HT011 – Bass Trapping for the Ultimate Home Theater

Episode #011
Host:  Mike Deckys
Guest: James Lindenschmidt – General Manager – RealTraps
Topic:  Bass Trapping for the Ultimate Home Theater

Download Full Transcript here

Mike:  On this episode of Smarter Home Theater we welcome James “JWL” Lindenschmidt.  He is an acoustics expert and general manager of Real Traps.  Real Traps is a high performance acoustic treatment manufacturer that is made right here in the United States. We would like to start off by welcoming you to the show James.

James:  Thanks a lot.  Glad to be here.

Mike:  We are glad to have you James.  Now I want to start off with saying all you have to do is search the online forums for Bass Trapping and pretty much the general consensus is that it is the first, best and easiest step to improve the sound quality of your home theater.  Why is that?

James:  It creates a space that is more favorable to listening to music accurately.  This would be the short answer.  There are quite a few acoustic problems that are in pretty much every room but especially in smaller rooms like the home theater.  One problem with a small room like that is the way that the bass behaves in the room.  When bass energy, which is basically omni-directional, (meaning when it leaves the speaker it goes in all directions at once, rather than what leaves the tweeter it is going to be very directional) it behaves a little bit differently in the room.  When you have that sort of energy in a small home theater kind of room the reflections of all that bass energy bouncing around the room causes all kinds of acoustic problems. They manifest usually as peaks and nulls in the room response. So some bass notes, like if you are listening to music and you have a good bass player playing something, some of the notes that he hits might excite the whole room and make the whole room seem to resonate.  Other notes will seem to disappear.  That is the main phenomenon that we want to correct with bass trapping.  It flattens out the response of the room and lets you hear things much more accurately.

Mike:  I see.  So let’s say you are watching a movie and you have a big car chase with a couple of explosions in there, what bass trapping is going to do is make the explosions be nice and clean and you are going to hear them just the way Hollywood intended you to.

James:  Exactly, because when you are listening to explosions you are going to want to hear clean explosions.  It does improve the accuracy of what you are hearing.  As you say, it will sound the way the sound mixers and the director intended it to sound more in your room.  A lot of times, and this is especially noticeable in home theaters as opposed to music, when you have something like an explosion that is a lot of energy and a lot of different frequencies. That sound is almost like white noise.  It almost sounds like static but at very low frequencies.  When you are exciting that wide of range of frequencies at once, like a cannon fire or explosion it is very easy to excite the room and create actual acoustic distortion. It sounds like distortion.  You can hear the room ringing.  It creates an almost unpleasant effect.  The bass trapping smoothes   that out.  It allows things to be heard much more accurately.  Probably the biggest thing treating the room does, not so much with bass trapping, but in general when it comes to the movies is that it really improves the intelligibility of the dialogue.  That is a huge benefit of treating the room.

Mike: So, basically, if you don’t properly bass trap your room you get a lot of ringing and poor sound quality in general?

James:  Yes it is a very uneven frequency response.  These reflections that are bouncing around the room, these sound waves, interfere with each other. Sort of the technical term for how it works is a phenomenon called cone filtering.  Which I imagine we will get into that in our conversations over the next couple of episodes.   What cone filtering does is that at some frequencies those waves that are interfering with each other add together and reinforce each other.  That is where you get the peaks. And other frequencies will cancel each other out.  These waves and peaks and nulls are very steep and very close together.  If you were to look at an analysis of it you might get something like, and this is absolutely typical in small rooms, a 10 db boost at a 70 hertz, and then at 75 hertz or really close to that you might have a 15db null.  So that is a difference of 25 decibels just by going from 70 to 75 hertz.  Again, it is hard to convey how poor this is for listening to music or to sound because we are used to it.  We are used to hearing all these anomalies because most of us don’t live in highly treated rooms or haven’t spent a lot of time in it.  It is sort of what we are used to so it is hard to convey how bad the frequency response can be. Most modern gear, like you buy a pair of speakers, or amplifier or blueray player or whatever it is, the frequency response on the audio is going to be to flat from the low end to the high end, compared to the jagged response you get from the room.  And the room imparts that response on all of your gear.  It is really an eye opening experience (and I remember it myself) the first time you actually sit down and listen in a treated room.  It is much more accurate, much smoother, less problems when you create a reflection free zone, which we will talk about later, you can get very good stereo imaging.  It almost sounds like you get the imaging of something like headphones when you are listening.   That very precise imaging that you can hear.  But, obviously it sounds a lot better because it is speakers in the room rather than the headphones on your ears.

Mike: Where can you put bass traps?  Do you put them in the corners, the side walls, the ceilings, in your theater risers, under your chairs?  Does it really matter?

James:  Yes it does matter.  Getting the traps into the room is probably more than half the battle.  Maybe 50-60% of the battle at least.  But placing the traps in the room you can definitely put the traps in a position where they do the most good.  Most of the times that is going to be in the corners of the room and that is certainly where I would start with bass trapping.  The way you can tell (and again it is sort of complicated the way bass wave acoustically behave in the room) where the bass traps should go is to put on something that has a lot of bass energy in it. On the Real Trap site we actually have something you can download.  It is an MP3 that you can download.  It is basically filtered noise, it is that pink noise..

