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Author Topic: PROOF-Whole vs half space sub loading  (Read 15838 times)

Mark Wilkinson

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Re: PROOF-Whole vs half space sub loading
« Reply #20 on: April 07, 2016, 11:59:13 am »


Acoustic loading when reflected back through the driver is (I always thought) counterintuitive.   

When the resistive acoustic load on the driver is increased say with a horn, the resistive portion of the impedance goes up, not down and so, the power delivered to the speaker with a fixed Voltage, actually goes down proportionally too.
                 


Hi Tom, trying to understand what you are saying...

So as a start, I thought to maybe just test the effect on impedance in going from full space to quarter space... or should I say hanging out on an open deck space, to sitting against kitchen wall space haha.

I don't have an easy to move small sub so I used Peter Morris's DIY60 MID section as a hopeful fill in.  (It only goes down to 100hz, so i knew whatever effect seen will be reduced.)

Anyway, against my expectations, impedance dropped moving into less space.  I would have thought less space increases load....????

Here's the plots I took.  The one on the left is outside, right is in the kitchen.  I included the real and imaginary components just to see if anything was there...
I don't get it....???
Thx,  Mark

edit: If the answer is less air mass with less space, I get it.   If not, still lost...







« Last Edit: April 07, 2016, 01:43:16 pm by Mark Wilkinson »
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Steve Bradbury

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Re: PROOF-Whole vs half space sub loading
« Reply #21 on: April 07, 2016, 12:20:36 pm »

Surely rather than use a fork lift to raise the loudspeaker and leave the microphone on the floor, it would be easier to leave the loudspeaker on the floor and raise the microphone?
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John Rutirasiri

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Re: PROOF-Whole vs half space sub loading
« Reply #22 on: April 07, 2016, 12:33:54 pm »

Surely rather than use a fork lift to raise the loudspeaker and leave the microphone on the floor, it would be easier to leave the loudspeaker on the floor and raise the microphone?

This is about how spatial loading of the sub affects the output.  If the sub is on the ground, then it's half-space (the floor being the dividing plane.)  If the sub on the floor is in a corner, then it's 1/8th space.

The reason for getting the subs off the floor and into the air is to simulate whole/full/free space -- eliminate acoustic coupling with the floor or wall. 

Putting a microphone in the air won't accomplish that.

John R.
« Last Edit: April 07, 2016, 12:36:17 pm by John Rutirasiri »
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Steve Bradbury

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Re: PROOF-Whole vs half space sub loading
« Reply #23 on: April 07, 2016, 12:53:46 pm »

Am I missing something where the principle of reciprocity does not apply in this instance? If I am could you provide links/references to any articles that support your claims.

Thanks.
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John Roberts {JR}

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Re: PROOF-Whole vs half space sub loading
« Reply #24 on: April 07, 2016, 12:55:19 pm »

Not to spam Ivans thread, but here is an interesting white paper from David Gunness
http://fulcrum-acoustic.com/assets/pdf/whitepapers/comments-on-half-space.pdf  exploring constructive and destructive interference from the ground bounce.

We often discuss these different spaces as if the sum from a bounce is flat, but it isn't always and can have audible frequency response effects.

I now return this discussion to the speaker geniuses (not me).

JR 
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Steve Bradbury

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Re: PROOF-Whole vs half space sub loading
« Reply #25 on: April 07, 2016, 01:05:57 pm »

From the second page of the article linked to above:

Interestingly and importantly, the position of the acoustical source and microphone can be
exchanged and the resulting response will be identical. For example, if either the source or the
microphone is on the boundary (or both), the response will be +6 dB at all frequencies. This is one
example of the principle of duality in acoustics.

Mr Gunness seems to agree with me. His referral to duality is the same as what I termed reciprocity.
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John Rutirasiri

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Re: PROOF-Whole vs half space sub loading
« Reply #26 on: April 07, 2016, 01:12:37 pm »

Am I missing something where the principle of reciprocity does not apply in this instance? If I am could you provide links/references to any articles that support your claims.

Thanks.

The loudspeaker (subwoofer in this case) provides the room excitation, not the measurement microphone.
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Steve Bradbury

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Re: PROOF-Whole vs half space sub loading
« Reply #27 on: April 07, 2016, 01:36:26 pm »

John Rutirasiri,

From your response I am assuming that you do not understand the principle of reciprocity. Basically it states that if you swap the positions of the source and receiver (microphone/ear) the result is the same, it doesn’t matter which way round they are.

The following is an extract from the book Reciprocity in Elastodynamics. It was one of the first results that came up with a Google search. I’ve simply copied and pasted it. There are probably better explanations out there if you do a search.

The first reciprocity relation specifically for acoustics was stated by von Helmholtz (1859). This relation caught the attention of and was elaborated by Rayleigh (1873) and Lamb (1888). Rayleigh (1873) briefly discussed the reciprocal theorem for acoustics in his paper “Some general theorems relating to vibrations.” In The Theory of Sound (1878, Dover reprint 1945, Vol. II, pp. 145–8), Rayleigh paraphrased this theorem as follows: “If in a space filled with air which is partly bounded by finitely extended fixed bodies and is partly unbounded, sound waves may be excited at any point A, the resulting velocity potential at a second point B is the same both in magnitude and phase, as it would have been at A, had B been the source of sound.” In this statement it is implicitly assumed that sources of the same strength would be applied at both places. In Rayleigh's book (1878) the statement is accompanied by a simple proof. A similar statement of the Helmholtz reciprocity theorem for acoustics can be found in the paper by Lamb (1888). Both Rayleigh and Lamb generalized the theorem to more complicated configurations, and in time the reciprocity theorem became known as Rayleigh's reciprocity theorem.

Most books on acoustics devote attention to the reciprocity theorem; see for example Pierce (1981), Morse and Ingard (1968), Jones (1986), Dowling and Ffowcs Williams (1983) and Crighton et al. (1992). A book by Fokkema and van den Berg (1993) is exclusively concerned with acoustic reciprocity.

As well as the above referenced texts, reciprocity is also covered in Olson’s Acoustical Engineering and Beranek’s Acoustics (both the original and new version) which most people interested in loudspeaker design should have a copy of.
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George Dougherty

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Re: PROOF-Whole vs half space sub loading
« Reply #28 on: April 07, 2016, 10:50:00 pm »

Am I missing something where the principle of reciprocity does not apply in this instance? If I am could you provide links/references to any articles that support your claims.

Thanks.

I think I understood Tom's post and, if so, then the answer is that yes there are differences that would make reciprocity not entirely faithful here.

In general, yes it holds true with all other things being equal. It gives rise to the practice of placing a HT sub in your listening position and then walking the room to find the loudest/best response and then placing the sub there to optimize response in the listening position.

In this case, Tom's contention that the proximity to the ground impacts the impedance and output at a fixed voltage would seem to indicate that moving the sub could produce differing results in both positions that wouldn't necessarily be matched by moving the microphone instead.
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Chuck Simon

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Re: PROOF-Whole vs half space sub loading
« Reply #29 on: April 08, 2016, 12:26:28 am »

In this case, Tom's contention that the proximity to the ground impacts the impedance and output at a fixed voltage would seem to indicate that moving the sub could produce differing results in both positions that wouldn't necessarily be matched by moving the microphone instead.

Now that makes sense to me!  Do those changes in impedance result is something we can actually hear?
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Re: PROOF-Whole vs half space sub loading
« Reply #29 on: April 08, 2016, 12:26:28 am »


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