You are here

Log in or register to post comments
dbowker
dbowker's picture
Offline
Last seen: 1 month 1 week ago
Joined: May 8 2007 - 6:37am
Taming a 40hz spike?

OK- so I'm hoping for some advice from either speaker designers or DIYers who have experience building speakers. I recently completed some DIY Pro-Ac Response 2.5s that sound every bit as good as the originals IMO, in some areas better maybe, but I think perhaps they are overpowering my room a bit. I noticed some lower bass bloat coming through in some recordings (not all, just ones with that area highlighted, like with a big bass drum).

I pulled out my old Stereophile test CD and played through all the tones and without even measuring (no equipment but my ears) and sure enough the 40hz tone jumped right out of the mix. All the other tones seemed entirely natural and of a similar volume level, if you get my meaning.

So... I don't think my otherwise highly supportive wife is going to go with any obvious room treatments, nor have the speakers more then the 24" out of the room they already are. But I'm willing to look into internal tweaks like different damping materials or tuning the port some more. Obviously the trick will be too affect that 40hz area and nothing else.

The room is 12x15 BTW with 9' ceilings, a heavy rug covering most of the floor, medium weight shades covering the windows, a decent amount of upholstered furniture and some other items to help with resonance. Anyway, suggestions are welcomed! Thanks!

Jan Vigne
Jan Vigne's picture
Offline
Last seen: Never ago
Joined: Mar 18 2006 - 12:57pm
Re: Taming a 40hz spike?

You need bass traps. Big bass traps. Or a parametric EQ.

struts
struts's picture
Offline
Last seen: 3 weeks 1 day ago
Joined: Feb 1 2007 - 12:02pm
Re: Taming a 40hz spike?

Sounds like your 1L mode which for a 15ft room will lie at 38Hz.

The problem is the room, not the speakers so I would advise against trying to fix the speakers. You will almost certainly do more harm than good if you start messing with port lengths etc.

There are basically three tools you can use:

  • Speaker placement
  • Room treatments
  • Parametric EQ

However, don't despair! A lot can be achieved with (i) - particularly in your case the distance from the port to the rear wall and it's relationship to the room length - and subtle applications of (ii). Remember that the 40Hz output from the speaker will be coming from the rear firing port so it may be possible to deploy room treatments behind the speakers where they are less conspicuous.

For further reading I would recommend the Rives Audio website as a great place to start. Check out their Listening Room to understand the 'magic numbers' for your room and then download their 'Top 10 Tips' white paper for suggestions how to tackle it.

Good luck!

PS I'm in a hotel room right now and it suddenly struck me, 15x12x9 is a pretty small room for full-range speakers like 2.5s. I would expect the amount of LF energy they put out to cause bass problems in a room that size. If the above suggestions don't work out for you I hate to say it but you could consider smaller speakers.

dbowker
dbowker's picture
Offline
Last seen: 1 month 1 week ago
Joined: May 8 2007 - 6:37am
Re: Taming a 40hz spike?

Thanks for the suggestions. As I said it's actually not a problem with about 70% of the recordings I've played so far, but a few can be annoying for sure. So are we talking absorbtion or diffraction as room treatments? Thanks again!

struts
struts's picture
Offline
Last seen: 3 weeks 1 day ago
Joined: Feb 1 2007 - 12:02pm
Re: Taming a 40hz spike?

Absorbtion. The more advanced products are tunable so you can almost surgically target quite narrow frequency bands. I have seen my dealer's 'room team' setting up a hotel room for a show and it is quite incredible what they achieve with only a couple of carefully selected treatments and a wealth of experience.

Room modes are determined by the dimensions of the room and so the only programme-dependent variable is how much content there is at the frequencies that excite them. If you mostly listen to chamber music, choral or acoustic guitar this will be very little for a 40Hz mode. But that once again begs the question why you selected a full-range transducer?

Apologies if I appear critical, I am not trying to be judgmental. I am just trying to offer you an outside, and completely dispassionate, perspective.

PS Can you diffract sound? Maybe you can. Have to confess I've never really thought about it!

dbowker
dbowker's picture
Offline
Last seen: 1 month 1 week ago
Joined: May 8 2007 - 6:37am
Re: Taming a 40hz spike?

