How should I tune up Solstice MLTL?

[kotofei]

Hello,

Recently, I built a pair of Solstice MLTL speakers using Parts Express kit. Speakers did produce sound; however, it was boomy at some frequencies. In addition, I had an impression that bass is wear and generally speakers are not "sweet sounding" with some music such as guitar and jazz.

This is my first speaker built, so I asked a friend who is much more experienced in this craft for help. He said they should be tuned up but first, an impedance measurements should be done.

We did this today, both for assembled cabinets and drivers taken out of them. Input was a signal from a generator at 1V RMS or 3V RMS, a 1kOhm resistor was connected in series with the generator and a True RMS Triplett 9045 DMM was used to measure voltage at speaker or woofer terminals.

What we got for assembled cabinets is a pronounced peak at 70 Hz that gradually goes down with decreasing of the input frequency up to 40 Hz and then a second, smaller peak at 26 Hz. Results are attached as EXCEL files and are virtually identical for both speakers.

For individual woofers, Morel TiCW 638 Nd, results were different if a woofer was measured in a cabinet or removed from it. In the first case, we found resonant frequency at 50 Hz; for the driver alone, it was approximately 60 Hz. Measurement results are also attached.

Now the question is: how to tune up the Solstices to lower the 70 Hz peak which is most likely is the reason for "boominess" and increase output at 40 Hz. A friend of mine said it might be very difficult if not impossible since the resonance frequency of the Morel woofer alone is 60 Hz.

I would be willing to try any and all suggestions since in the current state speakers are just not working right.

[djg]

How did you stuff them? Did you use any sound treatment other than the Acousta Stuf included with the kit?

[Wolf]

What you are seeing is fairly normal. This indicates a tuning of about 40 Hz, and a driver's Fs resonance will shift up in a cabinet vs alone.
This is normal,
Wolf

[Jeff B.]

Your measurements are representative of the Solstice design, which is a Mass Loaded Transmission Line design. The cabinet should be heavily stuffed in the top half and empty in the bottom half for best bass performance. However, you should do a little study on impedance curve and box tuning and what it all means, as an impedance peak at 70 Hz in no way indicates that there will be a peak in the response at that frequency. The actual Helmholtz tuning of the Solstice is a fairly low, at about 36 Hz, and this is where line output should be its strongest. It sounds more to me like you may be dealing with a room mode issue if you are getting higher midbass peak. You may want to trying playing a little with speaker placement.

[Paul K.]

Your impedance measurements shows clearly what Jeff described and is typical of virtually all TL configurations and typical of vented boxes. There will be two impedance peaks with the lower-frequency peak having a smaller magnitude than that of the higher-frequency peak, and the lowest impedance magnitude in the "valley" between these two peaks occurs at the system tuning frequency. Jeff's suggestions that you confirm how you stuffed the box and the possibility of a room mode issue are spot on. I have a room mode in the 100-125 Hz range that's excited by only the right-channel speaker, and when there is substantial content in the music in that frequency range, boomy bass often results.
Paul

[Paul K.]

To further illustrate I've attached 3 modeling graphs from an ML-TL design I'm currently working on. In the first graph the red line is the predicted system bass response in an anechoic environment (no room effects) and while ignoring baffle step losses. The red line in the second graph is the impedance characteristic, and in the third graph the red line is the response from the woofer with the blue line showing the response from the mass-loading port. There are differences specific to this design in comparison to Jeff's Solstice. The system tuning frequency is ~31 Hz, which is visible by the lowest impedance point in the valley between the two impedance peaks in Graph 2 as well as the large dip in the woofer's output at the same frequency in Graph 3. The red line in the first graph, though, clearly shows no peak in the system's output corresponding to the impedance peak that occurs at ~57 Hz.
Paul

[kotofei]

[Jeff B.]

Are you able to measure the Thiele /Small parameters of the woofers? Maybe the Qts is higher than spec on your woofers.

[kotofei]

Perhaps, need to look at what's required. I have generator, Oscope and stuff like resistors, caps etc.

The only thing I measured is Fs for woofers taken out of the box and it was around 57 Hz for TWO woofers measured. The manual specifies 45 Hz for "small signal" and 33 Hz for 1 V. Since woofers were fed with 1V RMS through 1 kOhm resistor the Fs should be close to "small signal" value and it isn't.

The other two are soldered in so I didn't do them.

Are you able to measure the Thiele /Small parameters of the woofers? Maybe the Qts is higher than spec on your woofers.

[Paul K.]

The impedance peak at the higher frequency of 70 Hz is not causing the louder sound at that frequency.. The third graph I posted is typical of all vented boxes and TLs (expect TLs that have been intentionally over-stuffed to make them perform like a sealed box by flattening the lower-impedance peak which eliminates all bass contribution from the port/terminus) because they are all inherently 4th-order designs. The port's output contributes to the total system output while, at the same time the woofer is generating less output, and when both outputs are correctly combined in phase and magnitude, the system output is as shown in my first graph. The mass-loading port is not intended to "compensate" for anything other than to provide bass reinforcement over the frequency range of its intended effect, while letting the woofer do less work, with the peak in the port's output coinciding with the valley in the woofer's output (called the minimum motion frequency of the woofer). Your Solstice impedance measurements pretty much show what Jeff described, a system tuning frequency of ~36 Hz, and since you apparently installed the Acousta Stuf as required, I would suspect your Solstice speakers are performing as designed.
Paul

Thank you everyone for suggestions and comments;

Regarding stuffing of the boxes: I used one pound of Acousta Stuf per box, included with the kit. I stuffed only the top half of the box, leaving the bottom half empty (found Jeff’s comments on a subject in a different Solstices thread and followed them). I didn’t use any other stuffing.

