How wide is a bass note?

[Islander]

I recently moved my speakers and furniture, in effect rotating the whole room by 90 degrees, to enable me to move the speakers further apart and have a more symmetrical layout for the surround speakers. There's been a big improvement in the soundstage and some improvement in fine detail, since the speakers are now almost 12 feet apart (center-to-center), instead of the 5 feet they had been. They're also now the same distance from the front wall (4 inches to one rear corner of the speaker, 13 inches to the other, toed in to the listening position) instead of 1 foot for one speaker and 10 feet for the other, since one was placed where the living room opens to the dining room and hallway.

As well, I now have the sub between the speakers, which should be a good thing. However, the speakers' bass response is less even than it was before, and adding the sub doesn't help. Where I had one small dip and a plateau or two, I now have a three big dips and a peak that wasn't there before. Adjusting the sub's phase made some improvement, but the response curve is still kinda lumpy.

My question is this: how significant are these dips and peaks? Does a single note occur at, say, 50Hz specifically, or does it sound from perhaps 40Hz to 60Hz? If the response is at reference level at 45Hz and 55Hz, but down 10dB at 50Hz, does it merit trying to get rid of the dip, or should I just ignore it unless it spreads over a 20 or 30Hz range?

[DrWho]

I think a google search on "room modes" would answer most of your questions. There is nothing about the size of the room that limits which frequencies are being reproduced by the subwoofer - but there is interference where the bass cancels itself out due to the reflections in the room. Try walking around the room - especially the corners and note the bass response. I bet in some places it's going to sound very boomy. Basically you're going to have to play with moving the subwoofer around until you get bass at the listening position.

I hope that answers your question - at least partially.

[Islander]

Thanks for your response, Mike, but it's not exactly what I'm looking for. I've been shifting my speakers and sub around and trying various phase and hi-cut settings for a week (28 200Hz-20Hz runs so far) and I'm trying to decide when to quit. It's more important to please my ears than my SPL meter. Most of the changes have produced incremental improvements, like closer to reference level at 6 points, but further away at 3 other points. So I try another change and get a few more improvements.

For example, moving the speakers a couple of inches further from the front wall evened things up a bit, but two more inches and eleven more inches did nothing, so I shifted them back to the "plus 2 inches" position. Luckily, the La Scalas slide around easily on the carpet, since my early models have flat bottoms.

To rephrase my question, when listening to most types of music, how important are narrow peaks and dips in the bass area? Does the average note actually spread across a few frequencies, so that narrow peaks and dips are almost inaudible? Thanks in advance if you or anyone else can shed any light on this.

[DrWho]

How exactly are you taking your measurements?

You're actually asking a very loaded question - most of which revolves around the issues of room acoustics. I think you'll find this article helpful:
http://www.ethanwiner.com/acoustics.html
Though it's geared towards acoustical treatment as the solution, the concepts still apply to speaker position. But generally speaking - looking solely at the frequency response is the wrong way to be looking at the time-domain issues you're hearing the effects of

Generally speaking - dips narrower than 1/3 octave are inaudible. Octaves work on a logorithmic scale - so a doubling of frequency is one octave. In the bass regions, a 50Hz to 100Hz jump is one octave. So roughly speaking, one note is 6Hz wide in that region. 500Hz to 1000Hz is also one octave, and one note is about 60Hz wide in that region.

Anyways, the reflections in the room causing the measured dips in the response are arriving slightly after the direct sound. Since most of these reflections occur "quickly", our ears hear the combined sound as a single sound - and in the case of destructive interference, we only perceive a partial reduction compared to any long term measurements we're taking.

Also, one of the dangers of basing things off measurements alone is the dramatic nature in which the frequency resposne changes with even slight changes in the mics position. There are some great graphs showing the effects here:
http://www.ethanwiner.com/believe.html

[Islander]

Ah, that clears it up a bit! Thanks. I've been doing measurements using a test CD and an RS meter mounted on a tripod. I can understand frequency response, but time-domain stuff is still a bit over my head. I think I first heard of that on this forum.

The sub-200Hz zone includes around 3 octaves, so I figured it was a significant part of the music spectrum, thus the time taken to try to get it straight. To show my level of ignorance, I thought a flat frequency response was the ideal situation, but some forum members seem to feel it's secondary to certain other factors. You guys are so far ahead that I'll have to get studying just to catch sight of your dust disappearing over the horizon. That's okay, I like learning new stuff.

Main thing is that the system sounds great to my ears. I'm just trying to wipe a few dust spots off the sound stage.

[DrWho]

Frequency response is still important - it's just that the factors contributing to the dips and peaks are the results of behavior that happen over the time-domain. It's just a fancy way of sayting reflections in the room don't arrive at the same time as the direct sound.

As far as your measurement technique...you might give this a try:
http://www.hometheatershack.com/roomeq/
It's free and effectively measures the impulse function of your speakers/room, and then provides you with spectral and waterfall plots in addition to a windowed frequency response plot. All you need is a computer with a decent soundcard - the help files are very good at walking through the entire process of setting it up. And I believe there are compensation files that you can download for using your SPL meter. It will be a lot faster and a lot more informative as to what's going on in your room. I've been there with the whole test tone and SPL meter thing and I think you would prefer this a lot more.

