Plugging only one rear bass reflex port?

[jcebedo11]

sorry for the misinformation. I used to do some box building back in the days (more than 10 years ago). From my memory that is what I remembered. Well at least we do know that a longer port results in lower tuning frequency whatever that number may be.

 

[Duke]

sorry for the misinformation. I used to do some box building back in the days (more than 10 years ago). From my memory that is what I remembered. Well at least we do know that a longer port results in lower tuning frequency whatever that number may be.

Hey sorry if I came across harsh, but kudos to you for not taking it that way. I have not yet attained infallibility either!

I run a lot of box design simulations, usually several times a week... your memory is GREAT for recalling that much from ten years ago.

 

[Frank Dernie]

sorry for the misinformation. I used to do some box building back in the days (more than 10 years ago). From my memory that is what I remembered. Well at least we do know that a longer port results in lower tuning frequency whatever that number may be.

I expect, since resonant frequency is proportional to the square root of the mass, doubling the port length would reduce the tuning frequency by around 70% and that the tuning of the enclosure will no longer be a "classic" match to the driver characteristic.
I have read that ATC tune their reflex enclosures lower than "classic".

 

[Glasvegas]

My shiny new Revel M105 arrived this afternoon, thanks to the persuasion of Amir's recent review as well as the good folks on ASR whose opinions I valued highly. My wallet will recover eventually.....

The M105s replaced the spots previously occupied by my Harbeth P3esr, and straight away I noticed the bass was booming a bit due to the rear bass reflex ports being only 33cm from the rear wall, and the right speaker is also quite near the corner of the room while the left speaker is next to the open side. Previously my Harbeth with its sealed box design was quite happy there.

I tried using the supplied foam plugs to seal the ports, but the bass was reduced too much so I took them out. Then an inspiration came, and I plugged in only the port to the right speaker, thinking the room corner is worse in trapping / distorting the bass, and guess what, it sounds pretty good now.

My question is - will I go to audiophile hell by plugging only one port? Has anyone else done the same?

Apart from the bass booming a bit, how do the Revels compare with the Harbeths?

 

[Archsam]

about ports and tuning frequency. If you double the length of the port and the port is the same diameter, the resonance frequency is cut in half. Im guessing the resonant frequency of your speakers are around 50hz. doubling the length is brings it now to 25hz. You can also change it to somewhere in between. Im guessing the one of the lengths or widths of your room to be 10 or 11 feet. If it is, your room as a resonant point of around 50hz. This is probably what is causing the boom, because both the port tuning and the room mode coincide together at around 50hz. I had a room with this very problem before.

Good guess on my room dimension! My living room measures 3390mm wide and 4065mm deep.

After playing with the paper tube length I settled on 6cm protrusion from the port. This is strictly guided by my hearing, and is a good balance between reducing the boom (and improved clarity noticeably) and retaining a decent amount of bass.

I speculate that once I altered the tube length and changed the port resonant frequency, as long as it is no longer interacting with the frequency of the room mode the result should be fine? My very rough estimate on the port tube inside the speaker is around 12cm in length. I can find a bigger piece of paper and experiment with a 12cm protrusion later on and see what happens.

 

[Archsam]

That has not been my experience, either modelled or measured. The relationship between port length and tuning frequency (or number of ports and tuning frequency) is not straightforward. Here's an online calculator, I can't vouch for its accuracy (it's not what I use) but you can plug in some numbers and observe the trends:

http://www.mobileinformationlabs.com/HowTo-1Woofer-Box-CAL Port lenth 1.htm

* * * *

As for plugging ports, in most cases it works just fine (theoretically you could run into chuffing at high SPL's). I try to incorporate multiple pluggable ports into most of my designs. Here's a photo of the back of a bass cab, and you can see that two of the four ports are plugged, which can help in a boomy venue:

View attachment 74890

I did a very very rough estimate using the calculation (based on what I can measure / estimate on the M105) and the frequency worked out to be VERY roughly 40hz:

 

[Archsam]

Apart from the bass booming a bit, how do the Revels compare with the Harbeths?

A few subjective observations based on 2 days of listening:

• No contest the M106 is a better all round speaker, especially with the quality and quantity of bass - the Harbeth simply is no match.
• Percussion instruments sounds more impactful on the M105.
• But perhaps because of a lack of sub bass in the Harbeth (and the room mode issues I am currently experiencing with the M105), there is a slightly better overall clarify with the Harbeth when it plays music that is not bass heavy e.g. jazz vocals, simple acoustic arrangements.
• Timbre and clarity of the midrange and treble are surprisingly similar between the two.
• The tweeter waveguide an wide dispersion pattern of the M105 really does make a big difference. Some songs can sound holographic on the M105 whereas the Harbeth can sound very good but spatially flat.
• Some vocals do sound better on the Harbeth - female vocals are more or less on par, but there are a few male vocals e.g. Kurt Elling, where the M105 sounded just a little lean, whereas the Harbeth sounded more full-bodied.
• I did a measurement on my Harbeth using REW a few weeks ago, there is a slight hump in the 750hz region, i wonder if this is what gives male vocals slightly more fullness?

