 
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
|
|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
203.212.152.21
In Reply to: Is a dip a room null? Paging Kenster, will ASC help? posted by WBWB on March 17, 2007 at 20:11:43:
Can a trap solve a null?The problem with both standing waves and suckouts related to room dimensions is room reflections. With standing waves, the reflections reinforce a modal frequency and create the standing wave. With suckouts. the reflection is out of phase with a particular frequency so it cancels it. Bass traps can help with both problems by absorbing sound that would otherwise be reflected. If the strength of the reflection is weakened because some of the sound is absorbed rather than reflected, standing waves get less reinforcement so they aren't as strong, and the suckout frequency isn't "sucked out" as much because the out of phase reflection has been weakened.
So yes, bass traps can help with suckouts caused by out of phase reflections.
Now you're talking a dip from 35 Hz to 76 Hz (or at least I think you are—your post said "dB" after those numbers but that doesn't make sense so I'm assuming you made a typo). That's a dip over an octave wide. Most problems due to reflection reinforcement or cancellation aren't an octave wide, they're much narrower than that. You haven't said how you made the measurements and if you used a test disc with test frequencies a third or a half octave apart, it's quite possible you have dips at the frequencies you measured and peaks in between. You really need to measure a lot closer than a third of an octave or half an octave intervals at frequencies below 200 Hz if you're really going to get a handle on what your room is doing, and you also have to take into account your speaker's response when you get below 100 Hz because the speaker's going to start rolling off at some point. In your case your speaker is apparently down 3 dB at 40 Hz so part of your dip could simply be a fall off in speaker response aggravating whatever the room is contributing to the dip.
Also reflections at these frequencies are very sensitive to where the measuring microphone is placed and you may simply have the mic at a location in the room where the bass response is noticeably lower than at other points. Obviously you should be measuring at the listening position but the listening position may not be in the position to give you the best bass responese.
There's a lot of factors that could be affecting your results here and you really haven't provided any information about room size or measurement technique so it's hard for anyone to comment. Bass traps will definitely help reduce the impact of room problems, whether they be peaks or troughs in the response, but it's hard to say how much of what you're reporting is room response, how much is deficiencies in measurement technique, and how much may be something else based on what info you're given us.
David Aiken
Follow Ups:
Dave we can use phase cancellation and knock out a lot of the problems too...bass traps are not always necessary.Alan
Yes, I realise that.My comments on phase cancellation only related to why bass traps, ie absorption, can reduce scale of the nulls caused by them.
David Aiken
Hi David,
Appreciate your very detailed reply.I am using the JBL Room Mode Correction (RMC) unit which comes with a test disc and a SPL meter. The SPL meter is mounted on a tripod at ear height at where I sit for listening. The test tone starts at 126 Hz and goes down to 20 Hz.
Sorry about incomplete data earlier, the detailed measurement between 76 and 35dB are as below:
85Hz = +1.5dB*
76Hz = -3dB
68Hz = -7dB
62Hz = -8dB
57Hz = -9dB
53Hz = -10dB
50Hz = -11dB
47Hz = -11dB
45Hz = -10dB
41Hz = -9dB
35Hz = -3dB
31Hz = +2dB*
(*Just for a more complete picture)Is my objective correct to raise this band between 41 to 76Hz to less than -3dB, since my speakers dip at 40Hz? And I was wondering what frequency tubetraps will raise this null. Does it mean I have to get a tubetrap that is effective down to 40Hz?
Thank you once again!
Well, at least the test frequencies are closer than third octave but they're still not particularly close, and not close enough to give a really good idea of what's going on at bass frequencies.Many problems related to room dimensions are narrower in bandwidth than 2 or 3 Hz which is the interval between most of those test frequencies, but the interval between others is 4 Hz on a couple of occasions and 6 Hz in one case. I don't know why they picked such an erratic set of intervals for their test tones but there is probably an awful lot going on in your room in the 'cracks' between those frequencies. It would be a very strange room if the whole picture between 35 and 76 Hz was one of depressed levels only.
One problem with these measurments is simply that we don't know what your 0 dB level indicates. I could restate the measurement you've given as ranging from +6 to -7 dB, simply by resetting the reference level from the 0 dB point on your meter to it's -5 dB point. The actual curve shown by the measurements would be exactly the same shape. The only change would be what level I used as a reference to calculate the + and - levels from. A room response of +6 to -7 dB over the frequency range you measured would not be considered a particularly bad result for an untreated room. The ups and downs in many rooms can be much worse than that.
Another part of the problem is the overall measurement range from 20 to 126 Hz. This is too narrow to rely on for room correction. What you want to achieve is a more uniform response over the 20 to 20 kHz range, the whole audible range. Room response is very uneven below 300 Hz and there's no guarantee that your 0 dB level, which I'm taking as the reference level for your measurement device over the 20 to 126 Hz range, is going to match the levels of the room above 126 Hz because you don't know what's happening there. You actually want things to match up with what's happening there, just as much or even more than you want to see a smoother response between 20 and 126 Hz.
Speaker test measurements tend to take the level at 1000 Hz as the reference point around which the + and - calculations of the speakers response are made and there's a good reason for that. 1000 Hz is roughly 5 octaves distant from both 20 Hz at one end of the audible range, and 20 kHz at the other end. Ideally what you should be aiming for is a rom response that match the 1000 Hz level across the whole 20 to 20 kHz range without worrying if you get a drop off in the 20 to 40 Hz octave where your speakers are rolling off relatively fast. Your problem in achieving that goal is that you don't know how the level at 1000 Hz compares to the levels over the range for which you have measurements.
