Speaker ports question

[steve4232]

Hi, can anyone please tell me why I should want to block the FRONT speaker ports to "tune" bass. My existing MA speakers are very close to a rear wall as I have not got the luxury of having speakers a foot or more out into the room (my living room just would not work or look right with them that far out). I understand the idea of blocking ports with foam bungs can help to attenuate bass boom where close proximity to walls is concerned. However, what purpose does blocking the front ports achieve? Surely it is the rear ports that might cause / suffer from over-load given that the wall can reflect sound back into the speaker? Or have I understood the purpose incorrectly? I can't see why I should want to block the front ports which fire into an open unrestricted space. Can anyone please explain the theory and best practice? Thanks!

 

[spiny norman]

In a sealed box speaker, the entire purpose of the box is to form an ‘infinite baffle’, stopping any of the output from the rear of the drive unit reaching the listener, as this can detract from the desired output from the front of the cone. Before box speakers became popular, this was achieved by mounting drive units in as large a baffle as possible: effective, but not entirely practical.

Bass ports work by allowing some of that rear, out of phase, output to escape from the enclosure, but in an controlled manner that allows it to reinforce, not reduce, the effect of the main forward output of the driver.

The other effect of a port is reduce the back-pressure on the driver, thus allowing it more excursion: in a sealed box, the fact that the air has nowhere to go, and thus has to expand and contract as the driver within that space moves, acts as a spring or damper on the driver unit, impeding its movement and thus limiting, its output.

A port reduces this back-pressure and damping, and the design of the port (length, diameter, profile, surface finish) can be adjusted to control the effect it has. Blocking the port will of course remove its influence, which can be useful if the bass is overbearing or too loose, but you don’t have to go the whole way and block it completely.

Different densities of foam plugs will have differing effects, as will (for example) inserting a foam ring in the port to reduce its diameter and add some resistance to air flow. One old idea used to be to insert a bundle of drinking straws into a port to slow the air flow by creating lots of small diameters rather than one big one.

All of this is something with which you can experiment: what works in one room might well create too little bass in another.

 

[steve4232]

Many thanks for that informative and detailed response. Does this mean that it shouldn't affect the sound which particular ports are filled with bungs? I was thinking about wall surface relections back to the rear ports but this might not be the case based on what you've said here? MA supply foam bungs that snugly fill each port. I like the straw idea but I'm not sure about the aesthetics of it!

 

[davedotco]

Spiney is quite correct.

In essence, blocking the bass port will increase the loading on the bass driver and reduce the overall bass output. If the speaker is a bit boomy, this may help.

A couple of more general thoughts.

Unless the speaker is going to be placed tight to a wall, (a few inches) it makes no difference whether the port is on the front or the back, in respect of the bass response, anyway.

All speakers placed close to a room boundary will suffer/enjoy a boosted bass response. The reason is simple enough, at bass frequencies the ouput of the speaker is more or less omnidirectional, at mid and high frequencies it is far more directional.

A solid rear wall will reflect some of the bass that has been radiated to the rear back to the listening position, thus adding to the bass radiated directly forwards. If the speaker has a good balance in this position then removing the wall will stop the reflection and only the forward radiated bass will be heard at the listening position. If the speaker was balanced before, it will sound bass light, and of course vice versa.

If you speaker booms close to a rear wall, then it was probably balanced to sound best in open space. Using the bungs may help but there are downsides to this, the real solution is a speaker whose balance is more suited to near wall positioning.

There is another issue with front mounted ports, that of noise. In some speakers the sound waves from the rear of the speaker cone may 'bounce' off the rear of the enclosure, escape through the post and be audible, frankly this is poor design and pretty rare.. At very high level the air moving through the port may be sufficient to cause a 'chuffing' noise, this will only occur at very high volumes and not normally an issue in home systems.

 

[spiny norman]

Many thanks for that informative and detailed response. Does this mean that it shouldn't affect the sound which particular ports are filled with bungs? I was thinking about wall surface relections back to the rear ports but this might not be the case based on what you've said here? MA supply foam bungs that snugly fill each port. I like the straw idea but I'm not sure about the aesthetics of it!

As m'learned friend has said, it's a matter of the effect of the port on the driver, and the effect of boundaries (such as a rear wall or even worse a corner) on both the speaker as a whole and the output from, say, a rear port.

Thus on speakers using both front and rear ports, bunging one or the other may have a very different effect, depending on the positioning of the speaker.

Agreed on the aesthetics of the straws, though I did once see one very neat idea used on a speaker with a large front port, where the owner had put over the front end of the bundle a piece of speaker grille cloth, held in place with a rubber band around the straws inside the port.

You can achieve similar effects to the bundle of straws by experimenting with a foam of a more open or closed cell structure, or a foam ring. Reading some of the reviews, it seems this is what B&W is doing with its latest 600 series speakers; they have a two-part foam bung comprising a ring with a plug made to fit inside it, so you can vary the effect by using the whole thing or just the ring.

Anyway, hope all that rambling helps: just my 2p's worth before the massed hordes of the WHSV review team no doubt leap in with more considered and detailed advice! :rofl:

 

[steve4232]

All very interesting stuff so thanks to both of you who have responded thus far.

The MA's include an instruction booklet that states that they are "at their best when positioned approximately 6" (15cm) from a rear wall". To my mind, and allowing for what has already been said here, this suggests that they might already be "balanced" when fairly close to a wall. As I have my 8's toed in slightly towards the listener sitting squarely (but comfortably with a glass of whatever tickles the fancy) in the centre of a three-seater settee, about 6/7 feet away, the distance from rear wall to rear of speaker cabinet obviously varies as it tapers off. It's probably between 3" at the closest corner and 6" at the opposite rear corner.

