Schu Posted February 20, 2018 Share Posted February 20, 2018 Can anyone (Chris or Roy) please explain what these terms... Directivity Index and Directivity Ratio... are and what they represent? I see them quoted in specification sheets and have no idea why that is important. Quote Link to comment Share on other sites More sharing options...
Chris A Posted February 20, 2018 Share Posted February 20, 2018 I'll take a stab. "Directivity factor" (Q(f))--is the ratio of the intensity (SPL) on the the main axis of the acoustic radiator to that intensity that would be produced by a point source--i.e., omnidirectional--acoustic source producing the same total acoustic power output as the (directional) source of interest. (from Beranek Acoustics) This is also apparently referred to as "directivity" (D)--the ratio of power density in the direction of the pattern maximum to the average power density at the same distance from the acoustic source (from antenna theory). [This is almost the same thing as "antenna gain", except that you must subtract the losses to get true antenna gain.] The directivity index (DI) is equal to 10*log(Q(f))--i.e., the logarithmic value of the directivity factor, expressed in dB. The term directivity ratio seems to be much less used in the acoustic domain, and appears to be synonymous with directivity factor or just directivity. Chris 1 Quote Link to comment Share on other sites More sharing options...
Chris A Posted February 20, 2018 Share Posted February 20, 2018 Why it's important is a much more interesting discussion. It turns out that, for movie theaters and PA use, directivity is important so that you can place the acoustic energy where you want it, to cover the audience seating in a more or less equal fashion. In small room acoustics however, the effect of directivity is a bit more involved with how the human hearing system works--i.e., psychoacoustics. It turns out that early reflections in-room get integrated with the direct arriving acoustic energy via the human hearing system. Sometimes this is called the "precedence effect" or "Haas effect". Directivity in loudspeakers allows the user to keep the ratio of direct to early reflected energy higher. It is the subject of many papers on acoustics in cinemas and small rooms. So the right amount of directivity is desired for the application (the room and its interior acoustics) that the loudspeakers will perform in. It can be too narrow or too wide, depending on the application and the desired effect. In my room for instance, 90 degree (horizontal) coverage of the loudspeakers in the corners and in the center seems to have the right amount of direct vs. early reflected levels when placed against the wall and corners of the room. This is very close to the principles that PWK supported in loudspeaker design. Other people like the sound of omni-directional (i.e., Bose 901-type) or dipole (electrostat or Magneplanar) loudspeakers. YMMV. If you mix loudspeakers with different directivities, or if the loudspeakers themselves have differing directivities vs. frequency, they will have different timbres and soundstage/imaging. Chris 1 Quote Link to comment Share on other sites More sharing options...
Schu Posted February 20, 2018 Author Share Posted February 20, 2018 TYVM... Quote Link to comment Share on other sites More sharing options...
MC39693 Posted February 21, 2018 Share Posted February 21, 2018 Chris, thanks for that note. I have a set of Heresy and a set of Martin Logan Clarity that I use in multi-channel in a small room 18x12x8 with a long side hall. I had to raise the Heresy and eq the room and increase the Clarity volume to get a sound I like. Clarity need to be away from the walls with that bipolar effect! Sometimes I have the Heresy behind (4 ft) the Clarity and the sound between them is fun! Sensitivity, sound strengths/weaknesses, placement issues...but the combo is good. Quote Link to comment Share on other sites More sharing options...
Deang Posted February 21, 2018 Share Posted February 21, 2018 Nice job Chris. Thank you. Quote Link to comment Share on other sites More sharing options...
Klipsch Employees Chief bonehead Posted February 22, 2018 Klipsch Employees Share Posted February 22, 2018 On 2/20/2018 at 7:33 AM, Schu said: Can anyone (Chris or Roy) please explain what these terms... Directivity Index and Directivity Ratio... are and what they represent? I see them quoted in specification sheets and have no idea why that is important. never really paid that much attention to those specs. i always am interested in what the coverage is in the hor and ver. i used to say.....constant directivity index does not guarantee constant coverage. constant coverage does guarantee constant directivity. 3 Quote Link to comment Share on other sites More sharing options...
mikebse2a3 Posted February 22, 2018 Share Posted February 22, 2018 12 minutes ago, Chief bonehead said: never really paid that much attention to those specs. i always am interested in what the coverage is in the hor and ver. i used to say.....constant directivity index does not guarantee constant coverage. constant coverage does guarantee constant directivity. Here are some comments by Roy about this subject with examples for those interested. 1 Quote Link to comment Share on other sites More sharing options...
