Untuned ports

[ficelles]

[quote name='alexclaber' post='1322743' date='Aug 1 2011, 08:13 AM']And why would you want that to happen? Just curious about the thinking - I think I might write an article about ports soon...[/quote]

Firstly because the drivers I was initially using are not recommended for sealed enclosures, secondly to increase low end efficiency & response.

Incidentally I initially got the port tuning wrong on my larger cab with the result that it was so resonant an amp wouldn't stay on the top...

For both my large and small cabs I now have untuned ports i.e. no pipe... the larger one I currently have vented at the rear, the smaller at the front.

ficelles

Edited August 1, 2011 by ficelles

[Bill Fitzmaurice]

[quote name='LawrenceH' post='1323079' date='Aug 1 2011, 08:52 AM']But apparently a so-called acoustic engineer knows better than my scientific colleagues at the cutting edge of hearing research, how the ears and brain work.[/quote]
An acoustical engineer must know how the ears and brain work. If he does not he isn't an acoustical engineer, he's a mechanic. If you've got nothing better to do than argue about the audibility of group delay got to DIYaudio and waste a few dozen pages in debate with Earl Geddes.

Edited August 1, 2011 by Bill Fitzmaurice

[Mr. Foxen]

[quote name='ficelles' post='1323177' date='Aug 1 2011, 03:21 PM']Incidentally I initially got the port tuning wrong on my larger cab with the result that it was so resonant an amp wouldn't stay on the top...[/quote]

I think would be more to do with poor cab construction than port tuning. Or possibly the hole compromised the structure.

[ficelles]

[quote name='Mr. Foxen' post='1323290' date='Aug 1 2011, 05:20 PM']I think would be more to do with poor cab construction than port tuning. Or possibly the hole compromised the structure.[/quote]

No, it was the port tuning... I took the tubes out and the problem went away. The cab is very solidly made and the structure is 100% sound. I've actually just been testing it with a tone generator and it's amazing how resonance-free the cab is now (which is more than I can say for my kitchen ).

ficelles

[stevie]

[quote name='Bill Fitzmaurice' post='1323280' date='Aug 1 2011, 05:13 PM']If you've got nothing better to do than argue about the audibility of group delay got to DIYaudio and waste a few dozen pages in debate with Earl Geddes.[/quote]
Lawrence isn't arguing about group delay: he's describing current thinking on the subject. There certainly are quite a few "real" acoustical engineers who admit they don't have a definitive answer to this one, Linkwitz and O'Tool included. You're the one insisting you have the answer. If you have some information, please share it.

It is a rather arrogant to tell another forum member to go elsewhere to discuss a topic. It's the equivalent of telling somebody to shut up. Why don't you go to DIY audio and talk with Dr. Geddes yourself - I believe he has done recent some work on group delay audibility. Oh, I forgot - that's one of the forums you're banned from isn't it? Shame.

[LawrenceH]

[quote name='Bill Fitzmaurice' post='1323280' date='Aug 1 2011, 05:13 PM']An acoustical engineer must know how the ears and brain work. If he does not he isn't an acoustical engineer, he's a mechanic. If you've got nothing better to do than argue about the audibility of group delay got to DIYaudio and waste a few dozen pages in debate with Earl Geddes.[/quote]

That is rather rich given that you were the one who managed to construct an argument from nothing, by putting words into my mouth. I struggle to see why you do this.


ficelles, as has been said before a port without an additional pipe is still tuned. The effective port length can usually be considered the width of the baffle with the hole in.
In general for a given box size, a port tuning is raised by either enlarging port surface area or decreasing port length. So you can have a short port with a deep tuning by keeping the diameter small. But there is no free lunch as the maximum air flow before it starts to get very noisy through this port is reduced. It'll stop behaving itself at higher volumes and you won't reach the full potential of the loudspeaker driver, which is why cabs for bass ideally have large, long ports.

As BFM has suggested the easiest way to estimate how different ports will behave in your cabinet without actually riddling it full of holes is to plug the measurements of the cab into appropriate modelling software and play around with the numbers. Your port length before probably resulted in a tuning that was too high, giving a hefty bump in the lower mids/upper bass - increase the length and the speaker will behave a bit more smoothly and go deeper.

