Low frequencies - omni-directional? - true or not?

[STHLS5]

We have all accepted that low frequencies below 80Hz to be omni directional. But is it true that low frequencies are really omni directional because they radiate in every direction or can there be another factor to it.

I humbly would say the statement that low frequencies to be omni directional is wrong. The more accurate statement should be low frequencies cannot be localized. But even then can this statement be true?

I have experienced difficulties in locating my ringing mobile phone if I place them in a different spot than where I normally put them. On Sunday, my phone slipped to the side of my car seat, and I had difficulties in locating where it was ringing from. I haven't measured the ringing tone frequencies but my guess would be around 5kHz. Now, we are also having difficulties localizing high frequencies. I hope you guys experienced this, too.

Isn't it a fact that all frequencies radiate in every direction? Can't we tell the direction of low frequencies played in an anechoic room? I believe we should be able to tell the direction where the low frequencies are coming from in an anechoic room Maybe, Alan has made some observation when testing the M40.1 in the woods.

If the we can pin point low frequencies in anechoic chamber then there must be another reason why low frequencies cannot be localised in our rooms.

ST

[BAS-H]

Nice question. I can't contribute any technical knowledge, so perhaps a practical analogy. You're walking home across the fields. There are storm clouds all around. You hear a distant rumble of thunder; there is no visible lightning. Can you localise the sound? Yes I'm pretty sure we can.

So we should be able to localise LF in an anechoic chamber.

But not in a listening room. I suggest this is because LF waves are reflected so much more efficiently than HF, making it impossible to distinguish between the direct and indirect waves.

So, is LF from a speaker omnidirectional? I also think this is false. LF cannot be localised? I think this is true. But not in free space.

[STHLS5]

Yes,that's what I thought about low frequencies in music room. The long but strong waves continue to bounce around in the music room making it difficult to locate. I experienced this when I moved my subwoofer to the lively living room. Perhaps, the long waves make it difficult for the ears to discern the intensity of direct and reflected sound loudness.

Now, how about other frequencies being omni direction? Why can we hear HF even when we stand behind the speakers? It may not be as loud as LF but we are still able to hear them. I believe the technical term used to refer LF being omni and HF being directional does not accurately describe the behaviour of sound waves. As I mentioned, all sound waves propagate in all directions. I maybe wrong here. I am still trying to figure out why it is sometimes difficult to locate my mobile phone. Is it possible that we sometimes expect the sound to from certain direction and that is confusing our brain.

In the case of the thunder, do you really hear them coming from the sky or are we assuming it must be from there?

ST

[Sebastien]

I'll go also with a practical experience. On the street where I live, there are sometime big trucks passing by. You know how they can generated powerful low frequencies. About them, we can state that they come from that direction. Is it because we know that the street is that way? Maybe. But there are also times where the ground is shaking and it is clear that the vibration comes from the street side.

In a listening room, I have experience with side woofers speakers (Audio Physic Virgo III), friend's Verity Audio Rienzi, back woofers and his REL sub, at a time he only had small monitor. In these three different experiences, it would be hard for me to tell you exactly the source where the low frequencies come from.

To conclude, I think like BAS-H that low frequencies waves are directional, but they are so large and so reflected that I can't localised them in a close environment. Outside is a different story.

Sebastien

P.s.: maybe a surfer is better in localise big waves...

[STHLS5]

I'll go also with a practical experience. On the street where I live, there are sometime big trucks passing by. You know how they can generated powerful low frequencies. ..

I think the low noise rumble is generated from the long exhaust pipe. I have observed that the bigger the car, especially luxury ones, the rumble is strong. Probably, it got something to do with the exhaust pipe design.

Anyone having difficulties locating ringing mobile phone? Experiment with different polyphonic ringtones. Why it is difficult to locate even higher frequencies? Something to do with the tones?

ST

[BAS-H]

I really don't know. Birdsong is another analogy. Compare the blackbird's mid-range dawn song with any small bird's HF warning call. Evolution has honed the song to leave the listener in no doubt as to what made it and from where, and the call to be as untraceable as possible. It works very well. Perhaps if a frequency is high enough, subsequent peaks reach both ears at the same moment. Hence, not locatable. Just guessing here, though.

[derekhughes]

Any frequency will be omni-directional in free air if the diameter of the radiating surface if small compared to the wavelength. If a subwoofer has a 12" driver, then at 100Hz the wavelength is around 10ft and so for all practical purposes the device itself is omni-directional. Once you place it in a room there are reflections to consider which mean that if you crossover above 100Hz problems arise with cancellations, but this is not directly due to the directionality if the device itself.

Our ability to detect the direction of a low frequency source is a different issue & is related to the spacing of our ears and the shading effects of our head. We can reliably tell direction when the phase and/or intensity difference between our ears is sufficient & this reduces as we reduce the frequency.

