The Perfect Solution for Home Theater Bass Optimization Here!

[andy_c]

I should add that early in the video, there's an error regarding the claimed output voltage for the miniDSP unbalanced 2x4. Its maximum output voltage is 0.9 Volts RMS, not 2 Volts RMS. There is an input jumper that allows the user to switch the input sensitivity of the device between 2.0 Volts RMS and 0.9 Volts RMS. With the former setting, an input voltage of 2.0 Volts RMS results in an output voltage of 0.9 Volts RMS, which implies a loss of 6.9 dB. With the latter setting, an input voltage of 0.9 Volts RMS results in an output voltage of 0.9 Volts RMS, implying a 0.0 dB loss. Regardless of the input jumper setting, the maximum RMS output voltage of the unbalanced miniDSP 2x4 is 0.9 Volts RMS. This is very important, because it means that if you want to drive your subwoofer amp to full power, that amp must have an input sensitivity of 0.9 Volts RMS or less. This is not very common, but can be found with careful selection.

The balanced miniDSP 2x4 puts out a maximum of 4 Volts RMS in the balanced mode, so those who use pro amps for their subs will have no problems with maximum output.

Another alternative is the recently released miniDSP 2x4 HD, which is unbalanced, but has a maximum output voltage of 2.0 Volts RMS, plenty to drive almost any pro amp one might use for a subwoofer. It also solves a problem with the standard miniDSP 2x4, which is a maximum delay of 7.5 msec. This is too low for many, perhaps most multi-sub applications. The HD version has a maximum delay of 80 msec, which is far more in line with what might be needed in practical installations, especially those with fairly large rooms, in which the subs have a distance difference from the main listening position of more than 8.45 feet.

One needs to be very careful with the choice of miniDSP products, especially when considering the unbalanced 2x4 (non-HD version). This has been the subject of many posts over several years. Even the balanced miniDSP 2x4, which has plenty of output voltage for any application, is nonetheless handicapped by the 7.5 msec maximum delay limitation.

 

[agarwalro]

Using multiple subwoofers, to address room modes and achieve uniform FR in multiple seats, seems fairly logical.

How does installing multiple subwoofers remove the need for bass traps? Isn't RT60 of 300ms the metric corresponding to "quick" and "clean" bass?

 

[agarwalro]

One needs to be very careful with the choice of miniDSP products, especially when considering the unbalanced 2x4 (non-HD version). This has been the subject of many posts over several years. Even the balanced miniDSP 2x4, which has plenty of output voltage for any application, is nonetheless handicapped by the 7.5 msec maximum delay limitation.

Thank you for bringing up light these important limitations that often get buried in the chatter.

 

[agarwalro]

Using multiple subwoofers, to address room modes and achieve uniform FR in multiple seats, seems fairly logical.

How does installing multiple subwoofers remove the need for bass traps? Isn't RT60 of 300ms the metric corresponding to "quick" and "clean" bass?

Bueller... Bueller... Bueller.

 

[andy_c]

How does installing multiple subwoofers remove the need for bass traps?

Your question is written in such a way as to assume the burden of proof rests with those who claim there is no "need" for bass traps. For this argument to make sense, one must have solid evidence that such a "need" has been established in the first place, when in fact nobody has done so. There may have been some cases for which the use of bass traps has been established as sufficient, but nobody has even come close to establishing that they are necessary.

In fact, the use of bass traps is problematic for a number of reasons. The ones that that convert acoustic energy to heat (acoustic absorbers) must have dimensions, including thickness, that are non-negligible compared to a wavelength. But the wavelength at e.g. 100 Hz is 11.26 feet. Other types of bass traps exist, but they have their own problems. There are a lot of questionable vendors of room treatment products, including one guy who has written a book claiming himself to be an "audio expert", when in fact he has no engineering, physics or mathematical background at all. The claims of such people ought to be rigorously investigated, not taken at face value.

I'd suggest getting Floyd Toole's book and reading chapter 13 for more information, as the answer can't be written in a forum post of reasonable length.

 

[agarwalro]

I bring it up since the reverberant nature of small rooms and need for dissipating acoustical energy is conveniently swept under the rug, in lieu of multiple subs and EQ.

No, bass traps and other acoustical panels are far from the end of and be all of dissipating acoustical energy to achieve better listening conditions in small rooms. For example, Siegfried Linkwitz suggests bipole speakers.

