Author Topic: Bass Null  (Read 527 times)

Online Dolby

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Bass Null
« on: May 17, 2018, 07:01:59 PM »
Is a null always caused by sound waves bouncing back from the back wall and cancelling out those coming from the front? So theoretically, if I had no back wall ... There'd no null?

Offline Michon

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Re: Bass Null
« Reply #1 on: May 17, 2018, 07:19:59 PM »
Yes, cancellation can happen between the front and back wall, but it also happens on other axes. For instance, side wall to side wall, and floor to roof. In practice one ends up with comb filtering all across the audio spectrum due to the destructive interference of reflections with the direct sound and reflections with each other.

The largest null in the bass might be caused between the cancellation occurring between two specific axes in the room, like the front to back, as you mentioned. One can predict by measuring the distance from the radiating point to various reflective surfaces where the largest nulls are likely to be.

Corner bass traps will reduce the null.

This is a helpful article on the topic;
http://ethanwiner.com/basstrap_myths.htm

The way to go about treating a room is quite standard - bass traps and broad band absorption at first reflection points. If one wants to go further than that you can add more absorption and diffusers at appropriate points to get the decay time equal across the audio spectrum.

The down side; bass traps have to be large to absorb low frequencies. I understand that tuned traps can reduce the size, but I wouldn't use a tuned trap if not also using broadband bass traps.
« Last Edit: May 17, 2018, 07:33:45 PM by Michon »

Offline Michon

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Re: Bass Null
« Reply #2 on: May 17, 2018, 07:42:40 PM »
To answer the second part of your question; if you had no back wall you would eliminate one reflective surface, so less reflections will exist in the room, which will reduce the amount of cancellation, but the reflections from all other surfaces will still be present and they will still cause nulls. The frequencies at which those nulls occur will depend on the dimensions of the room.

Having no back wall, and assuming infinite space beyond where the wall would have been, would be analogous to having an absorber with a 100% absorption coefficient across all frequencies on that plane, because no sound will reflect back from that plane.


If some of this is confusing the following article can offer clarity; http://ethanwiner.com/acoustics.html.
« Last Edit: May 17, 2018, 07:46:30 PM by Michon »

Online Dolby

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Re: Bass Null
« Reply #3 on: May 17, 2018, 07:53:04 PM »
Thanks!

Will read up.

So another hypothetical question :

What would a subwoofer do in an open field?

(Just easier for me to understand if I do to extremes)

Offline Michon

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Re: Bass Null
« Reply #4 on: May 17, 2018, 08:05:37 PM »
Thanks!

Will read up.

So another hypothetical question :

What would a subwoofer do in an open field?

(Just easier for me to understand if I do to extremes)

If not even a floor reflective surface is present it would theoretically radiate sound equally in all directions.
I suspect that the dispersion pattern might still differ a little between the side that has the driver on and the other sides, especially the one of the opposite side of the cabinet, but lower frequencies are a lot more omni-directional than high frequencies so you will end up with something that pretty much radiates sound equally in all directions. As the frequencies increase directionality, in fewer axes, will increase too.

Manufacturers use anechoic chambers to simulate open field environments.

As soon as you place the sub on the floor, i.e. a reflective surface, there will be a gain in bass. Then place the sub in the corner of two planes and there will be an additional gain in bass. Then place the sub in a three planed corner, like the corner of a room, there will be yet another gain in bass. If I understand correctly this gain will not necessarily be equal at all frequencies that the sub is playing, but will be dependent on the dimensions between the source and reflective surface, which quickly turns into a complex equation the more reflective surfaces are present. The gain in bass that occurs when as each additional reflective surface is introduces is roughly 3 dB if I recall correctly - I don't have a reference book on hand and am not sure of my memory at the moment.

Offline Michon

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Re: Bass Null
« Reply #5 on: May 17, 2018, 08:09:32 PM »
The reason why there is an increase in SPL when a reflective surface is added to the sub woofer is because the sound that would have disappeared into the infinite space of the "open field" in that direction is now no longer able to travel infinitely far away from the sub anymore, but instead is reflected and added to the rest of the signal travelling in the directions that remain.

Some caveats apply, like cancellation occurring dependent on distance between sound source and reflective points and how reflective at the applicable frequencies the surface is, but this is the general idea.

Then as you add more planes and eventually result in a room you end up with constructive and destructive interference occurring at different frequencies, again dependent on the dimensions of the room.

A parody could be made of the amount of time I spend looking at frequency response charts of loudspeakers, nit picking 2dB SPL or 3 dB SPL differences between drivers when listening rooms can easily cause 30dB SPL nulls at many places in the audio spectrum. There is some debate about the audibility of these severe nulls though, but it isn't exaggeration to state that the room has the most drastic influence on ones sound in the frequency domain - transducers fighting for first place I'd say. But it must be said audio isn't a phenomena that occurs on the axis of frequency response only.

An untreated room also adds "reverb" (perhaps not the right term as the lengths can be short), but the longer the reflections are present in the room the more the time domain of the audio gets "distorted".

