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Dip between 100 - 200 Hz

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Posted on July 4, 2018 at 17:41:33
George6
Audiophile

Posts: 182
Joined: November 18, 2012
 In one of the recent threads, Josh showed a response curve for his Timpani and I noticed quite a large dip just above 100Hz.
My 1.6 had a dip at 180Hz, and my 3.7i have one at 125Hz.

This probably accounts for comments about "lean" or "analytical" sound, but I'm wondering if there is a physical/electronic reason for this dip. Or does Magnepan simply voice their speakers this way?

If this has been discussed in an older thread, please point the way, as I couldn't find anything.

 

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RE: Dip between 100 - 200 Hz, posted on July 4, 2018 at 19:38:23
zulugone
Audiophile

Posts: 98
Location: Seattle, Washington
Joined: August 31, 2013



I had the same thought. My 1.7's are in a very different room than Josh's. From left to right my REW sweep shows similarities: the 40HZ Maggie bump followed by a second bump. I have a 5db drop between 2KH and 3KH and don't use a resistor. This can shift a little lower toward 1KH. Before my extreme damping I typically had a bump + dip between 125HZ and 175hHZ. You can see what's left of that. I have seen a similar pattern in other REW sweep posts.

In the many room positions I tried I had only one where the response curve declined in a line from higher on the left to lower on the right and only one where it was almost flat. All others showed the drop from about 2KH to 3KH.

 

RE: Dip between 100 - 200 Hz, posted on July 4, 2018 at 20:35:40
hahax@verizon.net
Audiophile

Posts: 4310
Location: New Jersey
Joined: March 22, 2006
 Frequency response of all speakers is affected by room position. In deed the same is true of live sound. Change your seat in a concert hall and the octave to octave balance changes. Decades ago Gordon Holt and Henry Kloss got into a difference of opinion of the sound of the KLH speaker in a Stereophile test. Gordon found it mildly colored. Henry argued Gordon was wrong. The next time Gordon went to the Academy of Music in Philadelphia for a concert he changed his seats from the regular ones. Darned if the new seat didn't sound like the KLH 12.

By the way this also says that sounding like live sound is not dependent on absolutely flat response. Both seats were of course live sound but the response of course varied between seats.

 

RE: Dip between 100 - 200 Hz, posted on July 4, 2018 at 22:15:34
zulugone
Audiophile

Posts: 98
Location: Seattle, Washington
Joined: August 31, 2013
 I would agree we all know that rooms affect the frequency response curve. Ihe question I read from the OP (and me) is 'is there any kind of standard Maggie FRC'? I have read comments on pro forums saying that some pro monitor manufacturers drop the FRC around the 2KH "harshness" region.

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 09:23:06
josh358
Industry Professional

Posts: 12332
Joined: February 9, 2010
 That dip is cancellation from the front wall reflection.

The back wave from a dipole is 180 degrees out of phase with the front wave. When it reflects off the front wall and back to the speaker, it cancels the front wave at the frequency at which the distance from the speaker to the wall plus the distance from the wall back to the speaker is equal to a wavelength, and you get that dip. In other words, twice the distance to the wall.

At a 5' distance from the speaker to the wall, the round trip to the speaker is roughly 10', so cancellation occurs in the 100 Hz region.

Monpoles also suffer from cancellation, but since their backwave is in phase with the front wave, it occurs when the distance is equal to half a wavelength. It also is more likely to occur from multiple reflections -- side walls, front wall, floor and ceiling. That's why it's generally recommended that monopoles be positioned at different distances from the side and front walls.

A dipole parallel to the front wall will be affected mostly by the front wall reflection, but the null will be very deep. If you angle the speaker in, it will start to illuminate the side wall and you'll get two reflections with shallower nulls. Single panel dipoles are generally use with toe in so the same rule applies as to monopoles, you want to split the difference between front and side walls to reduce the null.

(Things actually get a bit complicated at those frequencies since the wavelength is large compared to the room and treating them as specular reflections doesn't really work -- it becomes a matter of wave mechanics. So rooms and positions will be somewhat idiosyncratic in this regard. I'm using my woofer in a somewhat unconventional arrangement against the wall.)

Bass trapping can reduce the null by reducing the magnitude of the reflection. Another possible solution is to use multiple woofers at different positions.

I just got some Owens-Corning 703 FRK that I'll be using to make corner bass traps. Hopefully, that will reduce the null to the point at which it can be equalized out -- otherwise, I'll have to look at multiple driver solutions.

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 09:32:48
josh358
Industry Professional

Posts: 12332
Joined: February 9, 2010
 Enviably smooth midrange response!

You actually don't want speakers to be flat for two channel stereo -- they sound too bright if they are. This is because cardioid or close mics pick up a lot of direct sound and less reverberation, which has attenuated highs, and also because of the head-related transfer function of the ear -- stereo speakers are positioned at an angle at which sounds are bright.

If you're in a room in which the speakers measure flat, you want to use the tweeter resistor to bring the level down some as it is here.