If you could measure above 126 Hz you might, for instance, find that the response was measuring at around the -5 dB mark when the room's response smoothed out as it starts to do above 300 Hz, and that the -5 dB level approximated the level over most of the audible range. If that was the case, you wouldn't be wanting to raise the level over the frequency range shown in your actual measurements to -3 dB. You'd be wanting to raise the level at the more depressed frequencies to around -5 dB, but you'd also want to be lowering the level at those frequencies with levels above that to around -5 dB also. You wouldn't be worried in the slightest if the level at the 2 frequencies which did show levels of -3 dB actually dropped a further 3 dB in that process.
You're right in assuming that you want to get a smoother overall response, but you're wrong in assuming that simply elevating the lowest levels to a level much closer to 0 dB is the goal. The level you want to match is well above the frequency range shown by your measurements so you have no idea what target you should actually be aiming for. Getting things up to the -3 dB level you mentioned may sound fine, or it could actually end up giving you a response in the 20 to 126 Hz range that's too high to match your response at higher frequencies and you could end up with an excessive bass bloom as a result.
Having said that, you aren't totally lost. Bass traps, both the tube sort and the panel sort, are broad band devices and they work to even and smooth the overall response in the bass region. They will reduce the levels of peaks as well as raise the level of the troughs to some degree so bass trapping should reduce the difference between the highest and lowest levels in your measurement range. That's definitely desirable, and the sound of your music should improve noticeably as a result. Get the best traps you can afford or make, and get/make enough to treat all 4 room corners from floor to ceiling if you can (with the proviso, of course, that you can actually place traps in all 4 corners which isn't always possible).
What you should find after you've done that is that your measurements show less difference between the highest and lowest levels but don't worry too much if the measurements you get don't measure up to your expectations. Also don't worry about the scale of the numbers_ look at the difference between the highest and lowest levels measured. That difference should reduce. Your measurement device simply won't give you a good enough picture of what's going on to be totally reliable anyway and you have no idea whether the 0 db level is a suitable reference level or not. Rely more on your ears. What you should be hearing is better defined bass, cleaner and clearer bass, more articulate bass, in short bass that is much more supportive of the music in general. You may feel you get a little more bass extension as well. What you're chasing is a musical result, not a particular set of measurements and you really don't know what measurements you want to get because you have no idea of what's happening above 126 Hz and the frequency which should be your real reference point is actually a couple of octaves above 126 Hz.
You may well not be able to get a set of measurements that you think is ideal but you will still be surprised at how much audible improvement you notice musically. Measurements are great for showing how the room response has changed but you can't create a perfect room. Thankfully great sound doesn't require a perfect room and any improvement in room response in the bass region tends to have a much bigger subjective effect on your musical enjoyment than you would expect, at least in my experience.
David Aiken
Hi David,
In the JBL test-disc, there is a 200Hz to 20kHz pink noise for SPL meter calibration, as a reference 0 dB. And immediately the next track starts off with the 126Hz test tone and descending frequencies and so forth. That's how I measured all the dips that came about.I understand your advice and really thank you for your time and effort in giving me a good perspective on room calibration and the necessary treatment. I've really learnt a lot from you.
Cheers!
Ah, so you've referenced the readings to the level you got for 126 Hz. I wondered what was going on.Well, if the level you got for 126 Hz is comparable to the level you would get if there was, say, a 1000Hz tone on the disc, then it would make a good level. If 126 Hz happens to be the frequency of a standing wave in your room and the reading actually represents a peak in the room's response at that frequency, your string of lower readings from 35 to 70 Hz or so might actually look desirable. If the 126 Hz reading represents a trough in the room's response, you would have a very bad string of readings over the 35 to 70 Hz range. As I said, however, we don't know how your reference level of 0 dB at 126 Hz relates to what's happening above that range. Whatever frequency you use for a reference should be well up above the frequency range where the room's dimensions strongly affect the response you get and 126 Hz isn't high enough in the audible range to serve as a useful reference point.
Even though the test tones are one third octave apart, it would be worth while getting a hold of a copy of something like one of the Stereophile test discs which have tones ranging from 20 to 20 kHz. That would give you a much better idea of the room's overall response and a reference tone at 1000 Hz as well.
You could then replay the JBL disc using one of the tones on that disc which matches the frequency of one of the test tones on the Stereophile disc as the reference tone. Ideally you would adjust the volume when playing the JBL disc so that the reading you get with your meter is exactly the same as the reading you got for the identical frequency test tone on the Stereophile disc (note that the reading for that frequency on the Stereophile disc is almost certainly going to be + or - something in relation to the reference 1000 Hz tone). If you then overlay the readings from the JBL disc on the readings for the Stereophile or other test disc, you will have a bit more information on what's happening than the Stereophile or other disc gives. You may well find some of the Stereophile or other disc's reading's in the 35 to 70 Hz range elevated in comparison to JBL disc readings at other frequencies in the same range, and you will have a much better idea of where the sound you're getting over that range compares to the rest of the audible frequency range. That should also give you a reference point for what you want to achieve over the 20 to 126 Hz range of the JBL disc.
David Aiken
This post is made possible by the generous support of people like you and our sponsors:
Top of Page ]
[ Contact Us ]
[ Support/Wish List ]
[ Copyright © 2024, Audio Asylum, all rights reserved ]
