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The Complexities of Bass in Most Homes

Discussion in 'Acoustics' started by Flint, Nov 2, 2018.

  1. Flint

    Flint "Do you know who I am?" Superstar

    I still struggle with how to explain how bass is so different than midrange or treble in a room, but I tried in a thread about Heeman’s great speakers and room on his request.

    Here’s what I hope is a clearer and easier to understand explanation of the concepts I tried to convey in that other thread.
     
  2. Flint

    Flint "Do you know who I am?" Superstar

    Concept One

    Pressurization & Cavity Resonance:

    At very low frequencies in most rooms there comes a point the room is a pressure chamber, like the inside of a speaker enclosure. The pressure chamber of your room will also have an inherent cavity resonance which can be tuned by adjusting the "port" in the forms of open/closed doors, open/closed windows, HVAC vents, closets, etc. The point where the room becomes a pressurized cavity is defined by its volume, losses in the materials used, and resonant "vents" like doors or whatever. At the room's cavity pressure resonance there will be a long decay defined by the amounts of losses in the room. In larger rooms this resonance is likely at very, very subsonic frequencies, but in moderate and small rooms it can occur in the listening range. In my listening room it is at 12Hz when the door is open and 16Hz when the door is closed and can be heard/felt when I play that very rare content with enough deep bass energy to be audible and thus the sustain of the resonance is felt.
     
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  3. Flint

    Flint "Do you know who I am?" Superstar

    Concept Two

    Standing Waves:

    At frequencies where the wavelengths are mathematically aligned to the reflective barriers, such as wall to wall or ceiling to floor, there will be standing waves which create modes and nulls at various locations in the room (hence placement of subs/speakers and the listening position are important to these). There will also be a long sustain of the acoustic energy at those resonances which decays at a rate defined by the losses in the room or non-perpendicular reflections. Standing waves can be created between two walls, or transversely by reflecting off multiple walls at angles which allow for repeating cycles of the acoustic wave. All of these standing wave resonances are impacted by the losses in the room at the specific resonances which are impacting performance. This is why you cab buy special design bass traps which are tuned to a specific frequency, such as 70Hz (for the half-wavelength of a 8ft high ceiling), or similar based on known common dimensions. A bass trap absorbs these low frequencies and thus decreases the decay to silence when the room experiences a standing wave. That's a loss. Tuned bass traps focus on specific frequencies, while huge "superchunk" or insulation filled cavity style bass traps absorb all low frequency energy. That said, a bass trap which might be able to absorb large quantities of energy down to 50Hz, may not absorb much of anything at 30Hz, so the issue of the room becoming boomy below the effective range of a large "superchunk" style trap is still real. It gets pretty heavy to try to absorb all bass down to 20Hz in a normal sized room.
     
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  4. Flint

    Flint "Do you know who I am?" Superstar

    Concept Three

    Direct Reflections:

    Simple reflections (not standing waves) also play a critical role in the bass range. If you have a direct reflection where the sound travels a certain distance farther before reaching yor ears, the time delay between the original source sound and its reflection will cause a cancellation or amplification. Famous and well known examples are the floor bounce null and baffle step null. There are similar nulls based on the distance to the side walls, ceiling, and the wall behind the listener. Since the frequencies associated with these nulls tend to be in the main bass or upper bass range, a good absorber, diffuser, or angled reflector can reduce or eliminate the null from any specific reflection point if treated at that point.
     
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  5. Flint

    Flint "Do you know who I am?" Superstar

    Concept Four

    Proximity Effect:

    If a sound source or listener is placed within a certain distance of a large reflective surface the acoustic energy of frequencies who quarter wavelength is greater than that distance will be effectively reinforced, or “amplified”, by 6dB SPL. So, if a speaker is 2 feet from the side wall, acoustic sound below 141Hz will be reinforced by 6dB. If it is placed 6 feet from another wall, sound below 47Hz will be reinforced by 6dB. This also applied to the listener’s proximity to the nearby walls, floor, and ceiling. This is often referred to as “room gain” when audio experts talk about tuning subwoofers. In car audio it is usually called “cabin gain.” This is unavoidable in terms of instantaneous sampling or reproduced sound, such as measuring the source frequency response in a room. However, bass absorption can reduce the long term "sustain" effects of this principle.
     