What are you using for mains and subwoofage? The last time I was at Colter's place we were measuring the response of his lascalas and noted that a 100Hz crossover to his Ultra2 subs resulted in the flattest frequency response, but an 80Hz crossover sounded much better [] In fact, if you do any movie watching at all, I would recommend 80Hz regardless of the speakers and subs.

Anyways, I'm by no means the acoustics expert, but I've been trying to learn. I'm surprised nobody else has chimed in yet...

[Islander]

Man, you weren't kidding about meter/mic position and comb filtering! I did a test run today and tried moving the meter slowly sideways a couple of feet. At 50Hz I got from 64dB to 77dB! So I did the 2-foot-wide sweep for every tone and picked the middle value. The response doesn't look so bad now.

During some of the lowest tones (35Hz and 30Hz), I noticed a humming from the direction of the wall behind the sofa (on the opposite side of the room from the speakers), as a nasty peak was causing the whole room to resonate. I took a 4'x4' sheet of Cor-Plas (that plastic stuff that's just like corrugated cardboard) and leaned it against the wall in that area. It knocked 2dB off the peak, changing it from really annoying to slightly annoying.

I took a look at that link you provided and it looks pretty interesting, but my computer's at the other end of the apartment, about 50 feet from the receiver. Do I have to run a lead from the meter (or a mic) to the computer and another one from the computer's sound card to an input on the receiver? I haven't listened to much music on the computer since I set up the living room system.

As for hardware, I'm using a pair of La Scalas (with re-capped AA xovers and CT125 tweeters) and a Paradigm PW-2100 (10" driver and 400 watt amp). In the previous location on the short north wall, the response was smoother, but was down 16-17dB from reference level at 25Hz. I was fine with that, but now it's only down 5dB at 25Hz, so I got a bit more bottom just by moving everything to the longer west wall. Works for me.

[DrWho]

Ya, you need two runs from the computer. One sending the measurement
sweep from the computer to the receiver and then another one going from
a microphone to the computer.

If this isn't an option, then there
are ways of measuring without the computer by creating a CD with the
frequency sweep and then recording the output of the mic into some
portable source. You would then dump it into your computer, deconvolve
the sweep into an impulse response and then dump it into whatever
software you wanted to use (like Room EQ Wizard). This approach is a
bit more involved, but I think I can walk you through it (though I only
just learned how to do it....). If you (or anyone) is interested, I can
hunt down the necessary software, manuals and even provide the
frequency sweeps (it's all free). The only downside to this is that
your accuracy will be limited by that of the equipment you're using.
That's the nice thing about REW - it has a function where it can
measure the frequency response of your sound card and apply
compensations for it and your particular mic.

For what it's
worth, I'm on the 3rd floor of my apartment and have run cables out to
a friend's car in the parking lot to do measurements...granted, I have
a bunch of XLR's laying around, but the results I got seemed fairly
accurate. Anyways, all that to say that a 50 foot run is doable. It's
pushing it for unbalanced lines, but if you can crank the volume when
measuring you should be able to avoid the noise floor.

If I
haven't mentioned it already, you can get a calibrated mic and a
phantom power source for around $100 ($50 each). If your soundcard
isn't suitable for measuring, then you can just get an external
soundcard that provides phantom for only a little bit more. I was able
to get my entire setup for $110 including a 50 foot cable and free
shipping [Y] Hardly a major investment considering the amount of fun
I've gotten out of it.

What are the dimensions of your room? And
is it a sealed room or open to the rest of the apartment? A very simple
experiment and a good lesson in reflections is the concept of the 1/4
wavelength boundary cancellation:
http://www.peavey.com/support/technotes/soundsystems/boundarycancellation.cfm
With
that article in mind, it's interesting to think about dips you see in
the frequency response in terms of wavelength and distances of the
speakers from the walls. You can also run a mode calculator to get an
idea what frequencies are going to result in standing waves:
http://www.realtraps.com/art_tuning.htm
And here's a 1/4 wavelength calculator:
http://www.realtraps.com/sbirlbir.htm
For
what it's worth, these prediction programs aren't meant to be accurate
predictions of what is going on in a real room. They're just good for
giving you a feel for where you might expect problems, or why you're
experiencing a problem at a particular frequency...like your 30Hz boom
for example? I know I'm going to sound like a broken record, but that's
another reason why the time-based measuring methods are so cool....you
can create waterfall plots and see how the sound decays at each
frequency. The room modes will be extremely obvious as they will decay
much slower than other frequencies.

Room modes are a lot of fun
and this spring break I will be doing some demonstrations on small room
acoustics for Engineering Open House. We're hoping to have subwoofage
capable of 140dB+ and then have people stand in the middle of a null in
the room - then have them move to a peak to experience the importance
of acoustics [H] But I was going to recommend doing this - maybe not
measuring every frequency, but walking around the room and noting
extremely low dips. You should be able to find locations where the sub
is playing at 100dB+, but you're measuring something under 60dB on the
meter.