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 09:42:34
zulugone
Audiophile

Posts: 98
Location: Seattle, Washington
Joined: August 31, 2013
 My 1.7 backs are enclosed floor to ceiling in 703 eliminating most of the rear wave. Heresy I know. The most reduction in the 100+HZ bump + dip came when I "razored" and added aluminum angle.

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 09:51:23
zulugone
Audiophile

Posts: 98
Location: Seattle, Washington
Joined: August 31, 2013
 Most of the "smooth midrange response" came after "razoring" the entire back with 3M 2552 and adding aluminum angle. This also completely eliminated the need for my DWM. I probably spent more money on aluminum and damping tape than Magnepan spent on material for the 1.7s :|.

If have read the Fletcher-Munson wisdom and related wisdom and remain skeptical of their application to home stereo but I could just be thick.

https://en.wikipedia.org/wiki/Fletcher%E2%80%93Munson_curves

My old ears have built-in treble filters anyway.

It still looks to me (from several online posts) like at least the 40HZ Maggie bump and the lowering from 2KH to 3KH are a Magnepan design or artifact of design.

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 11:10:49
josh358
Industry Professional

Posts: 12332
Joined: February 9, 2010
 Well, Fletcher-Munson as applied to home stereo is typically by way of loudness compensation -- it makes up for the fact that if you play music at low levels, bass can fall below the threshold of hearing so the music starts to sound tinny. It is rarely properly implemented, which would require knowing the sensitivity of the speakers, the gain of the amplifier, and the original level of the recording.

Magnepan aims for a gentle downslope which as I said makes two channel stereo sound right. But that's just a starting point since your room also influences balance.

There's something called the BBC dip or Gundry dip and its purpose is to compensate for the change in power response at the crossover point between a large woofer and a small tweeter. Because the large driver is becoming directional at that frequency and the small driver has wide dispersion, the sound can become too "hot" in the crossover region. The BBC dip compensates for that to achieve a subjectively neutral response. So that may be what Magnepan is doing here.

There is also a fundamental flaw in two channel stereo in this region, because of interference between the two speakers.

Interesting that razoring improved your midrange smoothness!


 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 11:20:48
neolith
Audiophile

Posts: 4842
Location: Virginia
Joined: February 21, 2002
Contributor
  Since: December 2, 2004
 I talked to Wendell when I purchased my MG3.7i. He said the goal was to have a gradual (about 5 db overall) drop in the fs from 500 Hz to 20 kHz. So that is a design feature. The peak at the low frequencies is probably room related and not intentional.
Here is the fs that I did when I first set-up the speakers. Note the drop around 60 hz and the gradual fall-off above 500.

This is the current fs which includes the sub-woofer and some re-arrangement of absorption panels, etc. Now the dip is now replaced by a peak but the treble is largely flat. Like you I am relying on decrepit ears to attenuate the high end. FWIW my wife thinks the speakers are bright (although she does not describe it that way - more like "annoying, could you turn it down"). I may consider adding resistors to attenuate the treble so she is happier -- nah, that ain't happening. BTW the speakers run full range with the sub kicking in at about 34 Hz.

The point being that the anomalies in the lower end can be ameliorated with room treatments.




"Our head is round in order to allow our thoughts to change direction." Francis Picabia

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 11:54:58
Davey
 Any dips down in the low frequencies like that are caused by room effects and/or the measurement scheme.
Even if they wanted to engineer a dip into the response like that, it's not easy to do and requires a bunch of extra components in the crossover.

Dave.

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 13:08:04
emailtim
Audiophile

Posts: 5500
Joined: July 2, 2017
 "... The most reduction in the 100+HZ bump + dip came when I "razored" and added aluminum angle.
..."

How would you explain how the "razoring" eliminated the 100 Hz bump/dip?

I would think "razoring" would do more to remove smearing than to fix peaks/nulls.

TIA
.

2022/03/30 Historical Records CENSORED

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 13:30:20
George6
Audiophile

Posts: 182
Joined: November 18, 2012
 Thanks everyone for the interesting and helpful comments.
I can EQ the dip so that it is less apparent, and the overall frequency response does slope downwards as it gets higher, so not really a problem.
And some material sounds better with the dip.

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 14:13:37
josh358
Industry Professional

Posts: 12332
Joined: February 9, 2010
 Interesting. Maybe the razoring acted as a bass trap -- an interesting thought!

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 14:37:49
zulugone
Audiophile

Posts: 98
Location: Seattle, Washington
Joined: August 31, 2013
 Yea. My guess would be that the 2kHZ to 3kHZ drop might be by design. The prevalent 40HZ bump is either by design or an artifact of how the speakers are built. The 100HZ dip is less likely by design. For one non-definitive example my monitors in the same position are rated lower than the 1.7; have no 40HZ bump and are generally smoother for the same room treatment.

FWIW the response curve gets flatter with damping and with the tedious use of a tape measure and a plumb.