From here, http://www.linkwitzlab.com/rooms.htm section C4: [emphasis mine]

It has been suggested (R. Walker, BBC, 1998) that the reverberation time T60 over the 200 Hz to 4 kHz frequency range be adjusted to T60 = 0.3 (V/V0)1/3 where V0 = 100 m3 (11) with a tolerance of +/-50 ms which is allowed to increase linearly to +300 ms between 200 Hz and 63 Hz.

The room of Example 2 should thus have T60 = 300 ms +/-50 ms. This makes for a subjectively quite dead room, which is fine if the room is dedicated solely to Home Theater and surround sound, but is in my opinion a very overstuffed environment for normal living. It has the effect of making the reverberation distance xr = 1.04 m for the monopole and xr = 1.8 m for the dipole. At a viewing/listening distance of 2 m the direct sound is only about 6 dB below the reverberant level of the monopole which is good for sound clarity.

Instead, you could use a dipole, increase T60 to a much more livable 600 ms and have the same direct-to-reverberant ratio as for the monopole for which the specification was developed.

From the Note in Section C2:

Example 2
Take a room with L = 25', W = 16' and H = 9'

 

[agarwalro]

I'd suggest getting Floyd Toole's book and reading chapter 13 for more information, as the answer can't be written in a forum post of reasonable length.

I'll skip ahead to chapter 13 .

My spring reading is here .

 

[andy_c]

I bring it up since the reverberant nature of small rooms and need for dissipating acoustical energy is conveniently swept under the rug, in lieu of multiple subs and EQ.

No, bass traps and other acoustical panels are far from the end of and be all of dissipating acoustical energy to achieve better listening conditions in small rooms. For example, Siegfried Linkwitz suggests bipole speakers.

From here, http://www.linkwitzlab.com/rooms.htm section C4: [emphasis mine]

You were talking bass traps. In the article he says:

Room treatment can be very effective above 200 Hz, but the same result may be obtained more aesthetically with ordinary furnishings, wall decoration, rugs on the floor and the variety of stuff we like to surround ourselves with.

"Above 200 Hz" is not is not referring to "bass traps" at all.

Dipoles are of course one way to help reduce the effects of room modes, but they don't work into the frequency range normally covered by subwoofers. The "passband" of a dipole is not flat, but rolls off at the low end by 6 dB/oct, and must be boosted by 6 dB/oct at the low end to be flat. This restricts their applicable frequency range to the mid-bass region and above.

 

[agarwalro]

On the topic of controlling reverberent time, I'll take Dolby Labs word for it. This post has details on papers referenced, http://www.homecinemaguru.com/bass-traps-diffusion-absorption-what-should-you-do/

Basically, take the optimal RT60 at 500Hz from this chart (Note: Upper limit of 0.3s at 4000 cu.ft.) ,

Try to keep the ratio of the RT60 at a given frequency to the 500Hz RT60 and value as close to the bold line for rest of FR,

 

[-Jim-]

Gene, A very interesting presentation. I just bought a second sub and I am considering trying this approach but I can't seem to find the Dayton OmniMic for ~ $100 on their site. (There is a version 2 selling for closer to $300 as part of a "Computer Based Precision Room Measurement System") Is there another Mic you'd recommend for this at that price level?

I did notice someone above said a UMM-6 Dayton Audio Mic works well in this application, and at $90 it is quite reasonable.

I've become an Audioholics junkie. Your CES coverage was great. However I must be part of the great unwashed. Cutting holes in my ceiling for more speakers just won't happen if I want to stay married. My misses thinks having a couple of 5.1 systems are too much. And bouncing sound off the ceiling just seems bizarre.

Anyway, The Perfect Solution for Home Theater Bass Optimization video was great and now I can try a 5.2 system without having marital strife. If I only new what Mic was the right fit...

 

[gene]

I should add that early in the video, there's an error regarding the claimed output voltage for the miniDSP unbalanced 2x4. Its maximum output voltage is 0.9 Volts RMS, not 2 Volts RMS. There is an input jumper that allows the user to switch the input sensitivity of the device between 2.0 Volts RMS and 0.9 Volts RMS. With the former setting, an input voltage of 2.0 Volts RMS results in an output voltage of 0.9 Volts RMS, which implies a loss of 6.9 dB. With the latter setting, an input voltage of 0.9 Volts RMS results in an output voltage of 0.9 Volts RMS, implying a 0.0 dB loss. Regardless of the input jumper setting, the maximum RMS output voltage of the unbalanced miniDSP 2x4 is 0.9 Volts RMS. This is very important, because it means that if you want to drive your subwoofer amp to full power, that amp must have an input sensitivity of 0.9 Volts RMS or less. This is not very common, but can be found with careful selection.

