I use the ominimic v2 for my measurements. Would you reconmaned using a parametric to flatten the above mention problem area or try to fix the problem in the room? Thanks, Sam

I’m glad to see that my comments are well taken.

I hope I didn’t underplay the DSP card. The improvement our customers experience is usually very significant.

It seems to me that the solutions you have investigated doesn’t fully represent the offerings that are available. We are among a few companies that offers true time domain correction. It certainly doesn’t “fix the room” but it attenuates parts of the time domain distortions induced by the room in a very fundamental way.

The scope of the time domain correction in Audiolense is frequency dependant. You can correct a really long time window in the deepest bass. The speaker will then basically operate as an active bass absorber and speaker combined. A typical window is 0.25-0.5 seconds @ 20 Hz. In the other end of the spectrum we are limited to address microseconds; basically correcting the timing, phase and shape of the first arrival in the treble, perhaps with the earliest cabinet diffractions included.

It is highly variable how much time domain correction there is any point in doing. Sometimes it just sounds the same when you add more, sometimes there is a break-even point that varies from room to room. The time domain correction is very sensitive to measurement artifacts and measurement quality and also depending on good overall performance from the hardware (low distortion and high linearity). I don’t like to see a lot of grit and noise before the main spike of the impulse response when the user is aiming for a time domain correction.

I frequently use head phones for evaluation. I listen to the pure version, to a version convolved with various room + speaker responses and to versions convolved with the room/speaker response after correction. This has become an integral part of ongoing development but is also a valuable tool when customers need support. It enables me to hear something similar to what they are hearing. The coloration from room and speakers are usually very audible, even in rooms with studio grade acoustic treatment. The good news is; even with a moderate correction – as soon as you start to follow the music it is very easy to hear that the linear distortion (level & time errors) has been dramatically reduced in the corrected version. The room sounds the same but the speakers and the music sound clearly better, so to speak.

When I work on improving the correction algorithms I can apply almost insane quantities of time domain correction and get away with it. It works as a sort of magnifying glass. Then it is easy to hear that the corrected version has significantly less room coloration added than the uncorrected. This proves to me anyway that DSP has some capabilities towards negotiating room effects in a very fundamental way. In a real setup you have to settle for less time domain correction or you will get audible correction artifacts. The time domain correction doesn’t make the room disappear acoustically. But it usually changes the perceived sound quality towards a few dB more direct sound, and this is also measurable improvement.

The biggest win in using DSP is in any case always related to the frequency domain correction. That may not be as sexy on paper as time domain correction, but the frequency response of the direct sound and reflections combined has impact on a huge number of sonic qualities. Some of them are usually not associated with the frequency domain, such as sound stage and resolution. When I discuss DSP with acousticians, they usually emphasize the time domain behavior and I usually find myself defending the significance of the frequency response in the listening seat. In a way we are both right, since the biggest frequency domain problems is caused by (delayed) reflections.

The quality of various frequency domain corrections are highly variable, and you can almost talk about a tradition where mediocre frequency correction is established as the norm. I regard that as the main reason why DSP is underestimated by quite a few audiophiles. For some mysterious reason a lot of audiophiles seem to believe that negotiating baffle step issues, driver issues and Allison effect like issues with high power resistors, inductors and capacitors inside the speaker is as good as it gets. And that doing the same task in the digital domain with far greater resolution and flexibility, and without the heat buildup and distortion associated with passive crossover components is a less puristic solution. Go figure.

You need FIR filters with a long duration to achieve proper correction in the bass with good enough frequency resolution. Not all available solutions provides that. But the most important part is perhaps that the DSP unit filters and analyses the measurement in a psychoacoustically adequate way: A correspondence between how it looks on the graph and how it sound. And this is the hardest part. In theory you can dial in the sound by ear as long as you have enough frequency resolution, but in practice it is almost impossible. For the same reason you also need a calibrated microphone to dial in the system properly. I also noted that you assessed the ability to manually adjust the target in various offerings and I can’t overemphasize the significance of a custom target.

Acoustic treatment and DSP is a very potent combination. Acoustic treatment can negotiate all issues except the deepest bass very well. You can basically choose the decay pattern in the room by design. DSP can compliment and perhaps even do the heavy lifting in the lowest octaves time domain wise where the size of effective diffusers and absorbers takes on abnormous proportions, but dsp can only scratch the surface of the time domain for most of the frequency band.

When the time domain is cleaned by room conditioning the frequency domain becomes much smoother. The roller coaster like frequency response that was before is now reduced to a bumpy road. Part of the bumps will be due to a speaker that was a bit bumpy out of the factory and parts will be remaining of the before acoustic problems. DSP can then be used to almost completely eliminate the frequency bumps. And to tighten up the time domain behavior further. And take the system up to a sound quality level that is unattainable without both acoustic conditioning and DSP. A combination of acoustic treatment and DSP is a very potent one. That’s why I like to discuss these matters with people who covers the middle ground between DSP and acoustic conditioning.