The two polar opposites on the spectrum are a room in which all surfaces are parallel and 100% reflective and then a room in which all surfaces are 100% absorptive.
The average listening room is somewhere in between. Neither extreme is desirable, but it is easier for rooms to lean closer to the former than it is to the later. That said, it is difficult to put too much absorption in a small room.

This video can give you an idea of a completely untreated room vs a treated one:
Hearing is Believing - The Ultimate Small Mixing & Mastering Room
https://www.youtube.com/watch?v=dB8H0HFMylo

I have read some folks saying that say they don't like a lot of acoustic treatment, so subjective preference can apply here too.

Personally an ideal room for me would have a decay time across all frequencies below 0.4s and would address frequency response issues, such as bass nulls, to the extent to which they can.
« Last Edit: May 17, 2018, 08:39:36 PM by Michon »

Offline Michon

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Re: Bass Null
« Reply #6 on: May 17, 2018, 08:57:58 PM »
Thanks!

Will read up.

So another hypothetical question :

What would a subwoofer do in an open field?

(Just easier for me to understand if I do to extremes)


Notice that the higher the frequency the more directional waves become.
Each circular chart illustrates the direction of travel, from source, of a different frequency.

Online Dolby

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Re: Bass Null
« Reply #7 on: May 18, 2018, 08:28:31 AM »
Thanks !

Much appreciated :)

I've just bought a second, identical subwoofer thinking the spread would sort the issue and ... zero bass in my seat. Rumbles everywhere else in the room.

Before I bought bass traps, I wanted to understand what they did ie absorb the waves so they don't bounce back

Offline Michon

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Re: Bass Null
« Reply #8 on: May 18, 2018, 09:09:40 AM »
You're welcome. The thanks should largely go to Ethan Winer and the Master Handbook of Acoustics by F. Alton Everest.

I've just bought a second, identical subwoofer thinking the spread would sort the issue and ... zero bass in my seat. Rumbles everywhere else in the room.

If you are able to play with placement you might be able to get them into positions where the don't cancel exactly in your seat, but they could end up in funny places in the room at which point time alignment between drivers will be affected.

There is a method by which one places the sub woofer in the listening seat, play music and then walk around the room to find places were you are happy with the bass then place the subwoofer in those places. Doing this, I read, might move the bass null to a different position than your chair. I haven't done this myself.

Before I bought bass traps, I wanted to understand what they did ie absorb the waves so they don't bounce back

Beware of commercial sponge and foam corner "bass traps" that are only about 30cm deep. They often aren't able to absorb low frequencies.
If you are going to buy a commercial product ask if they can supply you with the absorption coefficient of the product at bass frequencies.

Ideally you want rigid fiberglass.

Either in a "SuperChunk".


Or a panel in the corner.


SuperChunk style vs. Real Traps Bass Corner panels
http://forums.musicplayer.com/ubbthreads.php/topics/2363873/SuperChunk_style_vs_Real_Traps

Do-it-Yourself Bass Traps By Ethan Winer
http://www.audioundone.com/do-it-yourself-bass-traps
« Last Edit: May 18, 2018, 09:13:55 AM by Michon »

Offline Rodney_gold

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Re: Bass Null
« Reply #9 on: May 18, 2018, 10:58:00 AM »
You need to move the listening chair out of the null , also try subs in corners if possible and switch the phase of one of them.
Try sitting closer to the back wall , will increase the  bass.
Have you actually measured the bass response at seated position to find out where the nulls and peaks are freq wise?
tube traps will be much better than flat or chunk traps
google ASC tube trap or DIY tube traps

« Last Edit: May 18, 2018, 11:01:48 AM by Rodney_gold »
Roon/tidal > Squeezebox touch> Trinnov St2 >2x Devialet D premiers>  Vivid audio Giya G1 spirits ..fully treated room

Offline ALF

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Re: Bass Null
« Reply #10 on: May 18, 2018, 04:20:29 PM »
what about the phase deference between the sub`s and the main speakers bass drivers, I am sure it cant be 100% in phase due to multiple amps and active crossovers. I am sure the out of phase bass drivers will keep you run around in circles, what work for me ones was to add a big cap before the main speakers that filters out say 180hz and down  :nfi: 
"the fostex thang".....

Online Dolby

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Re: Bass Null
« Reply #11 on: May 18, 2018, 07:15:06 PM »
Thanks for the help, guys.

I have lots of reading and experimenting to do

Offline Rodney_gold

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Re: Bass Null
« Reply #12 on: May 18, 2018, 08:42:33 PM »
Roon/tidal > Squeezebox touch> Trinnov St2 >2x Devialet D premiers>  Vivid audio Giya G1 spirits ..fully treated room

Offline Michon

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Re: Bass Null
« Reply #13 on: May 18, 2018, 10:05:46 PM »
tube traps will be much better than flat or chunk traps
google ASC tube trap or DIY tube traps
I haven't looked into these.
Better than flat or chunk traps sounds promising.


Start here
http://forum.blu-ray.com/showthread.php?t=48286
Thank you for this.

Offline Trompie67

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Re: Bass Null
« Reply #14 on: May 19, 2018, 03:24:30 AM »
*Insert profound or witty phrase here*