Intro
Denon and Marantz have added a new feature into their new line of receivers called Directional Bass Management. We briefly covered this in our Denon and Marantz comparison articles, but in this article I dive much deeper.
My goal is to investigate what directional bass is, how it is different from the standard bass management mode, and if there is a benefit in using this feature.
What is Bass Management
Bass management is the process of directing bass signals from the main loudspeakers to a subwoofer that is capable of handling low frequencies. This is because most loudspeakers are not able to reproduce bass frequencies efficiently or effectively, so a subwoofer is needed to handle them. By sending the low frequencies to the subwoofer, we allow the main loudspeakers and amplifiers of the main channels to play louder with less distortion.
In most audio processors, bass is managed by setting your loudspeakers to “small” and adjusting the crossover point, usually to around 80hz. The crossover point tells the system that frequencies below the crossover value should be sent to the subwoofers. This combines the bass signals from the other speakers with the original signal from the LFE channel and sends it to the subwoofers.
If you would like to learn more about setting crossover points, please check out our article called “What Crossover to Choose for Your Speakers.”
Mono/Standard Bass Management
Mono Bass Management, or standard subwoofer mode in Denon and Marantz receivers, sends all the LFE channel information and bass below the crossover point of the main speakers to all the subs. This means all subs receive the exact same signal. This is how most AV equipment handle bass management. It works with single or multiple subwoofers.
The prevailing theory is that bass frequencies are omnidirectional and cannot be localized, therefore it does not matter that subs placed in the front receive signals from the surround speakers. However, there is some debate over this idea which we will get into further in the article.
Directional Bass Management
The concept of directional bass management is slightly more complicated and harder to set up. Instead of all subs receiving the same crossover data from all loudspeakers, each subwoofer outputs bass below the crossover frequency of the main speakers that are placed near them. Meanwhile all of the subwoofers receive the same signal from the LFE channel, the .1 in the 5.1 channel layout.
Please note that directional bass mode is only available if you have two or more subwoofers connected.
With directional bass turned on you will be able to tell the processor or AVR where a subwoofer is located in a particular zone, as seen pictured above. Then each subwoofer outputs bass below the crossover frequency of speakers that are only placed in their zone. The picture below shows the signals that each sub would get.
Theoretical Background on Directional vs Mono Bass
To determine if directional bass benefits our system, we need to understand bass behavior. There are two contradicting studies that I’d like to discuss: one conducted in free air while another conducted in an actual listening space.
Bass in Free Air
Let’s first look at how bass would work without a room, aka a theoretically perfect acoustic space. For our example, we are going to assume two bass sources because the directional bass setting can only be used for multiple subwoofers. If we place two subs in an open field, which represents an anechoic space, one sub being front left and the other front right, and play them one at a time, then according to a study done by Nastasa, you would be able to locate where the bass is coming from down to 31.5hz. Moreover, you can perceive as little as 10 degrees of separation between speakers playing pure tones in this environment, according to the study.
However, it must be mentioned that I could not find how they controlled the mechanical noise or higher frequency distortion in the study, which can lead to localization cues. This may affect the results of the study if these variables were not controlled.
That being said, directional bass seems plausible within the scope of the study for enhancing the listening experience by localizing low frequencies in an anechoic environment, which was surprising to me.
The Original THX Study
While this study was never published, THX tested subwoofer localization in actual listening spaces – as opposed to an anechoic chamber – and found that bass was not localizable under 80Hz reliably. Since this is in stark contrast to the above research, you wonder whether localization cues are masked by room issues, or there were errors in either of the studies.
Let’s suppose both studies are correct! If so, issues within real listening rooms will mask localization cues. Let’s look at what those issues could be.
A Brief Lesson on Sound Masking
So how could localization cues be masked by issues within the room? Well, sound masking can happen in two dimensions:
- In frequency: a particular frequency is louder than another, therefore masking it. For example, if a particular bass note is louder than its multiples higher up, it may mask those higher frequencies. An uneven frequency response in a room might do this and affect reference playback.
- In time: if a particular note rings out longer, it can mask other sounds close to it in frequency or lower in level. A long RT60 / decay time in a room might do this. In addition, a long decay time can smear the sound image and make it less precise as certain notes / frequencies linger around.
