Front vs rear port?

KEW

KEW

Audioholic Overlord
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).
 
S

shadyJ

Speaker of the House
Staff member
I think the difference it makes is small. If the ports are on the front, turbulence is much more easily heard. I don't know what the port tuning frequencies are on the Philharmonic BMR, but if it is well into subwoofer frequencies, then it won't even matter for your purposes, since the ports won't really be used much.
 
3db

3db

Audioholic Overlord
Front porting speakers may be easier to place in a room compared to rear ported speakers. Like shady eluded, you may hear port noise with front ported speakers. That being said, Ive not once heard chuffing from my speakers ports which are located in the front.

With subs you may get chuffing which is asymptomatic of poor room placement or the room being too large for the sub.
 
B

Beave

Audioholic Chief
It's not just chuffing that can be audible with a port. Ports have higher-frequency resonances of varying amplitude (some speaker's port resonances are very well suppressed, while in some other speakers the resonances are quite high in amplitude). With a front firing port, care has to be taken to minimize these resonances, lest they be audible. With a rear firing port, they won't be heard much (because they're too high in frequency to diffract around the cabinet) from the front direction of the speaker (unless the wall behind the speaker is very reflective and the resonance is high amplitude, in which case it *might* be *barely*(?) audible).

So, front port allows more placement flexibility (a little closer to the wall behind the speaker). Rear port avoids issues of port resonance being audible. Carefully designed front port is best of both worlds. But the amount of space needed behind a rear ported speaker is usually only a couple of inches between rear of speaker and the wall for the port to function properly, so a rear port isn't as much of a placement issue as one might think.
 
D

Dennis Murphy

Audioholic General
It's not just chuffing that can be audible with a port. Ports have higher-frequency resonances of varying amplitude (some speaker's port resonances are very well suppressed, while in some other speakers the resonances are quite high in amplitude). With a front firing port, care has to be taken to minimize these resonances, lest they be audible. With a rear firing port, they won't be heard much (because they're too high in frequency to diffract around the cabinet) from the front direction of the speaker (unless the wall behind the speaker is very reflective and the resonance is high amplitude, in which case it *might* be *barely*(?) audible).

So, front port allows more placement flexibility (a little closer to the wall behind the speaker). Rear port avoids issues of port resonance being audible. Carefully designed front port is best of both worlds. But the amount of space needed behind a rear ported speaker is usually only a couple of inches between rear of speaker and the wall for the port to function properly, so a rear port isn't as much of a placement issue as one might think.
Beat me to it. those are all valid points. I'll just add a practical one. If the woofer is fairly potent, you'll probably end up with a large port. If you front-mount it, you either have to increase the height of the speaker significantly, or use a slotted port. I found that on the BMR a Precision double-flared port (Precision is a brand name) made a big difference not just in port noise but in overall performance compared with a slotted port. I could never fit the Precision on the front without an unacceptable increase in height and cabinet cost, and it would be absolutely impossible on the prefabricated BMR cabinets.
 
B

Beave

Audioholic Chief
Good point. Take a look at the Revel Performa3 lineup. The bookshelf speakers are rear ported, probably to keep them from being so large. They would have to be much taller if they were front ported. But the towers are front ported. There was room there, and with the narrowing cabinet in the rear, the ports make sense on the front.
 
TLS Guy

TLS Guy

Seriously, I have no life.
I agree with Dennis. At the frequencies the port is radiating the speaker is an omnipole anyway. If the speaker is properly designed the output of speaker and port should be in phase, as the tuning inverts the rear radiation of the cone.

In a small two way there is HF radiation, which rear porting helps conceal. If there is port chuff or chiff, then the port is too small in diameter, and needs to be longer and wider.
 
Bucknekked

Bucknekked

Audioholic Samurai
I agree with Dennis. At the frequencies the port is radiating the speaker is an omnipole anyway.
Beat me to it. those are all valid points. I'll just add a practical one. If the woofer is fairly potent, you'll probably end up with a large port. If you front-mount it, you either have to increase the height of the speaker significantly, or use a slotted port.
I can't add to the technical reasons one might one might choose to use front or rear ports, but I can add a practical example as I had to make the same choice with my Songtowers. Dennis Murphy is familiar with the Songtowers :)

When I was considering mine I called Jim Salk and asked about "front or rear ports" since that's a choice. Jim said as long as the placement to the closest wall is at least 2x the port diameter, the rear port tends to provide better solutions for more environments and fewer problems with chuffing etc etc. Since my listening room is pretty compact, making the right choice seemed pretty important.