Chris A Posted February 22, 2018 Share Posted February 22, 2018 An article that discusses how directivity and coverage are measured and plotted for those those that wish to understand a little more on this subject: http://www.doctorproaudio.com/doctor/cajondesastre/pdfs/brusi_understanding_directivity_game_numbers_doctorproaudio.pdf Toole's 3rd edition book (Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers & Rooms) discusses the importance of listening window and early reflections window curves, as well as the importance of controlling DI vs. frequency for loudspeaker designs. It's a rewarding read once you get through most of the snippets of discussion concerning directivity vs. frequency. Chris Quote Link to comment Share on other sites More sharing options...
Coytee Posted February 22, 2018 Share Posted February 22, 2018 9 hours ago, Chief bonehead said: i used to say.....constant directivity index does not guarantee constant coverage. constant coverage does guarantee constant directivity. What do you say today? 2 Quote Link to comment Share on other sites More sharing options...
Klipsch Employees Chief bonehead Posted February 23, 2018 Klipsch Employees Share Posted February 23, 2018 On 2/22/2018 at 5:29 AM, Coytee said: What do you say today? that you are still a bonehead.... 1 Quote Link to comment Share on other sites More sharing options...
mark1101 Posted February 23, 2018 Share Posted February 23, 2018 +1 Quote Link to comment Share on other sites More sharing options...
Deang Posted February 23, 2018 Share Posted February 23, 2018 So, he's not "Chief Bonehead", he's "Chief of the Boneheads". 1 Quote Link to comment Share on other sites More sharing options...
JohnA Posted February 23, 2018 Share Posted February 23, 2018 So what is a "good" directivity Index? If I see a DI of 8 dB, what good is it. It should vary by frequency. Or, what DI indicates a "spotlight" and what DI indicates a "Floodlight"? I'm betting this is getting too a quantification of long throw vs short throw, true? Quote Link to comment Share on other sites More sharing options...
mark1101 Posted February 23, 2018 Share Posted February 23, 2018 Just stand in front of the speaker. Quote Link to comment Share on other sites More sharing options...
babadono Posted February 23, 2018 Share Posted February 23, 2018 1 hour ago, Deang said: So, he's not "Chief Bonehead", he's "Chief of the Boneheads". You're catching on Quote Link to comment Share on other sites More sharing options...
Chris A Posted February 24, 2018 Share Posted February 24, 2018 20 hours ago, JohnA said: If I see a DI of 8 dB, what good is it. It should vary by frequency. There are several loudspeaker measures that (IMHO) should always be plotted as curves or spectrograms: input impedance directivity index Additionally, there are other measures that should be provided by loudspeaker manufacturers...but usually aren't. This is interesting because there is a lot of other data plotted in EASE data for cinema loudspeakers. These performance characteristics are always measured by the engineers producing the loudspeakers and really should be published, too: frequency response phase/group delay distortion (harmonic, AM and FM, and compression) impulse/step response I would add that sensitivity is also a measure that is frequently abused, too. Using a quasi-anechoic frequency response plot calibrated to 2.83v input at 1 metre is a very good substitute for sensitivity. It would be more useful to plot current input/frequency, too. The loudspeaker efficiency could be calculated instead of sensitivity. These data are usually published as "EASE" data by cinema loudspeaker manufacturers, including Klipsch. For instance, the sensitivity of the KPT-535 Jubilee vs. frequency from Klipsch's published on-line EASE data: Chris Quote Link to comment Share on other sites More sharing options...
Schu Posted February 24, 2018 Author Share Posted February 24, 2018 is directivity measured on a 2 dimensional plain or in 3 dimensions? it seems I have seen pictures of a speaker being suspended in mid air in a chamber as it was being measured. Quote Link to comment Share on other sites More sharing options...
Chris A Posted February 24, 2018 Share Posted February 24, 2018 ...and the horizontal beam width of the KPT-535 Jubilee: Quote Link to comment Share on other sites More sharing options...
Chris A Posted February 24, 2018 Share Posted February 24, 2018 10 minutes ago, Schu said: is directivity measured on a 2 dimensional plain or in 3 dimensions? A lot of manufacturers publish DI, which combines both the vertical and horizontal measures into one measure (as Roy was apparently referring to). When you look at beam width (i.e., coverage) in the horizontal and vertical directions, you get a very different picture than just DI: The KPT-535 Jubilee vertical beam width plot: and the plot of DI... I get a lot more information out of the beam width plots. Chris Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.