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Why do Front Ported Cabs sound better?


acidbass
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What's the logic behind this?

 

I have always preferred the sound of front ported cabs - much more trouser flapping bottom end spilling out the front of it and no issues with boominess when placed beside a wall etc.

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Port placement doesn't matter, as the radiation pattern from a port is 360 degrees. What does matter is the size of the enclosure. Larger enclosures go lower and louder. In most cases the reason for putting ports on the back is that the enclosure is too small to fit them on the front. Boom is also typical of too small enclosures, and it has nothing to do with port placement either.

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Front porting always sounds better to my ears in a live gig situation though.  The bottom end is surely more audible when it's coming from the front of a cab versus the back?  Especially when a cab is placed against a wall (which happens lots of times at indoor gigs)

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Google 'omnidirectional'. Not only is port radiation omnidirectional, cone radiation is as well up to about 200Hz. A port output can be choked off if the port is placed tight to the wall, but even an inch or two of space will eliminate that. Without a doubt you may hear a difference between front and rear ported, but it's not due to the port location.

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I agree that front-mounted ports sound better on bass guitar cabs - not so much on hi-fi cabs though. Theoretically they shouldn't, and even acknowledged experts in the field can't agree on this one. If the cabinet is large, you could make a case for a reduction in port volume from the port in the nearfield due to distance. Bill's first point is a good one but I suspect you might not get a completely satisfactory answer to this question.

Edited by stevie
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Honestly I doubt that they do sound better, I can think of at least one good reason why they ought to sound worse but none which would make them sound better. There is no theoretical reason i know of that would suggest a difference  and i doubt you'd measure any difference at a decent distance from the cab. You are then down to subjectivity/listening tests. The problem is proving a difference one way or the other. You'd need to build identical cabs with identical drivers and then carry out multiple double blind tests where neither the listener nor the operator knew which cab was which. The big problem is that our ears are connected to a big organic computer which processes the input from our ears and often functions to make us hear what we expect to hear.

 

I've spent many hours doing listening tests when designing and building speakers and you'd be amazed how often you can fool yourself that you can hear something that isn't there or fail to spot something that is obvious. My wife assures me that i often fail to hear things which are important to her but rarely miss something important to me :)  It isn't just our sight that is subject to illusions. To me ports on the back look wrong and I tend to avoid them even though I know there is no good reason to do this.

 

The reality is that I've never actually listened to the same driver in a box identical except for the port placement. Maybe you just prefer some speakers to others and as not many have ports on the back it's just coincidence that you didn't like those as much as your favourite cabs

 

So what could make it sound different but objectively worse? Firstly cutting a hole in the baffle weakens the part of the cab you have already weakened with the speaker cut out and the bit you need to be most rigid, you end up with a big heavy magnet fixed to a wobbly board. A weak baffle isn't good secondly the port itself has a series of resonances where the wavelength is a multiple of the port length. These are going to be more directional and could be audible.

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Yes, there are pluses and minuses for front and rear porting and we agree about the fallibility of making subjective judgements. Chienmortbb makes a good port about the physical attack of a well designed front port (in a well-braced cabinet). A lot of that is going to be dissipated if the port is vented to the rear. 

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It's not that simple. There's baffle, vertical column or not, cab size vs speaker resonance freq vs port size, phase coherence if there's a hf unit, etc. etc.

Fascinating matter!

 

Also "sounding better" is highly subjective. On stage I like a cab with good dispersion so I can still hear what I'm doing when moving about.

 

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16 minutes ago, stevie said:

Chienmortbb makes a good port about the physical attack of a well designed front port (in a well-braced cabinet)

 

As you chaps will no doubt be aware from my endless harping on about it, I'm a PJB owner and fan. They are front ported and do seem to have this quality. The cabinets are solidly made - the ports are not just holes cut in the baffle - and heavy for their small size, which must help. Mr Jones considers front porting to be best.

 

However, one of the nicest sounding cabs I've owned was an old American made Ampeg SVT210HE, which was rear-ported. I bought it new many years ago and I've always regretted selling it. I looked for another for a long time without success, sold it and a few weeks later one - not mine - came up in the classifieds. Sod's law. So I guess port location doesn't matter that much, provided the cab is well designed and made.