I will plug the rear ports and see what happens! I recall plugging the front ports when I first bought them and thinking that the sound appeared slightly "clipped" and less dynamic. I'll experiment and see.

 

[steve4232]

I have to say that bass isn't an issue for me really. They have oodles of bass but not to overwhelm or kill the music either. I like the sound and I'm not asking to solving a problem, more curious about the design and port issues / purpose. Some of you dedicated audiophiles might not like the sound though!

On the subject, is it normal to hear MORE bass, that might if anything be added to by room "boom", when you are standing to the sides of your setup? I notice how heavy the bass is more so when I am not sitting down and listening properly. When you move into the triangle formed on plan by the toed-in speakers and the listener, the bass appears to drop off slightly and when sat down in the centre between the speakers, the midrange and treble pull into focus and the sound "re-balances" totally. I'm almost certain the bass that arrives at the listener's ears is quite a bit less than that heard when walking to the sides of the setup / triangle. Is this a "typical" scenario?

I wonder if some element of low frequency is being cancelled out somewhere? I've heard that it can happen if speakers are toed-in but I have some doubt about this.

 

[tomlinscote]

With regard to finding (hearing) less bass at the seating position compared to other places in the room, I have this too. In fact in every house I gave owned/used and installed HiFi in, I have experienced the same problem. Having the speakers in phase and sitting an equal distance away from each should ensure the sound reaches me in phase too, but like you the bass response is diminished at the point of the triangle. Maybe room reflections are too blame, causing phase differences??

Over to the panel.........

 

[steve4232]

"Tomlinscote", when I say I hear more bass at the side of the listening setup, I really do mean a LOT more bass. When you sit down in the listening position it sounds like a completely different set of speakers: the bass "boom" reduces and the mid-range and high frequencies really come to the fore giving a much clearer and more focused sound. If the sound I hear at the sides of the setup was the only sound I could get from my speakers I'd have to sell them today!

BTW are you an ex-Tomlinscote pupil? Thanks for your observations too. I'm glad someone else experiences this.

 

[Vladimir]

I presume you have the MA RX8 on spikes. Screw in the back pair of spikes shorter than the front pair, thus making the speaker tilt back by a small angle. The more you tilt it, less bass boom will come from the floor, where your actual problem is, not the back walls. Too much tilt and your speakers may become unstable or just have the tweeter axis too high and speaker will feel as lacking HF.

LF is spherical and HF is directional.

 

[davedotco]

"Tomlinscote", when I say I hear more bass at the side of the listening setup, I really do mean a LOT more bass. When you sit down in the listening position it sounds like a completely different set of speakers: the bass "boom" reduces and the mid-range and high frequencies really come to the fore giving a much clearer and more focused sound. If the sound I hear at the sides of the setup was the only sound I could get from my speakers I'd have to sell them today!

BTW are you an ex-Tomlinscote pupil? Thanks for your observations too. I'm glad someone else experiences this.

Two things.

As I explained earlier bass is, more or less omnidirectional, mid and high frequencies are 'beamed' straight ahead. If you move 'off axis' the mid and high frequencies reduce, the bass does not. The result is a balance that has less mid and highs, or if you prefer, more bass.

The other issue is standing waves at the rooms major dimensions. As you move around the room you will get nore or less bass, depending on whether you are in a peak or a trough of the standing wave. A peak will give more bass, a trough very little. Hence the encouragement to experiment with speaker positioning, it can make a big difference.

Generally, speakers like the big MAs need plenty of space, the fact that you have acceptable bass that close to the wall is testimony to the grip and control of your amplifier, and maybe a bit of luck.

 

[davedotco]

I presume you have the MA RX8 on spikes. Screw in the back pair of spikes shorter than the front pair, thus making the speaker tilt back by a small angle. The more you tilt it, less bass boom will come from the floor, where your actual problem is, not the back walls. Too much tilt and your speakers may become unstable or just have the tweeter axis too high and speaker will feel as lacking HF.

LF is spherical and HF is directional.

There is some truth in this but floor standing speakers sit, well, on the floor. This should be taken into account by the design, floors are obligatory, back walls are not.

 

[steve4232]

"I presume you have the MA RX8 on spikes. Screw in the back pair of spikes shorter than the front pair, thus making the speaker tilt back by a small angle. The more you tilt it, less bass boom will come from the floor, where your actual problem is, not the back walls. Too much tilt and your speakers may become unstable or just have the tweeter axis too high and speaker will feel as lacking HF.

LF is spherical and HF is directional"

No, they're not on spikes as I have a wooden suspended floor (not solid concrete). However, they do also come with small round srew-in padded discs which MA recommend instead of spikes for wooden floors. These can also be adjusted in exactly the same way as you have suggested for the tilting effect. I will try it and see what difference it makes.

I have a thick piled rug in front of them and of course quite a lot of soft and other hard furnishings in the room. The one thing that I have noted is the complete lack of vibration within / across the floor itself. I'm interested to learn that most of the surface reflections are from the floor and not the walls.

 

[Vladimir]

Even the thickest wool rugs help very little. It is questionable if they shave of even 1dB. Those MA's have 2 bass drivers, one mid/bass driver and the port is practically on the floor. Worth a try with tilting as a more effective technique IME. Just lifting them up from the floor will slice of an audible chunk of boom, and some more with tilting.

In your case it's free and reversable modification so its worth a try.

 

[tomlinscote]

Ex staff Steve, ex staff!!!