I chose the rear port and even though my space is small, the ports have never provided me with anything I could hear (the omnipole affect TLS Guy mentioned) while in the listening position. They are invisible to my ear. I think that's the way its supposed to be: the port should provide some sound but its source should be invisible.
 
KEW

KEW

Audioholic Overlord
Beat me to it. those are all valid points. I'll just add a practical one. If the woofer is fairly potent, you'll probably end up with a large port. If you front-mount it, you either have to increase the height of the speaker significantly, or use a slotted port. I found that on the BMR a Precision double-flared port (Precision is a brand name) made a big difference not just in port noise but in overall performance compared with a slotted port. I could never fit the Precision on the front without an unacceptable increase in height and cabinet cost, and it would be absolutely impossible on the prefabricated BMR cabinets.
Great!
I wasn't looking forward to the extra cost or wait to get the custom Salk cabinets, but did not want either of those short term concerns to rule a long term decision.

Make mine Cherry!

(I'll email via your Philharmonic Audio email later today).
 
Swerd

Swerd

Audioholic Warlord
Great!
I wasn't looking forward to the extra cost or wait to get the custom Salk cabinets, but did not want either of those short term concerns to rule a long term decision.

Make mine Cherry!

(I'll email via your Philharmonic Audio email later today).
Nice choice Kurt :D!

For those interested, here is an example of what port resonance can look like in a frequency response curve. The speaker is an expensive ($3800/pair) 2-way bookshelf speaker with a front-mounted slot port. The measurements are from Stereophile.

The red trace shows the output of the port, with a prominent peak centered at roughly 700-800 Hz. This peak is about 10 dB quieter than the mid woofer's output at that range, and I'd expect, it can be heard, depending on the volume and musical content. In this example, the front-mounted slot port doesn't seem to be worth the effort.

I expect most speakers to have quieter port resonance, at least 20-25 dB below the mid woofer. If the port is rear-mounted, port resonance should not be heard.
 
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KEW

KEW

Audioholic Overlord
Nice choice Kurt :D!

For those interested, here is an example of what port resonance can look like in a frequency response curve. The speaker is an expensive ($3800/pair) 2-way bookshelf speaker with a front-mounted slot port. The measurements are from Stereophile.

The red trace shows the output of the port, with a prominent peak centered at roughly 700-800 Hz. This peak is about 10 dB quieter than the mid woofer's output at that range, and I'd expect, it can be heard depending on the volume and musical content. In this example, the front-mounted slot port is not worth the effort.

I expect most speakers to have quieter port resonance, at least 20-25 dB below the mid woofer. If its rear-mounted any port resonance should not be heard.

Aside from the bass hump, the aggregate is a pretty nice FR... if you don't include 500-2,000Hz!:rolleyes:

 
killdozzer

killdozzer

Audioholic Samurai
Lacking technical knowledge, I can tell you of experience in placement and using the plugs and semi-plugs on my speakers. Respecting the minimum distance from the rear wall stated by the manufacturer of my rear ported speakers proved to be too short. 2x port opening distance would measure in only a few inches, but I end up with 18" minimum distance to lose the boomines and get all the lows promised. I'm wondering what is the material of a rear wall that is taken into these distance measurements and whether these rear walls manufacturers talk about are insulated?

I ended up designing my own wooden speaker stands that would have three legs and the back leg would protrude backwards to assure the minimum of 18" distance.

I didn't find plugs helpful at all.
 
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shadyJ

Speaker of the House
Staff member
Aside from the bass hump, the aggregate is a pretty nice FR... if you don't include 500-2,000Hz!:rolleyes:

We have to keep in mind that the close mic testing that Atkinson does will exaggerate the bass frequencies. If that speaker was testing in the anechoic far field, that hump centered just above 100 Hz would not be as elevated.
 
KEW

KEW

Audioholic Overlord
Strangely enough, his port measurement stops right around the frequency range where a peak could develop. Is there a peak at 700 or 800Hz? How would we know?
Good observation!!!

B&W = big advertising dollars!
 
S

shadyJ

Speaker of the House
Staff member
Strangely enough, his port measurement stops right around the frequency range where a peak could develop. Is there a peak at 700 or 800Hz? How would we know?
If there was a significant amount of output from the ports in that region, it would have shown up in the far-field measurements, but they don't show anything like that. It there was a peak, it wasn't enough to make a difference.
 
B

Beave

Audioholic Chief
True (although depends on one's definition of 'significant'). But it's still strange that he plots the port output to 1kHz for some speakers and only to 500Hz for other speakers. If you look at the response of the B&W midwoofer, you can see a dip right around 1kHz. Is that where a (smallish) port resonance occurs? Maybe.
 

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