Bass traps for asymmetrical set-up in L-shaped apartment? - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 48 Old 08-26-11, 07:58 AM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

Here's the mdat file from last night. I hope that helps, and I can also try to alter the graphs tonight when I'm home from work. The distance from the tweeters to the mic is 8' 8" as measured with the Bosch laser measuring device, so it's pretty accurate.

Thanks,

Andrew
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post #22 of 48 Old 08-26-11, 12:14 PM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

Below is the ETC windowed for normal viewing.



Something is not right - in addition to the pre-causal behavior (energy arriving prior to the direct energy!!!). And I do not think the pre-causal energy is due to mechanically coupled and transmitted energy. The floors are not 'sprung' wood are they? But more than that, I have reason to doubt ~7 direct and indirect sources within ~-5 dBfs spread over ~38ms.

Also, for our purposes, can you measure from the center of the speaker baffle (not the tweeter) to the mic capsule to the proper precision. (The precision of the measurement is determined by the measuring device. The precision is to the nearest tenth of the smallest unit division...meaning, for instance, if the marking of a measuring device is marked to tenths of a unit, then we can estimate to the nearest tenth of that scale - meaning we are precise to the nearest 100th. Or if the device is marked only to integer units, then we can estimate precisely to the nearest tenth of the unit. Stating the measurement to less or greater precision than that determined by the unit sale is a mistake. This is important as the total calculation determined by the use of he measurement is determined by the least precise measured unit. So we will benefit from the most 'correctly' precise we can be... )

I know many are of the opinion that the tweeter possesses some magic quality, but to clarify this issue - the tweeter is ofter a concern ONLY in terms that its dispersion is normally the most limited (narrow). Thus in order to be 'on axis', as the tweeter tends to beam, we think interns of the tweeter being the limiting agent.

But here that is not our concern. We are concerned with the energy that is emitted by the loudspeaker. And in this frame of reference, the tweeter emits much less energy than the other drivers. Here we are concerned with what is often referred to as the acoustic center of the unit - similar to the center of mass or a unit's acoustic 'moment'. In other words, the 'point' from which all of the energy appears to radiate from the entire loudspeaker. And as we have not measured the actual acoustic center, we can use the geometric center of the loudspeaker's front baffle as the reference point.

Thus, the more precise we can be with that measurement, the better off we will be using that measurement summed with the data from the ETC, where the limiting precision will be that of this distance measurement from speaker baffle to mic.
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post #23 of 48 Old 08-26-11, 12:58 PM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

Thanks again for all detailed info. A lot of it is going over my head, but some it is starting to sink in. I will take a more accurate measurement when I get home from work. My speakers look like ones below, so should I measure from the mic to the point between the mid and bass drivers?

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post #24 of 48 Old 08-26-11, 01:32 PM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

Yes, that would be a better point from which to measure.

Also, using the picture as a source, note how the cabinet extends a bit in front of the speaker to the left side (viewing from the front).

This small jump discontinuity will function as a source of diffraction and reflection - causing energy to emanate from that cabinet corner as if it is yet another small speaker, further acting to superpose with the direct signal and thus harming intelligibility, imaging and localization.

Assuming one does not have the option to remove the impediment, it would be preferable to position the speaker such that the cabinet face was flush with the speaker baffle front in order to (try t0) act as an extension of the speaker baffle to minimize this effect.

I mention this simply as a point of illustration, as I know the picture may not be of your particular installation.
It seems everything is out to get us, doesn't it?
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post #25 of 48 Old 08-26-11, 03:28 PM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

Quote:
SAC wrote: View Post
Something is not right - in addition to the pre-causal behavior (energy arriving prior to the direct energy!!!). And I do not think the pre-causal energy is due to mechanically coupled and transmitted energy. The floors are not 'sprung' wood are they?
Sorry, but I just noticed the date on the mdat I posted was Aug 23rd. I meant to upload the file from the 25th, but was rushing before I went to work. I'll redo the ETC's tonight and post the graphs based on your recommendations.
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post #26 of 48 Old 08-26-11, 09:47 PM Thread Starter
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

Here's a couple new graphs with the distance from the mic to speaker baffle marked on them. I've also attached the mdat. I tried to measure as accurately as I could with the Bosch DLR130 laser measuring device, which is accurate to 1/16" or 1.5mm. However, it was hard to line the edge of the device up perfectly with the mic, and I might have moved it slightly when measuring.