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FWIW I use front porting on most of my designs because most people buy with their eyes, not their ears. How a rear firing port works as well as front firing can be better understood if you consider the case of sub woofers, which operate within the same frequency pass band as ports. SOP for those in the know is to aim them at a wall, or a corner. They work better that way than they do facing away from the wall or corner.

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20 hours ago, Bill Fitzmaurice said:

Google 'omnidirectional'. Not only is port radiation omnidirectional, cone radiation is as well up to about 200Hz. A port output can be choked off if the port is placed tight to the wall, but even an inch or two of space will eliminate that. Without a doubt you may hear a difference between front and rear ported, but it's not due to the port location.

 

A port at the rear might sound the same, but the fact it's at the rear means there is effectively an object in the way of it and your ear (i.e. the cabinet itself).  It's also further away from your ear.  Could either of these explain why front ported cabs sound better to me?

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When I had rear ported cabs a couple of drummers said they didn't like the sound because when we're set up in a line they'd hear more bass from the back of the cab and less sound from the front. Also in some of the gigs I do space is so tight that leaving 6" behind the cab is difficult. Front ported might not make a sonic difference but IMO it's a more convenient package.

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1 hour ago, acidbass said:

 

A port at the rear might sound the same, but the fact it's at the rear means there is effectively an object in the way of it and your ear (i.e. the cabinet itself).  It's also further away from your ear.  Could either of these explain why front ported cabs sound better to me?

The wavelengths that a port produces range from 12 to 25 feet. That makes an obstacle some 2.5 feet wide, as in a speaker enclosure, invisible to them. They go around it as if it wasn't there. Where the cone radiation is concerned those wavelengths also go fully around the cab, in the opposite direction, up to where the enclosure baffle is one wavelength in dimension. 2.5 feet is one wavelength at 450Hz. That explains why when you walk behind the cab, or when a drummer is behind the cab, the lows and midbass are still heard, but the mids and highs aren't. If your drummer isn't in front of the cab and wishes to hear your mids and highs and you have two cabs the best placement is to stack the cabs, with the bottom cab aimed at the drummer.

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2 hours ago, chris_b said:

When I had rear ported cabs a couple of drummers said they didn't like the sound because when we're set up in a line they'd hear more bass from the back of the cab and less sound from the front. Also in some of the gigs I do space is so tight that leaving 6" behind the cab is difficult. Front ported might not make a sonic difference but IMO it's a more convenient package.

 

It makes a sonic difference to some!

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17 hours ago, Bill Fitzmaurice said:

FWIW I use front porting on most of my designs because most people buy with their eyes, not their ears. How a rear firing port works as well as front firing can be better understood if you consider the case of sub woofers, which operate within the same frequency pass band as ports. SOP for those in the know is to aim them at a wall, or a corner. They work better that way than they do facing away from the wall or corner.

Discussions about ports seem to be contentious so here goes. While a corner facing rear port in subwoofers may have  a higher/different output a moved closed to the corner, it is a matter of taste as to whether it is better or not. In my experience of playing pubs and clubs in the UK, a rear firing port makes it much harder to get a balanced sound when placed close to a wall or in a corner.

Edited by Chienmortbb
smelling mi55taek5
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Its been a while since i read up on cab design, so rather than getting my books out can Bill and or Phil explain about phasing, as I believed that cab design and their porting - length and size (Diameter) of port was so that the air from behind the cone reached our ears in phase with the air from the front. If i am correct how dose this get achieved from both these opposite designs. I am in the Front port camp, pending the reply 😉.

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32 minutes ago, Stub Mandrel said:

How many people here have actually A/B tested otherwise identical cabs with front and rear porting?

 

Other than @Bill Fitzmaurice...

 

Well yes, but science doesn't matter when something sounds better to the customer in a real world environment.

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1 hour ago, deepbass5 said:

Its been a while since i read up on cab design, so rather than getting my books out can Bill and or Phil explain about phasing, as I believed that cab design and their porting - length and size (Diameter) of port was so that the air from behind the cone reached our ears in phase with the air from the front. If i am correct how dose this get achieved from both these opposite designs. I am in the Front port camp, pending the reply 😉.

 

The wavelength of a 40Hz audio signal is 330m / 40 = 8.25m. In other words, at the frequencies porting enhances (let's say sub 100Hz) the distance from front to back of the cabinet is only a small proportion of a wavelength, insufficient to cause destructive interference and hence beaming or phase issues and the cab is essentially omnidirectional at these frequencies.

