Front vs rear port?

[TLS Guy]

Cabinets will resonate sound, sure, but the question then becomes one of audibility. On a reasonably well built cabinet, it is simply not an issue. You don't have to go to great lengths to reduce these resonances into insignificance either. For example, B&W's matrix cabinets and Magico's cabinets are overkill and definitely not worth the weight penalty. If cabinet resonances are a serious problem they would show up on the frequency response graph and impedance response.

Shady, we are not talking panel resonance here. That can be suppressed with bracing like the matrix. We are talking about direct sound transmission without any movement of the cabinet walls.

If you enclose a speaker inside the best braced cabinet you would still hear it. That is why to isolate a recording studio, you need a room within a room.

We are talking direct molecular transmission of sound from air in the box, through the walls and then exciting the air again on the outside. There is much more of that than you think, and a considerable amount of reflected sound in the cabinet directly transmitted through the loudspeaker cone.

A cabinet does not have to resonate to transmit sound, not by a long shot.

That is the major reason d'etre of the "Get Rid of the Box" fraternity.

 

[KEW]

But sound through solids being easier than air? That's something new to me I'll have to go study up on a bit. I would not have guessed that.

I think you already "know" this, but the context is throwing you.

Easiest example I can think of is a tuning fork.
If I am three feet from you and strike a tuning fork you will hear it.
If I hold a board between the fork and your ears, it will be muted.
If I touch the fork to the board it will be amplified significantly (and this is often called a "sounding board").

So, on the one hand the board is an effective barrier and on the other it is an effective transmitter of sound.

The difference is the coupling/transmission of air molecules to wood (or whatever higher density material) molecules is not very efficient, while the coupling of the wood to air is more efficient.

The coupling of solid to solid is very efficient (tuning fork to board).

In the case of a speaker box, the driver is mechanically coupled to the box, so you have an efficient transmission of the sound vibrations into the box.

Another example is to consider a nerf ball (very low density ball). I can throw it at a wall and you would hear very little impact (if any). I can throw a baseball at that wall and it will be quite loud. That is loosely analogous to an air molecule vs a molecule of a solid material.

My analogies may not be perfectly accurate, but I think they are good enough for a practical understanding.

 

[Irvrobinson]

Shady, we are not talking panel resonance here. That can be suppressed with bracing like the matrix. We are talking about direct sound transmission without any movement of the cabinet walls.

If you enclose a speaker inside the best braced cabinet you would still hear it. That is why to isolate a recording studio, you need a room within a room.

We are talking direct molecular transmission of sound from air in the box, through the walls and then exciting the air again on the outside. There is much more of that than you think, and a considerable amount of reflected sound in the cabinet directly transmitted through the loudspeaker cone.

A cabinet does not have to resonate to transmit sound, not by a long shot.

That is the major reason d'etre of the "Get Rid of the Box" fraternity.

B&W published a paper on the subject I found some time back for a different thread:

https://www.comsol.com/paper/download/199487/cobianchi_paper.pdf

The radiation levels look so much lower than direct transmission + room reflections that it is difficult to believe it is significant with robust cabinets.

 

[Bucknekked]

I think you already "know" this, but the context is throwing you.

Easiest example I can think of is a tuning fork.
If I am three feet from you and strike a tuning fork you will hear it.
If I hold a board between the fork and your ears, it will be muted.
If I touch the fork to the board it will be amplified significantly (and this is often called a "sounding board").

So, on the one hand the board is an effective barrier and on the other it is an effective transmitter of sound.

The difference is the coupling/transmission of air molecules to wood (or whatever higher density material) molecules is not very efficient, while the coupling of the wood to air is more efficient.

The coupling of solid to solid is very efficient (tuning fork to board).

In the case of a speaker box, the driver is mechanically coupled to the box, so you have an efficient transmission of the sound vibrations into the box.

Another example is to consider a nerf ball (very low density ball). I can throw it at a wall and you would hear very little impact (if any). I can throw a baseball at that wall and it will be quite loud. That is loosely analogous to an air molecule vs a molecule of a solid material.

My analogies may not be perfectly accurate, but I think they are good enough for a practical understanding.

KEW
Good metaphors and examples illuminate rather than obscure. Your examples helped a great deal.
The tuning fork example illustrates the basic problem I was struggling with : the coupling issue.
Just when I think I'm getting a handle on some of the finer points of the hobby, along comes a new stream of information that causes me to reflect and reset a bit.

This topic was one I would never have given much thought, but I learned some good things today.

I suppose this explains why good speaker cabinets are heavy, or, at least very dense for their size.

