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Discussion Starter #1
I have some Dayton RS180S-8 and RS28f-4 drivers that I want to use for some HT speakers. My room does not accommodate floor-standing speakers positioned away from walls. I built a test enclosure that I have tested. The drivers have very nice response when located more than two or three feet away from a wall. In this configuration, I could make a really nice 2.5 way design with very flat frequency response. When positioned against the wall, however, I get some pretty severe cancelation between 150Hz and 250Hz. The enclosure depth is six inches. I'm not sure I see a good way to prevent this behavior. I considered a 2.5 way design using the second woofer to reinforce the attenuated frequency range. I am concerned that this will not help much because the same cancellation mode will apply to both woofers. I could se some ideas from anyone who has Experience with this issue.
 

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Best guess is that it is not a 1/4 wave cancellation from the front wall, unless the drivers are over 10" from the wall. Likely not enclosure resonance unless the enclosure has no bracing, but possibly room interaction? Is the dip in response in many locations?

JSS
 

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Thanks for the response. The enclosure has internal bracing and is very solid. Other measurements suggest that my issue is with the placement and not the enclosure itself. I have attached some pictures. The first one is with the speaker on the floor in my measurement room. The second is in my living room where I removed the furniture and repeated measurements to see if the issue is a room mode associated with my measurement room. I suspected not because the issue remains with gating conditions that should remove reflections from the walls (not the floor because it is too close). The speaker is 6 inched deep and sets almost against the floor in the measurement room. I get exactly the same response nulls measuring in both rooms of significantly different dimensions. One picture shows the speaker on top of a stool and a bucket. While these items do influence the results, they do not change the behavior I was trying to confirm. The first measurement was collected with the speaker in the center of the room and shows pretty nice response (excluding the baffle step response). There are two pictures of this measurement with and without smoothing. The second measurement was collected with the speaker against the wall. This measurement is also shown with and without smoothing. Against the wall, I get some pretty substantial disturbance between about 150Hz and 400Hz. The response null gets more narrow and lower in frequency as I move the speaker away from the wall. My original intent was to use the on-wall configuration with little or no baffle step compensation. Now, I am not so sure, unless I can figure out how correct the low-frequency response. Any suggestions are greatly appreciated.
 

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I thought this thread sounded familiar... it looks very familiar. Always good to ask a lot of quesitons!

HAve fun,
Frank, who uses the same alias on all forums
 

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I can only guess that these speakers will not sound of their best hard against a wall. Either put them in the room, or go back to the drawing board. Cheers, Mike.
 

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The main differences I see from the floor to the wall measurement is all the fiberglass surrounding the speaker on the floor position, absorbing any reflection that could cause a null. It does look like a placement issue. Absorptive treatments only 2" thick should solve problems, and wall rigidity (or lack of) can cause some problems as well, when compared to a concrete floor.

JSS
 

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I was just looking through my old posts and decided I should follow up and perhaps add some value if anyone ever reads these things. I eventually did complete an MTM project based on these initial experiments. I made three of them for the front and slightly different MT's for the rear in a 5.1 arrangement. I have attached a picture of the MTM. The shape was chosen because this is an on-wall design. The color was chosen to match the wall (otherwise not the best speaker color). This picture was taken before the grill was firmly attached, so the edges look a little wavy. That's fixed now. Also, the grill has a little more industrial look than most will like, but I wanted to be able to see the drivers. For a more refined look, the clips could be removed and the grill covered with cloth. I currently have these attached to a Pioneer Elite sc-lx801 and complimented by a subwoofer I made using a TC Sounds TC-1000.

I tried performing measurements in various locations to investigate the frequency response dips in the original posts. It turns out these were not directly related to the wall behind the speaker. It was related to the distance to the floor and ceiling. This problem becomes worse when pushed against the wall adding an additional boundary at the corner where the wall and floor meet. The problem was significantly worse in the basement (the dungeon where I am confined to do my speaker work) where the ceiling is much lower. Measuring in a room with high ceilings and mounting the speaker and microphone higher from the floor helps. I have attached some frequency response data for the MTM and the MT. In the measurement you can see I was very careful with placement when taking the MTM measurement. I was lazy for the MT measurement, so there is a dip at below 300Hz. I also subtracted 2.5dB from the MTM data because I was comparing character between the two configurations when I made the graph. The MTM efficiency is actually about 90dB (2.83V into 4 ohm).

REQ software was a great tool for this project and well worth the time to setup and calibrate. One note of caution, The PE EMM-6 microphone I used initially resulted in a crossover design that was not to my liking at high frequencies. I replaced it with the same microphone ordered from and calibrated by Cross Spectrum. Measurements with this microphone caused me to change my crossover design to something that sounded much better and more lively at the high end. I measured with the two microphones back to back in the same setup. The one ordered directly from PE had rise in response at high frequencies that deviated from the Cross Spectrum unit by 5dB, causing me to unnecessarily attenuate the high frequencies. When plotted in excel, the calibration data from PE showed similar response and was not consistent with data sheet frequency response graphs. Based on my experience (admittedly only one microphone) I would be careful trusting the PE calibration.

As for the comment above about the speakers not sounding their best against the wall, they are not as bad as you might think. This turns out to be the least of my worries. My room is too deep for the width, so stereo imaging suffers. I can fix this by moving closer the the speakers. Other than this issue, they sound pretty good. Overall I am quite pleased with the sound and impressed by these drivers. Based on this experience, I would not discourage people from wall mounting speaker projects in rooms where speakers can't be out in the room.

That's all, have fun.
 

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