Polk T15 Bookshelf Speaker Review

[pma]

I think it might be interesting to show that we might even read distortion from the burst recordings (top - voltage at speaker, bottom - acoustical output). Speaker distortion is much lower with bursts than with steady state sines. The amplifier does not care. Voltage was 21.42Vrms at the burst peak amplitude.

 

[infinitesymphony]

Flipping the polarity of the tweeter would produce better results.
If you tested only one speaker it may be that there was an error in production where they connected the tweeter out of phase, it happens sometimes with cheap speakers and no QC..

edit: though probably not worth your time and effort to retest.

Yes, when a tweeter is crookedly mounted and possibly wired incorrectly from the factory, those are data points about quality too.

I find it really sad that 2 of the budget lines that emphasized flat frequency response, Boston Acoustic and Infinity, are essentially discontinued.

Were they focused on flat response? I would like to think so, as at one point I was a BA fan. I found their modern VR speakers to be a little on the bright side. Wouldn't mind seeing anything with the Lynnfield aluminum dome tweeter tested.

 

[Thomas savage]

My last post before I step out of this absolute disaster of a thread.

Low cycle count tone-burst has been used for many decades, as it is non-destructive (mostly) and yet will determine the limits of speakers quite easily. 1.5kW is nothing into a medium sized speaker on a single or even several cycle tone-burst. It's all about duty cycle and effective power.

As for the rest of your post, I'll just leave it.

There's really good reasons many of us have decided to totally ignore the speaker "reviews" on ASR going forward.

 

[Blumlein 88]

I wonder what the FR would look like if the tweeter phase was reversed (or "unreversed" if it is already)?

I wonder what the FR would look like if we replaced the drivers with something better? Come on guys, this is a measurement based review, but not an old issue of Speakerbuilder. I think asking things like this is a bit much. Plus you are what the 15th fellow to ask for it or something. Plus I've been mystified at the volume of responses and interest in what was a bottom of the barrel speaker which performed like it. The point of measuring this is if you stumble across an unexpected gem. But when you get poor results no surprise, another data point and move on.

 

[DSJR]

I also commented on the possibility of the tweeters being out of phase, but then, I barely look on Audioholics which many of you people seem to know. The review aluded to there - and Amirm's comments, indicate the performance of this speaker is deliberate and intentional for it's given lowly market, where they're likely to be set either too low or rather to high on the wall, in which case they're not so bad. As with the *original* Harbeth M30 recently tested here, there is a back story as to why they do what they do which was rendered irrelevant as time (and the BBC itself) moved on to Dynaudio and Genelec for the most part I gather and, as a result of this, this design broke free of the BBC shackles, was more 'normalised' in balance and began to fly more universally. Polk isn't so well known in the UK, but I wonder if larger more expensive models they do wouldn't test rather better? I always remember old ads for them I saw, showed a youngish man in white lab coat fussing round them, so maybe they've always tried to present a more 'techie' image your side of the pond?

I must ask please, why the predilection for so many beer-budget boxes? Is this what people are buying en masse these days (there's barely a market left in the UK for this stuff unless it's recently discontinued old models sold via our 'Richer Sounds' chain - Diamond 220's which 'sound' basically very nice sold for £99 or so - half original price I think).

Too far for outsiders to send stuff, but I hope Amirm can get hold of more expensive speakers to test out as time moves on.

 

[amirm]

Flipping the polarity of the tweeter would produce better results.
If you tested only one speaker it may be that there was an error in production where they connected the tweeter out of phase, it happens sometimes with cheap speakers and no QC..

edit: though probably not worth your time and effort to retest.

Again, my measurements match that of James Larson. So there is no reason to doubt assembly.

 

[MZKM]

Again, my measurements match that of James Larson. So there is no reason to doubt assembly.

And AverageJoeAudiophile:

 

[Putter]

Yes, when a tweeter is crookedly mounted and possibly wired incorrectly from the factory, those are data points about quality too.


Were they focused on flat response? I would like to think so, as at one point I was a BA fan. I found their modern VR speakers to be a little on the bright side. Wouldn't mind seeing anything with the Lynnfield aluminum dome tweeter tested.

Boston Acoustic A25 from Stereophile. I'd say within +/- 3db to (60 hz - 20khz) although one could note an upper midrange (BBC? dip) and high treble emphasis.

