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

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.

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!

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!

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.

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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

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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...

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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

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

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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.