[Guest bassman7755]

[quote name='ficelles' post='1320497' date='Jul 29 2011, 02:46 PM']Untuned ports i.e. just a hole... anyone in favour / not in favour? Any rule of thumb for size?

ficelles[/quote]

If your asking is a random (rather than calculated) hole cut in your cab going to make it better I think the answer is "no". Most likely outcome is that you are going to create sharp peaks and/or dips in its response and possibly even damage the driver through over excursion.

[ficelles]

[quote name='LawrenceH' post='1323341' date='Aug 1 2011, 06:08 PM']ficelles, as has been said before a port without an additional pipe is still tuned. The effective port length can usually be considered the width of the baffle with the hole in.[/quote]

I know that, it's just my use of terminology - untuned to me means what you get from the basic enclosure with a hole, tuned (in my usage anyway) means you have added something, pipe, shelf or whatever, to tune for a specific frequency.

I would do the maths (I might even dig the formulae out of my old acoustics textbooks for fun one day) but ultimately my cabs seem to sound fine "untuned"...

I am intrigued as to why this topic has caused such dissent though! Am I right to assume that no-one in here has actually studied acoustics at college or whatever, or more specifically the acoustics of speaker cabinet design?

Hands up all those with Acoustics Degrees Not me btw...

ficelles

[ficelles]

[quote name='bassman7755' post='1323450' date='Aug 1 2011, 08:15 PM']and possibly even damage the driver through over excursion.[/quote]

Which in the event is exactly what happened with the tuned cab by selecting the wrong tuning, although the drivers - while useless for bass now - still work fine in a friend's guitar extension cab. Over-excursion folded the cones at the point of join with the rubber mount but there is still enough rigidity for non-bass use.

ficelles

Edited August 1, 2011 by ficelles

[Mr. Foxen]

[quote name='ficelles' post='1323578' date='Aug 1 2011, 09:37 PM']I am intrigued as to why this topic has caused such dissent though! Am I right to assume that no-one in here has actually studied acoustics at college or whatever, or more specifically the acoustics of speaker cabinet design?

Hands up all those with Acoustics Degrees Not me btw...[/quote]

BFM is qualified as far as In know, even if he doesn't have the paper, he has the knowledge, even if he is a bit 'attitude' about it.

[Balcro]

Maybe your original post was confusing. Saying are you in favour or not is probably asking the wrong question.
Most people on here these days, would expect you to have a ported enclosure. That's why both alexclaber and I
asked you "what's the purpose of a adding a hole?"
I think this thread has established that whether there's a hole or a port, a tuned frequency exists, whether it's
useful or not.

The usual response to questons such as yours is to recommend the use of winISD software. Use that and you can get the tuning, frequency response and power handling sorted out in a few minutes. Because of tools such as this you can achieve an optimium setting and you might avoid damaging drive units. In other words it would solve the first two sentences of your post #26.

Take a look at my post #18. The diameter of a 100mm drain-ppe is very close to one of the standard port diameter settings in winISD. It's also cheap and adaptable. The apparently odd-ball device in the link is actually a serious device used in hi-fi and would prove useful if you had a closed box loudspeaker and then cut a hole in it. It lets the air out for a purpose and is an alternative to a reflex port. The research on it goes back more than fifty years.

I have no idea why this sub-forum, not just this thread, that attracts such dissent, but it does. These days you half expect it, and then the protagonists veer off at a tangent. Whatever. As far as I'm aware Bill Fitzmaurice and Alex Claber are the people who know their physics. Some others will have lots of experience in the the entertainment industry and they may also know their physics, others have a passion for loudspeaker boxes. THey will have to tell you who they are.

Balcro.

[Dood]

Moderator in the house!

Guys, these threads can be very informative and helpful and would really like them to stay that way. I know that it can get a bit heated, by all means exercise your collective brains to benefit but please, keep off the low-blows and inflammatory comments.

[ficelles]

[quote name='Balcro' post='1323688' date='Aug 1 2011, 11:19 PM']The usual response to questons such as yours is to recommend the use of winISD software.[/quote]

Just tried it out... nice to see what looks like Delphi software still around. But the problem is it seems to be design rather than reverse engineering i.e. it tells you what you should have not what you have got, you can't adjust port length afaics. What is the Sd parameter? Can't find that for my BN12-300S and can't think what's in m2...

It's always interesting to read people's profiles btw

ficelles

Edited August 1, 2011 by ficelles

[Mr. Foxen]

WinISD pro Alpha is the one to use.