It is generally accepted that a frequency of around 100Hz is 'safe' for a sub to be difficult to locate as a separate source, but this also depends on positioning and being able to adjust the relative phase between it & the main speakers.

Derek

[Sebastien]

... spacing of our ears and the shading effects of our head. We can reliably tell direction when the phase and/or intensity difference between our ears is sufficient & this reduces as we reduce the frequency...

Thanks for your contribution Derek. I'm not familiar with the "shading effects". Could you elaborate on this please?

Sebastien

[STHLS5]

Any frequency will be omni-directional in free air if the diameter of the radiating surface if small compared to the wavelength.... If a subwoofer has a 12" driver, then at 100Hz the wavelength is around 10ft and so for all practical purposes the device itself is omni-directional... Our ability to detect the direction of a low frequency source is a different issue & is related to the spacing of our ears and the shading effects of our head....It is generally accepted that a frequency of around 100Hz is 'safe' for a sub ...

Thanks Derek for the explanation.

It looks like for all practical purposes SHL5 is omni directional for frequencies up to 18000Hz considering the tweeter’s diameter is only 25mm*{Mod's comment: see below}. That probably explains why I am able to hear higher test tones behind the speakers.(Not 18kHz! Which I can't hear even facing directly but probably up to 10kHz, I need to double check). There are other factors like the enclosures which likely to block most of the sound waves from radiating to the back.

However, this observation does not explain why and how we could hear the sound leaking from another person with tiny ear phones that comes with Ipod and MP3. How could we hear, usually the "chings and tings" from the ear phones when it is tightly plugged into the ears and pointing straight into the ear canals? And most importantly why are we hearing the upper frequencies rather than the lower one's, since the former are many folds longer than the radiating surface of the tiny ear phones speakers compared to the higher frequencies?


You have also mentioned about phase’s role in localizing sound source. Phase is something that fascinates me for a long time without really knowing how it works. Anyone with in-phase and out phase test tones will experience during the out of phase tones you can’t tell where the sound is coming from despite you know it is coming from the speakers. But this also gave me a clue as to why it is difficult to locate a ringing mobile phone. I am thinking it could be the phone designer intended the ringtones to be omni directional so that it could be heard loud and clear when it is ringing irrespective the speaker’s direction. Otherwise, if the speakers were directional then we would be having difficulties hearing them when it is placed with the speakers pointing to the opposite direction. Strangely, I couldn’t find anything on Google about this!

To Ben, thanks for your birdsong analogy. It gave me the right keywords to search google to find useful information about why it is difficult to locate bird’s alarm calls. It also gave another clue why mobile phones with gradual increase in ringing tone is difficult to locate. More info in James W. Kalat’s book explaning 3 cues that human use to localize sound.

ST

{Moderator's comment: NO! Completely incorrect to say SHL5 or any similar 2-3 way system is omnidirectional across the whole audio band. It isn't and if it were, it would be an acoustic disaster in an ordinary, untreated listening room. Remember the top-down plots of speaker directivity?}

[STHLS5]

..It looks like for all practical purposes SHL5 is omni directional for frequencies up to 18000Hz considering the tweeter’s diameter is only 25mm*{Mod's comment: see below}. ...

Please allow me to correct myself. I meant to say "..considering the smallest radiating surface is about 25mm and therefore it is possible for the sound to be omni-directional. Hence, explaining why higher frequencies can be heard at the back". Would have been appropriate here for the moderator to strike out the first line to avoid any confusion.

ST

[A.S.]

Please allow me to correct myself. I meant to say "..considering the smallest radiating surface is about 25mm and therefore it is possible for the sound to be omni-directional. Hence, explaining why higher frequencies can be heard at the back". Would have been appropriate here for the moderator to strike out the first line to avoid any confusion.ST

Ummm. I can't really understand your point. The fact is that no loudspeaker is completely omnidirectional - just as well - or the splashing of sound off the side walls (etc.) would sound mighty confusing to the listener at the sweet spot. Let's look at an "old fashioned" polar plot of the type that used to be published years ago.

Attached is a top-down view of a typical speaker. The speaker is (literally) mounted on a very slowly rotating turntable, taking perhaps a minute or two to turn 360 degrees. The measuring microphone is clamped in the usual position, say 1m from the tweeter at the start position i.e. on the reference axis. The test oscillator outputs one fixed frequency as the speaker completes its 360 degree rotation and as it rotates, the sound pressure generated by the loudspeaker system is collected by the microphone as a continuous voltage. That voltage drives the pen chart's ink pen mechanism which is synchronised with the turntable rotating the speaker box. Once back at 0 degrees, the oscillator frequency is reset and another rotation commences. Thus, we can clearly see how directional the speaker is by the shape of the pressure envelope or balloon.