[Islander]

Mike, I wanted to try one more thing before I got back to you. This morning, I moved the sub to the south wall so that it would be aimed toward the end of the room that opens toward the kitchen and dining room and did a test run. The 35 and 30Hz resonance was actually worse (I got a headache within a minute that lasted half an hour), but I was able to identify the culprit.

The four-by-five-foot window above the sub was really humming loudly, so I moved the sub back to its position between the speakers. I'm not about to Dynamat one of my living room windows, so I think I'll leave everything as is. Most music sounds really good, with the exception of some bass-heavy stuff that always sounds a bit unpleasant anyway. Since the lowest note on a 4-string bass guitar is around 40Hz, there's not much music below that.

The room is 18' x 19' x 8' high and opens out to 26' from the 19'. I've had the sub near the half-wall (9' wide, 8' high), near the long wall (19' x 8') between the speakers and near the short wall (18' x 8') and got the best results in the second location.

I considered using my old Yamaha EQ-70 to EQ down the problem area, but then I read an article in Ultra High Fidelity that clarified the "time" thing for me. UHF, a Canadian hi-fi magazine, has been running a series of articles by Paul Bergman, who really seems to have a clue. In this month's issue (number 78), the topic is "Taming Reverberation".

A quote: "In most rooms, perfectly flat response when you are seated right behind the recording console may not be so flat for the person sitting next to you, who may be the producer making the creative decisions, nor for the musician who comes and sits in the back of the control room to hear how she sounded.

A second problem is that time and amplitude interact in strange ways. By "time" I mean phase, and perhaps I can best explain it this way.

Let us say you measure a peak at 410 Hz. If we suppose that the speaker is without fault, then it must be the fault of the room. There is a standing wave at 410 Hz, and the acoustical energy stays around longer than it should, therefore seeming louder. However it is not truly louder. Rather, energy is being stored, and what you hear is old sound. If you EQ out the peak, you will actually have a dip at 410 Hz, which appears to be filled in by old information stored in the room."

So I conclude that with the adjustments at my disposal, namely speaker and sub positioning, sub level, phase and cut-off frequency, I've got it good enough for enjoyable listening and should stop tweaking and obsessing for a bit. Thanks for taking the time to point me in the right direction. Now it's time to listen to some music!

[DrWho]

Could you post a rough sketch of all the layouts you tried? Even something as rough as a rectangle drawn in MS Paint a few X's to indicate placements []

You mentioned that your window vibrates...does it go away if you press on it? If your windows are able to move freely like that, I might wonder how well they're sealing and keeping the cold air out? I've never heard of vibrating windows before! That's gotta get annoying.

Interesting snippets from that article you read. Another live sound engineer once told me that the only spot in the room that needs to sound good is the one where your paycheck is coming from. I've never thought of applying it to the studio situation - in fact, that might explain quite a lot of the poor decision making processes that go on in the studio. [^o)]

Along those lines, one of my most memorable lessons was mixing a live outdoor concert....we finished setting up the FOH / sound system 15 minutes after it was supposed to start and basically had to let the band start playing before we could do any sound check or system alignment. It was a hectic time and I sat there cranking the knobs and riding the faders trying to make it sound good. A good friend of mine, also a studio engineer and later a studio drummer, slapped me upside the head about 20 minutes into the show and told me to shut up and enjoy the music. I wasn't expecting to get hit upside the head, lol, but the lightbulb went on...I percieved everything as sounding bad because I had my critical ears on - trying to hear all of the flaws and trying to minimize them as best I could. I sat back, took a deep breath and changed my focus - I couldn't believe how much better everything sounded. It was the wierdest experience ever. I just wish it would happen more often when listening to music at home []

[Islander]

Mike, I haven't been ignoring you, but I've had no time to make some sketches yet. As for the window vibration, it's not loose in the frame, it's a solidly-mounted double-glazed window that hums loudly as the room resonates at 30Hz and 35Hz. I think it would take several people putting their hands on the glass to make it stop, since my one hand had little effect. And that was at just 88-90 dB. Reference level was 80dB.

Today I heard about Auralex products, so I may check out some of their bass traps. Have you heard of them? They're at http://www.auralex.com/

[DrWho]

Yea, Auralex makes a lot of products. I believe they're based out of Indianapolis (or somewhere close) so we were hoping to have them give some demonstrations to the Audio Engineering Society on campus. Here are some measurements taken of their product:
http://www.realtraps.com/data.htm

The real traps stuff is more expensive, but apparently more effective - Ethan Winer (the guy behind realtraps.com) seems like a pretty straight up guy, but I still wonder how much faith can be put into the measurements ("marketing"?)

If your issue is actually at 30,35 Hz then I don't think the classic corner bass traps are going to solve your problem. The wavelengths are just too huge. You would probably be better of building a helmholtz resonator, but apparently they gotta be rather huge to be effective at lower frequencies. It's basically a tuned surface or tube that resonates at a particular frequency in such a way as to be out of phase with the resonance.