From Sound and Vision
The A 25’s listening-window response (a five-point average of axial and +/–15-degree horizontal and vertical responses) measures +3.20/–2.55 decibels from 200 hertz to 10 kilohertz. The –3-dB point is at 73 Hz, and the –6-dB point is at 58 Hz. Impedance reaches a minimum of 5.07 ohms at 235 Hz and a phase angle of –45.60 degrees at 111 Hz. It's available as a refurb for $145 vs. Amir's $70 and more typically $100 or so for the Polk. I'd hesitate to recommend testing either BA or Infinity because they're essentially being discontinued. PSB might be a better candidate given that they do seem to have budget speakers that measure well.

 

[BYRTT]

A message for all those member suggestions that @amirm should flip polarity of tweeter there is technically reasons to forget it and @pma told in post 130 quoted below ...

Technically explanation is:
1. For symetrical slopes of odd order a polarity flip will not change anything on amplitude only system phase will change.
2. For symetrical slopes of even order a polarity flip will create a clean deep more than 30-40dB interference.
3 For asymetrical slopes of odd or even order that is engineered to sum perfect a polarity flip will probably either perform as in point 1 or point 2.

Conclusion: interference for T15 is so mild a 10-15dB nature its outside above technical facts so will mean pma's message below is correct that filter is technically too low cost and oversimplified so a polarity flip will do nothing in amplitude domain or transform issue to any other region.

It looks like crossover design problem. Probably oversimplified crossover circuit. Inverting tweeter phase would probably not fix the issue, but just transform it to another issue.

 

[AudioSceptic]

I wonder what the FR would look like if we replaced the drivers with something better? Come on guys, this is a measurement based review, but not an old issue of Speakerbuilder. I think asking things like this is a bit much. Plus you are what the 15th fellow to ask for it or something. Plus I've been mystified at the volume of responses and interest in what was a bottom of the barrel speaker which performed like it. The point of measuring this is if you stumble across an unexpected gem. But when you get poor results no surprise, another data point and move on.

Excuse me. I didn't realise I was the 15th, or whatever. So, what is the answer to my stupid question?

 

[BYRTT]

@AudioSceptic look post 169 there is technical reasons when interference is not a clean deep >30-40dB number its not a polarity fault ab fabric but probably a consequence of a low cost product.

 

[pma]

@AudioSceptic look post 169 there is technical reasons when interference is not a clean deep >30-40dB number its not a polarity fault ab fabric but probably a consequence of a low cost product.

Yes. Below is the inverted polarity issue.

 

[ROOSKIE]

Excuse me. I didn't realise I was the 15th, or whatever. So, what is the answer to my stupid question?

Hey, don't worry friend. This has been asked a lot but the answer is up for debate. Polk claims it was intentional. People are still upset and don't quite buy Polks claim because it is not in the manual. Fact is from my honest point of view I believe Polk and while I am not a fanboy at all, I have found their gear is engineered with intent. Most owners manuals suck. Most speakers have a design philosophy, that philosophy is not always (hardly ever in my experience) well described in the manual.
In a nutshell they sound better above or below the vertical axis, unlike most "monitor" style speakers. Most monitor style speakers sound much worse above your head, these sound better this way. Makes sense to me. These are for casual users to put up on a bookshelf and then they listen to background music while they still well below tweeter height. If you want to do that these are great speakers, that cost almost nothing.

Quoted from Audioholics
"
I contacted Polk Audio about this behavior and asked them about the intent of height placement regarding this response.
Their answer, from Stu Lumsden, VP of engineering:

​

We recognized that bookshelf speakers at this price are rarely placed on tall stands that position the speaker at the listener’s ear level. Usually the speaker is placed on furniture that results in positions slightly above or below ear level. And, as Audioholics astutely observes, this small speaker is a prime choice for surround channel and will likely be above ear level.

​

Let's emphasize here that this behavior was a deliberate design decision by Polk and not an accident or unintended side-effect. Let’s compare the horizontal dispersion at tweeter height versus horizontal dispersion at an angle 20 degrees below the tweeter to better see the consequences:"

 

[JustIntonation]

Didn't read yet that others had measured the same crossover dip. So it's indeed a very bad designed speaker.
Flipping the polarity would though give a better response unlike what some people wrote here.
Yes if you give a perfectly time alligned crossover a polarity flip you will get a null so it's not completely flipped in this speaker. But what some may not realize is that halfway from a null and a perfect flat response is only a -3dB dip for a Linkwitz-Riley crossover. The on-axis dip for this particular speaker will be much smaller than the -15dB that it has now if you flip the tweeter polarity as currently it's fairly close to a complete null axis on-axis right now.