You can see in this (good) example that at, say, 90 degrees off axis, the output at 10kHz is 15-20dB down compared to say 100Hz. If the frequency was increased to 20kHz, the balloon would be more like a pencil of sound, with almost nothing off axis (drawn in red - my guess). In fact, this is a somewhat simplistic plot and what is really interesting is what happens at the top end of the bass/midrange unit when, by definition it will be significantly beamy (due to its size etc.) yet thanks to the crossover it hands over to the bottom end of the tweeter where it will be far less directional than the bass unit. And subjectively blending that transition, dear readers, is 70% of the entire speaker's design effort in my experience.

[STHLS5]

Ummm. I can't really understand your point. The fact is that no loudspeaker is completely omnidirectional - ....

Thank you for the chart which actually confirmed my earlier statement that I am able to hear 10kHz test tones at the back of the speakers. I think when I used the word omnidirectional I am referring to sounds that can be heard at all directions. I am not saying it can be heard at equal loudness. Perhaps, the word omnidirectional is a precise technical term to describe equal loudness for all frequencies at all directions. That was not my intention.

ST

[A.S.]

Thank you for the chart which actually confirmed my earlier statement that I am able to hear 10kHz test tones at the back of the speakers. I think when I used the word omnidirectional I am referring to sounds that can be heard at all directions. I am not saying it can be heard at equal loudness.

I'll bet you one thing though .... if you took that same speaker outdoors and hoisted it up, you'd not hear any HF when standing around the back of the speaker. In other words, what you are hearing is the speaker's directional HF being bounced off hard non absorptive surfaces in the normal domestic listening room giving you the illusion that the speaker is more omnidirectional than it really is. This shouldn't be a surprise: all discussions we've had here about room treatment are concerned with absorbing just that HF splatter in the room - there is not much we can practically do to absorb LF with skimpy, thin absorbers.

[derekhughes]

There will be some difference in amplitude due to one ear being further from an of-axis source than the other, but presence of the head between the ear and the sound will increase this difference.
This is what I was referring to as the 'shading effect' of the head

Thanks for your contribution Derek. I'm not familiar with the "shading effects". Could you elaborate on this please?

Sebastien

[STHLS5]

I'll bet you one thing though .... if you took that same speaker outdoors and hoisted it up, you'd not hear any HF when standing around the back of the speaker. ..

That thought did occur to me. And I thought that was the only reason why I am hearing frequencies that I am not supposed to hear behind the speakers until Derek explained that "any frequency will be omni-directional in free air if the diameter of the radiating surface is small compared to the wavelength". Then the polar-plot chart clearly showed at 10kHz there's a drop of 17 to 18dB at 180 degrees. The test tone in the chart was fixed at 50dB so at the back it should be around 32dB. It is still within the audible level. And if the test tones were at the normal listening level of 75 to 85dB than at the back of the speakers it should be significantly higher than 32dB, and hopefully higher than the room's noise floor. I take it that the polar plot measurement was conducted in an anechoic chamber.

I do not have the means of doing a proper research in a quasi-anechoic environment like Alan is suggesting which should be a proper experiment conducted by qualified persons to put to rest if 10kHz sound can be heard at the back of a box speaker. Instead, I used a headset to run the experiment just to satisfy my curiosity. It may not have any scientific values but hopefully the science can explain the result of my observations.

First using 10kHz test tones, SE headsets held tight between inside of my fingers and cupped on a pillow. The initial sound of the headset facing me could be heard at about 1 meter away. But when I cupped it on the pillow with the headset’s speakers pointing to the pillow, the 10kHz could still be heard at around 30cm away. There was significant attenuation of the loudness but still audible. And it is louder when my head is 90 degrees off axis compare when facing directly at the headsets.

Interestingly, I also discovered that there was an initial loud “ting/ching” when the test tones start, which then disappears. The initial “ting/ching” appears in other test tones as well. The same “ting/ching” you hear when a person playing his headset very loudly. I am unsure if this explains the question that I raised earlier in the post as to why we could hear the “tings and chings” even when the headsets were plugged in tightly to the ears but this raises more questions about the micro speakers of headsets characteristics or maybe we hear a tone louder at the onset before our ears adjust to the loudness…

Please understand what I am doing here is a practical experiment as in a common situation that we face in ordinary living environment to see if it confirms to an already established theory and not doing an experiment to prove or disprove a theory.

ST

[honmanm]

Have a look at the measurement section of any Stereophile speaker review. They usually include both horizontal and vertical dispersion plots across the audio frequency range - though only for the frontal arc of the speakers.

The vertical ones are particularly interesting as they show how the effective frequency response changes with different ear heights vs. reference axis of the speakers.