The balanced miniDSP 2x4 puts out a maximum of 4 Volts RMS in the balanced mode, so those who use pro amps for their subs will have no problems with maximum output.

Another alternative is the recently released miniDSP 2x4 HD, which is unbalanced, but has a maximum output voltage of 2.0 Volts RMS, plenty to drive almost any pro amp one might use for a subwoofer. It also solves a problem with the standard miniDSP 2x4, which is a maximum delay of 7.5 msec. This is too low for many, perhaps most multi-sub applications. The HD version has a maximum delay of 80 msec, which is far more in line with what might be needed in practical installations, especially those with fairly large rooms, in which the subs have a distance difference from the main listening position of more than 8.45 feet.

One needs to be very careful with the choice of miniDSP products, especially when considering the unbalanced 2x4 (non-HD version). This has been the subject of many posts over several years. Even the balanced miniDSP 2x4, which has plenty of output voltage for any application, is nonetheless handicapped by the 7.5 msec maximum delay limitation.

Yes I was referring to the HD version of unbalanced unit which does have 2Vrms out. I wouldn't use a device that can't do 2Vrms for a sub channel. Sorry for the confusion.

7.5 ms of delay is a bit short but in my 26x27ft room, all I needed was 3ms relative to the other subs and it did the job nicely.

I will likely be adding an unbalanced HD version to my family room system in the coming months and may do a short followup article on it.

 

[gene]

Gene, A very interesting presentation. I just bought a second sub and I am considering trying this approach but I can't seem to find the Dayton OmniMic for ~ $100 on their site. (There is a version 2 selling for closer to $300 as part of a "Computer Based Precision Room Measurement System") Is there another Mic you'd recommend for this at that price level?

I did notice someone above said a UMM-6 Dayton Audio Mic works well in this application, and at $90 it is quite reasonable.

I've become an Audioholics junkie. Your CES coverage was great. However I must be part of the great unwashed. Cutting holes in my ceiling for more speakers just won't happen if I want to stay married. My misses thinks having a couple of 5.1 systems are too much. And bouncing sound off the ceiling just seems bizarre.

Anyway, The Perfect Solution for Home Theater Bass Optimization video was great and now I can try a 5.2 system without having marital strife. If I only new what Mic was the right fit...

Yes the UMM-6 is good for the price but I believe its only accurate down to 18Hz. I had Dayton send me one last week so I will test it and report back. Glad you're focusing on the bass via multi-sub rather than bouncing sound for Atmos. Good choice!

 

[gene]

I'm seeing many articles regarding the MiniDSP 2x4 (both balanced and unbalanced) having noise floor issues leading to audible hiss when idle. Any feedback on this.

I'm interested in the Balanced 2x4 since it'll be paired with a pro amp, maybe a Crown XLS2002 Drivecore 2.

I measured a good noise floor on the 2x4 and will publish test results in my formal review. I had no noise issues at all and I'm a stickler for that.

 

[gene]

So the XO for the front Velodyne subs is set to 250Hz, while the XO for the other subs is set to 80Hz ?

No only the bass drivers of my 8Ts are set to 250Hz since they have their own crossovers and the speakers are meant to be played as a single unit with their companion satellite. All 3 of the other subs are set at 80Hz.

 

[gene]

Using multiple subwoofers, to address room modes and achieve uniform FR in multiple seats, seems fairly logical.

How does installing multiple subwoofers remove the need for bass traps? Isn't RT60 of 300ms the metric corresponding to "quick" and "clean" bass?

Please read the article associated with this video. I discuss and show how multi-sub with SFM pretty much eliminates the need for LF bass trapping. It's also far more efficient since your not converting bass to heat via passive treatments.

 

[andy_c]

Isn't RT60 of 300ms the metric corresponding to "quick" and "clean" bass?

I've never seen this established in any sort of credible way with regard to performance in the modal region. In Toole's book, all references to reverberation time (which is a function of frequency) refer to signals whose frequency content is restricted to the statistical region, i.e. above the modal region. If you want to talk about "quick" and "clean" bass, these are subjective concepts. But if they make any sense at all, it would be to relate them to the time-domain behavior of the system in the modal frequency range. The criticism has been leveled that if one fixes the frequency domain behavior of a subwoofer system (e.g. by flattening the magnitude response), this does not necessarily fix the time-domain behavior. That's a fair point.