[LawrenceH]

[quote name='ficelles' post='1323578' date='Aug 1 2011, 09:37 PM']I am intrigued as to why this topic has caused such dissent though! Am I right to assume that no-one in here has actually studied acoustics at college or whatever, or more specifically the acoustics of speaker cabinet design?
ficelles[/quote]

I have certainly studied the basic physics sufficient to give a reasonable description of soundwave propagation and the low-frequency behaviour of these boxes. A lot of that side of things is really no more than first-year undergraduate science, if that. I think the dissent is more in the area of acoustic perception, i.e. how we hear. There is lots understood and lots more ongoing research in this area, including that by my colleagues over in Glasgow. Why is this so controversial? My take is that in addition to areas of active debate within the field, there is a lot of stuff floating around that's out of date, not to mention claims that are likely unfounded where financial interests of exotic hi-fi companies/marketing men are concerned. It does take a bit of time for that type of biological research to filter out and cross specialisms to the engineering/physics arena, and much longer before it becomes 'common' knowledge.

It's also worth bearing in mind that a lot of the research into hearing has been geared towards the interest of medicine rather than loudspeaker designers, and consequently perception of frequencies within the normal range of speech is prioritised. Add to that the fact that bass presents more practical complications to work with, read gets more expensive, and you see why it isn't an area that's focused on so much.

[ficelles]

[quote name='LawrenceH' post='1323765' date='Aug 2 2011, 01:19 AM']I have certainly studied the basic physics sufficient to give a reasonable description of soundwave propagation and the low-frequency behaviour of these boxes. A lot of that side of things is really no more than first-year undergraduate science, if that. I think the dissent is more in the area of acoustic perception, i.e. how we hear. There is lots understood and lots more ongoing research in this area, including that by my colleagues over in Glasgow. Why is this so controversial? My take is that in addition to areas of active debate within the field, there is a lot of stuff floating around that's out of date, not to mention claims that are likely unfounded where financial interests of exotic hi-fi companies/marketing men are concerned. It does take a bit of time for that type of biological research to filter out and cross specialisms to the engineering/physics arena, and much longer before it becomes 'common' knowledge.

It's also worth bearing in mind that a lot of the research into hearing has been geared towards the interest of medicine rather than loudspeaker designers, and consequently perception of frequencies within the normal range of speech is prioritised. Add to that the fact that bass presents more practical complications to work with, read gets more expensive, and you see why it isn't an area that's focused on so much.[/quote]

Interesting perspective, not sure the accepted acoustics viewpoint is the same but then maybe that's just my perspective Splitting hairs I know but is the study of the hearing mechanism really biology, isn't it physiology?

I'm off to re-read my psychoacoustics textbook... in theory I reckon can make a cab with a tiny bandwidth that [i]sounds like[/i] it can handle a low B if I can just get those darn harmonics right...

ficelles

[Bill Fitzmaurice]

[quote name='LawrenceH' post='1323765' date='Aug 1 2011, 08:19 PM']It does take a bit of time for that type of biological research to filter out and cross specialisms to the engineering/physics arena, and much longer before it becomes 'common' knowledge.[/quote]Indeed. If you want to see some screwy ideas check out the home theater sites, where some truly believe that there's benefit to speakers that run to 5Hz. That's where biology comes in. Below roughly 12Hz human beings lack the ability to hear anything, because 75 foot long wavelengths pass through muscle and bone with impunity. That fact, combined with the period of the wave, means that both sides of the tympanic membrane are exposed to the same pressure and phase. It does not vibrate, so no sound is heard, and as those wavelengths pass though the body they aren't felt either. What is heard and felt from systems capable of high output below 12Hz is harmonic content and air pressure fluctuation.
But it would be incorrect to assume that this is knowledge restricted to the medical community. Acoustical engineering isn't limited to loudspeakers and acoustics. One of the major sub-fields is the hearing aid industry, where knowledge of how hearing works is paramount.

Edited August 2, 2011 by Bill Fitzmaurice

[charic]

[quote name='ficelles' post='1323578' date='Aug 1 2011, 09:37 PM']I know that, it's just my use of terminology - untuned to me means what you get from the basic enclosure with a hole, tuned (in my usage anyway) means you have added something, pipe, shelf or whatever, to tune for a specific frequency.

I would do the maths (I might even dig the formulae out of my old acoustics textbooks for fun one day) but ultimately my cabs seem to sound fine "untuned"...

