Do we need to consider inter-aural differences when designing audio systems for cars?

The majority of cars being driven in the UK today have an off-centre driving position, this is to say, the driver is not sitting in the middle of the vehicle equidistant from the speakers making up the stereo payback system. Most drivers of cars in the UK sit off-centre to the right, contributing to the roughly 35% of right hand drivers in the world, leaving 65% of drivers facing the same situation of being off-centre, but in favour of the left side of their vehicle.

In a normal listening situation, the optimum position for a listener is to be an equal distance from all speakers. In a 2.0 stereo configuration, (this being a speaker setup consisting of two speakers, one for the left channel, and one for the right), the optimum listening position that will give the listener the best representation of what the artist and mixing engineer wanted you to hear, is to be equal distance from the speakers, and have the speakers equal distance from themselves. This forms an equilateral triangle where you, and the two speakers are the corners.

2.0 listening possition
It is understood that not everyone employs this level of discipline when listening to music. Music is often playing from a stereo in the corner of a room, or from a radio, and we as listeners are not always sitting down and listening. We are often moving around the room, between rooms, working, or just sitting somewhere that isn’t the optimum design location for the music. And this is okay, music still sounds good, and it cannot be expected for every listener to dedicate time to listen to a piece of music exactly how it was mixed by an engineer, although understandably people do, and this is great. But one listening situation does have us, as listens, sitting in one position for a fixed period if time, whilst driving.

Listening to music in a car

As discussed previously when we are sitting in a car we are off centre to the optimum listening position, meaning we are less likely to hear the music the way it was intended by the artist and mixing engineers.

car speaker diagram (1)

As seen in the diagram above, the listener is much closer to the speaker located at the front-right of the vehicle than any other. Whilst listening to music mixed in 2.0 stereo, the listener will  have a disrupted stereo image as information from the right speaker will arrive before information from the left.

The below measurements were made using a 2001 model Volkswagen Polo hatchback.

Distance measured from drivers position to speakers inside the car:

L – 128.5 cm (left ear)
R- 97.5 cm (right ear)
L rear – 119 cm (left ear)
R rear – 77 cm (right ear)

L front tweet – 120 cm (left ear)
R front tweet – 78.5 cm (right ear)
L rear tweet – 96 cm (left ear)
R rear tweet – 40 cm (right ear)

Height measurements from the ground the car is sitting on:

L – 19.5 cm
R – 19.5 cm
L rear – 18.5 cm
R rear – 18.5 cm

L front tweet – 96 cm
R front tweet – 96 cm

L rear tweet – 84 cm
R rear tweet – 84 cm

Average human head – 114.5 cm

If we focus solely on the listening position in relation to the L and R speakers, best representing the L and R in a home stereo set-up, we can see that there is an imbalance in the distance between the speakers and the listener of 31 cm between mid, and 41.5 cm between tweeters. Treating the collective door speakers as one loud-speaker, in fashion with home loudspeaker setups, give us an average of 36.25 cm difference in space between the L and R channels in relation to the listening position.

The reason stereo sound works well is the assumption of a phantom sound source between the two speakers. If a sound is played at the same volume, and at the same time from both L and R speakers, the source of the sound will appear to be coming from directly between the two speakers. When we delay the sound, or increase/decrease the volume in one channel we can move this phantom sound image between the two speakers.
Dr Francis Rumsey tells us that a delay of 1.1 ms in the L or R channel, will movie the phantom image 30° (full pan in a stereo set-up).
To calculate the inter-aural time difference for the VW Polo example we do the following;

ITD (inter-aural time difference)
SOS (speed of sound)
DLD – (distance to left ear direct)
DRD – (distance to right ear direct)

ITD = (0.36m) / 343.216 m/s
ITD = 1.04890215 ms

As the VW polo features a time difference of 1.05 ms between left and right doors, the listener wont be listening to the music as originally intended, and is very close to the 1.1 ms delay needed to shift the phantom source completely to one side of a 2.0 stereo image. But is it actually detrimental to the listening experience?

Rear door speakers

Geoff Martin states that between the rear speakers in a 5.1 configuration (Ls and Rs) as set out according to ITU-BS 775 (standard 5 channel speaker configuration), a time delay of only 0.6 ms is needed to make a sound source appear fully to one side. Due to the driving position in the VW Polo used for this example, and most cars with a standard 2 front and 3 rear seating position, the closest speaker to the driver is the rear right tweeter, followed by the right rear mid. Again counting the doors as a single speaker setup by seeing any given door as a speaker enclosure, the distance to the average point between the speakers in the rear right door to the driver is 58.5 cm. Using the same calculations we can see that the average distance to the rear left door from the driving position is 107.5 cm, meaning the inter-aural time difference between the two rear doors is 2.42 ms, far beyond the 0.6 ms needed to move the phantom source image in rear channels to one side of the mic completely.


Experiment Stimuli

I am currently running an experiment and collecting data for this investigationtion.


Check back later.


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