Someone mentioned Magico's in a previous reply (and being overkill). I have a friend that purchased a new set of Magico's at the end of last year when he retired. The most distinctive thing about them is the body of the speaker is a single piece of extruded aluminum. I didn't connect the dots at the time he told me about them but that would have to be an attempt to eliminate cabinet leakage/noise/resonance. That's a pretty expensive solution given they are $30,000 a pair.

Keep up the informative posts. I learn something and it makes my hobby a little more enjoyable.

 

[Bucknekked]

B&W published a paper on the subject I found some time back for a different thread:

https://www.comsol.com/paper/download/199487/cobianchi_paper.pdf

The radiation levels look so much lower than direct transmission + room reflections that it is difficult to believe it is significant with robust cabinets.

Irv,
Ok, I'm not even going to pretend I understood the mathematics of what the folks who wrote that paper were doing. I agree with your conclusion: noise from those Diamonds was probably not audible no matter how you look at it. I saved the paper because someday, somebody is going to piss me off on this very subject and I will be able to shut them up by telling them I understand the answer and the answer is in this very simple PDF.

Thanks in advance. I feel stupid and wise at the same time when I read papers like this one.

 

[Irvrobinson]

Someone mentioned Magico's in a previous reply (and being overkill). I have a friend that purchased a new set of Magico's at the end of last year when he retired. The most distinctive thing about them is the body of the speaker is a single piece of extruded aluminum. I didn't connect the dots at the time he told me about them but that would have to be an attempt to eliminate cabinet leakage/noise/resonance. That's a pretty expensive solution given they are $30,000 a pair.

There is an entire class of speakers that use cone drivers in CNC-machined billet aluminum cabinets, including YG Acoustics, Krell, and Magico, to name the ones that come to mind. I've heard examples of all of these speakers in peoples' homes over the past several years, and they're all good to very good (and they should be for their very high prices), but I have to admit that none of them struck me as substantially better than the Salon2s I currently use. While I still covet electrostatics now and then, I never came out of one of these sessions and coveted one of the before-mentioned brands.

 

[shadyJ]

Shady, we are not talking panel resonance here. That can be suppressed with bracing like the matrix. We are talking about direct sound transmission without any movement of the cabinet walls.

If you enclose a speaker inside the best braced cabinet you would still hear it. That is why to isolate a recording studio, you need a room within a room.

We are talking direct molecular transmission of sound from air in the box, through the walls and then exciting the air again on the outside. There is much more of that than you think, and a considerable amount of reflected sound in the cabinet directly transmitted through the loudspeaker cone.

A cabinet does not have to resonate to transmit sound, not by a long shot.

That is the major reason d'etre of the "Get Rid of the Box" fraternity.

I understand what you are saying, wide-band transmission of sound, but I still have to question its audibility. That cabinet would have to be pretty noisy to exceed the thresholds of auditory masking.

 

[3db]

TLS, lots of good information as usual. You mentioned that sounds travels faster in denser materials but you forgot to mention that sound also attenuates much faster in denser material. Ports aside for the moment, how much sound would actually propagate thru the walls of a speaker and what would the relative amplitudes be between the driver and the cabinet walls?

 

[3db]

??

 

[TLS Guy]

TLS, lots of good information as usual. You mentioned that sounds travels faster in denser materials but you forgot to mention that sound also attenuates much faster in denser material. Ports aside for the moment, how much sound would actually propagate thru the walls of a speaker and what would the relative amplitudes be between the driver and the cabinet walls?

You are correct it does, especially in less dense fibrous materials. Metal speaker enclosures are actually a terrible idea. MDF is actually about the best material we have got. It is fortunate it is cheap!

 

[<eargiant]

Some people need to get out there more and do some actual listening. Buy (or borrow) speakers, take them home and play for a while.

I’ve never heard nary a whisper from the ports (or cabinets) on my B&W 805D2. Yet I’ve had several speakers that some here fawn over (due to their supposedly impeccable measurements) chuff like old men running up a hill carrying a sack of bricks!!!

 

[TLS Guy]

Some people need to get out there more and do some actual listening. Buy (or borrow) speakers, take them home and play for a while.

I’ve never heard nary a whisper from the ports (or cabinets) on my B&W 805D2. Yet I’ve had several speakers that some here fawn over (due to their supposedly impeccable measurements) chuff like old men running up a hill carrying a sack of bricks!!!

You are correct. You need many more measurements other than FR to evaluate a speaker. As I have said many times you could produce an impeccable FR have a totally useless speaker.

 

[psbfan9]

What are the pros and cons of either option?

I get that there is little or no difference if a subwoofer is down-firing, front firing, or side firing, but do the frequencies of the typical speaker port not matter that it is firing into the back wall?