I am intrigued as to why this topic has caused such dissent though! Am I right to assume that no-one in here has actually studied acoustics at college or whatever, or more specifically the acoustics of speaker cabinet design?

Hands up all those with Acoustics Degrees Not me btw...

ficelles[/quote]

Hand Up - Kinda...

But not so much for speaker design.

I'm still working on a project from Uni which may oneday prove fruitful. I work with Microphone placement and how it affects the sound.

Sodding nightmare!

As for speakers, not really bothered to look into it.

[charic]

[quote name='ficelles' post='1323782' date='Aug 2 2011, 02:11 AM']I'm off to re-read my psychoacoustics textbook... in theory I reckon can make a cab with a tiny bandwidth that [i]sounds like[/i] it can handle a low B if I can just get those darn harmonics right...

ficelles[/quote]

Interesting thought and should be possible. Good luck in getting it to FEEL like there's a low B aswell. You might struggle to get the depth of sound at high volume.

[Balcro]

[quote name='ficelles' post='1323747' date='Aug 2 2011, 12:40 AM']Just tried it out... nice to see what looks like Delphi software still around. But the problem is it seems to be design rather than reverse engineering i.e. it tells you what you should have not what you have got, you can't adjust port length afaics. What is the Sd parameter? Can't find that for my BN12-300S and can't think what's in m2...

It's always interesting to read people's profiles btw :)

ficelles[/quote]

If you've got a Celestion BN12 look it up here.
[url="http://professional.celestion.com/bass/pdf/BN12-300S8.pdf"]http://professional.celestion.com/bass/pdf/BN12-300S8.pdf[/url]

However, be careful in all these sites because there's a variation in the way the T/S parameters are expressed.
Some will refer to Kgs and others will use grams, sometimes it's fractions of a metre, sometimes it's in cetimetres.
winISD has it's own. Then there are the country to country variations.

Thiele/Small parameters - [url="http://yu-ra.tripod.com/ts_parameter.htm"]http://yu-ra.tripod.com/ts_parameter.htm[/url]
Just one site in hundreds with the same info. There's another inside the "Help" area of winISD.

The Celestion description is part of the above problem. The easist way is to follow the winISD "getting started" info under the "help" button and just input the first four or five parameters and let it do the calculation for you. It will tell you what the Sd is. You've got the BN12 parameters, now measure the box. winISD tells you what you've got.

Balcro.

Edited August 2, 2011 by Balcro

[stevie]

[quote name='Mr. Foxen' post='1323591' date='Aug 1 2011, 09:48 PM']BFM is qualified as far as In know, even if he doesn't have the paper, he has the knowledge, even if he is a bit 'attitude' about it.[/quote]
We all have a soft spot for the gifted amateur, but an ‘acoustical engineer’ is not some honorary engineering title like a British Telecoms engineer or a FOH engineer: it is a recognized job description requiring a BSc or MSc degree or equivalent in physics or engineering plus several years of practical experience. (Look up the job ads). An MEng Acoustical Engineering, which you can study for at Southampton, will get you Chartered Engineer status (Professional Engineer in the US and Canada). In most countries, including the US as far as I can tell, you then need to pass a licensing exam to practice. Which is not surprising, as these guys can act as expert witnesses in court.

[alexclaber]

When it comes to the specifics of loudspeakers and acoustics I'm essentially self-taught. But I have a BEng from Bristol in Mechanical Engineering (and obviously the relevant A-levels before that) and the general principles of engineering apply directly to the specifics of loudspeakers. I find car suspension and chassis design curiously fascinating too - a complex resonant system like a loudspeaker and a structural challenge like a cabinet. And just like good speakers can best be designed by those that are good at listening as well as analysing (and I'd argue that bass cabs are best designed by those that are good at playing too!) so too cars are best designed by those that can drive them well and feel how on paper differences translate in the real world - Jaguar's (now retired?) suspension guru Mike Cross being a fine example.

Charic, if you understand mics, just reverse them and you'll understand speakers! An oversimplication but there's a very close correlation.

I've yet to see anything definitive on group delay but my current feelings are that well designed ported cabs are so much better at high SPL that they make far more sense for pro sound, even if at small signal levels sealed or TL cabs perform better. Few things are more frustrating to me than seeing a great loudspeaker designer produce something that performs really well at low levels in test rooms when in reality it'll be used in much louder situations in rooms with difficult acoustics - what a waste of effort and ability, like making a large family car that is really safe and comfy at low speeds but that wobbles and feels unstable at motorway speeds. However for studio/hi-fi designs I've been leaning towards sealed, TL and open baffle designs for some time now. Mr Meyer and Mr Linkwitz are both high up on my list of loudspeaker designers well worth taking notice of!

