Polk Audio RT15i: Topless - Literally!

[RegularStereoGuy]

Not topless as in, can't be beat, but topless as in, no treble content unless I am in a straight line from the fronts of the speakers.

Off axis, even 30-40° off axis, and they sound like cotton stuffed in my ears. Is that a characteristic of 'monitors'?

I bought them used at Goodwill a couple years ago, and their sound has been otherwise satisfactory.

 

[dfuller]

They shouldn't fall off like you mention.

 

[Penelinfi]

"monitor" gets thrown around a bit.

The t15 showed some interesting response that wouldn't be helped with horizontally off axis listening.

Or maybe your tweeters are somehow not working well. Or perhaps you're sensitive to normal treble roll off?

 

[RegularStereoGuy]

"monitor" gets thrown around a bit.

The t15 showed some interesting response that wouldn't be helped with horizontally off axis listening.

Or maybe your tweeters are somehow not working well. Or perhaps you're sensitive to normal treble roll off?

Well they are used, so maybe, their best use is behind me?

In any case, what could I do to diagnose the tweeters?

Could it be an issue in the crossovers inside the cabinets?

 

[Penelinfi]

I guess damaged cap could cause issue, but usually won't happen unless you really thrash the speaker.

Ideally you would try to measure the speaker output.

Depending on experience you can listen and compare with known speakers or songs etc.

The speaker may be working normally and it just sounds muffled due to the dispersion, frequency response of crossover and driver design

When did you notice this lack of high frequency thing?

 

[Old_School_Brad]

Not topless as in, can't be beat, but topless as in, no treble content unless I am in a straight line from the fronts of the speakers.

Off axis, even 30-40° off axis, and they sound like cotton stuffed in my ears. Is that a characteristic of 'monitors'?

I bought them used at Goodwill a couple years ago, and their sound has been otherwise satisfactory.

You could test each speaker to check if they produce sufficiently high frequencies. This can be done as easily as using a ruler and the KRK Audio spectrum app while playing white noise at a low volume. Compare each speaker.

 

[RegularStereoGuy]

I guess damaged cap could cause issue, but usually won't happen unless you really thrash the speaker.

Ideally you would try to measure the speaker output.

Depending on experience you can listen and compare with known speakers or songs etc.

The speaker may be working normally and it just sounds muffled due to the dispersion, frequency response of crossover and driver design

When did you notice this lack of high frequency thing?

They've always sounded kind of 'soft' to me, rolled off, compared to most other speakers I've listened to or owned. Remember, they were, two years ago, already a 15 year old discontinued Polk model.

 

[Petrushka]

Not topless as in, can't be beat, but topless as in, no treble content unless I am in a straight line from the fronts of the speakers.

Off axis, even 30-40° off axis, and they sound like cotton stuffed in my ears. Is that a characteristic of 'monitors'?

I bought them used at Goodwill a couple years ago, and their sound has been otherwise satisfactory.

Get an app like dB or HouseCurve.

 

[RegularStereoGuy]

Here are two spectros I ran through KRK as suggested by one of you, thank you! Noise is wave files generated in my DAW.

Position: normal seated position in listening room.

 

[RegularStereoGuy]

Crickets?

 

[No. 5]

Position: normal seated position in listening room.

Since you are interested in the function/performance of your tweeters, I suggest running that test again but with your microphone a foot or so from the front baffle (and halfway between the tweeter and woofer). Most speakers for home use will have a downward sloping frequency response when measured at the listening position, and that makes it harder to judge if the tweeters are lower in level than they're supposed to be.

 

[Haflermichi]

If the tweeters are blown (open) all that acoustical testing will not help you.
Simply pull them out and measure them with a multimeter.
They may even be labeled with an impedance value. Check that. If they measure open, they're gone.

 

[restorer-john]

Tweeters are easily damaged. And they don't have to be open circuit to be damaged. Long term overdriving can warp/damage the voice coil and even result in shorted turns causing low output. They could also have dried out ferrofluid which turns to a 'goo' that damps the tweeter to the point you can hardly hear them. The more it dries out, the worse it gets.

 

[RegularStereoGuy]

Since you are interested in the function/performance of your tweeters, I suggest running that test again but with your microphone a foot or so from the front baffle (and halfway between the tweeter and woofer). Most speakers for home use will have a downward sloping frequency response when measured at the listening position, and that makes it harder to judge if the tweeters are lower in level than they're supposed to be.

 

[RegularStereoGuy]

The latest: White(img1994) and Pink(img1995) Pink shows a dip around 10-11kHz.

Captured from less than 1m from one speaker, other speakers muted.

 

[No. 5]

Based on that screencap (img1994) the tweeters appear to still be functioning. But take that with a grain of salt; when I tested the iPad that I have, its internal microphone showed a rise in response with frequency. That said, omni mics typically do not have undulations in their responses until you get up to the diaphragm's high frequency resonance, so that there appears to be a broad lack of energy centered on 3kHz is interesting. Maybe the tweeter caps are drying out?

 

[RegularStereoGuy]

The frequency anomalies could be the result of my aim.

Attached is a rough diagram of the circuit board inside of one of these Polk Audio RT-15is

At the bottom, obviously, are the back sides of the speaker terminals (red and black).

Above that is some kind of copper coil about the dimensions of a cigarette butt.

Dead center is a sole mounting screw.

Upper RH is a white piece of circuitry.

And mid-left blue circle is a capacitor, flush-mounted to the PC board with a raised edge around
where it is supposed to go. Capacitor dimensions about 1/4" diameter and 1/2" tall, installed.


When I feel more brave about it, I will remove the entire assembly and report back a set of values on that dark blue
cap, and maybe a mfg's name.

 

[kemmler3D]

Crickets?

If the white noise measurement was taken close and on-axis this does look like something is wrong with the speaker. The treble above 3khz looks to be about 10dB below the midrange where it should be even. To me that says driver damage or crossover out of whack.

If that's the case, you would hear soft but maybe usable treble in normal use (10dB deficiency can be acclimated to, sort of, if you don't listen to anything else very often) but off-axis you're going to be losing another 6-20dB depending on frequency, at which point the treble is probably getting masked by other stuff, i.e. you're totally losing audibility and you can no longer ignore it.

How much were the speakers? Do you have any other properly working ones you could compare these to?

 

[RegularStereoGuy]

If the white noise measurement was taken close and on-axis this does look like something is wrong with the speaker. The treble above 3khz looks to be about 10dB below the midrange where it should be even. To me that says driver damage or crossover out of whack.

If that's the case, you would hear soft but maybe usable treble in normal use (10dB deficiency can be acclimated to, sort of, if you don't listen to anything else very often) but off-axis you're going to be losing another 6-20dB depending on frequency, at which point the treble is probably getting masked by other stuff, i.e. you're totally losing audibility and you can no longer ignore it.

How much were the speakers? Do you have any other properly working ones you could compare these to?

Got them at a Goodwill for less than $50 for the pair.

Most speakers I've listened to, before or since, have far more treble content, possibly even exaggerated.

If re-capping is all that is needed, I'd be willing to take that chance on them.