I read what you said earlier about the pre-causal behaviour. Are those the peaks on the graph before 0s? My apartment building is made of concrete, with parquet wood floors, and I live on a busy street. So there are often loud traffic noises outside. I thought that might have been the cause for some of the peaks, but I ran the tests a couple times and tried to wait for the traffic to die down, and the graphs look similar. So I don't think that's it.

I also hope I'm doing the measurements correctly. I have the ECM8000 mic pointed straight up and tilted forward about 20 degrees. The top of the mic is set to ear level when I'm in my listening chair, about 36". The measurement settings are "256k log sweep from 20 Hz to 300 Hz at -12.0dB taking 5.9s". I'm using the Tascam US-144mkII as the USB preamp.
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Bass traps for asymmetrical set-up in L-shaped apartment?-right-speaker-etc.jpg  

Bass traps for asymmetrical set-up in L-shaped apartment?-left-speaker-etc.jpg  

Attached Files aug26rew.mdat (693.5 KB) 

Last edited by streetcore; 08-26-11 at 10:01 PM.
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post #27 of 48 Old 08-27-11, 12:54 PM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

Why are you band limiting the sweep from 20 Hz to 300 Hz? Technically, we are not even driving the room with energy that will have substantial specular behavior.

Use full range sweeps! And leave off all smoothing for any responses. And process/convolve only unsmoothed responses.

Do not try to play with the measurement by thinking in the frequency domain.

As far as the graph, there should be more definition to the energy arrivals. And it is highly unlikely that you will have reflections persisting at 25ms (~283 feet of travel including bounded incidence) that are barely 2dBfs down from the direct signal (or ~-6.5 dBfs at ~44ms (after 49.7 feet of travel including bounded incidence!).

And with a severe lack of L/R symmetry, you should not have such a plethora of high intensity early arriving reflections except off the floor (especially without substantial energy being delivered that behaves specularly at those frequencies)

Plus something is amiss with the pre-causal signal arrival of ~-2dBfs at ~-2ms relative to the direct signal - unless the signal and drivers are THAT much (~2.26 feet) out of alignment....

Try a full range sweep. We'll figure this out!

Last edited by SAC; 08-27-11 at 01:34 PM.
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post #28 of 48 Old 08-27-11, 03:19 PM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

20-300 is easier to identify (with smoothing off), where any frequency response problems are more specifically rather than a full range sweep where it's a guess at best.

Bryan

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post #29 of 48 Old 08-27-11, 05:26 PM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

If one wants to examine the modal range from 20 Hz to 300 Hz, use the proper stimulus and response.

Truncation of sweeps has unintended consequences, and folks would do well to stop thinking solely in the frequency domain.

For insight into modal behavior from 0-300 Hz, generate the waterfall response driven with a full range signal and then window the response appropriately.

In order to generate the ETC response, drive the system with a fullrange stimulus.

There is a fundamental conflict in trying to use a gated signal from 20 Hz to 300 Hz to generate an ETC that provides insight in to specular energy behavior while restricting the stimulus energy to frequencies that behave (primarily) modally and expecting to see the full range (where our region of concern is actually from about 300 Hz to 10,000 Hz) specular energy behavior.

Thus, for an examination of the specular energy region, drive the space with energy spanning the full range of interest - at least 20-20,000Hz for instance (0-22Khz is even better). ( As an aside, I would normally also suggest using frequency weighted sweep were this feature available in REW. This makes good use of the pink noise behavior and improves the signal-to-noise ratio for the high frequencies simultaneously with more energy being applied to the low frequencies and less to the high frequencies in order to avoid the potential to blow up tweeters, where the difference between bass and treble in the linear plot amounts to only 18dB between 20Hz and 20kHz, in the octave band plot to only 10dB, and the mostly high-level mid-band range is even lowered a bit (5dB).)

I wouldn't recommend using a Porsche GT3 to tow a 26 foot long boat or house trailer either.

Last edited by SAC; 08-28-11 at 12:30 PM. Reason: grammar "in order to while avoiding" ?????? :S
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post #30 of 48 Old 08-28-11, 06:58 AM
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Re: Bass traps for asymmetrical set-up in L-shaped apartment?

Understood completely. That said, using a tool that's well supported here which does do sweeps is very easy and relatively accurate for assessing the basics of what's happening in the room. Yes - you can certainly run a full range sweep and then look at just the 20-300 area.

I am serious... and don't call me Shirley.

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