# Simple qn on sound

1. Jan 6, 2005

### gunblaze

Simple qn on sound....

:rofl:

It goes like this...
A person is standing on a bus-stop.An ambulance with it's siren on came approaching the bus-stop and then passing by it. The article says that when the ambulance is still far away, the distance between the person and the ambulance is still far away, therefore the wavelength is stretched over a great distance. thus making the sound frequency very low. But when the ambulance come closer to the bus-stop, the wavelength is compressed. Thus making the frequency very high.<True/False>

Note: This isn't the article's main content...It is just using it as an example...

Personally, i feel that it sounds rather "absurd". But i hope someone can really advise me on whether this statement is true or false & WHY? ... I appreciate all ans.Thx

...I thought only the amplitude will differ..?

Last edited: Jan 6, 2005
2. Jan 6, 2005

### cepheid

Staff Emeritus
^^^This bit, in particular, sounds like nonsense to me.

It sounds like they are trying to describe the Doppler effect, but getting it all wrong! The perceived frequency of a stationary source of sound depends only on the frequency of the source. A stationary ambulance 100m away would sound with the same pitch as if it were 10m away, as far as I know. *EDIT yes of course, you are right. The amplitude would be greater for the closer source.* As for moving sources, the perceived frequency changes depending on the relative motion (not the distance!) between source and observer/listener. If the ambulance is approaching, then the sound waves are compressed (distances between successive wavefronts are shorter), leading to a steady increase in the perceived frequency (and pitch of the sound). As the ambulance recedes, then the waves are elongated (distances between successive wavefronts increasing) leading to a steady decrease in frequency.

I have a related ambulance question. In Europe, they have those funky ambulance sirens that blast out two tones back and forth...and the Doppler effect is definitely perceptible because as an ambulance approaches, both tones seem to increase in pitch.

In North America, we have the up down cycling siren that makes a "weeeeeiouuuu" type of sound. For some reason, I can never perceive the Doppler effect. The weeeeiouuuu seems unchanged no matter whether the ambulance is coming or going. Why is that?

Does anyone understand what the hell I am talking about?

3. Jan 6, 2005

### Q_Goest

Cepheid, you've explained it much better than the article that was quoted.

Just a few additional thoughts though. The speed of sound through air is a constant. For example: Let's say the air isn't moving relative to the ground, then the speed of the sound waves coming at you is roughly 760 mph at sea level. This is true regardless of how fast the source is moving. So if the source is moving at 60 mph toward you, the sound waves are still coming at you at 760 mph, but the frequency is slightly higher because the source of the sound is closer to you when it emits the next sound wave. So the wavelength of the sound shortens (ie: higher frequency) and this shorter wavelength is percieved as an increase in pitch.

4. Jan 6, 2005

### tozhan

as for the 'funky ambulance', in the UK we have ambulance...nothing new, but they have 2 or 3 different sirens. first they do have that 'whhheeeiiiooo' noise and the one that has 2 tones. Both seems to work well as far as my sound perception goes! i can still tell there is a dopler effect with either siren. The only time you notice it will be when the ambulance passes you as this changes the propagation of the wavelength. Just out of interest the other siren is more of a screaming wolf whistle style and is usually used when they reach a junction which is packed with cars and they need a way through or, as is my experience, when they sneak quietly into the lane next to you and want a new heart attack victim... :)

5. Jan 6, 2005

### DaveC426913

"In North America, we have the up down cycling siren that makes a "weeeeeiouuuu" type of sound. For some reason, I can never perceive the Doppler effect. The weeeeiouuuu seems unchanged no matter whether the ambulance is coming or going. Why is that?"

I suspect it's because NA sirens are a continuous tone, sliding up and down the scale. Since the Doppler effect has the same ... effect ..., the two are indistinguishable.

Say the siren's tone when coming towards you slides continuously between middle C and high C - eight notes. The ambulance passes, reverse the Doppler effect. The tone slides between middle B to high B, still eight notes, but almost all the notes are the same. The only places where you can tell the difference are the missing high C and the added middle B. That's a small window for perception.

Compare that to the two-tone siren. It alternates between only two notes. Middle C and high C under the influence of the Doppler Effect become middle B and high B. There are zero similar notes - that's 100% of the tone in which there's a change in pitch.

6. Jan 6, 2005

### cepheid

Staff Emeritus
ahahaha...that's quite funny. In both of those situations here (approaching intersection/traumatizing drivers), our ambulances (Canada) instead blast their horn at you, a horn that is only slightly softer than that of a boat like the QE2, it seems, and definitely more garish.

btw Dave, that explanation was awesome. It makes perfect sense to me.

7. Jan 7, 2005

### gunblaze

But i thought that when the ambulance approaches, only the no of waves produced will be lessened but not the wavelengths of these waves???

8. Jan 7, 2005

### DaveC426913

"But i thought that when the ambulance approaches, only the no of waves produced will be lessened..."

If the number of waves got less, that would create a lowering in pitch. (Two waves over a one mile distance is a much longer frequency than 10 waves over a one mile distance).

But the pitch is highest as the source is approaching, so no.

9. Jan 7, 2005

### Q_Goest

Gunblaze, the number of waves produced is dependant on how fast some noise making device inside the siren is vibrating. Every time the noise making thing moves one way it pushes air along with it and makes a wave. Every time the noise making thingy moves back, it leaves a slight vacuum where it had been. By making a full cycle, one push on the air, and one pull (so to speak, not actually pulling) but after one such cycle, it starts over, push/pull, push/pull, each time moving the air and making a "sound wave". So the number of waves doesn't change unless the mechanical thing making the noise changes how fast it vibrates.

Imagine riding along inside the vehicle or riding right next to the siren in some way (sticking yer head out the moon roof). Your perception of the siren would not change, there would be no difference in tone. It's only how your vantage point changes in relation to the noise emitter that results in a change to the frequency.

10. Jan 12, 2005

### gunblaze

oh, ok...I get it!

Thx guys, really. thanks