 

[tuga]

A few subjective observations based on 2 days of listening:

• No contest the M106 is a better all round speaker, especially with the quality and quantity of bass - the Harbeth simply is no match.
• Percussion instruments sounds more impactful on the M105.
• But perhaps because of a lack of sub bass in the Harbeth (and the room mode issues I am currently experiencing with the M105), there is a slightly better overall clarify with the Harbeth when it plays music that is not bass heavy e.g. jazz vocals, simple acoustic arrangements.
• Timbre and clarity of the midrange and treble are surprisingly similar between the two.
• The tweeter waveguide an wide dispersion pattern of the M105 really does make a big difference. Some songs can sound holographic on the M105 whereas the Harbeth can sound very good but spatially flat.
• Some vocals do sound better on the Harbeth - female vocals are more or less on par, but there are a few male vocals e.g. Kurt Elling, where the M105 sounded just a little lean, whereas the Harbeth sounded more full-bodied.
• I did a measurement on my Harbeth using REW a few weeks ago, there is a slight hump in the 750hz region, i wonder if this is what gives male vocals slightly more fullness?

View attachment 74913

Bass and loud are cheap thrills, it's the resolution/clarity in the midrange and the smooth/grainless treble that are difficult to achieve.
But that don't come in a Spin...

The Harbeths require either long-wall setup in a wide-room or early-reflection zone treatment and full toe-in for flattest response at the listening spot.
Your Harbeth plot shows a nice in-room balance, did you use Variable smoothing?

Also, could you post a measurement of the M106 for comparison?

 

[Archsam]

Bass and loud are cheap thrills, it's the resolution/clarity in the midrange and the smooth/grainless treble that are difficult to achieve.
But that don't come in a Spin...

The Harbeths require either long-wall setup in a wide-room or early-reflection zone treatment and full toe-in for flatest response at the listening spot.
Your plot looks nice, did you use Variable smoothing?
Could you post a measurement of the M106s?

Spot on re: midrange / treble, cannot agree more.

I liked my Harbeth best with full toe-in as you mentioned. In fact I had each speaker just a degree or 2 from crossing at my listening position, so technically each speaker is pointing directly to me left / right ear.

The plot is an average of 7 measurements, I was following Julian Krause's Youtube tutorial and took measurements from the listening position plus fore/ aft, left / right, and high / low variations.

Will definitely measure the M105 when I have time in the near future.

 

[Rick Sykora]

I did a very very rough estimate using the calculation (based on what I can measure / estimate on the M105) and the frequency worked out to be VERY roughly 40hz:

View attachment 74912

Unmodified, the box tuning for the m105 is around 60 Hz. This is from Amir’s posted impedance measure, I have not modeled. So if looking to establish baseline, need to start there. There is a bit more to consider as port lengthening significantly can create a pipe resonance or chuffing.

If you get a result you think you like, can model in Bassbox Pro as a crosscheck.

 

[tuga]

Unmodified, the box tuning for the m105 is around 60 Hz. This is from Amir’s posted impedance measure, I have not modeled. So if looking to establish baseline, need to start there. There is a bit more to consider as port lengthening significantly can create a pipe resonance or chuffing.

If you get a result you think you like, can model in Bassbox Pro as a crosscheck.

It would be interesting if @amirm were to measure plugged-port on-axis response and impedance of those speakers which provide port plugs.
It is quite a handy feature for those of us with a small listening room and/or positioning constraints.

This is from Stereophile's assessment of the PSB Imagine B:

https://www.stereophile.com/content/psb-imagine-b-loudspeaker-measurements

 

[Archsam]

Unmodified, the box tuning for the m105 is around 60 Hz. This is from Amir’s posted impedance measure, I have not modeled. So if looking to establish baseline, need to start there. There is a bit more to consider as port lengthening significantly can create a pipe resonance or chuffing.

If you get a result you think you like, can model in Bassbox Pro as a crosscheck.

That is interesting - I had a closer look at the port length by flashing a light into the hole and used a pencil to gauge the depth, and the port length (up to the external opening where the flare begins) is close to 5.5".

Working backwards using the online calculator, if we start with a 60Hz tuning frequency I ended up with an internal volume of only 0.35 cu. ft. That seemed very low based on the external dimension, even with the curved body / thick 1" panels / drivers and internal bracing taking up room?

I am REALLY curious to see a cross section of this speaker now...

