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In Reply to: RE: Rear port pros and cons (not an unbiased opinion) posted by theaudiohobby on November 18, 2007 at 03:44:52
Dear TAH,
Your statement that a rear firing port makes the speaker more room sensitive with more ill effects is simply incorrect, why would it do that??
Our experience is that as you move the speakers closer to the room boundaries the effects of the room are reduced rather than the opposite and this applies whether the port is at the front or the back of the speaker, which as it happens that is also concurs with what most of the 1930's research that I have seen.
Sincerely,
Peter Qvortrup
Follow Ups:
"Our experience is that as you move the speakers closer to the room boundaries the effects of the room are reduced rather than the opposite..."
Let me see if I understand this (for although I use rear-firing ports, I don't claim to know as much about them as you do).
It seems to me that the acoustic environment up close to the room boundaries will be more similar from one room to the next than will the acoustic envioronment out away from the room boundaries. So, placing the port on the rear of the enclosure will put it as close as possible to the room boundaries with proper placement, and therefore give you more consistent low end from one room to the next. Is this correct?
Not sure if you noticed, but elsewhere in the thread I acknowledge that I'm copying you in using rear porting. By any chance have you considered making the port length user-adjustable, for applications where the user cannot take advanage of the corner placement your speakers are tuned for? For instance, a 3" diameter modular flared Precision Port with an overall length of about 9 inches would tune the AN-E to 29 Hz, which I think is its normal tuning frequency. By removing the center section the tuning is raised to about 34 Hz, and by using only the outer flare the tuning is raised to about 41 Hz. The sections can be held together with a single wrap of electrical tape instead of glued together, and the port itself affixed to the enclosure by four small screws - so it's user-adjustable. Hey I'm using your idea - the least I can do is offer you my variation on the theme.
Best wishes,
Duke
Dear Duke,
There are really two issues here,
A.) The placement of speakers close to room boundaries
B.) The subsequent use of the boundaries for bass reinforcement
All the early research I have seen from Western Electric, RCA, Klangfilm and others plus later work done by Philips (the early corner speakers), Klipsch, Voigt & Chave, Allison and even Beveridge, point to the fact that sound waves being propagated from a corner or close to the room boundaries have a far more similar behaviour in different rooms than if the speakers were placed away from the walls in the same rooms, admittedly this work was done in the early days of cinema, but there are no reasons why it should not apply to someone's living room, the laws of acoustics haven’t changed since then to my knowledge, and in my experience this still applies.
Just by way of one example, I took part in an experiment that was sponsored by Bruel & Kjaer, at the main Danish engineering university at Lundtofte in 1981, where 4 pairs of speakers were placed in different rooms in various positions, then measured and listened to, we used a pair of Quad ESL63s, B&W 801, a pair of JBLs the model of which escapes me and a pair of SNELL Type A/IIs.
By a not insignificant margin, the general consensus was that the SNELL A's where least affected by the different rooms when close to the walls and was, by a lesser margin, preferred overall, both on the basis of the measured performance and by the listeners, followed by the Quads, the 801s and the JBLs.
Just for the record, the A/II's are floor loading the woofer, a fact which was considered to be a major reason for their good general in room performance, their excellent dispersion was another.
Our experience clearly demonstrates that provided the speaker has a good, wide and even dispersion window, meaning it projects an almost even hemispherical waveform into the room (something which modern narrow baffle speakers do not, which easily explains why they do not work close to room boundaries) then near wall and corner placement even with a sealed box markedly improves the room to room consistency and general behaviour, I see several reasons for this,
1.) When a sound source (read, a speaker) is placed close to the room boundaries the frequency content of the signal is reflected in a more predictable and consistent manner from room to room, which greatly reduces room interference. When the speaker is close to the side walls it is not possible for the ear to separate the reflected and direct sound from each other, and results in a great reduction in room variability.
2.) What the near side wall placement also does is increase the in room efficiency of the speaker, this is because the off axis energy is now added to the overall sound output and because the frequency energy from a speaker which has good wide dispersion is almost the same at say 30 and 60 degrees, this energy does not alter or skew the overall frequency response, a fact which also explains why speakers with narrow baffles and deep cabinets do not display this ability even if the port is moved to the back of the cabinet, the dispersion is simply too uneven off axis.
3.) Having the port in the back, and dimensioning it to take into consideration the additional loading available from the rear wall/corner/floor (the floor by placing it close to the bottom of the speaker cabinet) not just improves the low frequency extension by up to 18dB, it also allows an unprecedented degree of adjustability of in room bass response. The speaker pressurizes the room in a completely different way in the corner position using this technique when compared to the way a woofer in even a much larger cabinet with either a front facing port or a sealed cabinet
All in all there are many good reasons why near room boundary placement enhances the performance of a speaker designed to be used in this position, so whilst I can see some reasons for why the audio industry standard measurement require free field on axis measurements I believe these were chosen out of convenience and for the sake of having an easily repeatable method, but I think it is overall a no less misleading way of “proving” speaker performance because we all know (and so did the engineers who originally decided on the anechoic measurement technique) that all speakers end up in a room with four walls and floor and a ceiling, so why did they not establish a technique which better reflects that?