No offense taken- I do listen to a range of music that certainly is "full-range", from jazz to classical to rock, but you'd be surprised how few recordings spend much time in my problematic area of 40hz. It seems that almost everything by Alison Krauss has a lot of acoustic bass guitar, stand up bass and a big kick drum, all of which hover around the 40hz range. They still separate out extremely well from one another, it's just that they take on more importance than they should! Perhaps I've overstated the problem, because otherwise these are the best sounding and most naturally detailed monitors I've ever owned. I'm probably the only one who even notices the lower bass bloom, which, above or below seems fine. So this is more to make them "perfect."

I'll look into bringing them out a bit more first, and then into true room tuning. I still might try adding a bit more material inside the cabinet to bring some some of the air flow- if it adversely affects any other tonal region it's easy to pull it back out, right? Thanks again!

struts
struts's picture
Offline
Last seen: 3 weeks 1 day ago
Joined: Feb 1 2007 - 12:02pm
Re: Taming a 40hz spike?

Sounds like you have a plan! Don't want to put you off but in my experience tonal balance is the easy (easier) part to dial in 'by the numbers'. The room team guys reckon that they can usually get to within +/- 2" on paper (we are not worthy)! The challenge then is often fixing the soundstaging without breaking the frequency response.

Of course they are usually starting with the third or fourth best option anyway because of domestic constraints...

However a quick read of the Rives stuff should enable you to make a very good first guess and you can just tune by trial and error from there. You trust your ears which is the key.

Should be fun and you'll hopefully be enjoying 'fine tunes' in no time. Good luck!

cyclebrain
cyclebrain's picture
Offline
Last seen: 2 years 6 months ago
Joined: Jun 16 2006 - 11:40pm
Re: Taming a 40hz spike?

It will always be something with your speaker/room interface. Moving your speakers will quite possibly fix your 40Hz rez problem. At the expense of exciting another mode. Often things will seem just fine untill you play that one song that excites your speaker/room/listening rez frequency. Also you could try changing your listening position.

ethanwiner
ethanwiner's picture
Offline
Last seen: Never ago
Joined: Sep 1 2005 - 2:26pm
Re: Taming a 40hz spike?


Quote:
Often things will seem just fine untill you play that one song that excites your speaker/room/listening rez frequency.


Exactly, and this is why it's a lot more efficient to move things, and assess the improvement from adding bass traps, by measuring rather than listening. Otherwise, you're dependent on the key of the music.

--Ethan

struts
struts's picture
Offline
Last seen: 3 weeks 1 day ago
Joined: Feb 1 2007 - 12:02pm
40 Hz spike? Get one of these!


Quote:
Exactly, and this is why it's a lot more efficient to move things, and assess the improvement from adding bass traps, by measuring rather than listening.


Couldn't agree more Ethan! And for what must be the ne plus ultra in efficiency beg, borrow or steal one of these. This is what my dealer's 'room team' uses and he lent it to me for a week when I was setting up my new system. It contains a signal generator that you plug into the preamp; you set it to generate pink noise which contains components at the same level all across the audible spectrum. Then you can literally 'see' the response at the listening position on the screen of the analyzer.

Compare this with the old 'one frequency at a time' method with an SPL meter and test CD, no contest! So if anybody wonders what I'd like for Christmas...

dbowker
dbowker's picture
Offline
Last seen: 1 month 1 week ago
Joined: May 8 2007 - 6:37am
Re: 40 Hz spike? Get one of these!

Thanks again guys for the further suggestions. Haven't had any time yet, other than to move said speakers out another 6" which already is helping some. I'll look into some measurement devices and take it from there.

ethanwiner
ethanwiner's picture
Offline
Last seen: Never ago
Joined: Sep 1 2005 - 2:26pm
Re: 40 Hz spike? Get one of these!


Quote:
And for what must be the ne plus ultra in efficiency beg, borrow or steal one of these.


Just to be clear, that type of analyzer is more suited for large spaces like churches and auditoriums than for rooms the size you'll find in most homes. Small room analysis requires finer resolution than third octaves, and also should be able to measure decay times at very small bandwidths. This article explains much more about the measuring requirements for small rooms:

http://www.realtraps.com/art_etf.htm

--Ethan

struts
struts's picture
Offline
Last seen: 3 weeks 1 day ago
Joined: Feb 1 2007 - 12:02pm
Re: 40 Hz spike? Get one of these!

Thanks Ethan, interesting reading!

As usual I get to 'the next level' only to find there's (at least) another one above that which I never knew existed

dbowker
dbowker's picture
Offline
Last seen: 1 month 1 week ago
Joined: May 8 2007 - 6:37am
Re: Taming a 40hz spike?