Editorial Note by Roland Jutai: If the decay time is uneven, the lingering frequencies will dominate the room and impart a particular character to the sound. In such cases, decay time can dominate as opposed to an uneven frequency response. This is why simply focusing on an even frequency response is psycho-acoustically incorrect and why Toole and Olive's research - while very useful - was limited. Unfortunately, more recent research into these effects by the likes of Dirac or Trinnov are proprietary. It doesn't mean there aren't some plausible theories we can establish.
Frequency Domain – Creating an Even Bass Response
The biggest interaction between the room and bass frequencies is the ability for the soundwaves to reflect off of the walls and for that reflection to interact with the original sound. Since bass operates below the Schroeder frequency, room modes usually dominate how we perceive the sound. This distorts the sound the listener hears, by causing peaks and valleys in the frequency response, making different frequencies louder and quieter across the room.
The Schroeder frequency is a term that describes the transition point between two different sound behaviors in a room. Below this frequency, sound waves form standing waves or room modes that create uneven pressure distribution across the room. Above this frequency, sound waves become more diffuse and produce a uniform reverberation field. The exact Schroeder frequency depends on the size and shape of the room, but in general smaller rooms will have a higher Schroeder frequency than larger rooms.
For frequencies below the Schroeder frequency, the wavelengths of the sound are so large as to exceed the length of room so absorption of the sound becomes challenging. Hence, rooms typically employ alternative methods to minimize these reflections and and resulting distortions.
Among the most effective approaches is incorporating multiple subwoofers positioned in different locations and equalized to operate in unison. This is basically what the mono bass management mode is trying to accomplish in AV receivers, whereby the different subwoofers are independently time-aligned then EQ’d either individually or together. This technique often yields a balanced frequency response across multiple seating positions in the room.
Removing one or more subs from this setup would create an imbalance and diminish its effectiveness. This is essentially what occurs with the directional bass mode, as only one subwoofer is utilized for that specific main channel bass signal. Consequently, the reflections and room modes that were previously mitigated by multiple subs resurface. This aspect raises my first – and primary – concern regarding this mode.
The above image is the before (left) and after (right) equalizing my 4 subwoofers across 4 positions. The response is much smoother and more linear. However, if I was to remove one of the subs, it would no longer be as smooth and I would have to equalize again.
Time Domain – Decay Time
The decay time could be another factor why THX could not localize bass below 80hz in real rooms. The decay time of a frequency is defined by how long it takes for the sound to drop by a certain level. We typically use a measurement called RT60 decay time to estimate this decay. Please note that RT60 is not necessarily the best measurement for this behavior in smaller rooms. However, it does give us a universal metric to talk about. Topt is likely more accurate in small rooms. See the REW documentation here for more.
The RT60 measurements shows us how long a particular frequency will persist for after that sound has stopped being produced. The target for RT60 changes per room and use case, but in general we can state that it is best to have a decay time between 0.3-0.6 seconds long for each frequency for a home theatre application. As you go lower in frequency, decay time will increase as this is a function of the room exciting, how much absorption the room has – as well as the speaker’s own time-domain performance. Each of these topics require deep dives to explain and maybe we will do that in the future, but in general each of these properties affect the decay time if not controlled or treated properly.
The image above shows a waterfall graph, which is a way to visualize RT60 decay time. The closer the green surface is to the front of the graph the longer the sound is present. Note how as frequency decreases, the longer the sound is in the room.
Exceeding the limit of acceptable decay time range can result in a lingering sound that creates a muddy and unintelligible sound field in the room. This effect is particularly prominent in frequencies within the modal region, where the interaction with the room leads to longer decay times. In extreme cases, the decay time becomes so prolonged that bass notes blend together, creating a continuous low rumble throughout the content. While this extreme scenario illustrates the worst-case outcome, even decay times within the accepted range can still hinder the localization of the subwoofer by not decaying fast enough for our brains to recognize. This aligns with the findings of the aforementioned studies, indicating that any real room (defined as a room with a decay time above 0.3 RT60) will experience a loss of bass localization.
Furthermore, it is important to highlight the interconnection between frequency-domain and time-domain performance. Failing to take decay time into account while concentrating solely on a flat frequency response can lead to a deteriorated sound quality within the room. Heavy EQ-ing accentuates specific frequencies, especially evident in the robust room modes at lower frequencies. By boosting a frequency through EQ, you introduce extra energy to that specific frequency. If the room is already resonating at that frequency, the additional energy intensifies the resonance, ultimately extending the decay time at that particular frequency. This is a balancing act that we have to do when designing and tuning our home theaters.