 

[Glasvegas]

A few subjective observations based on 2 days of listening:

• No contest the M106 is a better all round speaker, especially with the quality and quantity of bass - the Harbeth simply is no match.
• Percussion instruments sounds more impactful on the M105.
• But perhaps because of a lack of sub bass in the Harbeth (and the room mode issues I am currently experiencing with the M105), there is a slightly better overall clarify with the Harbeth when it plays music that is not bass heavy e.g. jazz vocals, simple acoustic arrangements.
• Timbre and clarity of the midrange and treble are surprisingly similar between the two.
• The tweeter waveguide an wide dispersion pattern of the M105 really does make a big difference. Some songs can sound holographic on the M105 whereas the Harbeth can sound very good but spatially flat.
• Some vocals do sound better on the Harbeth - female vocals are more or less on par, but there are a few male vocals e.g. Kurt Elling, where the M105 sounded just a little lean, whereas the Harbeth sounded more full-bodied.
• I did a measurement on my Harbeth using REW a few weeks ago, there is a slight hump in the 750hz region, i wonder if this is what gives male vocals slightly more fullness?

View attachment 74913

Interesting comparison, especially as the P3s cost £2k and the 105s are on sale for £799 at Nintronics. Seems like a bargain at that price.

I auditioned the P3s early this year expecting to be blown away, based on many glowing reviews - and the £2k price tag. Didn’t happen.

At £799 the 105s could be worth a try.

 

[Archsam]

Interesting comparison, especially as the P3s cost £2k and the 105s are on sale for £799 at Nintronics. Seems like a bargain at that price.

I auditioned the P3s early this year expecting to be blown away, based on many glowing reviews - and the £2k price tag. Didn’t happen.

At £799 the 105s could be worth a try.

This is exactly why I bought the M105 - at that discount price I just couldn't resist.

I paid £1500 for the Harbeth a few years ago so this is a perfect apple-to-apple comparison vs. the standard £1600 asking price for the M105. My gut feeling is that 75% of the time I prefer the M105, while 25% of the time the Harbeth sounds better (as per my observations in my previous post).

One thing I haven't mentioned yet, and it might actually be an important consideration, is listening at low volume at night. The M105 was actually quite difficult to listen to when you turn down the volume - at an acceptable SPL the bass was too prominent and I kept worrying about the noise disturbing my partner sleeping next room / my neighbours downstairs. The midrange / treble became too recessed when I turn down the volume in order to control the bass. This is where I am so grateful to have tone control on my Yamaha amp - I turned down the bass by 4-5dB and all is well again. If you live in a flat in the city this is an important consideration.

Edit - when I said the bass was too prominent in normal listening levels it was only in reference to the SPL disturbing others, the sound quality was fantastic otherwise.

 

[Rick Sykora]

That is interesting - I had a closer look at the port length by flashing a light into the hole and used a pencil to gauge the depth, and the port length (up to the external opening where the flare begins) is close to 5.5".

Working backwards using the online calculator, if we start with a 60Hz tuning frequency I ended up with an internal volume of only 0.35 cu. ft. That seemed very low based on the external dimension, even with the curved body / thick 1" panels / drivers and internal bracing taking up room?

I am REALLY curious to see a cross section of this speaker now...

.35 cubic feet would be typical for a speaker with that size woofer.

Mind you, you also need to subtract the vent volume, woofer volume and any other major parts (braces, crossover). You do not need to be really precise, a good solid ballpark figure should be good enough. It is an SB15 woofer. Happen to know the specific model?

 

[Glasvegas]

You purchased the 105s from Nintronics? How were they to deal with?

They also sell the 106s for £999. Too much choice!

 

[Rick Sykora]

Lol. Sorry just waking up...

Assuming you are accurate on the other specs and your box calculator is correct, the volume of .35 cu ft is correct.

One tricky part would be if the port is tapered.

 

[Archsam]

You purchased the 105s from Nintronics? How were they to deal with?

They also sell the 106s for £999. Too much choice!

Yep I bought mine from Nintronics via their ebay shop. Extremely quick in shipping - they arrived literally the next day. Only hiccup is I have not received the purchase receipt with the speakers. I called them yesterday and the chap on the phone said they will send it via email on the same day. I have not heard back nor seen the receipt yet. I will give them until Monday before chasing up.

 

[Archsam]

Lol. Sorry just waking up...

Assuming you are accurate on the other specs and your box calculator is correct, the volume of .35 cu ft is correct.

One tricky part would be if the port is tapered.

Port looks pretty straight. It appears to be a cardboard tube painted black.

 

[Archsam]

Also I have been using this song as my reference track to observe the sub-bass quality of a speaker:

The double bass starting 10 seconds into the track will reveal right away if the bass is boomy / elevated / smearing the mids and trebles.

Having played this song over lunch I have increased my paper tube protrusion to 7cm, and set my Yamaha's bass tone control to roughly -4dB. I will have a poke at the port length calculator later on but right now I'm liking the sound.