meuge Posted July 14, 2004 Share Posted July 14, 2004 I used the radioshack SPL meter to to check the frequency response of my setup and I came up with some really bad results. I need help identifying the problem, so i'd appreciate any input. For your info: The speakers are located slightly off center, about 1ft away from the long wall of a 10x17 room. The floor is covered by a rug, and the walls are painted concrete. The speakers are running off a Pioneer VSX-811S receiver acting as both pre/pro and amp. And here's the terrifying picture: Quote Link to comment Share on other sites More sharing options...
michael hurd Posted July 14, 2004 Share Posted July 14, 2004 Is the chart corrected? The radio shack spl meter has a known variance, with the most variance at the lowest frequencies. I only tested low frequencies, and did not correct them, when I tried test tones. I should take some time to repeat the test, and also to see how high a peak I am getting in the corners. I know that if I stand in the corner, the bass is quite pronounced. It looks like if you can get that low-end peak and trough cleaned up, the response will fit inside of a + - 10 db window. Quote Link to comment Share on other sites More sharing options...
meuge Posted July 14, 2004 Author Share Posted July 14, 2004 I will see if I can find the correction data for the Radioshack SPL meter, and if anyone has a file or a link, I'd appreciate if you could post it. Also, I've repeated the test and got results within +/- 1dB of my previous ones, so the results seem quite precise. As for corner gain, I reduced the angle of the speaker towards listening position from about 15-20 degrees to 10-15 degrees and the gain went down to about 4-5 dB in the corners of the wall closest to the speakers. However, while the bass bothers me, it's not my biggest worry, since I can cut it off when adding a subwoofer. What really bothers me is the huge dip in low mids (~150Hz). I have no idea what could be causing it. Any acousticians are welcome to share your conclusions, Quote Link to comment Share on other sites More sharing options...
dougdrake Posted July 14, 2004 Share Posted July 14, 2004 Radio Shack compensation chart: http://www.svsubwoofers.com/faq_rscomp.htm Quote Link to comment Share on other sites More sharing options...
redtop Posted July 14, 2004 Share Posted July 14, 2004 Meuge, If I am reading your chart correctly, your dip at 40 htz and 160 htz are direct multiples of each other. This usually means the waves in the room (and their harmonics..thus the multiples) are canceling each other out. This tends to be most pronounced at the lower frequencies since they are longer waves. If it was me I would try moving the speakers to different distances from the corners, including up the long wall. Also try different toe-in angles etc. From what I have read, concrete is the worst surface for low frequency because it is so good at reflecting them and causing the wave cancelations. What ever you do don't blame the Pioneer... Sorry had to add that... Redtop Quote Link to comment Share on other sites More sharing options...
meuge Posted July 14, 2004 Author Share Posted July 14, 2004 Yeah, I found the compensation chart and in the ranges I am interested in, the real values are still within 1.2-2dB of what's shown by the meter, so the boost in upper bass and the huge drop in low mids are definitely not artifacts of the meter. i cannot begin to imagine how to fix it, so I am ripe for ideas. Quote Link to comment Share on other sites More sharing options...
tankhokie Posted July 14, 2004 Share Posted July 14, 2004 yep, to echo the other poster, try moving the speaks around....not much, but just a few inches. i was surprised to see how much of a difference 4 inches in/out of a corner made to my graph. what ended all of my insane graphing/obsessing was taking readings on different days without changing anything...you will see differences. i think if i farted while making measurements it gave me different results. remember, in the end, the room won't be empty, your accoustics will change, so this is just to get "kinda" flat. all that and if you move your spl meter over a foot, you get different graphs too. i see it as a losing battle, so i don't worry too much about it now. Quote Link to comment Share on other sites More sharing options...
J.4knee Posted July 15, 2004 Share Posted July 15, 2004 Also try fixing your room as well with acoustic treatments. Edit: Contact Arto, concrete walls are highly reflective. Quote Link to comment Share on other sites More sharing options...
DrWho Posted July 16, 2004 Share Posted July 16, 2004 ---------------- On 7/14/2004 11:16:29 PM tankhokie wrote: yep, to echo the other poster, try moving the speaks around....not much, but just a few inches. i was surprised to see how much of a difference 4 inches in/out of a corner made to my graph. what ended all of my insane graphing/obsessing was taking readings on different days without changing anything...you will see differences. i think if i farted while making measurements it gave me different results. remember, in the end, the room won't be empty, your accoustics will change, so this is just to get "kinda" flat. all that and if you move your spl meter over a foot, you get different graphs too. i see it as a losing battle, so i don't worry too much about it now. ---------------- One trick I've picked up is to move the SPL meter around a bit in the near vicinity of the main listening position. Then record the largest value you measure as you move the meter (just make sure you hold the meter still when you take the measurment). The reason for doing this is to generate a graph that is more representative of what you actually hear. To begin with, your head is never going to be always in the same spot. Second, most of the sounds in music don't exist long enough to create standing waves...when playing test tones, there's plenty of time for this to happen. As far as your response...i don't think you're suffering from harmonics between the 40Hz dip and the 160Hz dip for the main reason that there is no dip at 80Hz. One thing I notice is a correlation between 160Hz, 1 foot from the wall, and the boundary cancellation effect. at 160Hz, a quarter wavelength is roughly 1.5 feet. The boundary cancelation effect states that a boundary 1/4 wavelength away from the source will cause a 6dB drop at that frequency. The reason being that the sound reflecting off the wall will be out of phase with the original sound (because travelling 1/4 wavelength to the wall and then another 1/4 wavelength back causes the reflected sound to be 1/2 wavelength behind, or 180 degrees out of phase). OK, enough mumbo jumbo...I think your best bet in fixing the response would be to move the speakers around. I'd suggest getting your speakers right up next to the wall, or bringing them to about 3 feet off the wall (which would also help tame your peaks). Just keep in mind that you're only going to be able to do so much with moving the speakers...the basic idea is to get them in a spot the provides the flattest response (without sacrificing the imaging). After that, you're gonna have to look into some room treatment. Take a peak at the architectural forum and read up on bass traps and acoustic panels. Quote Link to comment Share on other sites More sharing options...
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