 

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16 hours ago, Stub Mandrel said:

 

The wavelength of a 40Hz audio signal is 330m / 40 = 8.25m. In other words, at the frequencies porting enhances (let's say sub 100Hz) the distance from front to back of the cabinet is only a small proportion of a wavelength, insufficient to cause destructive interference and hence beaming or phase issues and the cab is essentially omnidirectional at these frequencies.

 

It's not quite that simple as the back wave starts out 100% out of phase and the port / box system puts a delay on it within the useful range of the port tuning. That was the question, how the heck it works.

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4 hours ago, Downunderwonder said:

It's not quite that simple as the back wave starts out 100% out of phase and the port / box system puts a delay on it within the useful range of the port tuning. That was the question, how the heck it works.

 

With proper design the port signal is in phase. WP explains better than me:

 

Quote

If this vent air mass/box air springiness resonance is so chosen as to lie lower in frequency than the natural resonance frequency of the bass driver, an interesting phenomenon happens: the backwave of the bass driver sound emission is inverted in polarity for the frequency range between the two resonances. Since the backwave is already in opposite polarity with the front wave, this inversion brings the two emissions in phase (although the vent emission is lagging by one wave period) and therefore they reinforce each other. This has the useful purpose of producing higher output (for any given driver excursion compared to a closed box) or, conversely, a similar output with a smaller excursion (which means less driver distortion). The penalty incurred for this reinforcement is time smearing: in essence the vent resonance augments main driver output by imposing a "resonant tail" on it. For frequencies above the natural resonance of the driver, the reflex alignment has no influence. For frequencies below the vent resonance, polarity inversion is not accomplished, and backwave cancellation occurs. Furthermore, the driver behaves as though suspended in free air, as box air springiness is absent.

 

The small additional delay for the rear port might actually bring the signal even closer into phase reducing the 'smearing' described above.

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Phase is a moderately complex idea, so for anyone not familiar.

 

Speakers (anything making a noise actually) move backwards and forwards travelling fastest in the central position then slowing at the ends of travel before turning round and going back the the other extreme where they slow and return. That creates pressure waves in the air that radiate out into the room. At the back of the speaker the pressure wave is reversed and the air is pushed in the opposite direction. It is 180 degrees out and if you let it mix with the air at the front you get cancellation and no sound. Many other things can delay the sound including distance and the air can be any amount 'out of phase' or not at all so you can get anything from double the sound from an in-phase sound to complete cancellation (which is what sound cancelling headphones do)

 

Any time delay in the sound leads to phase cancellation but the 'phase' is dependent upon the frequency of the sound. Higher frequencies go through more phase change than low frequency ones in the same time.

 

In a ported speaker the mass of air in the port is bouncing on the air in the cab; it is resonating if it is in action and like a bass string it only does this at one frequency (sort of) The trick of the designer is to set this up so it does this just as the speaker is dropping off in its frequency response. Once the port starts vibrating it pushes back at the air inside and if you get it right the cone of the speaker stops moving and all the sound output is coming from the port. So at the tuning frequency you aren't going to get cancellation/phase issues. The speaker isn't making much sound at that frequency. Above that frequency the port output falls really rapidly so phase cancellation ceases to be an issue. Finally all this is happening at the very lowest frequencies the cab can realistically reproduce so the phase shift isn't great and frankly our ability to discriminate sound at those frequencies is really poor. Any issues would be swamped by room acoustics as the bass bounces off every hard surface (floors, ceilings and walls) and takes many differing routes to your ears, each far greater than the distance from rear port to the front of the speaker.

 

That's why I don't think you can hear a difference and I'm completely with Bill on this one; if you want to sell a speaker put the port where people like to see it. People buy with their eyes.

 

That's the simple picture (I'm sure the experts are wincing here at some of the generalisations but I'm trying to be clear)

 

 

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17 minutes ago, Phil Starr said:

 if you want to sell a speaker put the port where people like to see it. People buy with their eyes.

 

Quite right. The better method is to take the eyes out of the equation via double blind testing. I can't recall any such testing ever being posted on a bass forum. If it was undertaken the results would likely be quite interesting, as in this case:

http://seanolive.blogspot.com/2009/04/dishonesty-of-sighted-audio-product.html

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