May 17, 2010

Bass test - answers

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Answers below

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1. At 80 Hz bass can be localised

a. By no one
b. By some
c. By everyone

2. At 120 Hz bass can be localised:

a. By no one
b. By some
c. By everyone

Tests to determine this have found that most can't localise bass below 120 Hz, but there are a small number who can with some difficulty. At 80 Hz, no one can.

3. The Shroeder frequency is:

a. The transition point below which bass uniformly pressurises a room
b. The transition point below which room modes are a problem
c. The point where a speaker's bass response is -6db

Typically in most rooms this will be around 200 Hz.

4. The most effective way to get a smooth in-room bass response at different positions is:

a. Place multiple subwoofers so that problem room modes are cancelled
b. Excite as many modes as possible using multiple subwoofers
c. Acoustic treatment (bass traps, tuned absorbers, etc)
d. EQ
e. All of the above except a
f. All of the above except b

The ideal is to do it all, but the goal is not to cancel modes as many would suggest. The idea of cancelling modes suggests misunderstanding the problem. The reason for the problem is that there aren't enough modes in the problem range - they are spaced too far apart. Imagine walking on a bed of nails. If they are spaced far apart then they will pierce your skin, but if they are all densely packed together so that they are all touching, they will form a hard surface you can actually walk on. The same is true for bass - few modes mean big peaks and dips. But as we keep exciting more and more modes, we eventually end up with a very smooth response.

5. Adding a second identical subwoofer will increase the maximum output:

a. By 3db
b. By 6db
c. By 12db
d. Impossible to say - depends on location

If they are placed next to each other, then 6db, but if spaced apart it's impossible to say.

6. Movie sound tracks are set up so that the subwoofer level is

a. The same as the main channels
b. 3 db louder
c. 6 db louder
d. 10 db louder

7. Subwoofer A has 1kw of power handling and 80 db 1w1m. Subwoofer B has 100w power handling and 90 db 1w1m. Both have adequate excursion and thermal handling to handle the full power output of their amplifiers.

a. Identical maximum output
b. Subw A wins due to power handling
c. Sub B due to lower power compression

On paper they are the same, but power compression will prevent the sub that relies on power from achieving it's potential.

8. A rumble filter is

a. A high pass filter which eliminates all bass below a certain point
b. A low cut filter which attenuates very low frequencies
c. A low pass filter which keeps the cone under control
d. Both a and b

High pass and low cut mean the same thing. It is not a brick wall filter, but one that attenuates below the corner frequency.

9. If a low frequencies were filtered with a second order (12db/octave) filter, below this point

a. Excursion will remain constant with lowering frequency
b. Excursion will be reduced but will still increase with lowering frequency
c. Excursion will be decrease with lowering frequency

10. In the bass range, in order to sound twice as loud, a sound must be increased in level by

a. 3 db
b. 6 db
c. 10 db

Surprised? Have a look at the equal loudness contours and you will see that 10 db only applies to the midrange. For bass, the lines are spaced closer together.

11. Two subwoofers are placed in different locations in a room, each having peaks and dips in different places from their interaction with different room modes. In one particular seating location, one has a peak where the other has a dip. Assuming there is no phase difference between them, the combined response of the two subs at that point will show:

a. An average of the two outputs where they cancel according to their respective levels
b. The dip will dominate the response and the peak of the other sub won't be able to couteract it
c. The peak will dominate the response and the dip will be eliminated

12. The smoothest in-room bass can be achieved by:

a. Dipoles
b. Monopoles
c. Multiple monopoles if correctly placed

Generally, monopoles and dipoles on average will perform about the same on smoothness. Certain situations may favour one over the other, but the best chance at smooth in-room bass is multiple monopoles.

13. Phase is significant at bass frequencies:

a. Because it has a big impact on spaciousness
b. As it relates to the transient response and thus subjective speed of the bass
c. To the extent that it impacts the steady state response
d. Both a and b

Our ability to perceive time domain behaviour in the bass range is very limited. Often a mis-diagnosis is made and superior performance of one sub over another is wrongly linked to transient response and phase issues. This is a common practice where attention is paid to the wrong thing while the important thing, the steady state in-room response is ignored. Getting the best result becomes a gamble. Measurements are needed or it's like playing pin the tail on the donkey.

14. Using EQ to fix room issues:

a. Works in one position only and will make all other positions worse
b. Is the only thing you need to do for the best bass apart from get a top notch sub
c. Tends to work for part of a room, but usually not the whole room and can make some positions worse

Is isn't the best thing sliced bread, but it does work and often for more than one seat. It won't probably work for a whole room.

15. When using EQ for dealing with room modes:

a. You can fill in dips if you have enough headroom
b. You can't fill in the dips - no matter how much boost you apply, they will remain

If you have a 12 db dip, then you can fill it with a parametric eq filter applying just the right boost. Done right the result wil be perfectly flat. The downside is that you are giving up massive headroom. It takes a lot of power.

16. When using boost:

a. 10 db of boost requires twice as much power
b. 3db of boost requires twice as much power
c. 6db of boost requires twice as much power

17. A subwoofer driver is placed into a sealed box with a Q of 0.707. It has an fs of 20 Hz. If no EQ is used, the anechoic -3db point will be:

a. 20 Hz
b. 40 Hz
c. 50 Hz

The sealed box will generally push the F3 up one octave above fs.

18. The same driver is placed into a vented box tuned at fs. In comparison to the sealed box, we can expect:

a. Lower extension but the same output
b. One octave extra extension and about 5 db extra output
c. Less extension but higher output

19. We now use a high pass filter to keep the cone under control below tuning. In order to ensure that the excursion below tuning will never exceed excursion above, we need:

a. A first order (6db/octave) slope
b. A second order (12db/octave) slope
c. A third order (18db/octave) slope
d. A fourth order (24db/octave) slope
e. None of the above - it will vary with driver parameters

So you might ask why third order filters aren't more common!

20. THD (Total Harmonic Distortion)

a. Is a useful measure which shows how accurate a subwoofer will sound
b. Is useful depending on the level - the ear is not sensitive below a certain threshold
c. Is an important measure - when considering the equal loudness contours they are more important than you might think
d. Is not critical for the bass range as our ears are not sensitive to bass distortion
e. Doesn't relate to perceived accuracy and is meaningless for subwoofers

1 comment:

  1. I learned alot working through this - thank you for taking the time to write this up.


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