Originally Posted by pbz06
I have free time today to mess around with my subwoofer. I have previously posted about a -10dB null around 35hz (where 30hz and 40hz are fine). After Audyssey XT32, it's about -5dB...I don't really notice it during movies, have plenty of nice clean/smooth bass and tactile response.
Anyway, I can move it around a bit more now but it will stay in the general same corner....how many inches/feet would I need to play with to try and remove that null? Is it even worth fussing about?
Peaks are usually more noticeable than nulls for a few reasons. One is simply that we seem to be more sensitive to peaks than to nulls but peaks are noticeable in 2 ways. First they can sound louder than sounds at other frequencies and second because the decay of the sound at a peak frequency takes longer than the decay of sound at other frequencies and that longer decay can audibly "muddy" the sound of the bass, making it appear to drag. On the other hand nulls are simply not noticeable unless there's actually a sound at that frequency which you expect to hear but don't hear. With music recordings you could have a deep null at a frequency which isn't a musical note and never notice the null because you never played anything at that frequency. Soundtracks to movies and TV shows are different because there's lots of sounds such as explosions, engines, etc which include frequencies which don't coincide with musical notes so if the null is at a frequency not associated with a musical note then there is a chance of noticing it when listening to non-musical sounds in a soundtrack. In the end, nulls are far less noticeable than peaks which is good because you can't fix a null by increasing the level of that frequency, the increase in power you throw at that frequency suffers the same cancellation problems that the frequency is getting from the room and your amp will run out of power before you manage to cancel the null.
Originally Posted by pbz06
I used my SPL meter only (not REW), after Audyssey. My meter is rated to 31.5hz and I fed it 35hz signal from tone generator and it confirms the approximate/estimated multieq app graphs. It's flat from about 40hz to 120hz.
One thing I noticed while testing just now, moving the subwoofer up to 2' didn't show any differences, but moving the mic a couple inches results in a flat graph for the sub....strange.
Not strange. Walk around your room while your sub is playing a constant sound, say a 40 Hz tone, and watch your sound meter response, it will vary up and down. There are areas in a room which are high pressure zones and areas which are low pressure zones. The meter will show a higher level in a high pressure zone than it does in a low pressure zone. Room corners, for example, are high pressure zones so you will almost certainly see a difference in level between the level you measure at the listening position and the level you measure in a corner. If you move the sub you don't change the location of the high and low pressure zones in your room, they're determined by the room's shape and size, so depending on the distance you were from the sub when you were making measurements you may well not see a change in level when you move the sub 2' but don't move the mic. On the other hand, when you move the mic you're moving it in what is essentially a pressure gradient and if you move it from a low pressure zone where the null is occurring into a higher pressure area where the null is absent then you will get a flatter result even though you didn't move the sub. Sometimes small changes in mic position can make a big difference and what that also means is that small changes in your listening position can make a difference so if you're having a problem getting enough bass, one of the things you can always consider is moving the listening position backwards or forwards a little. That can sometimes solve the problem.
BTW, pressure in the room is distributed in 3 dimensions so moving the mic up and down can result in differences in reading just as moving it forwards/backwards and left/right results in differences. That's why calibration measurements are made at ear height in the listening area, the level can change at heights above and below your ear level.