Bass Traps

[gene]

This month we have a guest writer for our column on acoustics. We would like to welcome Ethan Winer, who has been a professional musician and audio engineer for over 30 years. Ethan discusses the important of the room and how even the most expensive speaker systems fall victim to poor room acoustics. In this article Ethan discusses basic principles of how room acoustics influence loudspeakers low frequency response and ways to minimize deleterious effects via repositioning the loudspeakers, and/or listening position, low frequency parametric equalization, and construction of bass traps to counter nasty room modes.

Bass Traps

[Fixed Link]

 

[av_phile]

Very informative article there. Since I don't have a large room to use bass traps, a paramteric equalizer will do. Would a graphic equalizer with 1/3 octave increments between 20Hz and 160hz do?

 

[Rip Van Woofer]

I don't think you need a large room to use bass traps. In the picture, they're only a few inches thick and against the walls. There are also quarter-round shaped traps available that fit into corners. See echobusters.com for an example.

If you read the article closely, you'll see the point made that nulls are at least as big a problem a peaks, and that controlling bass reflections/resonance is the only way to fix those. So EQ has its place, but you should first fix the room and speaker placement as much as possible.

He's really right about nulls. In my small room, there is a big null around 170 Hz. I was able to tame it somewhat with speaker placement. I noticed it especially on acoustic small jazz combo recordings: as the bassist would ascend the scale, some notes would just disappear!

 

[ThA tRiXtA]

Link is 404'd Gene.

 

[gene]

Sorry about the bad link and thanks for noticing. Thats the problem when posting articles after midnight

Try again.

or go here:

http://www.audioholics.com/techtips/setup/acoustics/BassTraps.html

 

[av_phile]

I don't think you need a large room to use bass traps. In the picture, they're only a few inches thick and against the walls. There are also quarter-round shaped traps available that fit into corners. See echobusters.com for an example.

If you read the article closely, you'll see the point made that nulls are at least as big a problem a peaks, and that controlling bass reflections/resonance is the only way to fix those. So EQ has its place, but you should first fix the room and speaker placement as much as possible.

He's really right about nulls. In my small room, there is a big null around 170 Hz. I was able to tame it somewhat with speaker placement. I noticed it especially on acoustic small jazz combo recordings: as the bassist would ascend the scale, some notes would just disappear!

I've noticed that too. In some jazz materials where a running bass goes through a scale, not all seemed of the same strength. And the peaks in some were more obvious to me than the disappearance of a few notes. I did try using the My Disc test CD on the tracks with ascending and descending pure tones of low frequencies and did notice that not all frequencies gave out the same loudness. I probably should get an SPL meter for a more accurate reading than my ears can judge.

I agree with you, those pictured bass traps are slim and can be easily positioned even in a small room. But more likely in a room dedicated for HT or music. Mine is not. It's a fairly large bedroom cluttered with drawers, paintings, tables, figurines and other accessories on the sides with a large bed in the middle (that's my listening position ) I am totally clueless where to put them if i had even 2 or 3 of those panels. And if i had to remove all those clutter, the missis will probably throw me out with them.

 

[rgriffin25]

I am sure that most of you are aware of this... Each room has its own acoustical properties. With that being said... There is a reason why some notes dissappear, and it has little to do with your equipment. I am a classical bass player and I experience this phenomenom on a regular basis. I would say that I regularly play or practice in 5 different rooms. Two concert halls at school, a couple of different practice rooms, and in my basement at home. As you might guess I get a different sound in each room. When preparing for a solo or any performance for that matter, we take the room acoustics into consideration when preparing a piece. This is not always the case with jazz recordings, especially if they are not recorded in a acoustically treated recording studio. The lost notes can be blamed on several things... Room acoustics, people in the crowd, the microphone, or even the bass (instrument) itself.

I do agree if you notice a significant dip in sound at certain frequencies there may be some things you can do to correct this. I guess my point is that no matter how good your room sounds there will always be recordings where some notes get lost.

 

[av_phile]

Now things are getting more complicated. So dissappearing notes can be due to room accoustics as much as the recording itself. I guess a good LF test tone sweep from a CD test disc should be used to really evaluate room response.

 

[rgriffin25]

Yeah, I think using a test tone and a spl meter would be a great way to begin calibrating your HT. I wouldn't use various musical recordings due to the environment that they may have been recorded in.

 

[ThA tRiXtA]

I have an SPL meter and the Avia home theatre dvd and it has sweeping frequencies but they are not intermittent so you can take a reading, whats a good test disc that plays one frequency at a time so you can measure the response?

Preferably a disc that you can choose what frequency to play?

 

[ThA tRiXtA]

anybody home?

 

[Rip Van Woofer]

I think that means no one knows of such a disc. I don't anyway.

You could look for shareware PC based tone (signal, or function) generators -- run a cable from your sound card to your system, if that's practical. I have one for the Mac, a shareware program called SignalSuite.

 

[Wabbit_Swayer]

Ive got this mp3 pack that has each frequency tone from 30 to 96 i think. each tone is about 3 seconds long, and is an individual track.

 

[ThA tRiXtA]

ok thanks guys... I thought I read somewhere on this site that there was a disc that plays rated frequency tones for measurement but who knows I could have been half asleep when I wrote it!

 

[av_phile]

ok thanks guys... I thought I read somewhere on this site that there was a disc that plays rated frequency tones for measurement but who knows I could have been half asleep when I wrote it!

Sorry for overlooking your querries. yes there is a test disc with discreet centered frequencies. I use the Sheffield "My Disc" test CD that I bought more than 7 years ago. Some of the tracks have indexes for individual frequency tones. So if your player can read indexes, great.

 

[ThA tRiXtA]

so is that particular disc still available?

Is it a useful tool ?

Thanks AV Phile.

 

[av_phile]

I don't think it still is. But I suspect you may find one online at ebay or amazon. But I did see lately some other titles for CD and DVD testing that may contain some discreet frequencies for subwoofer testing. Just can't remember the titles. Avia comes to mind. Will let you know if i come across one.

Is it useful? For me it is. You won't be using it as often as your favourite CDs ofcourse. But playing it once or twice can reveal your set-ups strengths and weaknesses so you can address them. With an SPL meter on hand, they can be used to tweak you room accoustic conditions and assist you in speaker placement. It can also reveal your equipment's sonic character by using a headphone. Having one can bring you to a field day of experimentation, adjustments and calibrations.

Alternately, the THX calibration section on some THX certified DVDs (many Disney DVDs I believe) has a sonic sweep from 100hz to 20hz so you can check the frequency response of your room as your mains and sub crossover along these frequencies. Can be revealing as well.

 

[av_phile]

Oh, I sudenly remember, look for a CHESKY disc labeled Ultimate Demonstration Disc or something like that. Not sure if its DVD or CD.

 

[Greg B]

RealTraps Acoustic Bass Traps

Here's a link to an online RealTraps dealer:

http://www.proaudiotoys.com/Store/catalog.aspx?catId=301032