extreme, then there will be extra port compression and SPL will suffer further. But here the issue is correct design, not bracing.

In both sealed and vented boxes, bracing should be designed so that all sheets allow the sound waves to pass through. As a result, the bass will uniformly pressurise the entire box. The fact that sound waves have to pass through openings in the bracing does not mean SPL will be reduced.

Try this. Turn on some music with bass. Go outside the room with the door 2" open. Now open the door half way open. Does the SPL for the bass change at all? Of course not! It's the midrange and above that changes. Bass will pass through small openings easily. It is the same inside a box with bracing.

Second point: Basically, with a port, there are shock waves which travel in the box and more the ports are able to sense them, more they are effective.

"The more the ports are able to sense them?"

I have no idea what you mean by that!

This is what you need with a port. First you must have the correct box volume, as this will determine the efficiency of the port. If the box is too small, you won't get enough output from the vent. This also relates to tuning. The lower you tune, the bigger the box needs to be in order to get enough output from the vent. You can see this going on in WinISD or any simulation program.

Secondly, aerodynamics are important. The port must have enough cross sectional area that the air velocity is below 34 m/s which is the absolute maximum. The velocity that is ok at the inside and outside ends of the port termination is dependent upon aerodynamics, and must generally be a lot less than 34 m/s to avoid turbulent noise. Large flares help a lot, but even a large flare will typically need air velocity to be below 17 m/s.

If the air is forced to move at high velocity, the resistance will lead to compression, which means the SPL will be reduced slightly.

If the bracing in the box had very small holes to let the waves through, you might have problems. This would add resistance and could cause the box to act as if it were a different volume of air. The result would probably be loss of SPL, although not siginificant. If the holes are at least 3" I see no problems here.

Third point: If the port is not placed at the opposite side of the woofer (for example, woofer in the front and the port in the back), the port becomes more leaky and the ported box doesnt have as much SPL.

This is not true. It does not matter which side of the box the port is placed. It makes no difference. If the port is at the back and there is any audible turbulence, it may be less noticeable. The only SPL issue is that the vent should have sufficient clearance to any surfaces. In a very large box if the vent is a large distance from the driver, then effectively what you have is a sealed subwoofer with a bandpass subwoofer placed where the vent is located.

[Volenti]

Of course the air is moving in a sealed enclosure, granted it's not moving very far but it is moving, if it didn't move then adding fill (resistive damping) would have no effect.

If you add fill to a sealed enclosure you can lose up to 3db off the resonance peak. This is direct evidence of "stuff in box reducing spl" (though mainly at resonance)

You go silly with bracing in a sealed enclosure and you will add a very small amount of resistive damping into the enclosure that wasn't there before. Sure the difference it makes is unlikley to be audible (and it will be swamped by the positive aspects of the bracing) but that doesn't stop it from happening.

More evidence of this somes from the guys competing in car audio SPL comps, where they use very high Q enclosures to extract every last Db possible. Now they may be crazy but they aren't stupid, the top guys do not use extensive shelf style bracing in their enclosures, even though that is a very effective way of making an enclosure strong and stiff (which is what you would think would be best to achieve a high Q)

Instead they use multiple layers of wood and/or concrete with very sparing use of steel tube/threaded rod bracing where strength can't be achieved any other way. They do everything humanly possible to avoid putting anything in the enclosure. That's a lot of trouble to go to for something that supposedly doesn't effect SPL.

I'm not advocating that the resistive effects of bracing makes a noticable difference in most "normal" applications, it's just bad science to state that something has has no effect when it actually does, no matter how insignifigant, especially when in extreme suitations it does have a substancial effect.