Several common questions are often asked related to loudspeaker sound reproduction, such as:
1. Why does a loudspeaker sound different when moved to another room?
2. Why does my new bookshelf loudspeaker sound terrible at home? They were great in the showroom!
3. Why does the loudspeaker sound muddy/bassy inside a room? It was great when I listened to it outside.
There are other questions, but readers can get the point from the examples above. Often a loudspeaker is being evaluated for how well it reproduces sound, but the room seldom comes into the discussion. This article discusses how loudspeakers and rooms interact.
Outdoor vs. Indoor
Impulse response (IR) is the response of a system to an impulse, usually plotted in a time domain graph (amplitude vs. time). The input impulse has specific characteristics: it contains all frequencies, it has the same energy at all frequencies, and an infinitesimally short duration.
This perfect impulse is also called a Dirac function. It’s also good to understand that a loudspeaker is usually designed, measured and specified in a free space condition. The term free space condition simply means no reflections.
Figure 1 shows a free space condition where a hand clap is recorded. The time domain graph will show one spike at a certain time. This spike is the arrival of the direct sound.
A true free space condition does not include any reflections, including floor reflection. Therefore a theoretical free space will be a sound source hanging in the air, far (greater than 10 m away) from boundaries.
When a sound source is created in confined in a room, sound will bounce from walls, floor, ceiling, and furniture, etc. Figure 2 shows a more complex time domain graph which includes reflections (blue spikes).
Side Note: When a mathematical function called fast Fourier transform (FFT) or discreet Fourier transform (DFT) is done to an IR, one can see the frequency response of the loudspeaker’s IR or room’s IR. Frequency response is a frequency domain data (amplitude vs. frequency) showing the device’s output spectrum in response to a stimulus.
Like a microphone, our ears hear all sounds in a room: direct sound and room reflections. The room reflections will affect the frequency response of the loudspeakers.
Figure 3 shows a frequency response of the same loudspeaker first measured outdoors (floor reflection is removed by a time window) and then indoors.
The black curve in Figure 3 represents the true frequency response output of the loudspeaker without any walls/floor reflections. The loudspeaker’s frequency response is +/- 1 dB at 80 Hz – 12 kHz.
This loudspeaker is later measured in a studio as shown in Figure 4.
The blue curve in Figure 3 shows the frequency response inside the studio. It looks like a comb, and isn’t smooth anymore, instead containing a lot of spikes. This is what is commonly called a comb filter.
The frequency response shows acoustical distortions due to room reflections. A common misunderstanding is thinking that electronic corrections such as EQ or other electronic processors can be used to fix/eliminate room reflections.
Acoustical distortions are best handled with acoustical treatments, finding better source/listener positions, selecting appropriate loudspeakers (for bigger rooms) and lastly, electronic correction (EQ).
We can study the effect of room reflections by observing the IR of a room. Illustratively, Figure 5 shows a room’s IR.
Side Notes: The time for direct sound to arrive is called time of flight, and the time between direct sound to the first reflection is called initial time gap/delay.
Direct sound always arrives first (Figure 5 – red spike) and the X-axis is called relative time since the reflections are observed based on the direct sound arrival.
Any reflections arriving up to 50 milliseconds (ms) after direct sound are called early reflections.
Early reflections can provide strong frequency response coloration to the sound source if the relative level greater than -20 dB to the direct sound. Beyond 50 ms, the reflections’ perception is more separated from the direct sound.
Reflections arriving 50 ms after direct sound are generally considered as reverberations. Reverberation is usually diffused in energy. Diffuse means equal energy per cubic volume of air. Reverberation time exists in a large room. Late reflections are usually arriving 50 ms after the direct sound and are noticeable due to their relatively high level.
If a clear repetition of the direct sound can be perceived, this is called an echo. Although echo may not color the direct sound as strongly as early reflection(s), it creates time domain problems and can degrade speech intelligibility especially for commercial venues or other large rooms that require a sound system for communications.