Measurement question

[horatio2]

I am making measurements on my Khorn bass bins after having made some mods, and these measurements are being done outdoors, as I have brought the bins to the garage for the mod work, and bring them out into my yard for measurement.

 

Only one audible mod, here, in these measurements: I enclosed the sides with 'wings' similar to the last production model Khorns.  These are Speakerlab builds.  I have opened the throats from 3x13 to 5.5x13, but right now am running the 3x13 motorboards with K33Es.  So, I don't think throat issues are in play here.

 

I have used REW in the past, in my home listening room for measurements, and frankly, room resonances have always made this pesky, and this is why I wanted to make the measurements outdoors.

 

I have also measured the resonant frequencies of both bins, and these measure right around 42-43 Hz, which was a bit of a surprise for the K33E's.  I can detect evidence of what is likely sealing issues with one of the bins, and you can see this in the slightly different frequency responses as well (probably).

 

I have, in order to get decent signal to noise, run the sound levels up quite a bit, and I believe I am in a workable sensitivity range; levels are good, not clipping.  I am sure my neighbors are wondering what the hell I am doing, as I sweep those bins.... 

 

My question is, I see really interesting bass performance in these measurements:  have a look at the response curves for each bin in the attachment.  They show an unexpected behavior in the 20-40 Hz range.  And I can't really explain that too well.  I can tell you that I (surprisingly) hear output from these bins in the mid 20's of Hz, and it really gets strong by about 35-37 Hz, and proceeds along.   I can clearly see the mic picking up output when the bins are excited at the beginning of the sweep, and I can hear output as well.  So it seems to be there.  I just expected a rolling off starting around 38-42 Hz, and just the opposite is happening (and I don't think it is real, in spite of hearing output).

 

For those who've made these measurements in the past on their bins or on other systems, how do these look to you?

 

I am not sure I trust the numbers under 35 Hz.

 

Microphone is a Radio Shack analog meter (the later model), REW correction file applied, Behringer UAC 202 ADC/DAC, calibrated in REW.  Serviceable equipment for what I am doing, but clearly not lab quality gear.

 

I will be undertaking a similar measurement upon fitting of the CB158 loaded 6x13 motorboards, including measuring the resonant frequencies of the bins.  Probably should break those in a bit before making the runs, but we'll see.

 

I plan on uploading a full summary of the measurements for inspection and comment when these are completed.

 

-M

[Don Richard]

Microphone placement issues? Probably best to put on ground, but try different locations.

Edited October 24, 2015 by Don Richard

[Chris A]

1) Does your computer that you used have an internal microphone--such as a laptop?

 

2) Was this an upsweep?  If so, can you post the distortion plot from the measurement from which you got that plot, above?

 

3) Where was the microphone from the front face of the speaker, from the ground plane, and in terms of nearfield reflections from walls?

 

Chris

[horatio2]

Both these posts were very useful.  Turned out that microphone placement was the problem.  Measurements were being done outdoors, and so reflective structures were some distance and sparse in number, and size. 

 

I do not have an internal microphone on the laptop, so this was purely the external mic.

 

I did not get the distortion plot (I will have to look to see if this information is somehow saved with the sweep).  These are upsweeps.

 

When I place the mic on the ground about 4 feet from the bass bins the responses looked much more normal and could be qualitatively and quantitatively (e.g., peaks/frequencies could be matched up) compared to curves I have collected for klipschorn responses.

 

I have not finished this work, but when I do, I will post it and will look forward to your feedback!

 

Thanks,

 

Mike

[Chris A]

I lost a lot of time in measurement.  There were momentary gliches in REW, Windows, or microphone/mixer connections that caused the microphone input to switch from the external ECM8000 (Behringer) to the internal default microphone in the laptop.  I still have a sticky note to remember to check every time that I run a sweep.

 

If you saved the sweep data, you've got the distortion plot: it's accessible via the row of buttons at the top of the graph display within the window - the third button from the left, next to "SPL and Phase", and "all SPL" buttons.  The scale is settable when the cursor is in the graph window, and the vertical scale is found next the top of the vertical axis, horizontal scale is settable next to the horizontal axis on the right. Each harmonic distortion order is plotted separately, up to 10th harmonic.  These can help to show you where the bass bin is really rolling off, and whether or not you're only listening to harmonics.

 

Placing the microphone 1 meter or less in front of the speaker on centerline usually works the best, and placing it on the floor will remove almost all reflections that cause dips the frequency response sweep in the 200-400 Hz region.

 

Chris

Edited October 26, 2015 by Chris A

[horatio2]

Chris:

 

Thanks for the tips- I greatly appreciate this, as I have spent quite a bit of time on measurements as well (in room though, while these present measurements are all outdoors), prior to this current exercise, and came to the conclusion that I could not necessarily depend on these in an absolute sense, but rather, could use them for relative effects upon making changes to this system.  Maybe a bit more than that, but really, the emphasis for me was on relative change.  Because of this, I have tended to use my ear for absolute purposes: if it measures poorly but sounds good, I tend to de emphasize the absolute nature of the measurement, in other words.  Same if it measures great but sounds awful.