Reflections as Additional Sound Sources
Sound masking of localization cues can also be the result of the reflections acting like additional sound sources. This can happen if the reflections are close to the same sound pressure level (SPL) to the original sound wave.
This make the bass sound like it is coming from everywhere at once versus being localized to one point. This effect gets more and more prominent, the lower the frequency gets.
Unfortunately, there is no way of helping this issue unless you employ complicated and expensive bass absorbers. Therefore, if we do not control the bass reflections to where there is a significant difference in level from the main source, then the bass may be coming from one sub, but it will still sound like it is coming from everywhere in the room even with directional bass turned on, further diminishing the effectiveness of directional bass.
Along with the high level reflections we also have to take into account that we are not listening to pure tones or one frequency. The combination of complex sounds may hide the directionality of bass, especially if there are different bass signals coming from each direction, masking the effect even further.
Possible Reasons for Subwoofer Localization
Some may suggest that they can localize their subs; however, there may be other processes at work. Let’s have a look at some of the issues that will make it easier to localize your subs.
Mechanical Noise
A subwoofer is a mechanical system with moving parts and air. Sometimes the moving parts can make mechanical noise that is localizable due to the movement of the cone, or air rushing out of ports.
Box Resonance
Due to the natural resonant frequency of the bass enclosure, sound will be produced at each harmonic of the fundamental resonant frequency and may give the illusion of being able to tell where the bass is. This can happen as well if you have a ported subwoofer: as more air is pushed out of the port, it can produce audible noise, such as chuffing, which can be localizable.
Noise Transfer
Bass frequencies have the potential to induce resonances in larger objects such as walls, tables, sofas, picture frames, and other sizable furniture. This occurs because a portion of the sound energy is absorbed by these objects, resulting in their movement. The vibrations produced by these objects could generate sounds at frequencies that our brains can detect and localize, creating the illusion that the bass originates from those objects.
One way to help this issue is to decouple you subs from the floor. To delve deeper into the topic of subwoofer decoupling, we recommend reading our detailed review of the SVS Soundpath Subwoofer Isolation System, where this aspect is discussed with greater detail.
Distortion
Distortion can also come into play if you are pushing your sub near its limits. These distortions result in higher than bass frequencies being produced, and therefore, possibly being able to localize the sub.
Additional Issues to Consider
Overall Output
Having multiple subs playing the same signal equals an increase in overall output. However, if directional bass is only sending a signal to one of the subs, then you are losing that benefit which may cause an imbalance in sound level. This has apparently been taken into account by the programming in the receiver, so a louder signal is played if the signal is going to one sub, versus sending the same signal to multiple subs.
My counterpoint to this is that not all systems can handle an increase to a single sub’s output to match 2 or even 4 subs. So I think that this solution is only a fix if you have capable subs with plenty of headroom.
Audio Mixes
Be aware that not all content will be optimized for use with directional bass. Audio engineers sometimes automatically send main channel bass signals to the LFE channel, so turning on directional bass may have no affect on the experience for some multi-channel content.
Mismatched Subs
You may not want to consider using the directional bass mode if you have subwoofers that are of different makes and models. Each subwoofer will have its own sound signature, just like a loudspeaker, and it may cause the sound to change in timbre as it moves through the sound field with directional bass turned on.
Dirac ART/Other Active Room Treatment Systems
One hypothesis why Marantz and Denon may have included this feature is for the future implementation of Dirac Live Active Room Treatment. With Dirac Live ART, the processor will need to know where the subs are located in the room to actively cancel out the time domain and phase issues. This would likely be needed for all active room treatment software so it is very much possible that this feature is implemented so that Dirac can plug into it in the future.
Should We Use Directional Bass?
Directional bass, although a cool concept, may not be useful until you have a near perfect acoustic space, and even then, it may not offer much benefit. Because bass frequencies being so hard to control in timing and phase, our brains have a hard time localizing the source. If we can improve the bass reflections to near anechoic levels, an RT60 time at or below 0.2 seconds, without creating other issues then maybe directional bass will have a significant impact. So it is entirely possible that once we employ active room treatment, Dirac or otherwise, this might make more sense.
More research will have to be done on this topic for us to know for sure. For now, I would consider holding off on buying a new receiver for this feature alone, but if you own one already, I would experiment which setting you prefer and run that. After all, it is your system and you should be the one enjoying it the most with what settings sound best to you.
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