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  6. Flint

    Flint "Do you know who I am?" Superstar

    Summary and Conclusion

    So, all four of these physical principles which affect the sound are defined by the room.


    One saving grace is that when modern standards for home building were established, the use of gypsum board on studs was decided upon in part because of the bass dampening properties of the materials and construction methods. The drywall thickness and the distance between studs effectively creates a vibrating panel which absorbs acoustic energy in the room. This was desired by architects and regulators mostly to improve the well-being of living in a house built with this style of construction as it mellows the typical noise in a house. Sounds such as closing doors, talking, playing the console radio, dropping a box, cooking, and so on is less annoying on your ears in a house build with stud and drywall surfaces rather than hard stone, concrete, plat boards, and so on. If you have ever visited a medieval castle you’ve experience how terribly annoying even talking can be. The ancient architects dealt with room noise pollution by mastering the art of diffuse reflections and this created the gorgeous reverb decay sought after by recording engineers when creating music for playback. But it would be far too expensive to build modern homes with solid concrete walls and ceilinging which also incorporated broken surfaces, nooks, strange angles, and so on, to get a more smooth and natural reverberant decay of sound.


    This is why I comment so often about “most rooms are not 100% reflective.” Modern drywall based rooms are quite reflective in the low treble to upper bass, but in the lower bass the walls will vibrate and flex and thus absorb some of the energy, which increases the rate of decay. Most designer put soft furnishing in living spaces, such as couches, rugs or carpet, drapes/curtains, and so on which effectively control a good portion of the treble and upper midrange. Then large, odd-angled furnishings such as tables, couches, chairs, cabinets, floor lights, and so on diffuse sound in the midrange and treble. So, houses which are bare and scarcely furnished tend to be more difficult to spend time in over the added decay time of sound. Likewise, a densely furnished room may seem heavy and dead to one’s ears which can also be disconcerting over long periods of time if you are interacting with people.

    Anyway, bass is a tricky beast, and getting everything right in an existing house is a matter of luck in the available space being more appropriate for sound reproduction, the amount of bass trapping you can install, the placement of the speakers and the listener, and so on.

    And, yes, you can measure this will things like RT60, Waterfall Plots of the room showing the decay from different frequencies, and standard “bass decay” plots. One simple test almost anyone could do is to play back a two to four cycle sinewave signal through your speakers at specific frequencies in the bass and record them a computer, then view the recording and see how long the sound continues to resonate in the recording compared to the two cycle pulse from the source file. This will expose the limitations of the speaker and the room simultaneously.

    Keep in mind, measuring the in-room frequency response, especially in the bass range, cannot effectively visualize things like sustain or resonances. Frequency Response Curves do show nulls and peaks caused by reflective surfaces, and using Pink Noise Response Curves can show the strongest resonant frequencies, but they do not show how sound decays.



    This was my attempt to explain the many complexities of the issues with reproducing bass in most homes.


    Let me know if there are questions.
     
    Zing likes this.
  7. Flint

    Flint "Do you know who I am?" Superstar

    One of the last things to point out is how we hear bass... in fact, we don't hear it at all until it is very loud.

    Take a look at the Equal Loudness Curve:
    [​IMG]

    As you can see, we humans don't even realize there is any sound at 30Hz until is it at least 60dB SPL, but then it will be masked by sounds above 80Hz to which we are significantly more sensitive. This is why I tune my system for a balanced tonality at 90-95dB SPL, because at those levels I can hear the bass fairly well and as such I tend to do my critical listening at those levels.

    Since it takes so much acoustical energy to even perceive the bass, if you listen at moderate levels (like below 85dB SPL) you have to either turn up the bass much louder than the midrange (as most do with their subwoofers) or you have to just give up on hearing the deepest output from your system until you do turn it up. I choose the latter. But, when you are playing it loud enough to hear the low bass, you are filling your room with a very large amount of energy and exciting all the pressurizations, standing waves, reflection angles, and so on - and putting whatever bass absorption material you have to the test. If all your bass traps reduce reflections by 3dB each time the pressure wave strikes it, then going from 95dB SPL to the perceptual limit of, say, 70dB SPL takes dozens of wave cycles. And, you didn't cover the entire room in bass trapping, so a solid 3dB of drop isn't going to be a reality. Parts of the room, very likely the majority of the room, is going to still be reflecting the bass with only the panel vibrations from the drywall and energy escaping through openings, cracks, and holes causing losses.

