Dolby TrueHD

7.1 vs 7.2 Surround Sound – The Difference a Point Makes


Engineers and Their Naming Things

Quadraphonic

When surround sound was first introduced, it came out as “quadraphonic“, which was essentially just double stereo: four speakers, one in each corner of the room, to immerse yourself in sound.

Then along came Dolby.

Dolby (who pioneer just about everything in the audio world) decided, in 1982, that the speakers should have more than just left and right, they should have a difference from front to back as well. They called it Dolby Surround.

Having the limited capacity for sound signals on the media and hardware of the time, they split the usual stereo signal, by using what they termed “matrix encoding” (basically two signals, one wire).

Fast forward to LaserDisc and DVD, and the birth of Dolby ProLogic.LaserDisc

With new media came enough space for five speakers (front left, front right, center, surround left, surround right) to have their own information, and, more importantly, the addition of the “.1“, making 5.1.


What is the “.1”?

To find out the difference between 7.1 vs 7.2 surround sound, you need to know what that “.1” means.

The “.1” in 5.1, 7.1, and 9.1 is the LFE, or Low-Frequency Effects, channel. This is where the internal crossover of the device powering the speakers splits off frequencies lower than which most speakers can respond and sends them to the subwoofer.

SubwooferHumans can hear frequencies between 20 Hz and 20 kHz, or 20 to 20,0000 vibrations per second. The majority of subwoofers can operate between 20-200 Hz, with some going as low as 18 Hz.

Subwoofers give you the frequencies that you would otherwise be missing with just front, surround and center speakers.

Even some of the most incredible floor-standing speakers only operate as low as 24 Hz, where some of the cheapest subwoofers will respond to 22 Hz.

You should aim at using a subwoofer between 20 and 80 Hz if your speakers can pick up the rest.

Click Here to read our review of an excellent subwoofer.


The Jump From 5.1 to 7.1

When 7.1 arrived (after a short-lived 6.1), it was possible to have the usual 5.1, plus two added “rear surround” speakers. The initial surround speakers in 5.1 are meant to be placed to the sides of the listening position and slightly behind. The rear surrounds are meant to be the mirrors of the front speakers, behind the listening position and to the left and right.

To handle the signals for 7.1, which required the added space of Blu-Ray discs, Dolby released Dolby Digital Plus and Dolby TrueHD.Blu-Ray

Digital Plus includes compression to make the information take less space on the media, where TrueHD is the full, lossless information, and meant to be exactly as it was recorded.


So, Why is “.2” Better?

After reaching 7.1, technology continued on to 9.1 and 7.2. The “.2” stands for 2 LFE outputs (they are the same track, just an added output). This means two subwoofers.

The reason for the upgrade from the previously universal 5.1, 7.1, and 9.1 is that with a single subwoofer, it is possible to “localize” the subwoofer, or figure out where it is in the room. This also means that there is one very good quality bass zone, or “sweet spot“, while other areas of the room are of degraded quality.

With two LFE channels and two subwoofers, placed strategically in the room (we recommend diagonal front and rear corners), the modal waves of bass frequencies converge and blanket much more of the room, and make it near impossible to locate the subwoofer by sound alone.

You get the feeling you are meant to get, anywhere in the room, with two subwoofers.


For more information on where technology has gone since (like Dolby Atmos), click here.

For our review of the Klipsch C-310ASWi and C-308ASWi subwoofers, click here.


4 thoughts on “7.1 vs 7.2 Surround Sound – The Difference a Point Makes

  1. I never knew that humans could hear frequencies between 20 Hz and 20 kHz, or 20 to 20,0000 vibrations per second. I thought we had far more evolved ears than that.

    So you recommend that getting a subwoofer gives you the frequencies that you would otherwise be missing with just front, surround and center speakers. I always wondered what a subwoofer’s purpose was, and now I know – thank you very much.

    It sounds like it will be a valuable addition to my home theatre system.

    1. If you have never had one, the difference is incredible. The frequencies you get from a subwoofer aren’t just heard, they are FELT. With the right subwoofer, and the right receiver, explosions and helicopters flying overhead and just about any loud noise starts to feel like you are deeper into the experience of movie-watching.

      Also, if you only listen to music on stereo speakers (even some of the higher end ones), the addition of a subwoofer will leave you wondering how you listened for so long only hearing half of what a song is truly supposed to sound like.

      A subwoofer isn’t just a valuable addition, it is a necessity for a proper entertainment system.

  2. Interesting writeup. I’ve been working with Dolby surrounds for a while and I’ve gathered few experience. All speakers have different rating frequencies and the change in a brand is easy to see. For example, one speaker provides a speaker for a bookshelf range between 55 and 20,000 Hz, a second with a range between 40-20,000 Hz and a few speakers for tower ranging from 35 to 20,000 Hz and even between 30 and 200,000 Hz. There is therefore a consequence that the frequency at the bass end decreases as the speaker food chain (inside the same brand). But some speakers that are much more expensive really have frequency specs that seem much worse.

  3. Thanks for writing this on 7.1 vs7.2 surround sound and the difference a point makes. I must commend you for a job well done for taking your time to explain each and every things about it and even some of the things I don’t know about Dolby and I have been using this product over the years because it one of my favorite 

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