I’m happy to know that my measurements are pretty normal and representative of the Solstice design. However, the problem is not in measurements: after all, we don’t listen to impedance.
Jeff noted that “an impedance peak at 70 Hz in no way indicates that there will be a peak in the response at that frequency”. Unfortunately, I hear it.

THE 70 HZ PEAK IS AUDIBLE IF I MOVE MY EARS CLOSE TO WOOFERS.

During tests, I moved each speaker from its “regular” position to the middle of the room and listened to it during tests. What I heard was a relatively strong tone from a woofer at 70 Hz and much weaker tones lower and higher than it!

Another reason why I don’t think the high midbass peak is a room mode issue is that my old speakers, Vandersteen 2CE Signature didn’t have any boom at this frequency although they occupied same position.

Paul, the third graph in your post suggests the low point in response from a woofer could be corrected with the broad peak of approximately the same frequency from the mass-loaded port. Is that true? Is this a way speakers are tuned?

In this case my mass-loaded port apparently doesn’t compensate for the 70 Hz peak in the “normal” or “listening” speaker position.

Is it a way to measure the speaker response other than purchasing a fancy microphone and software, such as with some I-Phone app?

I probably need to make such measurements and then post them here. Something has to be done with the speakers; they do many things well and we like their appearance but listening to them now is just not pleasant and sometimes unbearable.

[kotofei]

The impedance peak at the higher frequency of 70 Hz is not causing the louder sound at that frequency.. The third graph I posted is typical of all vented boxes and TLs (expect TLs that have been intentionally over-stuffed to make them perform like a sealed box by flattening the lower-impedance peak which eliminates all bass contribution from the port/terminus) because they are all inherently 4th-order designs. The port's output contributes to the total system output while, at the same time the woofer is generating less output, and when both outputs are correctly combined in phase and magnitude, the system output is as shown in my first graph. The mass-loading port is not intended to "compensate" for anything other than to provide bass reinforcement over the frequency range of its intended effect, while letting the woofer do less work, with the peak in the port's output coinciding with the valley in the woofer's output (called the minimum motion frequency of the woofer).

Your Solstice impedance measurements pretty much show what Jeff described, a system tuning frequency of ~36 Hz, and since you apparently installed the Acousta Stuf as required, I would suspect your Solstice speakers are performing as designed.
Paul

If this is so AND if it's nothing I can do to dump this 70 Hz sound, it's very sad. This speaker odyssey would be my biggest flop since 8 or so years in the audio hobby, both in terms of broken expectations and money/time invested.

I wonder if Parts Express has any "money back" policy.

[corradizo]

If say get a mic or at least an spl meter and measure the speaker to plot it's response. Impedance doesn't indicate how high or low a frequency is played. It's just telling you how power hungry your speaker is at a given frequency. That kit is not cheap, another 100 on a call calibrated mic would be a worthwhile investment imo. YMMV.

[Navy Guy]

Perhaps, need to look at what's required. I have generator, Oscope and stuff like resistors, caps etc. The only thing I measured is Fs for woofers taken out of the box and it was around 57 Hz for TWO woofers measured. The manual specifies 45 Hz for "small signal" and 33 Hz for 1 V. Since woofers were fed with 1V RMS through 1 kOhm resistor the Fs should be close to "small signal" value and it isn't. The other two are soldered in so I didn't do them.

How long have you had the speakers? Did you perform any break in?

[isaeagle4031]

If this is so AND if it's nothing I can do to dump this 70 Hz sound, it's very sad. This speaker odyssey would be my biggest flop since 8 or so years in the audio hobby, both in terms of broken expectations and money/time invested.

I wonder if Parts Express has any "money back" policy.

How did you determine this resonance at 70Hz? As Jeff said, I suspect you have a room node that is being excited by the speakers placement in the room. Close to a wall and/or corner is very likely to cause this.

[Jeff B.]

If this is so AND if it's nothing I can do to dump this 70 Hz sound, it's very sad. This speaker odyssey would be my biggest flop since 8 or so years in the audio hobby, both in terms of broken expectations and money/time invested.

I wonder if Parts Express has any "money back" policy.

I assure you that the Solstice was designed for the flattest most extended response from these woofers. There was no peak in the system measurements in my samples as I designed and built them. I also was able to listen to another pair this past spring in Fort Wayne and they sounded correct in the bass too. Your only measurement that you have provided is an impedance sweep, which essentially matches the impedance I published for the speakers. This would indicate that the bass is performing as designed and I would not expect any peaks in the response. I hate to belabor this point, and I don't know if you have guessed at the peak being at 70 Hz or if this is pretty accurate in terms of frequency, but this is a problem frequency for a lot of rooms since it corresponds to the standing wave between the floor and ceiling in a room with 8' ceilings. A lot of systems and rooms have a 70 hz peak. Maybe the fact that the Solstice can provide clean 30 Hz output is energizing the room more than your previous speakers did. I don't know. I can only say that the impedance plot you provided would lead me away from the speaker as the source of the problem. I'm not just defending my design, but I am honestly going by what you provided.