    So, it is complicated.

    This is, by the way, it is never a good idea to fill a room with midrange/treble absorption. If you do that, the bass end of the spectrum will resonate for seemingly an eternity while the treble and midrange vanishes almost instantaneously. The result is deafeningly dead feeling room with a huge, fat, boomy, unbearable bass. It all needs balance - no frequency range should be 10x more dead than another.
     
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  8. Haywood

    Haywood Well-Known Member Famous

    That is really interesting stuff and it explains some of what I've been experiencing in my new room. The bass is a lot tighter and cleaner than it was in the old place and I think it might be due to the first issue of cavity resonance. My sub is in the front right corner of the room. The front two-thirds of the left side of the room is open to the dining room, which then opens to the kitchen. The entire back of the room is mostly open to the entryway, which transitions into a stairway and a hallway. This is broken up by a half-wall on either side of the wide room entrance, which is also flanked by pillars. In other words, the room is neither square nor contained.

    Am I understanding this correctly?
     
  9. Flint

    Flint "Do you know who I am?" Superstar

    Yes.... among other things, you have fewer large barriers containing the energy (pressurization) and reflecting the energy (standing waves). This is, in essence, an acoustically "large room" with only some walls & barriers creating acoustic sustain in the bass.
     
  10. Flint

    Flint "Do you know who I am?" Superstar

    Here's an example of a bass decay chart I pulled off the web (I'll make one of my various rooms later):

    [​IMG]

    Note that in this room, there is a huge resonance at about 24Hz and another at 62Hz and a smaller one at 120Hz.
     
  11. Flint

    Flint "Do you know who I am?" Superstar

    Also, and I hope it is fairly self-evident, all of the principles above are why I instinctively ere to placing subwoofers tight into corners when initially setting up a room. By sticking the subwoofer into the corner you get the most of the room gain (proximity effect), pressurization, and all the standing waves and reflections are equally excited.
     
  12. PaulyT

    PaulyT Behind the Curtain Staff Member Administrator Moderator Superstar

    Thanks for the info, @Flint. Great stuff!
     
    Flint likes this.
  13. Flint

    Flint "Do you know who I am?" Superstar

    So... depending on the room, I expect to see these characteristics reflected in the audible performance of a good system:

    Approximately:
    0Hz up to 20 - 35Hz = Below a certain frequency point the room will just pressurize and cannot be treated well. Acoustic energy behaves as pressure.

    20 - 150Hz = Depending on the room dimensions, there will be ringing and sustained standing waves which create spikes and nulls in the frequency response and also resonate at those same frequencies. Treatments include bass traps, non-parallel walls or ceilings, and carefull application of EQ. Acoustic energy behaves as a wave.

    60 - 500Hz = Placement of speakers and the listener in relation to the large reflective surfaces of the room will create spikes and nulls at the listening position, but not energizing the entire room. This can be treated by changing the placement of the speaker and/or listener, huge bass range absorbers, large diffusers, and large angled reflectors. Acoustic energy behaves like a ray.

    0 - 60Hz = Proximity effect will increase the gain of the speakers output in the room. This can make the sound fatter, boomier, or heavier in the bass. Treatment is easiest with a simple shelf EQ, or turning down the subwoofer. Acoustic energy acts like pressure wave.

    As you can see, there is a ton of overlap in what each room acoustic characteristic does. So, treating one form of acoustic modification in the room could help with another or make another issue worse. I've been around it so long, I just play the balancing game in my head of how to get the least worst impact from each aspect mentioned above. Or, I will try to use the characteristic as an advantage, like cramming a sub into a corner to take full advantage of the proximity effect.

    There are modelling software packages which can predict how a room will behave based on dimensions and speaker/listener placement, few of the affordable or free apps are good at taking into account the difference in losses from carpet vs. rug, different drywall thicknesses, insulation in the walls, added absorbers, diffusers, or reflectors, and so on. The modelling apps can be useful in targeting the obvious frequencies of concern, but they are rarely accurate.
     
    Last edited: Nov 6, 2018
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  14. heeman

    heeman Well-Known Member Famous

    Thanks for all the great information Flint!!
     
    Flint likes this.
  15. team_tsp

    team_tsp Active Member

    Awesome posts @Flint, learned a lot! Having it broken down into the separate concepts really helps
     
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