For global EQ, what determines whether fixing the frequency-domain performance using EQ also fixes the time domain? Both fixes will occur simultaneously if the frequency domain behavior prior to EQ is minimum-phase, and a minimum-phase EQ is used to apply correction. Using multiple subs, one can eliminate non-minimum phase behavior (e.g. suckouts in the frequency response) by virtue of the "filling in" effect of additional subs.

The minimum-phase concept was originated by Bode in his classic 1945 book. The normal IIR filters used for correction are minimum-phase. So one question becomes, "is the response I"m trying to fix minimum-phase to begin with?". If there were a way to measure that, we could answer the question. Can one determine this?

Yes, there is a way to measure whether a system is minimum-phase in a given frequency range with REW using the excess group delay measurement. Details of this, along with further explanation of the minimum-phase concept can be found in the REW documentation here.

 

[nickwin]

Great article Gene! Im utilizing 2 subs, bass traps, and DSP EQ, and its brought me bass bliss! The bass traps cleaned up some ringing that EQ and placement alone could not realistically address. In my small room it was a worthy trade off for a slight loss in efficiency.

When you talk about "Subterranean Bass" how low do you need to go to get the full effect? 16hz? 14hz? 12hz?

How much room gain are you getting in your reference theater at 12-16hz?

 

[-Jim-]

Yes the UMM-6 is good for the price but I believe its only accurate down to 18Hz. I had Dayton send me one last week so I will test it and report back. Glad you're focusing on the bass via multi-sub rather than bouncing sound for Atmos. Good choice!

Gene,

Great that you are taking a look at the UMM-6. Maybe look at the MiniDSP UMIK-1 as well? (Or maybe a video on all low priced options on YouTube?) The UMIK-1 too seems to fall off at a little less than 20 Hz but for those who can't pry the $$ from the Family budget to get down to 15 Hz., maybe either will suffice? My birthday is this week and I was going to ask for a MiniDSP and a Mic so I could start mucking about with 2 Subs. Maybe I'll ask for a rain check until you comment on the Mics.

 

[Floyd Toole]

andy_c said:

"I've never seen this established in any sort of credible way with regard to performance in the modal region. In Toole's book, all references to reverberation time (which is a function of frequency) refer to signals whose frequency content is restricted to the statistical region, i.e. above the modal region. If you want to talk about "quick" and "clean" bass, these are subjective concepts. But if they make any sense at all, it would be to relate them to the time-domain behavior of the system in the modal frequency range. The criticism has been leveled that if one fixes the frequency domain behavior of a subwoofer system (e.g. by flattening the magnitude response), this does not necessarily fix the time-domain behavior. That's a fair point."

We tend not to talk about the decay of modes as reverberation time, but you can if you like. Reverberation is normally considered to be the decay of reflected sounds in a venue, and that happens above the transition frequency.

As for the decay of modes after equalization we must first specify that we are addressing specific modes, one at a time, using high resolution measurements and specific parametric filters tuned to the center frequency of the mode, with a bandwidth appropriate to its Q and we are reducing resonant peaks, not filling acoustical interference dips - a BAD thing to do because then you have added a high-Q resonance that is radiated everywhere in the room to fill a dip that exists at a specific location. This is something that is not well done by the automated room-EQ systems. It requires human intervention at the present time. The good news is that in reality most rooms exhibit only a small number of, sometimes only one, problematic modes.

In my book I show frequency and time-domain performance of modes in Figure 13.20 (positional EQ), 13.21 (positional vs electronic EQ), Figure 13.22 (2-sub mode cancelling), and 13.24 (straight EQ). In all cases flattening the frequency response has shortened the ringing. In the multi-sub Sound Field Management scheme, Figures 13.18 and 13.19, the resonances are substantially eliminated and the bass is, yes, "tight".

Conclusion: room modes at low frequencies behave as minimum-phase phenomena, so long as they reveal themselves above the average response. When a prominent resonant peak is attenuated by whatever means, including bass traps, the time domain is improved. Obviously, at lower levels there will be non-minimum-phase events, but they are not audible problems - the resonances are.

Bass traps work. They are just big and ugly. I have chosen not to diminish the space or degrade the visual impressions of my rooms with them and so have focused on other solutions.

 

[andy_c]

Whoa, I didn't know you posted here, Floyd. Thanks for the response!