 

[tktran303]

Justintonation,

this is the thingwhat I’m trying to get at.

Hours (cumulative) spent debating and theorising and juxtaposition and miscommunication or potentially insulting someone.

All would/could have been resolved or avoided if Amir had help with; or could delegate someone to unscrew only 3 screws holding in the tweeter and and switching the tweeter cable. Then a single on-axis re-measure would settle it once and for all.

Maybe it an issue with this Klippel NFS? After the on-axis measurement, Does it just churn through and keep doing all the measurements, as a batch process, even if there’s a gross problem eg. Kali IN-8 (damaged woofer)?

I guess these robotic machines can’t replace humans (just yet)

Here’s an example of alternative process of publishing, from one of my favourite reviewers
http://noaudiophile.com/Vanatoo_Transparent_One_Encore/

 

[BYRTT]

Didn't read yet that others had measured the same crossover dip. So it's indeed a very bad designed speaker.
Flipping the polarity would though give a better response unlike what some people wrote here.
Yes if you give a perfectly time alligned crossover a polarity flip you will get a null so it's not completely flipped in this speaker. But what some may not realize is that halfway from a null and a perfect flat response is only a -3dB dip for a Linkwitz-Riley crossover. The on-axis dip for this particular speaker will be much smaller than the -15dB that it has now if you flip the tweeter polarity as currently it's fairly close to a complete null axis on-axis right now.

Ha ha shall we bet you can flip to eternity agree its technically bad design but it cost almost nothing and in raw material that nothing was concentrated on looks, in reality imagine that woofer run up to 11,9kHz where on axis response plus DI makes a dip, then a super tweeter for the rest, we can ascertain at amirm's impedance plot that port is tuned 60Hz, now extract something ala that and lets say thats a ironcore inductor introducing a 6dB tilt at 200Hz to massage woofers nice breakup bells then we get a estimate for woofer in the brown curve.

 

[amirm]

Hours (cumulative) spent debating and theorising and juxtaposition and miscommunication or potentially insulting someone.

All would/could have been resolved or avoided if Amir had help with; or could delegate someone to unscrew only 3 screws holding in the tweeter and and switching the tweeter cable. Then a single on-axis re-measure would settle it once and for all.

Maybe it an issue with this Klippel NFS? After the on-axis measurement, Does it just churn through and keep doing all the measurements, as a batch process, even if there’s a gross problem eg. Kali IN-8 (damaged woofer)?

It takes the same effort to re-test this speaker that it would to measure another one. Actually more since you want me to open and modify it first. The robotic system needs reconfiguration for every measurement which means 15 minutes in a cold garage.

My best throughput is one review/measurement a day. There is no more to be had. Any repeat work costs review of other gear which has no data for which its owners are hammering me to test. The couple of electronic bits I tested recently have been sitting here as much as two months!

 

[amirm]

Spin data added to the review.

 

[Bear123]

I think the argument that this speakers performs better off axis does not hold any water. I think at this point in the game, we should alll realize that a speaker should have both good on and off axis response in order to qualify as a well designed speaker and to have good overall sound quality. If you want to argue that being off axis is the correct listening angle and gives you a better response, this then changes the incredibly bad on axis response, to an incredibly bad off axis response, which, either way, makes for a poorly designed speaker that will not sound good! Of course no one expects SOTA measured performance from a $70 speaker, but we know the AJ speakers can be picked up new all day on e-bay and various places for basically the same price. Even at $100 for the Pioneers, there is no reason to pay $70 for junk when $100 gets you darn good performance.

So basically, if someone has $100 budget, get the Pioneers. And if you only have $70, go mow someone's yard and get the Pioneers.

 

[Bear123]

It takes the same effort to re-test this speaker that it would to measure another one. Actually more since you want me to open and modify it first. The robotic system needs reconfiguration for every measurement which means 15 minutes in a cold garage.

My best throughput is one review/measurement a day. There is no more to be had. Any repeat work costs review of other gear which has no data for which its owners are hammering me to test. The couple of electronic bits I tested recently have been sitting here as much as two months!

I agree completely, don't waste any more time on this one. I'm interested to see the Ascend....see what all the hubbub is about the Raal tweeter. If I get around to finishing assembly, I have a $100 DIY design that I'm hoping will beat the Pioneer in a much smaller design that would be perfect for desktop. (Jeff Bagby's Quark)