I am planning to order a pair of Philharmonic BMR's and want to make sure I choose wisely!

Also, might as well throw in the option of plugging the port (a sub will likely be in play).

I want to take this a little further.

What about the shape of the port? Are there advantages to a slit port over a round port?
Most times whether it's a standard round or slit port it is on the bottom. Is there a disadvantage to placing the port on top?

 

[killdozzer]

I want to take this a little further.

What about the shape of the port? Are there advantages to a slit port over a round port?
Most times whether it's a standard round or slit port it is on the bottom. Is there a disadvantage to placing the port on top?

Great question. And another thing, the placement of the rear ported speakers is something that is taken as a sure disadvantage, so what about the speakers that have a "downfiring port" (on the bottom of the speaker, facing down in a tower type speaker, meaning it will hardly ever get far from the surface):

 

[KEW]

The round port is the most efficient port shape since you are trying to maximize laminar airflow for a given perimeter or area of hole.
The slot is not as efficient as a round hole, but is often a good option if you have more room for the hole in one direction than the other. Seldom will you see a slot on the bottom or back of a speaker because there is usually plenty of room for a standard round hole.
The disadvantage of placing a slot or port on top is usually the tweeter (or mid-range and tweeter in a 3-way) is located at the top and you would be much more likely to get port leakage of sounds from these higher frequency drivers!

Here is an example of a speaker with a slot at top and bottom, but the slots are as far as possible from the tweeter! You can see that using round holes on the front would force the cabinet to be taller than it otherwise needs to be.

 

[TLS Guy]

I want to take this a little further.

What about the shape of the port? Are there advantages to a slit port over a round port?
Most times whether it's a standard round or slit port it is on the bottom. Is there a disadvantage to placing the port on top?

Slit ports are usually tunneled slot vents with turns. This is done to get enough area to keep the vent air velocity below 20 meters/sec. I this case the port is too long for tube vent.

In terms of flared vents, this does reduce vent air velocity a little, but the port also has to be corrected for this and end up a little longer then the equivalent non flared port.

As stated before because the sound is omnidirectional at port frequencies it does not matter where the port exits.

 

[TLS Guy]

The round port is the most efficient port shape since you are trying to maximize laminar airflow for a given perimeter or area of hole.
The slot is not as efficient as a round hole, but is often a good option if you have more room for the hole in one direction than the other. Seldom will you see a slot on the bottom or back of a speaker because there is usually plenty of room for a standard round hole.
The disadvantage of placing a slot or port on top is usually the tweeter (or mid-range and tweeter in a 3-way) is located at the top and you would be much more likely to get port leakage of sounds from these higher frequency drivers!

Here is an example of a speaker with a slot at top and bottom, but the slots are as far as possible from the tweeter! You can see that using round holes on the front would force the cabinet to be taller than it otherwise needs to be.

The drivers are probably in their own cavities.

 

[TLS Guy]

Great question. And another thing, the placement of the rear ported speakers is something that is taken as a sure disadvantage, so what about the speakers that have a "downfiring port" (on the bottom of the speaker, facing down in a tower type speaker, meaning it will hardly ever get far from the surface):
View attachment 23043

I think that is a Polk LSiM707. The drivers, which incidentally seem based on the old KEF B139 design, are tuned differently. One driver looks to be a conventionally ported alignment, the other seems to be some type of coupled cavity tuning.

Polk love these complex tuning arrangements, which to my ears are not successful. The idea is to extend the frequency over which loading is assisting the drivers. I have never heard one of these improve, things, in fact to the contrary.

If you really want to do that, then a properly designed reverse tapered and damped TL is the solution. That does get you the desired result.

 

[Beave]

The round port is the most efficient port shape since you are trying to maximize laminar airflow for a given perimeter or area of hole.
The slot is not as efficient as a round hole, but is often a good option if you have more room for the hole in one direction than the other. Seldom will you see a slot on the bottom or back of a speaker because there is usually plenty of room for a standard round hole.
The disadvantage of placing a slot or port on top is usually the tweeter (or mid-range and tweeter in a 3-way) is located at the top and you would be much more likely to get port leakage of sounds from these higher frequency drivers!

Here is an example of a speaker with a slot at top and bottom, but the slots are as far as possible from the tweeter! You can see that using round holes on the front would force the cabinet to be taller than it otherwise needs to be.

Tweeters generally have little or no backwave that enters the cabinet because they have their own rear chamber. Midranges often have their own rear chamber or, as TLS points out, their own cavities within the cabinet.

 

[killdozzer]

Thank you TLS!

If you really want to do that, then a properly designed reverse tapered and damped TL is the solution. That does get you the desired result.

I've seen these in some Fostex speakers.