Introduction
Speaker placement is critical for good sound in a space. Imagine a speaker is a spot light. Now imagine the walls, floors and ceiling in a room are mirrors. Can you imagine the “hall of mirrors” effect that happens when you turn the spotlight on? The same thing happens with sound. In this post we’ll have a look at a graphical simulation of sound in a large hall and the effect speaker placement can have.
JBL's CBT Column Speakers
JBL manufacturers a well regarded range of column speakers with CBT (Constant Beamwidth Technology) which has a vertical array of speakers to provide sound at different distances and heights. Each speaker is angled slightly differently to target a specific distance. The result is a uniform coverage of sound from close up to far away.
The CBT Calculator
JBL has created a software tool that can help visualise the sound energy as a heatmap coming from a CBT speaker in a customisable room environment. Different CBT speakers can be simulated and two angle / height combinations can be quickly compared with a radio button.
The following screenshot shows the sound energy of a JBL CBT 100LA speaker in a hall 16m deep, 7M high with a ceiling halfway up at 4M from the rear wall ending at 4M before the front of the hall (see the black horizontal line).
The speaker is placed from 3M to 4M above the floor with a downward angle of -9 degrees.
Now see the difference when the speaker is moved just 1M downwards so the speaker is placed from 2M to 3M above the floor with the same downward angle of -9 degrees.
Visually we can see there is a lot more sound energy in the distance between 0M (directly underneath the speaker) and 6M away from the front wall.
In the bottom left hand corner is a table showing the SPL (sound level) of the speaker at different frequencies and Mic positions. The mics are placed every 2M away from the front wall on the floor.
In the first screenshot table we can see the Mic 1 reading at 2M is 73.2 dB in the second column under the frequency range 1000-4000Hz, commonly found in human speech.
The Mic 2 reading is 85.2 dB at 16M away from the front speaker at floor level. This is a difference of 12 dB! (this is the equivalent of being more than twice as loud!).
Compare this to the second screenshot with the speaker just 1M lower. The Mic 1 reading is 79.2 dB, Mic 2 at 4M reads 85.6 dB, Mic 3 at 6M is 88.5 dB with Mic 6 at 85.5 dB showing much more consistent sound energy from 4M onwards and just a 6dB drop at 2M.
What does this Mean?
Placing a speaker too high drastically changes the sound energy close to the speaker at the bottom. In other words, it’s much quieter directly under the speaker than further away from it. Imagine a focused spotlight pointing to the rear wall – it would be difficult to see the light from right under the spotlight compared with further away from the spotlight.
Translating this to the real world, imagine prayers in a Mosque. The front row at 2M away from the front will have the worst sound in both scenarios. If the speaker is placed too high up as in the first example above, sound is bad right up to 8M away from the front. This is the equivalent of approximately 8 rows! In fact, we get 85.1 dB 12M away from the front in the first scenario vs 85.6 dB at 4M away from the front in the second scenario. This means moving the speaker 1M up has halved the volume at the 4th row and the 8th row will be twice as loud as the 4th row!
Moving the speaker 1M up from 2M to 3M halves the volume 4M away
Conclusion
By moving a speaker just 1M higher can have disastrous effects on sound quality. We must ensure that speaker positioning and infrastructure takes precedence about all other aesthetic considerations such as lights, air conditioners and other decorative furniture.
It is highly recommended that Sound is designed into a building at the planning stage by experts such as us to ensure a building sounds as good as it can be.