OK- I think I fixed the problem, or at least minimized it to the point I don't notice it. So first I moved the speakers out a bit, then tweaked some furniture positions some and lastly tweaked the air flow of the port. I didn't want to cut it off or anythings so what I did is this: I took out the ports and wrapped the inner end with some soft cloth (staining cloth which is like an old soft T Shirt basically but two layers). What this did was let air through but take some of the force out. From what I can hear it did not markedly affect any other tonal range than the troublesome 40hz range other than slightly take some of the 20hz energy. It's still there but some of the ear pressure I felt before hand is reduced. Not a big deal to me and a fair trade off from a genuinely annoying spike in the 40hz range.

The other idea I have is constructing a constrained layer damped panel to mount off the back of speaker baffle. It'd only need to be slightly smaller than the width of the back face and maybe a bit longer vertically. It could stand off the face about 2". As a former industrial designer it seems a logical solution to a common problem. You want all the air flow but not all that bass energy hitting your back wall. If you could effectively diffuse and absorb the majority of it but keep the air flow open you're all set. What do the pros here think of this idea?

I know most people would hesitate to permanently alter their speaker, but I made these anyway so I'm OK with modifications. But the beauty of it is that it would hardly be visible at all unless you went around to the back so it works with the ever present WAF. As a bachelor in years past I made some pretty effective home made bass traps and reflection baffles, but hell would freeze over before that kind of thing made it onto our living room walls, no matter how finished and professional they looked.

Oh well, if I work these back baffles may I'll post a DIY set of instructions and pictures for those looking for a middle ground solutions. In the meantime, like I said, I think I pretty much got my problem tones tamed.

KBK
KBK's picture
Offline
Last seen: Never ago
Joined: Sep 30 2007 - 12:30pm
Re: Taming a 40hz spike?


Quote:
No offense taken- I do listen to a range of music that certainly is "full-range", from jazz to classical to rock, but you'd be surprised how few recordings spend much time in my problematic area of 40hz. It seems that almost everything by Alison Krauss has a lot of acoustic bass guitar, stand up bass and a big kick drum, all of which hover around the 40hz range. They still separate out extremely well from one another, it's just that they take on more importance than they should! Perhaps I've overstated the problem, because otherwise these are the best sounding and most naturally detailed monitors I've ever owned. I'm probably the only one who even notices the lower bass bloom, which, above or below seems fine. So this is more to make them "perfect."

I'll look into bringing them out a bit more first, and then into true room tuning. I still might try adding a bit more material inside the cabinet to bring some some of the air flow- if it adversely affects any other tonal region it's easy to pull it back out, right? Thanks again!

Get a pair of pantyhose. First try a single layer held over the inner end of the port, with an elastic. Try two layers if that does not work. Try this trick if it is indeed a resonance in the speaker system itself. You are looking to slow the peak acceleration (modulation) of the 'block' of air in the port, just a tad. Fill the port with a foam plug to do a 'check', first - to be sure if it is the box or the room. The pantyhose filter will slightly flatten out the 'peak' of the system resonance, without damaging the rest too much. Like anything, it's a trade off.

edit: I see you got it already. I was replying from reading the first page.

In one of my earlier incarnations, I was the box designer for a car audio shop. I took the job to get the opportunity to design and execute what ended up being about 2-3000 different woofer boxes. It taught me that ported boxes don't work. Even though about 99.9% of the boxes I designed and installed were ported.

dbowker
dbowker's picture
Offline
Last seen: 1 month 1 week ago
Joined: May 8 2007 - 6:37am
Re: Taming a 40hz spike?

Thanks KBK- I'm glad to see I came to the same conclusion that you suggested! This is first ported boxed I've built, so that fact that I came upon a similar tuning solution as someone who's built over a 1000 bodes well. A few weeks on I'm still quite satisfied with how it worked out, and having played many different albums by now I'm confident it fixed the problem while NOT causing any new ones. It's really a good tweak for the many folks out there with ported speakers that may be sending out too much bass energy for their room. A pretty common problem I bet!

In the past I've added better damping materials to other, commercially manufactured speakers (especially subs) and have gotten some good mileage out the effort. I definitely encourage folks to try out such tweaks, as long as it's something that can be taken away if they don't like the result!

  • X
    Enter your Stereophile.com username.
    Enter the password that accompanies your username.
    Loading