Ports, passive radiators and room coupling


2011-02-27 12:56 am
Basically my dilemma boils down to room coupling.

Its basic knowledge that passive radiator models the same as port for ventd enclosures, with the mian difference being a somewhat steeper rolloff below the fs, as opposed by considerable space taken by ports tuned low.

What I could not find much info on is the difference in room coupling of these. The higher the velocity of a speaker (or a sound source) the poorer it couples to the room, which is why classic 8" midbass dirvers give a perception of much greather authority than 4-5" long throw midwoofers, even if these extended lower on paper, at least from what I read.

In my case I have a 5" midbass which will be augmented by a sub. I was wondering how much authority would a 5" have at 80-100 Hz when I thought that I could provide a larger radiating area with less excursion by adding a passive radiator. I found two SB PRs, a 6" and 5x8", but in both cases these would peak at around 60 Hz, where there is realtively little output from the midbass and its about 10 db below sub levels. So basically simulation tells me that I would not gain much LF output, but perhaps I would gain some more authority in the bass?

What is Your experience in PR vs vents in term of room coupling? Does larger radiating area of a PR make a subjective difference in how speakers load the room?
If you can accept a 6" passive radiator, why not use just one 6" midrange driver?

A wide bandwidth 6" like the SB_Acoustics SB17NAC35-4 in a sealed 0.5cuft volume will produce fast and clean transients from a Qtc ~0.62 tuning, plus -F3 = 72Hz bass for easy integration with a woofer. The SB17NAC35-4 has 86db @ 2.83V efficiency. A 2kHz crossover allows smooth directivity transfer to the tweeter. The size and shape of the front baffle, along with the in-room placement, will determine how the crossover must be adjusted for smooth SPL .


2011-02-27 12:56 am
The decision to go with a 5" midrange was rather complicated. Speakers in mention will be used as fronts in a home theater system. I will be using the speakers with a PC based active crossover, mostly in 2.1 setup for listening to music, or as a 5.1 system driven by a 7.1 source, with front speakers taking 4 channels mid + tweeter). But I will also run movies from my TV and perhaps a PS/Xbox in the futur, in which case AVR will do crossover duties. In this case I will use 5.1 source and run mid without lowpass, as tweeter will only work if PC is used a source. So i needed a mid than can also act as a widebander. Also, I wanted to get the power response in the vertival axis as good as I can get. I can accomodate a 7.5" driver, but I use the available space to offset drivers to one side and have some directivity control. So its a long story and exactly on the topic of PRs and ports :)
The higher the velocity of a speaker (or a sound source) the poorer it couples to the room, which is why classic 8" midbass dirvers give a perception of much greather authority than 4-5" long throw midwoofers, even if these extended lower on paper, at least from what I read.

Now where did you read that? Sounds bollocks to me.

Imho the most discerning element in room coupling is the placement of the source. Put a speaker in the bottom corner of your room and listen in another corner. Overwhelming bass will be yours. Now place both speaker and ears in the exact middle of your room (vertical too) and gone is your bass.

And relative small differences in source placement do and will matter. So the passive radiator or port on the back or near to the floor in your speaker does a hellufalot better than that bass-mid that you try to keep away from boundaries (to reduce early reflections).

The only caveat is that you are not listening to your speaker but to your room response. And that is a complete different chapter of your basic acoustics handbook.


2011-02-27 12:56 am
The part on velocity is just my creativity but if You think about why would a larger cone area lead to improved room loading then it seems like a logical conclusion. More cone area means less travel over the same cycle, like 20 Hz, hence lower acceleration and velocity.

I got the whole idea of room loading from reading the forums on the topics like many smaller woofers vs one large, with conclusions giving the nod to larger woofer as a more efficient way of getting low bass (usually lower cost and smaller cab), and ofc this room loading thing.

Also, recetly I read on Troels Gravensen's site this passage

"You may think that in order to produce decent bass, you need speakers with a linear response down to 30 Hz. Now, 30 Hz would be nice - if the room allows - but I wish it was possible to demonstrate to all first-time diy'ers what it sounds like when we compare a 6" bass driver with -3dB @ 30 Hz to a 12" bass driver with -3 dB @ "only" 50 Hz. Most people would probably agree that bass can mean a lot of things."

While it does not clearly introduce the idea of room loading Im sure its also elsewhere on his site, I just can't find it at the moment. This why I started wondering about the difference in perceived response from PRs, which have far larger radiating area than ports, whereas in ports we get higher air velocity.

What You suggest is using the room as a waveguide - increasing sensitivity by limiting the space which driver loads. I agree its a very good strategy and I plan to use it myself for my 8" sub, but it does not address the radiating area thing.
Actually, to my knowledge, the radiation resistance of a large source with a given frequency will be lower than that of a smaller source at the same frequency. Now at f=30Hz that won't make big differences, but all else being equal, the smaller driver will actually be more efficient (read: louder).

And no, I did not refer to waveguides, but to the (often misunderstood) fact that low frequency reproduction in an average room and single or double sources is dominated by 'eigentonen' or room modes: standing wave patterns defined by room dimensions. In normal rooms that effect is dominant up to 150 to 200Hz.
The radiation resistance makes a difference but not the velocity.

Ignoring transducer specifics, the efficiency goes as the acoustic resistance, which is about the same for two different sized pistons at HFs (large ka.) They will differ depending on size at LFs. But all this is free field anyways, but in a small room the room dominates the radiation impedance of any source. Hence "coupling to the room" in mostly room dependent, not source size dependent. There will be a small factor for size, but mostly its the room and placement.