[ficelles]

[quote name='alexclaber' post='1324104' date='Aug 2 2011, 12:27 PM']Charic, if you understand mics, just reverse them and you'll understand speakers! An oversimplication but there's a very close correlation.[/quote]

A transducer by either name... ever tried using a speaker as a microphone? Not overly efficient but it works...

ficelles

[charic]

[quote name='alexclaber' post='1324104' date='Aug 2 2011, 12:27 PM']Charic, if you understand mics, just reverse them and you'll understand speakers! An oversimplication but there's a very close correlation.[/quote]

Thanks Alex

I understand enough to get by with speakers (and turned one speaker into a very poor microphone* for fun) but they never fascinated me in the way microphone placement and choice does/did.

Unfortunately currently persuing software dev career so its not moved much for a while however some pretty nifty software may become available at somepoint




*Although it didn't sound to bad for KickDrum lowend

[LawrenceH]

[quote name='ficelles' post='1323782' date='Aug 2 2011, 02:11 AM']Interesting perspective, not sure the accepted acoustics viewpoint is the same but then maybe that's just my perspective Splitting hairs I know but is the study of the hearing mechanism really biology, isn't it physiology?[/quote]

Well, physiology is part of biology. At least the physiologists I work with would count themselves as biologists! Physiology as an active research subject often goes by the name neuroscience these days. From a biological/physiological perspective, you can break human hearing down into three major components for study. First, soundwave propagation through the ear/body. Second, signal transduction. Third, signal processing - a complex area. The reason this is the hot topic is because if you can't hear it then it doesn't matter. It's comparatively easy measuring physical characteristics and, say, comparing them to a reference. Harder to tell what's going on in the head and appropriate tests employing sufficient subjects are complex and expensive.

[quote name='alexclaber' post='1324104' date='Aug 2 2011, 12:27 PM']When it comes to the specifics of loudspeakers and acoustics I'm essentially self-taught. But I have a BEng from Bristol in Mechanical Engineering (and obviously the relevant A-levels before that) and the general principles of engineering apply directly to the specifics of loudspeakers.[/quote]

It's interesting, at least to me, that the behaviour of speakers from a physical perspective is a relatively mature subject, with the issues more in the application of that science through engineering. Whereas the science of hearing is much less developed, with lots of fundamental issues outstanding. Most designs are, ultimately, subjectively measured because even though we can characterise their output we can't completely model the ear/brain side of things. I think a lot of people don't actually understand the (blurred, but definitely substantive) difference between an engineer and a basic scientist. A scientist can understand the principles behind how a speaker works...but designing a good speaker based on those principles is another matter entirely requiring a different tool set.

Btw I'd certainly agree with Alex that the output advantages of a properly ported cab make them easily worth choosing over sealed units for high-power PA/bass cabs. I don't think anyone has claimed otherwise here. But it is possible to royally cock up a cab with haphazard porting, as ficelles has already discovered - and some problems will only manifest themselves as you reach higher volumes. Thinking back to your problem before ficelles, when you added the pipe it's possible you introduced a port resonance which would be a harmonic sitting above the actual port tuning frequency. This can sound a bit 'orrible.

[quote name='Bill Fitzmaurice' post='1323793' date='Aug 2 2011, 03:33 AM']But it would be incorrect to assume that this is knowledge restricted to the medical community. Acoustical engineering isn't limited to loudspeakers and acoustics. One of the major sub-fields is the hearing aid industry, where knowledge of how hearing works is paramount.[/quote]
I don't think what I said disagrees with this, though the hearing aid industry is not really very concerned with deep bass. One of the major aims of study for medical research into hearing is indeed generation of data that can be used to improve hearing aid design (though actually, according to my colleagues there is a often a bigger lag than you might expect between research publication and market product. Medicine is inherently conservative where possible). As for the home theatre buffs, I think for them it's not so much about hearing per se, they just like being vibrated you can certainly feel it when the floor is vibrating at 5Hz

I would be fascinated to know how many speaker design companies are actually employing or regularly consulting with researchers into audio perception. I'm sure it's more common these days but historically it is a subject that has been dominated by engineers and physical scientists.

Edited August 2, 2011 by LawrenceH