[ad#Google Adsense]

Mike: Is it the test tone CD that I saw?

James:  Actually that is a good one.  That one goes up each individual frequency.  It starts at 40 hertz for a second and then it plays 41hertz, then 42 hertz, all the way up to 200.  That is one test cd, but the one I am actually talking about is filtered noise.  It is a little bit different.  It is pink, or white, noise (static) everything that is above 100 hertz, maybe 200 hertz has been filtered out.  That bass sound [  ] that is only in the bass energy.  If you put that on that is giving you bass energy at all frequencies in the room.  When you have that playing it is sort of annoying to listen to but it is useful because when you walk around you can really hear where the bass gets louder at different points in the room.  Usually that is going to be on the corners of the room and by corner I mean most rectangular rooms have 12 corners.  The wall-wall corners, the wall-ceiling corners, the wall-floor corners and any of them are valid places to put a bass trap.  The acoustics don’t generally know which way is up obviously.  Corners behave the same no matter what sort of orientation they are.  If it is a good solid corner it is probably going to be a bass build up there.  So by using tests like that you can just be sure that when you place the traps in your room that you are putting them in the position where it is doing the most good.  When you are walking around the room listening to this annoying low frequency thing you want to find the spots where the bass gets louder.  And wherever the bass gets louder that is where the bass is accumulating and it is a good place to put a bass trap because there is more bass there for the bass trap to absorb, if that makes any sense.

Mike:  Yes it does.  Now, if you look online you can see that there are many different shapes of bass traps available.  There are the wedges that actually fit into the corners that, like you said, are a good place for a bass trap.  There are the panels that you can put in like a triangle in the corner.  Is there a shape that works best?

James:  It is not as much about shape as it is about thickness of the panel, the material the panel is made from and the surface area in the room.  The coverage area of the room is also important especially at the lower frequencies of under 100 hertz, very low bass frequencies like that.

Mike:  So what materials would be a good bass trap material?  You see a lot of people will make a bass or acoustic trap and fill it with fiberglass.  Is fiberglass a good sound trapping device?

James:  Yes generally it is extremely good.  In fact in our Real Trap stuff we use a lot of rigid fiberglass.  It is a great dense form of fiberglass that comes in sheets as opposed to the pink fluffy stuff you can pick up at your hardware store.  The pink fluffy stuff still does absorb bass energy but you need so much of it as it is not very dense.  In fact a quick and easy solution, if you are not worried about appearance you just want to improve the sound you can actually stack the big rolls of the pink fluffy insulation, still in their plastic packaging, in the corner.  It is a quick fix.  That will help flatten the bass.  Yes fiberglass is good and one of the more common materials.   Rockwell people use. There is a newer material that is a form of insulation that is made out of recycled blue jeans.  It is cotton insulation.  There are a few different types out there.  I personally worked with the Ultra Touch Cotton.  That stuff works pretty well too acoustically.  The key thing is that any material that you use, you can’t just pick up any kind of insulation and think it is going to absorb sound.  It has to have absorptive properties to it, too.  In fiberglass, Rockwell, Acoustic Cotton all have that.  If it is good at absorbing sound most of the time it will be shown in the specs of the material.  If you go to Home Depot, or wherever you go, they should have a flier that will tell you how much this material absorbs and at what frequencies.

Mike: Is it possible to get too much bass trapping in a room?

James:  That is sort of a complicated question and I am going to give you a controversial answer and say no, you cannot have too much bass trapping.  However you can have (and it is very common) too much absorption at higher frequencies.  A lot of people when they build bass traps like that (get a piece of Rockwell or Cotton) and cover all the corners and put it all through the room and that stuff does absorb bass but it does also absorb treble all the way up through the audio spectrum. If you get too much of it in that way it is definitely possible to make the sound too dead.  That is sort of an unusual kind of space to be in.  One common form of acoustic treatment is to put foam on the walls and just cover all the wall surfaces with foam, like an interesting pyramid foam.  The problem with that approach is that foam that is that thin is not going to do hardly anything at all for bass frequencies. And it is going to absorb a lot of treble.  So you don’t address the bass with that approach at all.  And you make the room dead at higher frequencies and you are left with a room that is really dead sounding and still the bass is out of control.  It is boomy in spots and disappears in others.  It is a very artificial and unpleasant way to listen to anything.  Whereas with bass trapping it is important to strike a balance.  One of the innovations of the Real Trap stuff is that it is actually a hybrid design.  Our bass traps give you more bass trapping with the bass frequencies which is where most rooms need the most help.  And they are semi-reflective at high frequencies.  They still absorb some but they are not fully absorptive as raw material would be.  That curve was engineered very deliberately because it allows you to get the most help where you need it the most and the bass frequencies without making the rooms sound too dead.

Mike:  Alright, well, thanks James we appreciate you taking the time to talk about bass trapping.  If you could be so kind, because we are running a little bit long here to come back and on another episode we can talk about first reflections.

James:  Sounds great.  I am looking forward to it.

Mike:  Alright.  Thanks again.  Talk to you then.

James:  No problem. Thank you.