Do these impulse graphs of the lower woofer and upper woofer make you believe they are the same polarity or opposite?
Note: These were taken with the mic ~4-6" away from the lower then ~4-6" away from the upper with no mid-tweeter playing. The lower was playing while taking the upper measurement and vice versa.
Forgot you had asked about a gated measurement in one of your e-mails. Here is the right speaker at the LP gated for 4ms before the first large peak (as seen in the impulse) to 5ms after that peak.
FYI after another battery check on the woofers, although the impulse graphs I posted earlier make me think they are out of phase, the battery check shows them in phase.
Appreciate all the help so far.
I’d be concerned if they did look identical. Remember this is a 3.5 way design, and an extra 4.7 mH inductor is used on the lower woofer adding additional phase rotation. I don’t see any phase concerns…
With respect to your gated plot: The dip around 350 Hz is much less noticeable and more importantly, the mid/tweeter response no longer looks attenuated with respect to the woofer response. Response irregularities from adjacent boundaries often make it difficult to obtain ‘clean’ plots as the impulse is corrupted by the reflections. What is surprising is how much the room modal issues can affect measurements, yet the speakers will not sound compromised.
To make my measurements, I generally move the speaker in near the center of the room, and angle it so it is not orthogonal with the walls. I also throw a stack of eggcrate foam on the floor between the speaker and the mic to ameliorate the floor dip. Try this at a 1 meter distance to the mic. Put your start gate immediately prior to the impulse and your stop gate 5mS later.
Nearfield measurements are wonderfully free of boundary issues as the acoustic energy of the direct signal is many times larger than the reflected energy. Unfortunately they also do not show the diffraction effects of the baffle, and therefore are of limited use for accurate room measurements or crossover design.
Welcome to the Forum, Curt!
Understood, but theoretically all speakers should have the same problem – a dip in response like Pete’s, caused by floor bounce - but obviously they don’t. So either it’s typically compensated for in the design (especially with a tower speaker, where the distance between the drivers and the floor is a know factor that will obviously be the same in every room), or else Pete’s problem is either because of a deficient design, or else it’s being caused by something else.
Nulls are typically narrow and deep, not like what Pete’s seeing, which is more accurately a trough or depression, not a null. So Pete’s problem could probably be addressed to some extent by equalization.
Hi Wayne! Thanks for the welcome!
Theoretically all speakers do have the same problem. One only needs to look a one of John Atkinson’s in room plots of a speaker response in Stereophile to see the boundary mode issue is prevalent in most traditional designs. –And this is after he averages a grid of 12 separate measurement positions to mitigate the effects of those very same room modes on the response.
In a passively crossed design we have precious few tools to address response dips, but some attention to details can minimize its effects: The floor dip frequency of each driver is easily determined by its given driver height above the floor and a given listening distance. In a 3 way, a common method is to use crossover frequency that equates to the mean dip frequency between the woofer and the mid, thus partially compensating for the issue by manipulation of the transfer function of the individual driver’s transition bands. In a 2 way, this option is obviously not available to us, and in the case of a bookshelf size speaker, even the height above the floor is not a given, denying any chance of compensation. When you consider the effects of the wall reflections will be equal to or greater than the floor bounce, and how arbitrary those distances will be, the issue seems hopeless. -Yet we’ve nonetheless managed to enjoy our speakers regardless of room position. How can this be? While I’m certainly no expert in this area, I presume there is some psychoacoustic signal processing done in our hearing centers that allow us to hear ‘through’ these response aberrations. -Of course, these boundary aberrations are best controlled by careful placement of the speakers in the room and room treatments. After all, it isn’t the speaker that is broken, it’s the room that’s causing the issue.
My LspCAD software contains a relatively simple room sim program. Based on the computation time, it likely only takes into account axial modes, but not tangential or oblique modes, although to its credit the relative attenuation of these modes generally make the axial modes predominant. The program is interesting in that you can select/deselect each boundary surface and see the effects each has on the summed response. Selecting more than one boundary makes those sharp nulls disappear, and the resultant summation of the 6 boundary modes make for a response that is both unsettling to look at and difficult to interpret. –And similar to what Pete has posted.
At this juncture I’d like to reiterate that we are basing our presumptions of floor dip issue on speculation. This is one possible explanation for Pete’s measurements, but certainly not the only possible explanation. As I’ve suggested before, measurements are a 3D slice of a 4D world, and these measurements themselves are fraught with numerous potential pitfalls in interpretation and execution: Last week I was running harmonic distortion sweeps on a group of tweeters and found they all exhibited high 3rd order distortion between 3K and 5K, but otherwise apparently measured normal. Disappointed in the tweeters, I went so far that evening as to spec out a different tweeter for the design. However, I did a retest the next day and in checking the equipment over, found the errant HD plots caused by a low battery in the mic preamp.
Sorry for the loquacious response. I’ll try to be more succinct from now on…
John, my apologies. I just saw this is the REV forum, so we’re not very on topic, -but could be if it turns out to be a procedural issue with measurement. I did take a few minutes to look over the REV overview: It looks quite impressive. It appears full featured and robust with all the tools necessary. Kudos for giving away your intellectual property for the good of audiophiles everywhere!