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 14:45:13
zulugone
Audiophile

Posts: 98
Location: Seattle, Washington
Joined: August 31, 2013
 My explaining credentials are limited and I would not trust them myself. Overall from 100HZ to 400HZ where the DWM's "filled in" the FRC got smoother with aluminum angle, bracing and razoring. I also spent more time with a tape measure and a plumb so causes are somewhat mixed. The sonic result in any case is great. I use my sub much less now except for movies..

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 14:50:34
zulugone
Audiophile

Posts: 98
Location: Seattle, Washington
Joined: August 31, 2013
 I wasn't very scientific about measuring the effects of the aluminum angle vs. the brace to the wall vs.the razoring. 3M doesn't say anything about frequency of vibrations. https://www.3m.com/3M/en_US/company-us/all-3m-products/~/3M-Damping-Foil-2552/?N=5002385+3293194280&rt=rud

 

RE: Dip between 100 - 200 Hz, posted on July 5, 2018 at 15:41:29
josh358
Industry Professional

Posts: 12332
Joined: February 9, 2010
 It's the sort of thing that could work, a membrane on a viscoelastic damping layer, positioned at a point at which the wave has high velocity.

 

high frequency tilt down, posted on July 6, 2018 at 16:57:44
DrChaos
Audiophile

Posts: 2063
Location: San Diego
Joined: July 13, 2009
That high frequency tilt down is what is proper psychoacoustically, known for literally decades. 1 dB per octave: 500,1000,2000,4000,8000,16000. That's 5dB.


https://www.innerfidelity.com/content/acoustic-basis-harman-listener-target-curve



Flat in-room at the listening position averaged is wrong. (flat anechoically might be good but it's exceptionally difficult to measure that with large planars).

Wife's ears and brain are right.

 

RE: Dip between 100 - 200 Hz, posted on July 15, 2018 at 22:54:22
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 I am going to guess that there is a bending mode of the overall panel and frame that the aluminum angle bracing eliminates - or at least shifts to a different frequency. I have no idea what the freq range of it would be, but we can model a flexible board anchored at the bottom to calculate an estimate. A Plumb alignment can also eliminate early floor reflections and to the extent the angle brace makes that happen, then that may reduce floor to ceiling resonance contributions in the 100-140 hz to near nothing.

 

RE: Dip between 100 - 200 Hz, posted on July 16, 2018 at 06:58:48
zulugone
Audiophile

Posts: 98
Location: Seattle, Washington
Joined: August 31, 2013
 I have no idea how to measure resonance. Using test tones or music with low frequency I can feel the significant drop in vibration between the speaker element and the frame with a finger as I added aluminum and damping tape. I plumb the speakers side to side and front to back and then shim the front feet to tilt back. I have not been able to affect the frequency response curve with absorption on the concrete + thin carpet floor. I have reduced a 400 HZ dip with 703 on the ceiling (which has a metal furnace duct in just the right place).

 

Dip between 100 - 200 Hz - Changed XO points, posted on August 6, 2018 at 08:16:13
emailtim
Audiophile

Posts: 5500
Joined: July 2, 2017
 FWIW, I had an in-room dip with my 20.1's at the lower XO point. The dip appears at the listening positions, not measuring the speaker @ 3ft so I would attribute it to the room.

I diminished it by changing the lower XO points. The passive OEM stock XO points are 108Hz 18dB and 330Hz 6dB. I am currently using 200Hz 18dB and 300Hz 6dB which measures much better than all of the other combinations I was able to test with the configurable XO I have on hand.

The 20.1 manual mentions that the XO points may vary within a specified range as follows:

"... Set the high pass section at 200-300 Hz, 6dB per octave. Due to standing waves at the crossover frequency, the low pass section may require adjustment from as low as 90Hz (12dB Bessel) to 150Hz (15dB Butterworth). Typical settings are in the middle of this range. Optimization of the low pass can be done by ear (with music) or spectrum analysis. ..."

UPDATE: One other point. One of the guys (I think he was from GIK) said that a 6" thick bass trap with a 6" air gap behind it approaches the efficiency of a 10" thick trap without the extra expense of the additional 4" of 703.
.

2022/03/30 Historical Records CENSORED

 

RE: Dip between 100 - 200 Hz - Changed XO points, posted on June 19, 2020 at 05:23:05
WrineX-Joppe
Industry Professional

Posts: 10
Location: Nederland
Joined: June 19, 2020
 The dip might be the back wall. i got the same in my DIY planars. although outside there still will be a dip. usually this is caused by the resonance.

if resonance is 50Hz there will be a dip near 100hz if you compare it to the peak at 50hz. what i think happens the res creates a peak. if you work out the response with that peak ,after resonance the ouput will be lower, hence a dip. since the resonance is never damped away in magnepans. they use it to extend the low end. the dip at +- double the resonance is actually the frequency (or a bit higher) where the baffle cancellation front to back starts to kick in. notice that any crossover of a magnepan bass driver is much higher then the 100-300hz one might expect from an open baffle speaker.

if the resonance would be damped away completely the speaker would drop of at this 200 mark.

so to make a perferct bass planar resonance should be much lower, and probable allot more surface area or wider baffle to compensate for the losses.

 

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