My view of this is that as with many other bench mark tests of audio performance that what was sought was not the best way of getting a real measure of performance, but the easiest way of measuring something that could be used as a marketing tool, and off course because the overwhelming majority of speaker manufacturers use this measure to design their speakers they have no interest in rocking the boat or designing speakers which do not conform to the standard tests, because it would mean comparing poorly with their direct competitors, but does the fact that most speaker manufacturers uses the same method to design and measure their speakers mean that this method is the best available or produces the best end results??
If it not the job of the magazines to question the practices of the industry they write about, rather than pander to its need to sell its products, what is their job??
In my view it is the main job of the press to criticize and point out poor practice and that applies not just to audio (may I say here that many brave and committed journalists die every year exposing deplorable practices in many parts of the world and their sacrifices should be recognised and are not remotely comparable to the minor transgressions of their audio brethren), but magazines like Stereophile and I should say for the record that Stereophile are by no means the only or the main culprit here, should be pointing out the weaknesses in the methods used by the industry they are there to be policing on behalf of their readers in the hope/expectation that it would encourage the industry to improve its processes and as a result its product over the longer terms.
I just don’t see this happening, not in the 30 odd years I have worked on my "project", the magazines are almost greater slaves to convention than most manufacturers, they jump every new bandwagon almost before any manufacturers have had time to embrace it, thus forcing most larger manufacturers to embrace technology which in many cases turns out to dud wasting vast amounts o their readers money in the process, not really a commendable record is it?
Wolves should not be game keepers and perhaps this lack of ability/willingness to criticize explains the slow death of most of the printed magazines as readers turn their back to them and start looking elsewhere?
Anyway, enough of this hope it put a little more information on the bone so to speak!
Sincerely,
Peter Qvortrup
It seems to me that the acoustic environment up close to the room boundaries will be more similar from one room to the next than will the acoustic envioronment out away from the room boundaries.I do not believe that this is not borne out in practice, more correctly rooms of dissimilar dimensions will have dissimilar acoustic environments and vice versa. To give an extreme example a room measuring 12 * 18 * 7 room and one measuring 30 * 25 * 20 will have vastly disimilar acoustic xteristics even along room boundaries, specifically the eigentones even along the room boundaries will be vastly different as it is dimension dependent. I can think of a few reasons why having a speaker flush to the wall i.e. on a plane or a close approx, might be beneficial owing to diffraction effects, but that is whole 'nother issue.
Music making the painting, recording it the photograph
Oh I definitely understand that different rooms have different acoustic characteristics, especially in the bass.
I've had the exact same speaker be boomy in one room and then anemic and wimpy in another, when the room dimensions were within about 20% of each other and the relative placement within the rooms as similar as possible, but not close to any walls. In this case, re-tuning the speakers for corner placement and placing them in (or very near) the corners resulted in more satisfactory and consistent bass in both rooms.
What I'm saying is that I think the low-frequency presentation would vary less from one room to the next if the speakers were consistently placed where they'd recieve a lot of boundary reinforcement (like in the corners). This is based on too few observations to draw a definite conclusion, hence my asking Mr. Qvortrup - whose experience with such placement vastly exceeds my own.
On the other hand in my experience soundstaging is more three-dimensional when the speakers are moved out away from the walls, so like most things in audio apparently there are tradeoffs involved.
Duke
A actual citation from that period that I can get hold of in the national library will help and clarify where you are coming from.
Music making the painting, recording it the photograph
Dear TAH,
From my recollection it was not I who made the unsolicited claim about rear ported designs' it was you, so why don't we start by you providing the required references as to why rear ported designs have more room problems and other acoustic ill effects and spend some time analysing that material first, we can then move on to related matters.
I am perfectly happy to provide material to prove my end of the discussion once I have had an opportunity to see the material that backs up your claim.
Sincerely,
Peter Qvortrup
I should have guessed....just another day on the salesman trail...here was I thinking that you will use the opportunity to provide some clarification.Music making the painting, recording it the photograph
Wait you said this:
"but it comes at significant price, the speaker is much more placement sensitive with the associated ill-effects."
I have never heard this from any source except you. Why is it so hard for you ONCE to simply state a book and a page number where you got this information from? You stated it BEFORE Peter made a post.
You made a CLAIM FIRST
He said he would give you the information IF you could simply back up your claim. You are clearly the one weaseling out of it and the fact that the other poster is a manufacturer does not change anything.
You made a claim that should be very easy to back up since apparently you are an acoustics engineer wannabe so why is it so hard now.
I mean I asked you to show me how a horn speaker and a panel were more the same than they were different -- surely you could back up that claim with references but alas BS as usual.
Dear TAH,
Guess what, you are just another arm chair general with no army.
When you make a claim for which there appears to be little or no justification the burden is on you to bring something to the table that backs up what you say, so how about bucking up and backing your negative comments and claim about rear ported designs up, rather than, as usual when challenged, trying to take the fight to someone else's yard to divert attention to the fact that you have nothing by way of support for what you say?
TAH, we have been here before and this is your standard tactic, you make some unsolicited claim negating some aspect of a design, but without even the slightest scrap of evidence to back up what you say, and then you expect me or someone else to take what you say serious?
Sincerely,
Peter Qvortrup
We were actually having an intelligent discussion until joined the thread to offer a sales pitch.
Music making the painting, recording it the photograph
Okay you have now made another claim.
Please show the sale's pitch?
You slag a design with zero evidence but he's at fault and making a sales pitch. What weird little lonely life are you living.
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