Very good points and I generally agree with everything you have said. For reference I am not anti room correction. I use it in my personal system as a compliment to traditional acoustic treatments. I would write some small clarifications on your points but they would be quite theoretical and maybe not so interesting for people who read this zine…

Of course the article was intentionally written to try and be black and white. I wanted to provide a simple to the point response to the marketing bs from the room correction companies who say that their magic box can fix all the acoustical issues of our living spaces. I am convinced that someone at these companies has a good understanding of what can and cannot be fixed but that it gets lost or glossed over as it goes through the marketing department. It is refreshing to hear from someone in this domain who openly acknowledges that room correction can’t fix everything.

For those with further interest in the topic I would direct them to the room correction series on my website at http://www.acousticfrontiers.com/whats-new/2010/7/1/room-correction-a-primer.html and http://www.acousticfrontiers.com/whats-new/category/room-correction.

Speaker Boundary Interference: The frequency plot in the article is very representative of problems we often see amongst our customers. It seems like very few speaker manufacturers are successful in getting the deep bass and the lower midrange right at the same time. But this is a situation where digital sound correction can do a really good job.

If you boost a sharp depression, like the one around 80 Hz in the graph, with 6 dB, you will meassure and hear 6 dB more output of direct sound and reflections combined. It doesn’t matter that the caused by out of phase reflection doesn’t make a difference. As long as there is sound there it can be amplified.

This is a speaker with full range output and it will likely handle a lift of 15 dB around 80 Hz with ease if that’s what the doctor orders. A good correction will not eliminate the 80 Hz dip completely. The wide depression between 80 and 160 Hz will basically be gone, but a thin crack at 80 Hz will most likely remain.

The biggest benefit of using DSP is usually to get the overall frequency balance right. But there are a few pitfalls and limitations involved:

1) The freauency response – such as the one we see in the frequency plot is the stationary response of the system. If you play a 80 Hz note for a long period, it will settle on 68 dB output. But the signal may be much stronger before it settles. This is often the case. The dips aren’t relly as deep as the response plot shows. If you correct the dips 100% you end up with temporary peaks and get the hollow sound that everybody talks about. This is a general problem that can lead to overcorrection unless the measurement is properly analyzed.

2) A lot of people treat EQ and FIR based correction as if they were basically the same animal. Correcting such deviations with precision, using traditional EQ is however close to impossible. You get too little here and too much there, and possible a hollow sound because of boosting a few frequencies that are already strong. You need good frequency resolution and a flexible approach to make a mirror image of the response you wish to correct. A precise FIR based correction is what you need to obtain a really good result.

3) Correcting deep dips and high peaks and sharp wrinkles in general will require a filter that has more ringing than beneficial. You want to keep the filter as short and as simple as possible. So you get the best result by reducing the peaks and dips significantly rather than eliminating them completely. The best DSP solutions does this by default.

Strong early reflections: The strong early reflections will blur the stereo image and distort the timbre. A correction doesn’t remove the reflections, but it does restore the stereo image and the timbre very well. Strong early reflections will sometimes also have a negative impact on the midrange clarity even after the timbre is corrected. This lack of clarity is on my list of uncorrectable items.

Long decay: I agree with what is stated in the article. Too long decay tends to mask the ambience on the recording and other low level cues that are really important for a state of the art presentation of the music event. DSP falls short in this department from around 100 Hz and up. You can tighten up the bass but you can’t tighten up the decay.

A lot of the ongoing discussions of acoustic treatment vs dsp is limited by too much black or white thinking. A correction that is well done will always sound better than no correction. If you want to achieve the best sound possible you need good acoustics and DSP. And the icing on the cace is that a good time domain correction can compliment acoustic treatment, bringing the waterfall under control in the two bottom octaves.

I find the label “room correction” somewhat misleading. It is the mother of false claims and the father of straw men. What you can do with DSP is to optimize how the speakers interacts with the listener(s) in a given room. Nothing more and nothing less. The positive impact of Audiolense is usually dramatic when the speaker plus room behaves as the measurement in this article shows. The improvements are usually more subtle, but still significant in a well treated environment such as a monitoring studio.

Be aware that the differences between various dsp offerings can be massive. There are no unifying standards for how to interpret a measurement or how to make a correction. Two different DSP solutions are likely to perform different even though they seem to use the same principles.

Best Regards,
Bernt Ronningsbakk
Juice Hifi

Live end dead end was a concept that came out of the studio world. It was not a treatment scheme designed for home audio reproduction but for the control room. I don’t think it has any particular relevance to us as the priorities in a home environment are much different from those in a control room. Even the speakers we use are designed differently from those used in control rooms – off axis response is very important to the quality of home audio reproduction since we listen in reflective spaces whereas if I am a designing a speaker for a control room with absorption for all off axis sounds then that parameter is much less important than say maximum SPL.

Nyal

Any opinion about what is to be gained, or not, from the live end/dead end concept?

Jim – your speakers are pretty close to the sidewalls; normally I recommend at least 4ft away from the sidewalls or if they are closer then definitely experimenting with acoustic treatment. Depending on your personal preference for soundstage focus vs spaciousness then absorption (better focus) or combined absorber / diffusor panels (better spaciousness) work best here.