 

I must make some additional measurements before I post all of this, but I will also be picking up a better calibrated microphone as well, because I want to reliably measure north of 5-8 KHz and see how this system is behaving on the top end (e.g., after re-integrating the bass horns with the Altec 288 + EV HR6040 CD Keele-type top hat.  I am using Le Cleach crossover parameters, and crossing the bass horn out at just over 300 Hz.

 

The early take away is this:  The Legend CB158 measures about +3dB stronger in response pretty much throughout the measured range, with the K33E showing to advantage by about +3dB in the 350-500 Hz range (kind of as expected, since this was the reason for the 3x13 slot change in the early 60's).  This was pretty much what I was expecting by going to a driver with greater motor strength and sensitivity, and moving the throat slot to 5.5x13.  I also saw this response steadily improve with repeated measurement evidently betraying breaking in of the suspension.  First sweep was almost nearly on top of the K33E response recorded immediately before changing the woofer and motorboard.  But by the 4th or 5th measurement pass (doing 2 sweeps per measurement), the response stabilized to what I described above.  I am not too concerned about response much above 300-350 Hz, as I am not trying to push this horn beyond that point anyway, and my desire was to take advantage of a higher strength, more sensitive driver and see how this performs.

 

Vb resonant point with the CB158 was up 1 or 2 Hz over the K33E, but I did not go back and measure this again after breaking in; I suspect it may have dropped back down 1 or 2 Hz.  I say this because of the steadily improved bottom end with repeated sweeps.  To me this suggests that the suspension is loosening up to its operational design point and settling in.  And, this driver has a published Fs of about the same number as the K33E, so I expected it to be similar to the K33E Vb resonance.   Both (K33E and CB158)  are resonating around 41-42 Hz by the way.

 

 

Mike

[Chris A]

Driver break-in is real and measurable, as well as audible.  I personally saw Roy use at least a 30 minute break-in period for compression drivers at very low frequency (below the LF crossover point) at a several volts peak-peak drive level to break them in.  I assume that this is the procedure used for woofers in the Klipsch lab, too.  The FR on the compression drivers changed a significant amount in the 30 minute period, then seemed to stabilize.

 

Another important REW plot that you'll find is the "SPL+phase" plot.  It will show you the magnitude of the time alignment issues that some here talk about.  After you get all your woofer measurements completed and you start to think about things to do, consider this:  the width of the crossover band between the midrange horn+driver and the bass bin+woofer will likely be pretty wide (more than an octave), as well as the crossover band between the tweeter and midrange--which is typically even wider--almost two octaves for most stock Heritage loudspeakers.  These two areas will sound a little weird when you run a sweep through those two regions. 

 

If you pull out the tweeter from inside front of the cabinet and place it on top of the top hat, centered at the back of the cabinet directly over the midrange driver, then run a sweep, you'll hear the difference that time alignment of drivers makes - and it's not subtle.  You'll also see the correction in the phase curve between the tweeter and midrange, and that's not subtle, either.  This is the cheapest big-ticket fix for Klipsch Heritage that exists, IMO.

 

Chris

[horatio2]

Chris:

 

Great tip on the use of the SPL + Phase feature of REW!  I use a miniDSP (since 2011) and use a delay for the Altec 288.  I have not fine tuned this, and intend to, all the more since you have pointed out the REW feature.  There is a bit of a delay modification, too, for the Le Cleach crossover topology I am using as well.  But the time alignment is crucial if you want these all horn systems to image.  Getting a handle on directivity is also a part of this, hence my interest in the 1st gen EV family that started it all.

 

Right now, I just run this as a two-way, the 288 reaching out to around 14-15 KHz.  Probably good enough for my ears at my age (!)  I might play around with doing a three way, but will need to go to another crossover product for this, as my miniDSP only does two way for my needs.  I may just push ahead and do the recent DA8 box, because this will get me up to 4 way, uses a Sharc processor, and runs 96 KHz with the plugin I would need.  Has deeper delay adjustability, too, relative to my miniDSP.

 

My basic interest and direction on all this is to take a classic, time-honored design, return it to its purest form, taking advantage of the superior CD horn technology, the improved drivers of today, and the very high quality amplifiers and digital crossovers now available, at a reasonable cost permitting biamp/triamp.quadamp setups.  The crossover advances on the digital front go a very long way in addressing many of the shortcomings, as they can do the CD horn emphasis, correct for time alignment and enable one to pick drivers without much regard to their impedance characteristics.  And the better ones do room correction, too.  What's not to like?  Taken together, these bring a Klipschorn system tightly into focus, and deliver a level of performance that is formidable by today's standard.  This is why I have been messing around with getting the stronger driver in there, and opening the throat up to near the original design, and addressing the principal sore spot of response beyond 300 Hz by sidestepping it altogether.

 

-M